Shale gas: hydraulic fracturing is not as bad as it is painted. Fracking or hydraulic fracturing: technology, history, equipment What is oil production by fracking

British researchers analyzed the method of hydraulic fracturing (HF, a method of intensifying the work of oil and gas wells) from the point of view of its safety for the environment, economy and society. As a result, the hydraulic fracturing method was placed seventh out of nine energy sources. Perhaps a similar study will be carried out in America - in the only country in the world where the hydraulic fracturing method in oil production is now considered one of the main ones.

Low Security

Hydraulic fracturing is a controversial process in which high-pressure water, sand, and chemicals are injected into a formation, resulting in fractures that facilitate oil and/or gas production.

In order to evaluate the consequences of using hydraulic fracturing in the UK, a group of scientists from the University of Manchester ranked energy sources (among them coal, wind, sunlight), assessing the safety of their use from the point of view of the environment, economy and society. Scientists placed the hydraulic fracturing method at the seventh position in the rating.

Scientists report that in order for the hydraulic fracturing method to be as safe as wind and solar energy, it is necessary to reduce its negative impact on environment as much as 329 times.

The researchers made various forecasts for the future and determined that the situation in which the fracturing method will account for 1, rather than 8 percent of electricity generated in the UK, is more favorable.

Fracking in context

Scientists say that most of the research related to hydraulic fracturing is aimed at studying its impact on the environment. These studies are mainly conducted in the USA. British experts argue that the socio-economic aspect has not been sufficiently studied. They call their research project the first work that examines the impact of hydraulic fracturing on the environment, economy and society.

“This allows us to evaluate the safety of using the method as a whole, without focusing only on one aspect like transport, noise or water pollution, which are now actively discussed in the study of shale gas,” Adiza Azapadzhik, professor at the University of Manchester, told The Independent.

In some states, the hydraulic fracturing method is banned, and at the moment America is the only country that uses it on a large scale. Perhaps the British study will encourage American experts to conduct their own analysis. If the safety of hydraulic fracturing is rated as low in America, then politicians may turn to less hazardous energy sources.

V modern industry hydraulic fracturing (HF) is an effective method of influencing the bottomhole area of ​​a well. This method is necessary to increase the productive return from an oil or gas field, the degree of absorption of injection varieties of wells, and also as part of groundwater isolation work. The process of hydraulic fracturing itself includes the creation of new fractures and the increase in existing ones that lie in the bottom hole rock. The impact on fractures occurs by adjusting the pressure of the fluid supplied to the well. As a result of hydraulic fracturing, it becomes possible to extract valuable resources located at a remote distance from the wellbore from the well.

From the history of hydraulic fracturing

Developments to increase the productivity of oil production from finished wells were carried out in the States already at the end of the 19th century: then a method of stimulation was tested by means of an explosion of nitroglycerin, which broke up solid rocks and made it possible to obtain valuable resources from there. In the same period, tests were carried out on the development of the bottomhole zone using acid, and the latter method was actively used in the 30s of the last century.

During the use of acid to stimulate well productivity, it was found that increasing pressure can lead to formation fractures. This began the development of the idea of ​​hydraulic fracturing, and the first attempt was made already in 1947. Despite the failure, the researchers continued to develop the method, and their work was crowned with success two years later. In the 1950s, the United States increasingly began to develop using the method of hydraulic fracturing, and by the last third of the 20th century, the number of such operations exceeded one million only in America itself.

Hydraulic fracturing as a well development technique was also used in the USSR: the first attempts were made in 1959. After that, the popularity of this method began to fade, since wells began to be developed in Siberia, which, even without additional manipulations, ensured uninterrupted production of oil and gas in the required volumes. Since the late 80s, the technique has become widespread again, when the former deposits ceased to produce the same amount of valuable resources, but could not yet be considered completely exhausted. Currently, the technique of hydraulic fracturing is used throughout Russia, as well as in other states.

Varieties of hydraulic fracturing

In the modern field of resource development, two types of hydraulic fracturing are distinguished:

• Proppant hydraulic fracturing. With this method, a special wedging material is used. During the procedure, the proppant is poured in so that the cracks created by pressure do not reconnect. This type of method is well suited for sandstones, siltstones and other terrigenous rocks. Hydraulic fracturing with proppant is the most commonly used.
• Hydraulic fracturing using acid. This method is more suitable for carbonate rocks, and the cracks that are obtained by a combination of pressure increase and the addition of a fracturing fluid do not need additional reinforcement, as in the first case. The main difference between acid fracturing and conventional fracturing with the same acid is the amount of material and the degree of pressure.

Regardless of the type of treatment, the success of hydraulic fracturing depends on a number of factors. First of all, the object for the implementation of the method must be selected taking into account its features, types of reservoirs, as well as the depth and intensity of development. The choice of technology depends on the conditions in which the well is located. When applied correctly, the efficiency of oil recovery in a treated well becomes much higher.

The process of hydraulic fracturing

Hydraulic fracturing is advisable to carry out for wells with low productivity, which occurs due to the natural density of the layers or when the quality of filtration decreases after the opening of the next layer.

The processing process takes several stages:

• Study of the well, during which its absorption capacity, pressure resistance and other parameters are determined.
• Well cleaning. For this, drainage pumps are used and the wellbore is washed so that the filtration properties in the bottomhole area are sufficient for further work. Also, the well can be treated with hydrochloric acid so that the conditions for the formation of fractures from rupture are optimal.
• Descent into the well of pipes for supplying fluid to the bottomhole. The casing string is equipped with a packer and a hydraulic anchor so that the pressure does not deform the pipe. The mouth is equipped with a head for connecting equipment that is necessary for pumping flushing fluid.
• The hydraulic fracturing itself is performed by injecting fluid until cracks appear in the formation. Immediately after hydraulic action, it is required to pump liquid at high speed.
• The mouth is blocked, the well is not touched until the pressure decreases.
• Well flushing after hydraulic fracturing and development.

At a shallow depth, hydraulic fracturing can be carried out without tubing pipes or without a fuse. In the first situation, injection is carried out through casing pipes, and in the second, it can be organized along the ring around them. This technique minimizes pressure loss when a very thick liquid is used in the process. In addition, for some wells, multi-stage fracturing is carried out, in which different layers get cracks, due to which their permeability increases greatly.

To determine the location of the fractures themselves, the method of radioactive logging is used. This technology allows you to find out exactly where the gaps are, with the introduction of ordinary and charged sand.

The "shale revolution" is obviously taking over the minds of politicians and businessmen all over the world. The Americans hold the palm in this area, but, apparently, there is a possibility that the rest of the world will soon join them. Of course, there are states where there is practically no shale gas production - in Russia, for example, the main percentage of political and business elites are rather skeptical about this undertaking. At the same time, the matter is not so much in the factor of economic profitability. The most important circumstance that can affect the prospects of such an industry as shale gas production is the consequences for the environment. Today we will study this aspect.

What is shale gas?

But first, a little theoretical digression. What is a shale mineral that is extracted from a special type of minerals - The main method by which shale gas is extracted, the consequences of which we will study today, guided by the positions of experts, is fracking, or hydraulic fracturing. It's set up like this. A pipe is inserted into the bowels of the earth in an almost horizontal position, and one of its branches is brought to the surface.

In the process of fracking, pressure is built up in the gas storage, which causes shale gas to escape to the top, where it is collected. The most popular extraction of the mentioned mineral has gained in North America. According to some experts, the industry's revenue growth in the US market over the past few years has amounted to several hundred percent. However, unconditional economic success in terms of developing new methods of producing "blue fuel" may be accompanied by huge problems associated with the extraction of shale gas. They are, as we have already said, ecological in nature.

Harm to the environment

What the US and other energy powers should, according to experts, pay special attention to when working in such an area as shale gas production is the consequences for the environment. The most important threat to the environment is fraught with the main method of extracting minerals from the bowels of the earth. We are talking about the same fracking. It, as we have already said, is a supply of water into the earth's layer (under very high pressure). This kind of impact can have a pronounced negative impact on the environment.

Reagents in action

Technological features of fracking are not the only character. Current methods of extracting shale gas involve the use of several hundred varieties of reactive, and potentially toxic, substances. What does this mean? The fact is that the development of the corresponding deposits requires the use of large volumes of fresh water. Its density, as a rule, is less than that characteristic of groundwater. And therefore, light layers of liquid, one way or another, can eventually rise to the surface and reach the mixing zone with drinking sources. However, they are likely to contain toxic impurities.

Moreover, it is possible that light water will return to the surface contaminated not with chemical, but with completely natural, but still harmful to human health and the environment, substances that may be contained in the depths of the earth's interior. An indicative moment: it is known that it is planned to produce shale gas in Ukraine, in the Carpathian region. However, experts from one of the scientific centers conducted a study, during which it turned out that the layers of the earth in those regions that are supposed to contain shale gas are characterized by an increased content of metals - nickel, barium, uranium.

Technology miscalculation

By the way, a number of experts from Ukraine urge to pay attention not so much to the problems of shale gas production in terms of the use of harmful substances, but to the shortcomings in the technologies used by gas companies. Representatives of the scientific community of Ukraine in one of their reports on environmental issues put forward the relevant theses. What is their essence? The conclusions of scientists, in general, boil down to the fact that shale gas production in Ukraine can cause significant damage to soil fertility. The fact is that with those technologies that are used to isolate harmful substances, some materials will be located under arable soil. Accordingly, it will be problematic to grow something above them, in the upper layers of the soil.

Ukrainian bowels

There are also concerns among Ukrainian experts about the possible consumption of drinking water reserves, which can be a strategically significant resource. At the same time, already in 2010, when the shale revolution was just gaining momentum, the Ukrainian authorities issued licenses for shale gas exploration to companies like ExxonMobil and Shell. In 2012, exploration wells were drilled in the Kharkiv region.

This could indicate, experts believe, the interest of the Ukrainian authorities in the development of "shale" prospects, probably in order to reduce dependence on the supply of blue fuel from the Russian Federation. But now it is not known, analysts say, what are the future prospects for work in this direction (due to well-known political events).

Problem fracking

Continuing the discussion about the shortcomings of shale gas production technologies, one can also pay attention to other noteworthy theses. In particular, some substances can be used in fracking. They are used as fracturing fluids. At the same time, their frequent use can lead to a significant deterioration in the degree of rock permeability for water flows. In order to avoid this, gas workers can use water that uses soluble chemical derivatives of substances similar in composition to cellulose. And they pose a serious threat to human health.

Salts and radiation

There were precedents when the presence of chemicals in the waters in the area of ​​shale wells was recorded by scientists not only in the calculated aspect, but also in practice. After analyzing the water flowing into the sewage treatment plant in Pennsylvania, experts found a much higher than normal level of salts - chlorides, bromides. Some of the substances found in water can react with atmospheric gases such as ozone, resulting in the formation of toxic products. Also, in some layers of the subsoil located in areas where shale gas is produced, the Americans discovered radium. Which is, therefore, radioactive. In addition to salts and radium, in the waters that are concentrated in areas where the main method of extracting shale gas (fracking) is used, scientists have discovered various kinds of benzenes and toluene.

legal loophole

Some lawyers point out that the environmental damage caused by American shale gas companies is almost legal in nature. The fact is that in 2005, a legal act was adopted in the United States, according to which the fracking method, or hydraulic fracturing, was withdrawn from the monitoring of the Environmental Protection Agency. This department, in particular, ensured that American businessmen acted in accordance with the provisions of the Drinking Water Protection Act.

However, with the adoption of a new legal act, US enterprises were able to operate outside the Agency's control zone. It has become possible, experts say, to extract shale oil and gas in close proximity to underground sources of drinking water. And this is despite the fact that the Agency, in one of its studies, concluded that the sources continue to become contaminated, and not so much during the fracking process, but some time after the work is completed. Analysts believe that the law was passed not without political pressure.

Freedom in Europe

A number of experts emphasize that not only the Americans, but also the Europeans do not want to understand the dangers of shale gas production in the potential. In particular, the European Commission, which develops sources of law in various areas of the EU economy, did not even begin to create a separate law regulating environmental issues in this industry. The agency limited itself, analysts emphasize, to just issuing a recommendation that does not actually bind energy companies to anything.

At the same time, according to experts, the Europeans are not yet too keen on the earliest possible start of work on the extraction of blue fuel in practice. It is possible that all those discussions in the EU that are connected with the "shale" topic are just political speculations. And in fact, the Europeans, in principle, are not going to develop gas production unconventional method. At least in the near future.

Complaints without satisfaction

There is evidence that in those areas of the United States where shale gas is being produced, the consequences of an environmental nature have already made themselves felt - and not only at the level of industrial research, but also among ordinary citizens. Americans living next to wells where fracking is used began to notice that tap water had lost a lot of quality. They are trying to protest against shale gas production in their area. However, their capabilities, according to experts, are not comparable with the resources of energy corporations. The business scheme is quite simple. When there are claims from citizens, they form by hiring environmentalists. In accordance with these documents, drinking water must be in perfect order. If the residents are not satisfied with these papers, then, as reported by a number of sources, the gas workers pay them pre-trial compensation in exchange for signing non-disclosure agreements on such transactions. As a result, the citizen loses the right to report something to the press.

The verdict will not burden

If litigation is nevertheless initiated, then decisions that are not made in favor of energy companies are in fact not very burdensome for gas companies. In particular, according to some of them, corporations undertake to supply citizens with drinking water from environmentally friendly sources at their own expense or install treatment equipment for them. But if in the first case the affected residents, in principle, can be satisfied, then in the second - as experts believe - there may not be much reason for optimism, since some can still seep through the filters.

The authorities decide

There is an opinion among experts that interest in shale in the US, as well as in many other countries of the world, is largely political. This, in particular, may be evidenced by the fact that many gas corporations are supported by the government - especially in such an aspect as tax incentives. Experts assess the economic viability of the "shale revolution" ambiguously.

Drinking water factor

Above, we talked about the fact that Ukrainian experts question the prospects for shale gas production in their country, largely due to the fact that fracking technology may require spending large amounts of drinking water. I must say that similar concerns are expressed by experts from other states. The fact is that even without shale gas, it is already being observed in many regions of the planet. And it is likely that a similar situation may soon be observed in developed countries. And the "shale revolution", of course, will only help accelerate this process.

Ambiguous slate

There is an opinion that shale gas production in Russia and other countries is not developed at all or, at least, does not occur at the same pace as in America, just because of the factors we have considered. These are, first of all, the risks of environmental pollution with toxic, and sometimes radioactive, compounds that occur during fracking. It is also the probability of depletion of drinking water reserves, which may soon become a resource, even in developed countries, in terms of importance not inferior to blue fuel. Of course, the economic component is also taken into account - there is no consensus among scientists on the profitability of shale deposits.

Currently, hard-to-recover reserves are widely involved in development oil confined to low-permeability, poorly drained, heterogeneous and dissected reservoirs.

One of the effective methods for increasing the productivity of wells penetrating such formations and increasing the recovery rate oil of these, is hydraulic fracturing (HF). Hydraulic fracturing can be defined as a mechanical method of impact on a productive formation, in which the rock is ruptured along planes of minimum strength due to the action on the formation of pressure created by the injection of fluid into the formation. The fluids that transfer the energy required for fracturing from the surface to the bottom of the well are called fracturing fluids.

After rupture, under the influence of fluid pressure, the fracture increases, its connection with the system of natural fractures not penetrated by the well, and with zones of increased permeability; thus, the reservoir area drained by the well expands. A granular material is transported into the cracks formed by the fracturing fluids, which fixes the cracks in the open state after the excess pressure is removed.

As a result, the production rate of producing wells or the injectivity of injection wells increases manifold due to a decrease in hydraulic resistance in the bottomhole zone and an increase in the filtration surface of the well, as well as an increase in the final oil recovery due to the involvement in the development of poorly drained zones and interlayers.

The hydraulic fracturing method has many technological solutions, due to the characteristics of a particular treatment object and the goal to be achieved. Hydraulic fracturing technologies differ primarily in the volume of injection of process fluids and antlers and, accordingly, in the size of the fractures created.

The most widely used local hydraulic fracturing as an effective means of influencing the zone of wells. In this case, it is sufficient to create fractures 10–20 m long with injection of tens of cubic meters of liquid and units of tons of proppant. In this case, the well flow rate increases by 2.3 times.

In recent years, technologies for creating relatively small fractures in medium- and high-permeability formations have been intensively developed, which makes it possible to reduce the resistance of the bottomhole zone and increase the effective radius of the well.

Hydraulic fracturing with the formation of extended fractures leads to an increase in not only the permeability of the bottomhole zone, but also the coverage of the formation by the impact, and the involvement of additional reserves in the development oil and raising oil recovery generally. At the same time, it is possible to reduce the current water cut of the produced products. The optimal length of a fixed fracture with a formation permeability of 0.01...0.05 µm2 is usually 40...60 m, and the injection volume is from tens to hundreds of cubic meters of fluid and from a few to tens of tons of proppant.

Along with this, selective hydraulic fracturing is used, which makes it possible to involve in the development and increase the productivity of low-permeability layers.

To get involved in industrial development gas reservoirs with ultra-low permeability (less than 10 µm 2) in the USA, Canada and a number of Western European countries, they successfully use the technology of massive fractures.

Experience of using hydraulic fracturing abroad

For the first time in oil In practice, hydraulic fracturing was carried out in 1947 in the USA. The technology and theoretical ideas about the hydraulic fracturing process were described in the work of J. Clark in 1948, after which this technology quickly became widespread. By the end of 1955, more than 100,000 hydraulic fracturings had been performed in the USA As the theoretical knowledge of the process improved and technical characteristics improved equipment, fracturing fluids and proppants, the success of operations has reached 90%. By 1968, more than a million operations had been performed in the world. In the United States, the maximum well stimulation operations by hydraulic fracturing was noted in 1955 - approximately 4500 hydraulic fracturing / month, by 1972 the number of operations decreased to 1000 hydraulic fracturing / month, and by 1990 it had already stabilized at the level of 1500 operations / month.

The technology of hydraulic fracturing is primarily based on knowledge of the mechanism of fracture initiation and propagation, which makes it possible to predict the fracture geometry and optimize its parameters. The first fairly simple models defining the relationship between the fracture fluid pressure, the plastic deformation of the rock and the resulting length and opening of the fracture met the needs of practice as long as hydraulic fracturing operations did not require large investments. The introduction of massive hydraulic fracturing, which requires a large flow rate of fracturing fluids and proppant, has led to the need to create more advanced two- and three-dimensional models that allow more reliable prediction of treatment results. Currently, pseudo-3D models, which are a combination of two well-known two-dimensional models that describe crack growth and fluid flow in it in two mutually perpendicular directions.

The most important factor in the success of the hydraulic fracturing procedure is the quality of the fracturing fluid and proppant. The main purpose of the fracturing fluid is to transfer from the surface to the bottom of the well the energy necessary to open the fracture and transport the proppant along the entire fracture. The main characteristics of the fracturing fluid-proppant system are:

Rheological properties of "clean" liquid and liquid containing proppant;

Infiltration properties of the fluid, which determine its leakage into the reservoir during hydraulic fracturing and proppant transfer along the fracture;

The ability of the fluid to carry the proppant to the fracture ends in suspension without premature settling;

The ability to easily and quickly carry out the fracturing fluid to ensure minimal contamination of the proppant pack and the surrounding formation;

Compatibility of the fracturing fluid with various additives provided by the technology, possible impurities and reservoir fluids;

Physical properties of the proppant.

Technological fracturing fluids must have sufficient dynamic viscosity to create high conductivity fractures due to their large opening and efficient proppant filling; have low seepage leakage to produce cracks required sizes at minimal cost liquids; ensure a minimum decrease in the permeability of the formation zone in contact with the fracturing fluid; ensure low pressure losses due to friction in pipes; have sufficient thermal stability for the formation being treated and high shear stability, i.e. shear stability of the liquid structure; easy to be taken out of the reservoir and hydraulic fractures after treatment; be technologically advanced in preparation and storage in field conditions; have low corrosivity; be environmentally friendly and safe to use; have a relatively low cost.

The first fracturing fluids were oil based, however, since the late 1950s, water-based fluids have been used, the most common of which are guar gum and hydroxypropyl guar. Currently, over 70% of all hydraulic fracturing in the US is performed using these fluids. Gels on oil basis are used in 5% of cases, foam with compressed gas used in 25% of all hydraulic fracturing. To increase the efficiency of hydraulic fracturing, various additives are added to the fracturing fluid, mainly anti-filtration agents and friction reducing agents.

Fracturing failures in low permeability gas formations are often caused by the slow removal of the fracturing fluid and its blocking of the fracture. As a result, the initial flow gas after hydraulic fracturing, it can be 80% lower than the steady-state over time, since the increase in well production occurs extremely slowly as the fracture is cleaned - over weeks and months. In such formations, the use of a mixture of hydrocarbon fracturing fluid and liquefied carbon dioxide or liquefied CO is especially important; with the addition of nitrogen. Carbon dioxide is introduced into the reservoir in a liquefied state, and is carried out in the form gas. This makes it possible to accelerate the removal of fracturing fluid from the formation and prevent such negative effects, which are most pronounced in low-permeability gas reservoirs, such as blocking the fracture with fracturing fluid, deterioration of phase permeability for gas near a fracture, changes in capillary pressure and rock wettability, etc. The low viscosity of such fracturing fluids is compensated during hydraulic fracturing operations by a higher injection rate.

Modern materials used to fix cracks in the open state - proppants - can be divided into two types - quartz sands and synthetic proppants of medium and high strength. The physical characteristics of proppants that affect fracture conductivity include strength, grain size and particle size distribution, quality (presence of impurities, solubility in acids), grain shape (sphericity and roundness), and density.

The first and most widely used fracture fixing material is sand, which has a density of approximately 2.65 g/cm 2 . Sands are commonly used in hydraulic fracturing where compressive stress does not exceed 40 MPa. Medium-strength are ceramic proppants with a density of 2.7...3.3 g/cm 3 used at a compression stress of up to 69 MPa. Heavy-duty proppants, such as sintered bauxite and zirconium oxide, are used at compression stresses up to 100 MPa, the density of these materials is 3.2...3.8 g/cm 3. The use of heavy-duty proppants is limited by their high cost.

In addition, the so-called supersand is used in the USA - quartz sand, the grains of which are coated with special resins that increase strength and prevent the removal of crumbled proppant particles from the fracture. The density of supersand is 2.55 g/cm3. Synthetic resin-coated proppants are also produced and used.

Strength is the main criterion when selecting proppants for specific reservoir conditions in order to ensure long-term fracture conductivity at the reservoir depth. In deep wells, the minimum stress is horizontal, therefore, predominantly vertical fractures are formed. With depth, the minimum horizontal stress increases by approximately 19 MPa/km. Therefore, in terms of depth, proppants have the following areas of application: quartz sands - up to 2500 m; medium strength proppants - up to 3500 m; high strength proppants - over 3500 m.

Recent studies conducted in the United States have shown that the use of medium-strength proppants is cost-effective even at depths of less than 2500 m, since the increased costs due to their higher cost compared to quartz sand are offset by gains in additional oil production by creating a proppant pack of higher conductivity in the hydraulic fracture.

The most commonly used proppants with granule sizes are 0.425...0.85 mm (20/40 mesh), less often 0.85...1.7 mm (12/20 mesh), 0.85...1.18 mm (16/20 mesh), 0.212...0.425 mm (40/70 mesh). The choice of the desired proppant grain size is determined by a whole range of factors. The larger the granules, the greater the permeability of the proppant pack in the fracture. However, the use of coarse fraction proppant is associated with additional problems when it is transferred along the fracture. The strength of the proppant decreases with increasing grain size. In addition, in weakly cemented reservoirs, it is preferable to use a proppant of a finer fraction, since due to the removal of particles from the formation, the coarse-grained proppant package gradually becomes clogged and its permeability decreases.

The roundness and sphericity of the proppant granules determine the density of its packing in the fracture, its resistance, as well as the degree of destruction of the granules under the action of rock pressure. Proppant density determines proppant transport and placement along the fracture. High density proppants are more difficult to maintain in suspension in the fracturing fluid as they are transported along the fracture. Fracture filling with high-density proppant can be achieved in two ways - using high-viscosity fluids that transport the proppant along the length of the fracture with minimal settling, or using low-viscosity fluids at an increased injection rate. In recent years, foreign firms have begun to produce lightweight proppants characterized by reduced density.

Due to the wide variety of fracturing fluids and proppants available in the US market, the US oil Institute (API) has developed standard methods for determining the properties of these materials (API RP39; Prud "homme, 1984, 1985, 1986 - for fracturing fluids, and API RP60 - for proppants).

At present, the United States has accumulated vast experience in hydraulic fracturing, with increasing attention being paid to the preparation of each operation. The most important element of such training is the collection and analysis of primary information. The data required for the preparation of the hydraulic fracturing can be divided into three groups:

Geological and physical properties of the reservoir (permeability, porosity, saturation, reservoir pressure, position gas and oil and oil-water contacts, rock petrography);

Fracture geometry and orientation characteristics (minimum horizontal stress, Young's modulus, fracture fluid viscosity and density, Poisson's ratio, rock compressibility, etc.);

Properties of fracturing fluid and proppant. The main sources of information are geological, geophysical and petrophysical studies, laboratory analysis of the core, as well as the results of a field experiment, which consists in conducting micro- and mini-hydraulic fracturing.

In recent years, a technology has been developed for an integrated approach to hydraulic fracturing design, which is based on taking into account many factors, such as reservoir conductivity, well spacing, fracture mechanics, fracturing fluid and proppant characteristics, technological and economic limitations. In general, the hydraulic fracturing optimization procedure should include the following elements:

Calculation of the amount of fracturing fluid and proppant required to create a fracture of the required size and conductivity;

Technique to determine the optimal injection parameters, taking into account proppant characteristics and technological limitations;

A complex algorithm that allows optimizing the geometric parameters and conductivity of the fracture, taking into account the productivity of the reservoir and the well spacing system, providing a balance between the filtration characteristics of the reservoir and the fracture, and based on the criterion of maximizing profit from well treatment.

The creation of an optimal hydraulic fracturing technology implies compliance with the following criteria:

Ensuring optimization of field reserves development;

Maximizing the depth of proppant penetration into the fracture:

Optimization of fracturing fluid and proppant injection parameters;

Minimization of processing cost;

Profit maximization by obtaining additional oil and gas. In accordance with these criteria, the following stages of hydraulic fracturing optimization at the facility can be distinguished:

1. Selection of wells for treatment, taking into account the existing or projected development system, ensuring maximization oil production and gas while minimizing costs.

2. Determination of the optimal fracture geometry - length and conductivity, taking into account the formation permeability, well spacing system, well distance from gas- or oil-water contact.

3. Selection of a fracture propagation model based on the analysis of the mechanical properties of the rock, stress distribution in the reservoir and preliminary experiments.

4. Selection of proppant with appropriate strength properties, calculation of proppant volume and concentration required to obtain a fracture with desired properties.

5. Selection of a fracturing fluid with suitable rheological properties, taking into account the characteristics of the reservoir, proppant and fracture geometry.

6. Calculation required amount fracturing fluid and determining the optimal injection parameters, taking into account the characteristics of the fluid and proppant, as well as technological limitations.

7. Calculation of the economic efficiency of hydraulic fracturing.

Joint efforts of the American gas Research Institute (GRI) and the largest oil and gas US companies (Mobil Oil Co., Amoco Production Co., Schiumberger, etc.) have developed a new technological complex that includes a mobile equipment GRI for testing and quality control of the hydraulic fracturing operation, a GRI rheology tool, FRACPRO 3D fracture "design" software, reservoir stress profiling tools and microseismic techniques for determining fracture height and azimuth.

The use of new technology makes it possible to select the fracturing fluid and proppant that best suits specific conditions, and control the propagation and opening of the fracture, the transportation of proppant in suspension along the entire fracture, and the successful completion of the operation. Knowledge of the stress profile in the reservoir allows not only to determine the fracture pressure, but also to predict the fracture geometry. With a high stress difference in the reservoir and in impermeable barriers, the fracture propagates to a greater length and lower height than in a reservoir with an insignificant difference in these stresses. Accounting for all information in a three-dimensional model allows you to quickly and reliably predict the geometry and filtration characteristics of the fracture. Approbation of a new hydraulic fracturing technology at six gas US fields (in Texas, Wyoming and Colorado) showed its high efficiency for low-permeability reservoirs.

In some cases, hydraulic fracturing occurs at significantly lower pressures than the initial stresses in the formation. Formation cooling as a result of injection into injection wells of cold water, which differs significantly in temperature from the formation, leads to a decrease in elastic stresses and hydraulic fracturing in injection wells at bottomhole pressures used in waterflooding. Studies conducted at the Prudhoe Bay field (USA) showed that the half-length of fractures that appeared in this way ranges from 6 to 60 m. hydraulic break.

When performing hydraulic fracturing in deviated wells, the direction of which deviates from the fracture plane, there are problems associated with the formation of several fractures from different perforation intervals and with the curvature of the fracture near the well. To create a single flat fracture in such wells, a special technology is used based on limiting the number of perforations, determining their size, number and orientation with respect to the directions of principal stresses in the reservoir.

In recent years, technologies for the use of hydraulic fracturing in horizontal wells have been developed. The orientation of the fracture relative to the axis of the well is determined by the direction of the horizontal wellbore relative to the azimuth of the minimum principal stress in the reservoir. If the horizontal wellbore is parallel to the direction of minimum principal stress, then transverse fractures are formed during hydraulic fracturing. Technologies for creating several fractures in one horizontal well have been developed. In this case, the number of cracks is determined taking into account technological and economic restrictions and is usually 3.-.4.

The first field experiment to create multiple fractures in a deviated well was conducted by Mobil in the 60s. Fracturing in oil horizontal wells were carried out in fields in the Danish part of the North Sea. On the gas field in the North Sea (Netherlands) in a reservoir with a permeability of 1-10 -3 microns 2 in a horizontal well created two transverse fractures.

The largest project was carried out on gas Solingen field in the North Sea (Germany), characterized by ultra-low permeability (10-6 ... 10 -4 µm2), average porosity of 10...12% and average formation thickness of about 100 m. In a horizontal well with a length of 600 m, four transverse fractures, the half-length of each of which is about 100 m. The peak flow rate of the well was 700 thousand m 3 / day, at present the well is operating with an average flow rate of 500 thousand m 3 / day.

If the horizontal section of the well is parallel to the direction of maximum horizontal stress, the fracture will be longitudinal with respect to the axis of the well. A longitudinal fracture cannot give a significant increase in the production rate of a horizontal well, but a horizontal well with a longitudinal fracture itself can be considered as a very high conductivity fracture. Given that the increase in conductivity is the determining factor in increasing the flow rate of wells with fractures in medium- and high-permeability formations, when developing such formations, it is possible to use hydraulic fracturing in horizontal wells with the formation of longitudinal fractures. Experimental work to determine the effectiveness of longitudinal fractures, carried out at the Kuparuk River field (Alaska) in four horizontal wells, showed that productivity increased by an average of 71%, and costs by 37%. In all cases, the choice between designing vertical wells with hydraulic fracturing, horizontal wells or horizontal wells with hydraulic fracturing is based on an assessment of the economic efficiency of a particular technology.

Pulse fracturing technology allows creating several fractures radially extending from the wellbore in the well, which can be effectively used to overcome the skin effect in the bottomhole zone, especially in medium and high permeability formations

Hydraulic fracturing of medium- and high-permeability formations is one of the most intensively developing well stimulation methods at present. In high-permeability formations, the main factor in increasing well production due to hydraulic fracturing is the width of the fracture, in contrast to low-permeability formations, where such a factor is its length. Used to create short wide cracks

proppant tip screening (TSO-tip screen out) technology, which consists in pushing the proppant first of all to the end of the fracture by gradually increasing its concentration in the working fluid during treatment. Deposition of proppant at the end of the fracture prevents its growth in length. Further injection of proppant-bearing fluid leads to an increase in the fracture width, which reaches 2.5 cm, while in conventional hydraulic fracturing, the fracture width is 2–3 mm. As a result, the effective fracture conductivity (the product of permeability and width) is 300...3000 µm 2m. To prevent proppant runaway during the subsequent exploitation wells, TSO technology is typically combined with either a tar-coated proppant that sets and resists viscous friction during prey, or with gravel pack, when the proppant is held in the fracture with a filter (Frac-and-Pack). The same technology is used to prevent crack propagation to water oil contact. TSO technology is successfully applied at the Prudhoe Bay field (USA), in the Gulf of Mexico, Indonesia, and the North Sea.

The creation of short wide fractures in wells penetrating medium and high permeability formations gives good results with a significant deterioration of reservoir properties in the bottomhole zone as a means of increasing the effective radius of the well; in multilayer sand reservoirs, where a vertical fracture provides a continuous connection of thin sand interlayers with a perforation zone; in reservoirs with migration of fine particles, where sand is prevented by reducing the flow velocity near the wellbore; v gas formations to reduce the negative effects associated with flow turbulence near the well. To date, more than 1 million successful hydraulic fracturings have been performed in the United States, more than 40% of the well stock has been processed, resulting in 30% of the reserves oil and gas transferred from off-balance sheet to industrial. In North America, growth oil production as a result of the use of hydraulic fracturing, it amounted to about 1.5 billion m 3.

In the late 1970s, with the creation of new durable synthetic proppants, a rise in the field of hydraulic fracturing began on gas and oil deposits of Western Europe, confined to dense sandstones and limestones located at great depths. The first half of the 80s coincided with the second peak period in hydraulic fracturing operations in the world, when the number of treatments per month reached 4800 and was mainly aimed at tight gas collectors. In Europe, the main regions where massive hydraulic fracturing has been and is being carried out are concentrated in the fields of Germany, the Netherlands and Great Britain in the North Sea, and on the coast of Germany, the Netherlands and Yugoslavia. Local hydraulic fracturing is also carried out in the Norwegian fields of the North Sea, in France, Italy, Austria and in the countries of Eastern Europe.

The largest work on massive hydraulic fracturing was undertaken in Germany in gas-bearing seams located at a depth of 3000...6000 m at a temperature of 120...180 °C. Basically, medium and high-strength artificial ones were used here. in Germany, several dozens of massive hydraulic fracturing were carried out. The proppant consumption in this case was in most cases about 100 in a third of cases - 200 t/well, and during the largest operations it reached 400...650 t/well. The length of the cracks varied from 100 to 550 m, the height from 10 to 115 m. In most cases, the operations were successful and led to an increase in the flow rate by 3...10 times. Failures in individual hydraulic fracturing were mainly associated with high water content in the reservoir.

Fixation of hydraulic fractures in oily layers, as opposed to gas-containing, was carried out mainly with the use of sand, since the depth of these formations is only 700 ... 2500 m, only in some cases medium-strength proppants were used. On the oil in the fields of Germany and the Netherlands, the proppant consumption was 20...70 t/well, and in the Vienna Basin of Austria, the optimal proppant consumption was only 6...12 t/well. Both old and new production wells were successfully processed with good isolation of neighboring intervals.

Gas UK deposits in the North Sea provide about 90% of the country's demand for gas and maintain a dominant role in gas supply until the end of the century. Proppant consumption during hydraulic fracturing in gas-bearing sandstones, located at depths of 2700.-.3000 m, was 100 ... 250 t / well. . Moreover, if at first the cracks were fixed either with sand, or with medium- or high-strength synthetic proppant, then since the beginning of the 80s, the technology of successive injection of proppants into the crack, differing both in fractional composition and in other properties, has become widespread. According to this technology, 100...200 tons of sand with a grain size of 20/40 mesh were first pumped into the fracture, then 25...75 tons of medium-strength proppant with a grain size of 20/40 or 16/20. In some cases, the three-fraction method was successfully used with sequential injection of proppants 20/40, 16/20 and 12/20 or 40/60, 20/40 and 12/20.

The most common variant of two-fraction hydraulic fracturing consisted of pumping the main volume of sand or medium-strength proppant of type 20/40, followed by injection of medium- or high-strength proppant of type 16/20 or 12/20 in the amount of 10...40% of the total volume. There are various modifications of this technology, in particular, good results are obtained by initially pumping fine-grained sand type 40/70 or even 100 mesh into the fracture, then the main amount of sand or proppant type 20/40, and completing the fracture with strong coarse-grained proppant 16/20 or 12 / twenty. The advantages of this technology are as follows:

Fixing the fracture with high-strength proppant in the vicinity of the well, where the compressive stress is the highest;

Reducing the cost of the operation, since ceramic proppants are 2...4 times more expensive than sand;

Creation of the highest conductivity of the fracture in the vicinity of the bottomhole, where the fluid filtration rate is maximum;

Prevention of proppant flow into the well, provided by a special selection of the difference in the grain sizes of the main and finishing proppants, in which grains of smaller size are retained at the boundary between the proppants;

Blocking with fine-grained sand of natural microfractures branching from the main, as well as the end of the fracture in the formation, which reduces the loss of fracturing fluid and improves the conductivity of the fracture.

Proppants injected into different areas of a fracture can differ not only in fractional composition, but also in density. In Yugoslavia, massive hydraulic fracturing technology has found application, when a light, medium-strength proppant is first pumped into a fracture, and then a heavy, higher-quality, high-strength proppant.

Lightweight proppant stays in suspension in the fluid transporting it longer, so it can be delivered to a longer distance along the fracture walls. The injection of a heavier high-quality proppant at the final stage of hydraulic fracturing allows, on the one hand, to provide compressive strength in the area of ​​the highest stresses near the bottomhole, and on the other hand, to reduce the risk of failure of the operation at the final stage, since the light proppant has already been delivered to the fracture. Massive hydraulic fracturing carried out in Yugoslavia. are among the largest in Europe, since at the first stage 100...200 tons of light proppant were pumped into the fracture, and at the second stage - approximately 200...450 tons of heavier proppant. Thus, the total amount of proppant was 300...650 tons.

As a result oil crisis of 1986, the scope of work on hydraulic fracturing decreased significantly, but after stabilization of prices for oil in 1987 - 1990 an increasing number of fields are planned for hydraulic fracturing, while increased attention has been paid to the optimization of hydraulic fracturing technology, the effective selection of fracture and proppant parameters. The highest activity in conducting and planning hydraulic fracturing in Western Europe is observed in the North Sea at gas deposits in the British sector and in oil-bearing Cretaceous deposits in the Norwegian sector.

The importance of hydraulic fracturing technology for Western European fields is proved by the fact that booty a third of stocks gas here it is possible and economically justified only with hydraulic fracturing. For comparison, in the USA, 30...35% of hydrocarbon reserves can be extracted only with the use of hydraulic fracturing.

The specifics of the development of offshore fields determines the higher cost of well stimulation operations, therefore, in order to ensure higher reliability in 1989-1990. The decision was made to phase out the use of sand as a proppant in British fields in the North Sea. Especially for a long time and widely used sand as a proppant in Yugoslavia, Turkey, countries of Eastern Europe and the USSR, where they had their own equipment for hydraulic fracturing, but there was no sufficient capacity for the production of expensive synthetic proppants. Thus, in Yugoslavia and Turkey, medium-strength proppant was used only for fracture completion, and the main volume was filled with sand. However, in recent years, due to the creation of joint ventures, the expansion of the sale of proppants by Western manufacturing companies to direct consumers, and the development of their own production, the situation is changing. In China, hydraulic fracturing is carried out with the injection of bauxite proppant of its own production in a volume of up to 120 tons. It is shown that even a low concentration of bauxite provides better fracture conductivity than more high concentration sand. There are broad prospects for the use of hydraulic fracturing technology in the fields of North Africa, India, Pakistan, Brazil, Argentina, Venezuela, Peru. In the fields of the Middle East and Venezuela, confined to carbonate reservoirs, acid fracturing should become the main technology. It should be noted that in most third world countries natural sand is used as a proppant, the use of synthetic proppants is envisaged only in Algeria and Brazil.

In the domestic oil production Hydraulic fracturing began to be used since 1952. The total number of hydraulic fracturing in the USSR during the peak period of 1958-1962. exceeded 1500 operations per year, and in 1959 it reached 3000 operations, which had high technical and economic indicators. Theoretical and field-experimental studies on the study of the mechanism of hydraulic fracturing and its effect on well flow rates date back to the same time. In the subsequent period, the number of hydraulic fracturing operations decreased and stabilized at about 100 operations per year. The main hydraulic fracturing centers were concentrated in the fields of the Krasnodar Territory, the Volga-Ural region, Tataria (Romashkinskoye and Tuymazinskoye fields), Bashkiria, Kuibyshev region, Chechen-Ingushetia, Turkmenistan, Azerbaijan, Dagestan, Ukraine and Siberia.

Hydraulic fracturing was carried out mainly for the development of injection wells during the introduction of in-loop waterflooding and, in some cases, for oil wells. In addition, hydraulic fracturing has been used to isolate bottom water inflows in monolithic wells; while a horizontal hydraulic fracture created in a preselected interval was used as a water barrier. Massive hydraulic fracturing was not carried out in the USSR. With the equipment of the fields with more powerful equipment for water injection, the need for widespread hydraulic fracturing in injection wells disappeared, and after the large high-yield fields of Western Siberia were put into development, interest in hydraulic fracturing in the industry practically disappeared. As a result, from the beginning of the 70s to the end of the 80s in the domestic oil production hydraulic fracturing has not been used on an industrial scale.

The revival of domestic hydraulic fracturing began in the late 80s due to a significant change in the structure of reserves oil and gas .

Until recently, only natural sand in the amount of up to 130 t/well was used as a proppant in Russia, and in most cases 20...50 t/well was pumped. Due to the relatively shallow depth of the treated formations, there was no need to use synthetic high-quality proppants. Until the end of the 1980s, when performing hydraulic fracturing, mainly domestic or Romanian oil was used. equipment, in some cases - American.

There is now ample potential for the introduction of large-scale hydraulic fracturing operations in low-permeability gas-bearing formations in the fields of Siberia (depth - 2000...4000 m), Stavropol (2000...3000 m) and Krasnodar (3000...4000 m) regions. Saratov (2000 m). Orenburg (3000...4000 m) and Astrakhan (Karachaganak field (4000...5000 m)) regions.

V oil production Russia pays great attention to the prospects of using the hydraulic fracturing method. This is primarily due to the growth trend in the structure of reserves oil share of reserves in low-permeability reservoirs. More than 40% of the industry's recoverable reserves are located in reservoirs with a permeability of less than 5-10-2 µm2, of which about 80% are in Western Siberia. By the year 2000, such reserves in the industry are expected to grow up to 70%. Intensification of development of unproductive deposits oil can be carried out in two ways - by compacting the well pattern, which requires a significant increase in capital investments and increases the cost oil, or an increase in the flow rate of each well, i.e. intensification of use as reserves oil, as well as the wells themselves.

World experience oil production shows that one of the effective methods of intensifying the development of low-permeability reservoirs is the hydraulic fracturing method. Highly conductive hydraulic fractures make it possible to increase the productivity of wells by 2...3 times, and the use of hydraulic fracturing as an element of the development system, i.e., the creation of a hydrodynamic system of wells with hydraulic fractures, increases the rate of recovery of recoverable reserves, increases oil recovery due to the involvement of poorly drained zones and interlayers in active development and an increase in flooding coverage, and also allows you to put into development deposits with a potential well flow rate 2...3 times lower than the level of cost-effective prey, therefore, convert part of the off-balance reserves into "commercial" ones. The increase in well production after hydraulic fracturing is determined by the ratio of the reservoir and fracture conductivity and the size of the latter, and the well productivity index does not increase indefinitely with the fracture length, there is a limit value of the length, exceeding which practically does not lead to For example, with a reservoir permeability of about 10-2 µm2, the limiting half-length is approximately 50 m.

For the period 1988-1995. more than 1,600 hydraulic fracturing operations have been performed in Western Siberia. The total number of development objects covered by hydraulic fracturing exceeded 70. For a number of objects, hydraulic fracturing has become an integral part of the development and is carried out in 50...80% of the production wells. Thanks to hydraulic fracturing at many facilities, it was possible to achieve a cost-effective level of well flow rates for oil. The increase in flow rates averaged 3.5 with fluctuations for various objects from 1 to 15. The success rate of hydraulic fracturing exceeds 90%. The vast majority of well-operations were carried out by specialized joint ventures using foreign technologies and on a foreign equipment. Currently, the volume of hydraulic fracturing in Western Siberia has reached the level of 500 well-operations per year. The share of hydraulic fracturing in low-permeability reservoirs (Jurassic deposits, Achimov pack) is 53% of all operations.

Over the years, certain experience has been gained in conducting and evaluating the effectiveness of hydraulic fracturing in various geological and physical conditions. Extensive experience in hydraulic fracturing has been accumulated at JSC Yuganskneftegaz. Analysis of the effectiveness of more than 700 hydraulic fracturing performed by JV "YUGANSKFRAKMASTER" in 1989-1994. on 22 layers of 17 fields of JSC "Yuganskneftegaz", showed the following.

The main objects of hydraulic fracturing were deposits with low-permeability reservoirs: 77% of all treatments were carried out on objects with formation permeability less than 5-10-2 µm2, of which 51% - less than 10-2 µm2 and 45% - less than 5-10 µm2.

First of all, hydraulic fracturing was carried out on an inefficient well stock: on inactive wells - 24% of the total volume of work, on marginal wells with a fluid flow rate of less than 5 t/day - 38% and less than 10 t/day - 75%. Anhydrous and low-water (less than 5%) well stock accounts for 76% of all hydraulic fracturing. On average, over the period of generalization for all treatments as a result of hydraulic fracturing, the fluid flow rate increased from 8.3 to 31.4 t/day, and for oil- from 7.2 to 25.3 t/day, i.e. 3.5 times with an increase in water cut by 6.2%. As a result, additional oil production due to hydraulic fracturing amounted to about 6 million tons over 5 years. The most successful results were obtained during hydraulic fracturing in pure oil facilities with a large oil-saturated thickness (Achimov pack and B1 formations of the Prirazlomnoye field), where the fluid flow rate increased from 3.5...6.7 to 34 tons/day with an increase in water cut by only 5...6%.

The experience of hydraulic fracturing of discontinuous formations, represented mainly by individual reservoir lenses, was obtained at the TPP "LUKoil-Kogalymneftegaz" at the Povkhovskoye field. Interlayers of the discontinuous zone are penetrated by two adjacent wells at an average distance of 500 m only in 24% of cases. The main task of regulating the development system of the Povkhovskoye field is to involve the discontinuous zone of reservoir 1 in active work and accelerate the rate of reserves development along it. To this end, in the field in 1992-1994. carried out by JV "KATKONEFT" 154 hydraulic fracturing. The success of the treatments was 98%. At the same time, a five-fold increase in production rate was obtained on average for the treated wells. The volume of additionally produced oil amounted to 1.6 million tons. The expected average duration of the technological effect is 2.5 years. At the same time, additional booty due to hydraulic fracturing per well should amount to 16 thousand tons. According to SibNIINP, by the beginning of 1997, 422 hydraulic fracturing operations had already been carried out at the field, the success of which was 96%, the volume of additionally produced oil- 4.8 million tons, the average increase in well flow rate - 6.5 times. The average ratio of fluid flow rate after hydraulic fracturing in relation to the maximum flow rate achieved before hydraulic fracturing and characterizing the potential of the well was 3.1.

At the fields of TPP "LUKoil-Langepasneftegaz" during 1994-1996. 316 hydraulic fracturing operations were carried out, in 1997 - another 202 hydraulic fracturing. Processing is carried out by own forces and JV "KATKONEFT". Additional oil production amounted to about 1.6 million tons, the average increase in flow rate was 7.7 tons per day per well.

In 1993, pilot work on hydraulic fracturing at the fields of OAO Noyabrskneftegaz began, 36 operations were performed during the year. The total volume of hydraulic fracturing by the end of 1997 amounted to 436 operations. Hydraulic fracturing was carried out, as a rule, in marginal wells with low water cut, located in areas with degraded reservoir properties. After hydraulic fracturing flow rate oil increased by an average of 7.7 times, liquid - by 10 times. As a result of hydraulic fracturing, in 70.4% of cases, water cut increased on average from 2% before hydraulic fracturing to 25% after treatment. The success of treatments is quite high and averages 87%. Additional oil production by the end of 1997, from the production of hydraulic fracturing at JSC Noyabrskneftegaz exceeded 1 million tons. Dowell Schiumberger is one of the world's leading well stimulation companies. Therefore, her work on hydraulic fracturing in Russian fields is of great interest. This company prepared the draft of the first Soviet-Canadian experiment on massive hydraulic fracturing at the Salymskoye field. For example, in one of the wells in a reservoir with a permeability of 10^ µm^, a fracture with a half-length of 120 m was designed with a total height of 36.6 m. After hydraulic fracturing in the Bazhenov formation in the summer of 1988, the well began to 17 days decreased to 18 m^/day. Prior to hydraulic fracturing, the inflow was "non-overflowing", i.e. the liquid level in the well did not rise to its mouth.

In 1994, Dowell Schiumberger performed several dozen hydraulic fracturing operations at the Novo-Purpeiskoye, Tarasovskoye and Kharampurskoye fields of JSC Purneftegaz. In the period up to October 1, 1995, 120 hydraulic fracturing operations were carried out at the fields of OJSC "Purneftegaz". The average daily flow rate of treated wells was 25.6 tons/day. Since the beginning of hydraulic fracturing, 222.7 thousand tons of additional oil. Data on well flow rates approximately one year after hydraulic fracturing: in the second half of 1994, 17 operations were carried out at the fields of Purneftegaz OJSC; average well flow rate oil before hydraulic fracturing was 3.8 t/day, and in September 1995 -31.3 t/day. For some wells, a decrease in water cut was noted. The introduction of hydraulic fracturing made it possible to stabilize the falling oil production for NGDU "Tarasovskneft".

An analysis of the results of the introduction of hydraulic fracturing in the fields of Western Siberia shows that this method is usually used in single-selected production wells. The generally accepted approach to assessing the effectiveness of hydraulic fracturing is to analyze the dynamics oil production only treated wells. At the same time, flow rates before hydraulic fracturing are taken as base, and additional booty calculated as the difference between actual and base booty for this well. When making a decision to conduct hydraulic fracturing in a well, the effectiveness of this measure is often not considered, taking into account the entire reservoir system and the arrangement of production and injection wells. Apparently, the negative consequences of the use of hydraulic fracturing, noted by some authors, are associated with this. For example, according to estimates, the use of this method in certain areas of the Mamontovskoye field caused a decrease in oil recovery due to a more intensive increase in water cut in some treated and especially surrounding wells. An analysis of the hydraulic fracturing technology at the fields of OJSC "Surgutneftegas" showed that failures are often associated with an irrational choice of treatment parameters, when the injection rate and volumes of process fluids and proppant are determined without taking into account such factors as the optimal length and width of a fixed fracture calculated for given conditions; burst pressure of clay screens separating the reservoir from the overlying and underlying gas- and water-saturated layers. As a result, the potential of hydraulic fracturing as a means of increasing prey, the water cut of the extracted products increases.

Experience in acid hydraulic fracturing is available at the Astrakhan gas condensate a deposit, the productive deposits of which are characterized by the presence of dense porous-fractured limestones with low permeability (0.1 ... 5.0) and porosity 7 ... 14. The use of hydraulic fracturing is complicated by large depths operational wells (4100 m) and high bottomhole temperatures (110 °C). In progress exploitation wells, the formation of local depression funnels and a decrease in reservoir pressure in some cases to 55 MPa from the initial 61 MPa occurred. These phenomena may result in condensate dropout in the bottomhole zone, incomplete removal of fluid from wellbores, etc. To improve the filtration characteristics of the bottomhole zone of low-rate wells, massive acid treatments are periodically carried out with injection parameters close to hydraulic fracturing. Such operations make it possible to reduce working drawdowns by 25...50% of the initial ones, slow down the growth rate of depression funnels and the rate of decrease in wellhead and bottomhole pressures.

Hydraulic fracturing at the Astrakhanskoye field is carried out using a special equipment firm "FRAKMASTER". The technology of work, as a rule, was as follows. Initially, the well injectivity was determined by injection of methanol or condensate. Then, in order to equalize the injectivity profile and create conditions for treating less permeable areas with an acid composition and connecting the formation to work, gel was injected throughout its entire thickness. A mixture of hydrochloric acid with methanol or a hydrophobic acid emulsion ("hydrochloric acid in a hydrocarbon medium") was used as an active fluid reacting with the formation. During interval hydraulic fracturing, high-permeability zones or perforation channels were clogged either with gel or balls with a diameter of 22.5 mm together with the gel. The moment of hydraulic fracturing was recorded on the indicator diagram by a sharp increase and subsequent drop in pressure with a simultaneous increase in injectivity. It is possible that already existing fractures opened in some wells, since the fact of hydraulic fracturing was not noted on the indicator diagrams, and the pressures corresponded to the fracture opening pressure gradient. The practice of hydraulic fracturing at the Astrakhan gas condensate field has shown its high efficiency, subject to the correct choice of wells and technological processing parameters. A significant increase in production rate was obtained even in cases where several acid treatments were carried out on the well before hydraulic fracturing, the last of which turned out to be ineffective.

The highest efficiency of hydraulic fracturing can be achieved when designing its application as an element of the development system, taking into account the well placement system and assessing their mutual influence with various combinations of treatment of production and injection wells. The effect of hydraulic fracturing is manifested differently in the operation of individual wells, so it is necessary to consider not only the increase in the production rate of each well due to hydraulic fracturing, but also the influence of the relative position of wells, the specific distribution of reservoir heterogeneity, the energy capabilities of the object, etc. Such an analysis is possible only on the basis of three-dimensional mathematical modeling the process of developing a section of a reservoir or an object as a whole using an adequate geological and field model that reveals the features of the geological heterogeneity of the object. Via computer model development process using hydraulic fracturing, it is possible to assess the feasibility of hydraulic fracturing in injection wells, the impact of hydraulic fracturing on oil and gas recovery and the rate of development of reserves of the development object, to identify the need for re-treatment, etc. In the industrial implementation of hydraulic fracturing, it is first necessary to draw up a project document that would justify the hydraulic fracturing technology, linked to the reservoir development system as a whole. When conducting hydraulic fracturing, it is necessary to provide for a set of field studies at priority wells to determine the location, direction and conductivity of the fracture, which will make it possible to make adjustments to the hydraulic fracturing technology, taking into account the characteristics of each specific object. There is a need for systematic supervision of the implementation of hydraulic fracturing, which will make it possible to take prompt measures to increase its effectiveness.

The factors determining the success of hydraulic fracturing are the correct choice of an object for operations, the use of hydraulic fracturing technology that is optimal for given conditions, and the competent selection of wells for treatment.

Basic concepts of the hydraulic fracturing method

Definition. Hydraulic fracturing is a process in which fluid pressure acts directly on the reservoir rock until it is destroyed and a crack occurs. Continued exposure to fluid pressure expands the fracture down from the fracture point. A proppant such as sand, ceramic beads, or agglomerated bauxite is added to the injected fluid. The purpose of this material is to keep the created fracture open after fluid pressure is released. This creates a new, more spacious inflow channel. The channel integrates existing natural fractures and creates additional well drainage area. The fluid that transmits pressure to the formation rock is called the fracturing fluid.

Problems solved during hydraulic fracturing

During hydraulic fracturing, the following tasks should be solved:

A) creating a crack

B) keeping the crack open

B) removal of fracturing fluid

D) increase in reservoir productivity

Creation of a crack

A fracture is created by pumping fluids of a suitable composition into the formation at a rate that exceeds its absorption by the formation. The fluid pressure increases until the internal stresses in the rock are overcome. A crack forms in the rock.

Holding the crack open

Once the fracture has begun to propagate, proppant (usually sand) is added to the fluid and carried into the fracture by the fluid. After the fracturing process is completed and the pressure is released, the proppant keeps the fracture open and therefore permeable to formation fluids.

Fracture Fluid Removal

Before you start prey from the well, the fracturing fluid must be removed. The degree of complexity of its removal depends on the nature of the applied fluid, the pressure in the formation and the relative permeability of the formation to the fracturing fluid. The removal of the fracturing fluid is very important because, by lowering the relative permeability, it can create obstacles to the flow of fluids.

Improving reservoir productivity

Prior to designing a process, a feasibility study should be carried out.

Purpose of hydraulic fracturing

Hydraulic fracturing has two main objectives:

one). Increase reservoir productivity by increasing the effective well drainage radius. In reservoirs with relatively low permeability, hydraulic fracturing is the best way to increase productivity.

2). Create an inflow channel in the near-wellbore zone of disturbed permeability.

Reservoir permeability disturbance is an important concept to understand because the type and extent of the fracturing process is designed specifically to correct this disturbance. If it is possible to create a fracture filled with proppant passing through the damaged zone and bring the pressure drop to the normal value of the hydrodynamic pressure gradient, then the productivity of the well will increase.

Violation of the permeability of the productive formation. Usually, a reservoir permeability disturbance is identified with “skin damage”, that is, with a bottom hole zone permeability disturbance. However, this value cannot always be determined through measurements or skin calculations. It is common to take a skin factor (a factor that determines the degree of reservoir damage) equal to zero to indicate that there is no damage to the reservoir, but this does not actually mean that there is no damage. For example, an acid treatment may penetrate deep enough into the formation over several meters in the upper part of a 20-meter perforation interval that positive skin removal can be detected in surveys. However, in this case, the positive part of the interval can be partially clogged with mechanical impurities or drilling solution. The true potential productivity of this well could be many times greater than its production at measured zero skin.

Reservoir permeability may be affected by physical or chemical factors or their joint action: clogging of pores with a solution, changes in the wettability of the formation due to the intrusion of water from an external source. Ordinary water barrier caused by excess liquid absorption is a kind of permeability failure. A similar result causes formation water intrusion from another zone or from another section of the reservoir.

Here are some forms of formation permeability disturbance:

one). Particle intrusion drilling solution.

2). Filtrate intrusion drilling solution.

3). Cement filtrate intrusion into the formation.

4). Perforation discrepancy in size, number and penetration depth of holes.

5). Destruction of perforation and compaction of the parent rock.

6) Trash in the completion fluid or kill fluid that penetrates the formation or clogs perforations.

7). Invasion of completion or kill fluids into the formation.

eight). Plugging the formation with natural clays.

9). Deposition of asphaltenes or paraffins in a formation or perforation.

10). Salt deposits in the formation or perforation.

11). Formation or injection of an emulsion into the reservoir.

12). Injection of acids or solvents with mechanical impurities or deposition of mechanical impurities in the formation.

All this can lead to a decrease in productivity, and in severe cases - to a complete cessation. prey from the well. Some types of stimulation can help.

Effect of disturbed permeability on well productivity. Most types of permeability disturbance lower the initial permeability of the formation. The effect of this decrease on productivity depends on the depth of damage to the zone surrounding the wellbore.

If, for example, there is a 50% decrease in permeability in a 5 cm thick layer, then this will lead to a decrease in productivity of only 14%. If the decrease in permeability covered a 30-cm layer, productivity will decrease by 40%. A 75% reduction in permeability in a 30cm section would result in a 64% loss in productivity. Therefore, a well that should produce 100 cubic meters per day, but the permeability of the reservoir within a radius of 30 cm from the wellbore is only 25% of the initial prey, oil will be only 36 m3 / day.

Reservoir models (both mathematical and physical laboratory models) can be used to study the effect of formation damage on productivity. It is important to remember that no effort should be spared to minimize the depth and severity of formation damage.

Low permeability. Initially, hydraulic fracturing was introduced as an economic means of increasing gas production from reservoirs with relatively low pressure. In low-permeability (up to 10 md) formations, a high-permeability channel (100 - 1000 darcy) of inflow is created. This provides large drainage areas, into which hydrocarbons are slowly replenished from a reservoir with very low permeability. Thus, all the energy of the reservoir is used to the maximum. The bearing capacity of the formation fluid has a significant impact on the expected results of hydraulic fracturing of various types and sizes.

Fracture direction.

The rupture crack may be oriented in a horizontal or vertical direction. The type of fracture that can occur under specific conditions depends on the stress in the reservoir. The rupture occurs in the direction perpendicular to the lowest stress.

vertical break. Most wells are vertically fractured. The rupture crack forms two wings oriented at an angle of 180° to each other.

vertical gap

Horizontal break. A horizontal fracture occurs in a well if the horizontal stress is greater than the vertical stresses.

Horizontal gap

Fracture fluids

The most important part of hydraulic fracturing design is the selection of the fracturing fluid. In doing so, the following factors should be considered:

Compatibility with formation and formation fluids.

1) Violation of reservoir permeability

During hydraulic fracturing, fluid is absorbed in the zone adjacent to the fracture surface. Due to the increased liquid saturation of the invasion zone, the relative permeability of the formation fluid decreases. If the formation fluid permeability is low and the fracture fluid is even lower, this can lead to complete blocking of the flow. In addition, there may be heaving clays in the formation that swell upon contact with the fracturing fluid and reduce permeability.

2) Violation of the permeability of the sand plug

The permeability of a sand plug, as well as liquid intrusion zones, can be compromised by liquid saturation. The inflow through the fracture can also be limited by the presence of residual mechanical impurities or polymers in the sand plug after exposure.

3) Reservoir fluids

Many liquids tend to form emulsions or precipitate. Laboratory tests should be carried out in order to avoid risk when choosing the right chemical components.

Price.

The spread in cost for different fracturing fluids is quite different. Water is the cheapest, while methanol and acids are quite expensive. The cost of the gelling component should also be taken into account. In any case, it is necessary to compare the benefits of treating the reservoir with appropriate fluids and chemicals with their cost (Table 11).

Table 11

Comparative cost of various liquids (USD)

Fracture fluid name	Price 1 cubic meter	The cost of 1 cubic meter gelling component	Total cost
THICKENED WATER		66,00	66.00
POLYMER WATER		126,00	126,00
THICKENED REFORM	250,00	94,00	344,00
TWO-PHASE LIQUID	50,00	66,00	116,00
METHANOL+CO2	350,00	150,00	500,00
POLYMERIC METHANOL	400,00	210,00	610,00
LIQUID CO2	300,00		300,00
ACID 15%	380,00	200,00	580,00
ACID 28%	750,00	250,00	1000,00

Types of liquids

Water based fluids. Water-based fracturing fluids are used in most treatments today. Although this was not the case in the early years of hydraulic fracturing, when fluids oil basis were used in virtually all treatments. This type of liquid has a number of advantages over liquid on oil basis.

1. Water-based fluids are more economical. The basic component - water is much cheaper than oil, condensate, methanol and acid.

2. Water-based fluids give more hydrostatic effect than oil, gas and methanol.

3. These liquids are non-flammable; hence they are not explosive.

4.Water based fluids are readily available.

5. This type of liquid is easier to control and thicken.

Linear fracturing fluids. The need for thickening the water to help transport the proppant, reduce fluid loss, and increase the width of the fracture was obvious to early investigators. The first water thickener was starch. In the early 1960s, a replacement was found - guar glue - a polymeric thickener. It is used even today. Other linear gels are also used as fracturing fluids: hydroxypropyl, hydroxyethyl cellulose, carboxymethyl, xanthan and in some rare cases polyacrylamides.

Connecting fracturing fluids. They were first used in the late 1960s, when hydraulic fracturing received a lot of attention. The development of this type of fluid solved many of the problems that arose when it was necessary to pump linear gels into deep, high temperature wells. The coupling reaction is such that the molecular weight of the base polymer increases to a large extent by binding together the various polymer molecules into a structure. The first bonding liquid was guar glue. A typical bonding gel in the late 1960s consisted of 9586 g/m3 guar binder with borite antimony. The antimony environment was relatively low pH in the fracturing fluid. The boron environment was from high rate pH. Many other fluids of this type have also been developed, such as aluminum, chromium, copper, and manganese. Additionally, in the late 1960s and early 1970s, a CMC (carboxymethylcellulose)-based binder and some types of hydroxytylcellulose-based binder began to be used, although the latter was expensive. With the development of hydroxypropyl guar and carboxymethylhydroxyethyl cellulose polymers, a new generation of connectors has also been developed. The polymer molecules of the connector tend to increase the thermal stability of the base polymer. It theorizes that this thermal stability results from a reduction in the thermal instability of the molecule as a result of its most homogeneous nature and some protection from hydrolysis, oxidation, or other depolymerization reactions that may occur. Connector polymers, although they increase the apparent viscosity of the fluid by several orders of magnitude, do not necessarily cause pressure friction to increase by some degree during pumping operations. These systems have recently been replaced by retarding connector systems.

Slow connection systems. Noteworthy is their development in the 1980s when they were used as fracturing fluids with controlled bonding time, or delayed bonding. Bonding time is defined as the time for the base fluid to have a uniform structure. Obviously, the connection time is the time required to achieve a very large increase in viscosity and the liquid becomes homogeneous. A significant amount of research has been done to understand the importance of using fluid connection systems. These studies have shown that retardant joint systems exhibit better joint fineness, give greater viscosity, and increase thermal stability in the fracturing fluid. Another advantage of these systems is reduced friction during injection. As a result of this, retardation connector systems are used more than conventional connector systems. The main advantages of using connection systems over linear fluids are described below:

1. They can achieve much higher viscosities in fracturing compared to gel loading.

2. The system is most effective in terms of fluid loss control.

3. The connecting systems have better thermal stability.

4. Connecting systems are more cost effective per foot of polymer.

Liquids on oil basis. The easiest to oil based on gel rupture, possible today, it is the reaction product of aluminum phosphate and the base, typical soda aluminate. This addition reaction, which converts the created salt, which gives the viscosity to diesel fuels or holding back to a highly gravitational crude system. Aluminum Phosphate Gel Improves More Raw oil and increase thermal stability.

Aluminum phosphate can be used to create a fluid with improved high temperature stability and good proppant carrying capacity for use in high temperature wells: over 127°C. The main disadvantage of using liquids for oil basis is fire and explosion hazard. It should also be noted that the preparation of liquids on oil basis requires a lot of technical and quality control. The preparation of a water-based liquid greatly facilitates the process.

Alcohol-based liquids. Methanol and isopropanol have been used as components of water-based and acid-based fluids, or, in some cases, as saline fracturing fluids for many years. Alcohol, which reduces the surface tension of water, has been specifically used to remove water obstructions. In fracturing fluids, alcohol has found wide use as a temperature stabilizer since it acts as an oxygen retainer. The polymers increased the ability to thicken pure methanol and propanol. These polymers including hydroxypropyl cellulose and hydroxypropyl guar have been replaced. Guar gum raises the viscosity 25% higher than methanol and isopropanol, but also leaves a residue. In water sensitive formations, hydrocarbon based fluids are preferred over alcohol based fluids.

Emulsion fracturing fluids. This kind of fracturing fluid has been used for many years. Even some of the first fracturing fluids on oil basis, were outwardly oil emulsions. They have many disadvantages and are used in a very narrow range because the extremely high friction pressure is a result of their inherent viscosities and their lack of friction reduction. These fracturing fluids were invented in the mid 1970s. Cost Efficiency oil emulsion implies that the injected oil can be taken back and sold. These emulsions were very popular when raw oil and condensate cost $19 - $31 per m3. Use of type emulsions oil in water" decreased directionally with an increase in the price of oil.

The following types of fracturing fluids are also known in world practice:

Foam based fluids, energy fracturing fluids where nitrogen and carbon dioxide are used gas, soluble in water.

Rheology of liquids

The rheological properties of liquids include properties that describe the flow of liquids, their absorption, carrying capacity, etc. such as viscosity. The viscosity of the fracturing fluid greatly affects how the fluid is absorbed by the formation rock: less thick fluid is lost than non-viscous fluid. The following is a classification of fracturing fluids.

1) Newtonian fluids. For such fluids, there is a linear relationship between shear stress and shear rate. Examples: water, unthickened raw oil, reformed.

2) Non-Newtonian fluids. Bingham plastics are the simplest variety of non-Newtonian fluids. As with Newtonian fluids, there is a linear relationship between shear stress and shear rate. However, some, not infinitesimal, shear stress is required to excite the flow of these fluids. Example: foam.

Calculation of viscosity in a rectangular fracture:

E=P+5.79x10-3 xQ/HW2 (centipoise)

where P is the plastic viscosity (centipoise)

Q-flow rate during injection (m3/min)

H-height of crack (m)

W-crack width (mm)

3) Liquids obeying a power law. Such fluids exhibit an "apparent" viscosity that changes with flow rate (shear rate). The "apparent" viscosity decreases as the shear rate increases.

4) Supercritical fluids. When using fracturing fluids with a high CO2 content (fracturing with a mixture of methanol and CO2, fracturing with liquid CO2), fracturing occurs at a pressure and often at a temperature that is higher than the critical parameters for CO2. In this range, as the pressure increases, the density and viscosity increase, the rheology of the liquid becomes difficult to describe.

Viscosity measurement.

Viscosity is usually measured using a Fann rotational viscometer or a Marsh funnel.

Shear rate at standard viscometer revolutions (Table 12).

Table 12

Viscometer revolutions	Shear rate
	1022

Liquid filterability control

The fracturing fluid efficiency value indicates how much fluid is absorbed by the formation in relation to the amount of fluid that creates a fracture. For example, if the fluid efficiency is 0.65, this means that 35% of the fluid is lost and only 65% ​​of the fluid forms the fracture volume. Simply put, the lower the fluid loss, the higher its efficiency. However, it should be remembered that although excessive filtration is undesirable, low loss will not be beneficial unless sufficient proppant is added to the fluid to properly prop the fracture. Lower fluid leakage will also prevent the fracture from closing quickly and allow the proppant to fall out of suspension.

To quantify fluid losses, the filterability coefficient is used, which takes into account the reservoir rock, fluid properties, and fracturing fluid parameters.

Carrying capacity of the fluid against the proppant.

The proppant carrying capacity is a function of pumping, viscosity, sand concentration, and friction against the fracture face. During hydraulic fracturing, both the vertical and horizontal components of the velocity vector act on the proppant. The horizontal component is usually much larger than the vertical component, due to which the proppant moves with the fluid. Once the pump is stopped, the proppant will settle until the fracture closes.

Polymer-crosslinked fluids have a very high viscosity and form an almost perfect suspension with the proppant, which makes it possible to fill the entire volume of the fracture with proppant. In low viscosity systems, such as liquid CO2, turbulence is used to create a suspension of proppant particles.

Friction.

During hydraulic fracturing, up to half of the power of the mechanisms concentrated on the site can be spent on overcoming friction in the tubing. Some fluids exhibit greater frictional force than others. In addition, the friction is higher, the smaller the diameter of the pipes. Consideration of fluid friction and flow requirements in fracturing design is as important as pressure limitation or reservoir compatibility. Based on information from a large number of hydraulic fractures, pressure plots have been generated to assist in designing the energy requirements of the process.

Safety.

When choosing a fracturing fluid, in addition to the high pressure hazards present in any hydraulic fracturing, the flammability and toxicity of the fluid should also be considered.

Removal and determination of the amount of liquid.

Well return to prey after hydraulic fracturing requires careful planning. If the pressure at the bottom of the well is not enough for the well to start producing on its own, you can gasify liquid, thereby creating additional energy and lowering the static pressure. Some fracturing fluids, like liquid CO2 or foams, are removed very quickly and with determination of their volume.

Propping materials (proppants)

The wedging is performed in order to maintain the permeability created by hydraulic fracturing. Fracture permeability depends on a number of interrelated factors:

1) proppant type, size and uniformity;

2) the degree of its destruction or deformation;

3) the amount and method of proppant movement.

Some of the most common proppant sizes are:

Table 13

Properties of proppants

1) Dimensions and uniformity

As the limiting particle size of the material decreases, the load it can withstand increases, which contributes to the stability of the permeability of the proppant-filled fracture.

At zero closure stress, the permeability of the ceramic proppant is 20/40. One reason for this is the more uniform, compared to sand, sphericity of the ceramic particles.

A significant content of fine particles (dust) in the sand can significantly reduce the permeability of the fracture. For example, if 20% of 20/40 proppant particles pass through a 40 screen, the permeability will decrease by a factor of 5.

The permeability of 10/16 sand is approximately 50% higher than that of 10-20 sand.

American Oil Institute (API RP 56) .

2) Durability

With an increase in the fracture closure stress or horizontal stress in the reservoir rock matrix, a significant decrease in proppant permeability occurs. As can be seen from the proppant long-term permeability graphs, at a closure stress of 60 MPa, the permeability of the 20/40 "CarboProp" proppant is significantly higher than that of conventional sand. When the closure stress is higher than that of ordinary sand. At a closure stress of about 32 MPa, the particle size curves for all common sands drop rapidly. The strength of sand grains varies depending on the place of origin of the sand and the limiting particle sizes.

3) Thermochemical stability

All proppants used should be as chemically inert as possible. They must withstand aggressive liquids and high temperatures.

4) Cost

The cheapest proppant is sand. High-strength proppants, such as agglomerated bauxite or resin-coated sand, are much more expensive. Their applicability should be assessed on the basis of an individual economic analysis for a given well.

Permeability test.

When choosing the required types and sizes of proppant, it is very important to determine its permeability. Previously, radial filtration chambers were used in proppant testing. However, some fundamental difficulties - phenomena associated with flows that do not obey Darcy's law, and very low, unmeasurable pressure drops did not allow obtaining reliable test results. The imperfection of radial chambers led to the development of linear filtration chambers.

long-term permeability.

The principal disadvantage of the API technique is that it only provides results for short-term permeability. In the fields, it was found that the forecast booty rarely corresponded to reality. There are many reasons for this, but the main reason was the overly optimistic short-term permeability data used in the forecast.

Proppant types.

The first material used to hold the crack open was siliceous sand. As technology advanced, it became clear that some types of sand were better than others.

In addition, artificial proppants have been created that are suitable for use where natural sands are unsuitable.

1) Ceramic proppants

There are two types of ceramic proppants: agglomerated bauxite and intermediate strength proppants. The permeability of the latter is close to the permeability of agglomerated bauxite, while their density is lower than that of bauxite, but slightly higher than that of sand.

Agglomerated bauxite is a high strength proppant developed by Exxon Production Research. It is made from high quality imported bauxite ores. The manufacturing process involves grinding the ore into very fine particles, converting the primary ore into spherical particles of the desired size, and firing them in a kiln at a temperature high enough to cause the agglomeration process. The final product usually contains 85% Al2O3 . The remaining 15% are oxides of iron, titanium and silicon. Its specific gravity is 3.65 compared to sand's 2.65. Agglomerated bauxites are used mainly in deep (deeper than 3500 m) wells.

2) Intermediate density ceramics

These proppants differ from agglomerated bauxite primarily in their composition. Their alumina content is lower, the silicon content is higher, and the specific gravity is 3.15. At pressures up to 80 MPa, they are close in permeability to agglomerated bauxites. Therefore, in most cases, due to the lower cost, they replace bauxites.

3) Low density ceramics

These proppants are made in the same way as other ceramics. Their main difference is the composition. They contain 49% Al2O3, 45% SiO2, 2% TiO2 and traces of other oxides. The density of these proppants is equal to 2.72, that is, they are the most common proppants due to their price, strength, density, close to the density of sand.

Calculation of hydraulic fracturing

Make a plan for hydraulic fracturing, select working fluids and evaluate process performance for the following conditions:

Operational well (table 14), deposits.

Table 14

INDICATOR	SYMBOL	VALUE	DIMENSION
Well depth		2100
Bit diameter		0,25
Revealed formation thickness		13,5
Average permeability		9,8*10-8
Modulus of elasticity of rocks		2*1010	Pa
Poisson's ratio		0,25
Average density of rocks above the productive horizon		2385,2	kg/m3
Fracture Fluid Density			kg/m3
Viscosity of fracturing fluid			Pass
Sand concentration		1200	kg/m3
Download rate		1,2*10-2	m3/s

1. Vertical component of rock pressure:

Rgv \u003d rgL \u003d 2385.6 * 9.81 * 2100 * 10-6 \u003d 46.75 MPa

2. Horizontal component of rock pressure:

Рg \u003d Рgv * n / (1-n) \u003d 46.75 * 0.25 / (1-0.25) \u003d 15.58 MPa

In such conditions, during hydraulic fracturing, the formation of a vertical fracture should be expected.

We will design a hydraulic fracturing with a non-filterable liquid. As a fracturing fluid and a sand carrier fluid, we use thickened oil with the addition of asphaltin, density and viscosity are given in the table. We accept the sand content (see table 4.), for wedging the fracture, we plan to pump about 5 tons of quartz sand with a fraction of 0.8-1.2 mm, the injection rate (data in table 4.), which is much higher than the minimum allowable when creating vertical fractures .

During hydraulic fracturing, a sand-carrier fluid is continuously pumped in a volume of 7.6 m3, which is also a fracturing fluid.

To determine the parameters of the crack, we use the formulas arising from the simplified technique of Yu.P. Zheltov.

3. Determine the pressure at the bottom of the well at the end of hydraulic fracturing:

Rzab / Rg * (Rzab / Rg-1) 3 \u003d 5.25E2 * Q * m / ((1-n2) 2 * Rg2 * Vg) \u003d 5.25 * (2 * 1010) 2 * 12 * 10-3 *0.2/(1-0.252)2*(15.58*106)3*7.6) = 2*10-4

Rzab \u003d 49.4 * 106 \u003d 49.4 MPa

4. Determine the length of the crack:

l \u003d (VzhE / (5.6 (1-n2) h (Rzab-Rg))) 1/2 \u003d (7.6 * 2 * 1010 / (5.6 * (1-0.252) * 13.5 * (49.4 - 15.58)*106))1/2 = 31.7 m

5. Determine the width (opening) of the crack:

w = 4(1-n2)*l*(Pzab-Rg)/E = 4*(1-0.252)*31.7*(49.4-15.58)*106/1010 = 0.0158 m = 1.58 cm

6. Let's determine the distribution of the sand-carrying fluid in the crack:

L1=0.9*l=0.9*31.7=28.5 m

7. Let's determine the residual width of the crack, taking the porosity of the sand after its closure m=0.2:

W1 \u003d wno / (1-m) \u003d 1.58 * 0.107 / (1-0.3) \u003d 0.73 cm

8. Determine the permeability of a fracture of this width:

Kt \u003d w21 / 12 \u003d 0.00732 / 12 \u003d 4.44 * 10-6 m2

Hydraulic fracturing will be carried out through tubing with an inner diameter of d = 0.076 m, isolating the reservoir with a packer with a hydraulic anchor.

Let's determine the hydraulic fracturing parameters.

1. Loss of pressure due to friction during the movement of the sand-carrying fluid along the tubing.

Rf = rn(1-no)+rpes*no = 930*(1-0.324)+2500*0.324 = 1439 kg/m3

Reynolds number

Re \u003d 4Qrzh / (pdmzh) \u003d 4 * 12 * 10-3 * 1439 / (3.14 * 0.062 * 0.56) \u003d 516.9

Hydraulic resistance coefficient

L=64/Re=64/633.7=0.124

According to Yu.V. Zheltov, in the presence of sand in the liquid at Re>200, early flow turbulence occurs, and friction losses at Re=516.9 and no = 0.324 increase by 1.52 times:

16Q2L 1.52*0.124*16*(12*10-3)2*2100*1439

Рт = 1.52l¾¾¾ rzh = ¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾ = 26 MPa

2p2d5 2*3.142*0.0765

2. Pressure to be created at the wellhead during hydraulic fracturing:

Ru \u003d Rzab-rzhgL + Pt \u003d 49.4-1439 * 9.81 * 2100 * 10-6 + 26 \u003d 45.9 MPa

3. The hydraulic fracturing fluids are pumped into the well by pumping units 4AN-700 (Table 15.)

14,6

Required number pumping units:

N \u003d RuQ / (RaQakts) + 1 \u003d 45.9 * 12 / (29 * 14.6 * 0.8) + 1 \u003d 3

Where Ra is the working pressure of the unit;

Qa - supply of the unit at this pressure

kts - coefficient of technical condition of the unit depending on the service life kts = 0.5 - 0.8

4.Volume of liquid for squeezing the sand-carrying liquid:

Vp \u003d 0.785 * d2L \u003d 0.785 * 0.0762 * 2100 \u003d 9.52 m3

5. Duration of hydraulic fracturing:

t \u003d (Vzh + Vp) / Qa \u003d (7.6 + 6.37) / (14.6 * 10-3 * 60) \u003d 19.5 min.

Technique and technology of hydraulic fracturing

Hydraulic fracturing technology includes the following operations: well flushing; running high-strength tubing with a packer and an anchor at the lower end into the well; piping and pressure testing to determine the injectivity of the well by pumping liquid; injection of fracturing fluid, sand carrier fluid and squeezing fluid through the tubing into the formation; dismantling equipment and putting the well into operation.

According to the technological schemes of conducting, a single, directed (interval) and multiple hydraulic fracturing is distinguished.

With a single hydraulic fracturing, under the pressure of the injected fluid, all the layers opened by perforation are simultaneously exposed, with a directed fracturing, only a selected layer or interlayer (interval), which, for example, has an underestimated productivity, and with multiple hydraulic fracturing, each individual layer or interlayer is affected sequentially.

The design of hydraulic fracturing technology basically boils down to the following. With regard to specific conditions, the process flowsheet, working fluids and proppant are selected. With a single hydraulic fracturing, based on experience, 5-10 tons of sand are taken. The concentration of sand in the media set depending on its holding capacity. When using water, it is 40-50kg/m3. Then, the amount of sand-carrying liquid is calculated from the amount and concentration of sand. Based on experience, 5-10m3 fracturing fluid is usually used. The volume of the displacement fluid is equal to the volume of the casing string and pipes through which the sand-carrying fluid is injected into the reservoir.

The minimum fluid injection rate should be at least 2 m3/min and can be estimated in the event of the formation of vertical and horizontal fractures, respectively, by the formulas:

.

where Qhor - min. expenses, l/s; h – formation thickness, cm; Wvert, Wgor - width vert. and mountains. cracks, cm; µ - fluid viscosity, mPa x s; Rt is the radius of the horizon. cracks, see

The formation hydraulic fracturing pressure is set according to experience or is calculated according to the formula:

RGRP=pr + sp

where rHF - zab. fracturing pressure; рr =Hrпg – rock pressure; sp is the tensile strength of the reservoir rock under conditions of all-round compression; H is the depth of the formation; rp - average density of overlying rocks, equal to 2200-2600 kg/m3, on average 2300 kg/m3; g is the free fall acceleration.

Injection pressure at the wellhead:

RU = rHF + Δрtr - rs

where Δрtr – pressure losses due to friction in pipes; rs is the hydrostatic pressure of the liquid column in the well.

If the injection pressure pU is greater than the allowable wellhead pressure pUdop, then an anchor packer is installed on the tubing above the roof of the productive formation. The allowable pressure pUdop is taken as the largest of the two pressures calculated by the Lame formula and using the Yakovlev-Shumilov formula.

In sedimentary rocks subvertical fractures are usually formed, the length of which reaches a few tens of meters, and the opening is several mm, less often cm. Hydraulic fracturing causes an increase in production rates by 1.5-2 times or more. To increase the efficiency of hydraulic fracturing in carbonate rocks, it is combined with acid treatment of rocks. Fracture pressure is difficult to predict theoretically, since it depends on many factors: stresses in the rock, its strength, already existing fracturing, formation dip angle, etc. Usually, the overpressure is selected empirically and ranges from 0.1 to 1.5 (average about 0.8) hydrostatic pressure.

For hydraulic fracturing, the well is suitably equipped. High-performance pumps are connected to its mouth, capable of developing the necessary overpressure. Tubing pipes are lowered inside the casing pipes, equipped at the bottom of the packer (Fig. 1). The annulus of the casing string above the hydraulic fracturing interval must be securely cemented.

If all technological requirements and favorable conditions for hydraulic fracturing are observed, its effect is undeniable.

Special units and technical means used in hydraulic fracturing

The organization of hydraulic fracturing consists in the preparation of appropriate reagents as a hydraulic fracturing fluid and its subsequent injection into the productive zone at low flow rate and under high pressure in order to wedging the rock, resulting in a fracture as a result of hydraulic action. First of all, a clean fluid (buffer) is injected into the well to initiate fractures and promote it in the formation. After that, the suspension continues to develop a crack.

The hydraulic fracturing fluid is prepared on a well cluster, immediately before it is injected into the reservoir. The hydraulic fracturing fluid preparation system includes: a sand carrier, a tank with oil or diesel fuel, mixing unit (blender). The strapping of the system has a 1.5-fold margin of safety.

Before the start of hydraulic fracturing, equipment and piping are pressurized to operating pressure. Direct control of hydraulic fracturing (pumping units) is carried out through computer center, which has automatic protection against possible accidents (gusts of strapping). In the event of an accident, the computer center automatically turns off the pumps, the piping check valves close the reverse flow of fluid near the well and in front of each pumping unit. Relief of pressure is made in the vacuum installation which is included in the package equipment hydraulic fracturing and permanently included in the piping. The same vacuum unit collects liquid residues in the piping and pumps after hydraulic fracturing in order to prevent spills on the soil during line dismantling. The pressure is released from the annulus into the TsA-320 tank, which is permanently connected to the wellhead through a X-mas tree cross.

For the production of hydraulic fracturing, the following technique is used (on the example of the considered field area):

1. KRAZ-250 TsA

2. Ural-4320 fire truck

3. Kenward sand truck

4. Kenward chemical van.

5. Kenward blender

6. Kenward pumping unit

7. Kenward cement aggregate

8. Kenward Pipe Carrier

9. Ford 350 laboratory

10. UAZ-3962 ambulance van

11. K-700 vacuum unit

Kenward technique equipped special filters that capture emissions.

Underground equipment used in hydraulic fracturing.

The well is killed with a special saline solution, which is prepared at the solution unit.

The applied technology excludes hit of solution on a surface of the soil and the nearest reservoirs. When preparing a well for hydraulic fracturing, in order to exclude possible releases of killing fluid and well production, the wellhead is equipped with Hydril preventer units.

In preparation for hydraulic fracturing, a tubing string with a diameter of 89 mm is lowered into the well for fluid injection. The annulus (casing string and tubing 89 mm) is sealed with a packer installed in the hydraulic fracturing zone. The packer setting is checked by pressure testing the annulus with water to the operating pressure of the casing string through TsA-320.

The wellhead for hydraulic fracturing is equipped with two Hamera gate valves (working and back-up).

Fracturing fluid and proppants.

For hydraulic fracturing, it is best to use a fluid that does not contain an aqueous phase. According to the technology, diesel fuel should be used, but more often it is used oil(as more accessible and relatively cheap product) with a gelation activator and a destructor, as well as a surfactant - a friction reducer. The ratio of special additives depends on the temperature of the object (formation) of subsequent treatment. Thus, the ROG-4 system is used for high (more than 80 ° C) temperature conditions, ROG-5, respectively, for low ones. Each of these types of liquid, depending on the temperature of the medium, has optimal rheological properties. A certain permanent system is used to measure fluid parameters and regulate its values ​​with special additives determined on the basis of computer calculations carried out at the well. The structured fluid is optimal for the transfer of fixing material, moreover, it practically does not interact with the rock and fluids that saturate it. The absence of an aqueous phase in its composition excludes the possibility (during the destruction of the gel) negative impact on the nature of saturation of the formation medium in contact with it. The physical properties of a liquid are characterized the following indicators: density - 0.85 t/m3, viscosity - 90 MPa.s, consistency coefficient - 0.3. To fix the crack, a high-strength (withstands a pressure of at least 70 MPa) artificial thermal product (proppant) of aluminosilicate composition is pumped. The material used is practically the same size (20/40 mesh), the grains are quite perfect, round, the average sphericity coefficient is 0.9. This provides a high filtration capacity (about 200 darcy) even with the densest packing and an external pressure of 50 MPa.

Criteria for selecting wells for hydraulic fracturing.

For hydraulic fracturing, preference is given to wells that meet the established criteria listed below. The latter in combination make it possible, with a high probability, to intensify oil production. Depending on the initial permeability of the reservoir and the state of the bottomhole zone of the well, the criteria are grouped into the following two positions.

1. Low-permeability reservoirs (fracturing provides an increase in the filtration surface), while the following criteria must be observed.

1.1. effective formation thickness of at least 5 m;

1.2. absence of wells in production gas from gas caps, as well as injected or contour water;

1.3. the productive formation subjected to hydraulic fracturing is separated from other permeable formations by impermeable sections, more than 8-10 m thick;

1.4. the remoteness of the well from GOC and WOC should exceed the distance between production wells;

1.5. cumulative selection oil from the well should not exceed 20% of the specific recoverable reserves;

1.6. dissection of the productive interval (subjected to hydraulic fracturing) - no more than 3-5;

1.7. the well must be technically sound, as a condition operational columns and the adhesion of cement stone to the column and rock should be satisfactory in the interval above and below the filter by 50m

1.8. formation permeability not more than 0.03 µm2 at viscosity oil in reservoir conditions no more than 5 MPa.s.

2. Hydraulic fracturing in medium and low permeability reservoirs for stimulation oil production due to the elimination of increased filtration resistance in the bottomhole zone.

2.1. the initial productivity of the well is significantly lower than the productivity of the surrounding wells;

2.2. the presence of a skin effect on HPC;

2.3. well production water cut should not exceed 20%;

2.4. well productivity should be lower or slightly different from the design baseline.

As follows from the above, the above criteria allow for a versatile preliminary expert assessment of each well from a technical, technological, and geological and commercial standpoint.

With their rigorous execution, it is highly likely that the technological success of hydraulic fracturing operations and the corresponding receipt of additional oil production. The realizable volume of the latter must certainly compensate material costs for hydraulic fracturing.

Hydraulic fracturing technology.

On the example of the fields of JSC "Tomskneft" we will consider the technology of hydraulic fracturing.

The technology of the process is as follows. Packing in progress operational strings 15-20 meters above the top of the perforation interval, the packer interval is selected according to the MLM diagram.

The wellhead is equipped with AU-700 wellhead fittings. The annulus is pressed to a pressure of 15 MPa in order to check the tightness of the packer. In the future, during the process, the pressure on the annulus is at the level of pressure testing in order to reduce the load on the rubber cuffs created by the pressure under the packer during the process.

For hydraulic fracturing, 8 pumping units are used, and 6 of them are engaged in the process, 2 operate in idle mode.

The injection of the emulsion is carried out at burst pressure with a total capacity of the units of 1.8 m3/min. A fixing material with a concentration of 150 kg/m3 is fed into the flow of the injected liquid, which gradually increases and in the last 20 minutes is 500 kg/m3. Sand is preliminarily packed into sand mixers USP-50 and fed to the suction pipe 4AN-700 by the TsA-320 unit. After the sand supply is stopped, a displacement fluid of 20 m3 is pumped at a rate of 2.4 m3/min.

The valve on the buffer is closed after the process, the wellhead is equipped with a pressure gauge and the pressure drop curve is taken from it, the interpretation of which allows determining the fracture radius.

Of the equipment, sand mixers and TsA-820 and AN-700 units were used, which allow raising the pressure at the wellhead to 45-60 MPa. However, at pressures of 60 MPa, the AN-700 units were operated at the limit of their capabilities, i.e. at significant depths and a dense reservoir, technical limitations arise in terms of pressure, and, accordingly, fluid flow.

When these values ​​are reached, hydraulic fracturing usually occurs. The indicated pressure range was predetermined by the difference in lithological and physical, and mainly, by the strength characteristics of the layers and stresses in the rock. Therefore, the fractures created by hydraulic fracturing are oriented in the vertical direction.

According to domestic technology, a special composite liquid is used to carry out the rupture and transfer of the material fixing the crack, where 30-43% oil and 1.5-3.0% emulsifier. The type of emulsifier used, in turn, depended on the outside temperature.

ARNA polyemulsion is characterized by increased physical characteristics: density 1.18-1.24 t/m3, viscosity - 120-150 MPa.s, consistency coefficient - 0.8. The increased viscosity and consistency of the fluid was envisaged to ensure the transfer of the sand used to fix the fracture, the volume of which is constant and is about 20 tons. The maximum concentration of sand in the fluid reached 500 kg/m3. In order to better open fractures and prevent sand from falling out on the bottom of the well, a high pumping speed was required, which turned out to be technically feasible at a level of only 2.4 m3/min.

Imported quartz sand was used as a proppant.

The use of domestic technology during hydraulic fracturing did not give satisfactory results, therefore, at present, the Vah Frakmaster Services joint venture is being carried out at the fields of the hydraulic fracturing area using foreign technology and using more advanced equipment.

According to foreign technology, a special pumping station is used for injection. equipment: three-cylinder ejector plunger pumps with replaceable hydraulic part (from 3" to 71/2"), developing pressure up to 100 MPa and flow rate 2.5 m3/min.

Theoretical (experimentally confirmed) dependences of the geometrical dimensions of the fracture were established: length x height (fracture propagation area), width on viscosity, amount of fluid injected, pressure and injection rate. Their rather complex interrelation is reflected and solved at the level of computer simulation both before the work on the well and in the process.

The pumps provide a high fluid pumping speed of 5.5 m3/min, and with a relatively low proppant density (1.6 t/m3), a sufficiently high (up to 1000 kg/m3) concentration of the transferred fixing material is maintained during the operation.

After a certain estimated time, as the transition (under the action of the destructor) from the gel-like state to a more mobile liquid state, the injected fluid is gradually removed from the fracture.

From the foregoing, it follows that the applied JV "Vah Frakmaster Services" and specially treated fluids specialized only for hydraulic fracturing, fixing material, as well as equipment and technology favorably differ from the domestic one in many respects. Together, this provides greater initial and cumulative gains. oil production. The following main factors are considered to be advantageous:

The absence of an aqueous phase in the hydraulic fracturing fluid;

High filtration properties of the fixing material, provided by the sphericity of the grains and the homogeneity of the fraction;

Technological and technical ability to carry out hydraulic fracturing with a specified length and width of fractures. It has been theoretically established that long (up to 300 m) fractures are formed at low rates of hydraulic fracturing fluid injection (about 2.5 m3/min). For the formation of relatively short and wide fractures, twice the fluid injection rate is required. The presence of long fractures is known to contribute to unwanted premature water breakthroughs.

In addition to the above, there is also a significant difference in the sequence of operations when putting a well into operation. So, immediately after the hydraulic fracturing, according to foreign technology, the well is tested for spouting through various nozzles in an increasing sequence of their diameters: 2, 4, 8 mm; this ensures a smooth increase in drawdown in the bottomhole zone, accompanied by the removal of the hydraulic fracturing fluid, strengthening by the rock pressure of the proppant in the fracture and putting the development object into operation. As follows from the above, during the entire process of hydraulic fracturing, the water phase is not introduced into the reservoir environment of the bottomhole zone from the outside, which favors the movement and extraction oil phases.

Another method is hydraulic fracturing using domestic technology. Immediately after hydraulic fracturing, the well is killed with saline solutions, followed by packer break and tubing retrieval. Then the pumping station comes down equipment and starts exploitation wells. Thus, according to domestic technology, the entire process from the beginning of hydraulic fracturing to the subsequent start-up of the well is almost always accompanied by the presence of an aqueous phase in the bottomhole zone and fracture.

It is well known that the well killing process has a negative effect on the productivity, and the degree of this effect is proportional to the time of fluid exposure to the formation zone. In the field under consideration, a saline solution is used to kill wells and, depending on the formation pressure in the well area, the density usually fluctuates around 1.18 t/m3 (mineralization - 300 g/l).

In field practice, the solution is not properly filtered, so a lot of foreign substances of sandy-clay composition are pumped into the well. Their content is so high that it often causes failure of the pumping equipment. Hence, it is easy to imagine the degree of clogging of permeable interlayers in the perforation interval, hydraulic fracture and the inevitable decrease in well productivity due to this.

Evaluation of the technological efficiency of hydraulic fracturing

According to the currently accepted classification modern methods increase oil recovery hydraulic fracturing belongs to the group of physical methods.

Technological efficiency of application of magnification methods oil recovery characterized by:

Additional oil production by increasing oil recovery formation;

Current additional oil production due to the intensification of the selection of fluid from the reservoir;

Reducing the volume of produced water. Additional mined oil for a specified period of time is determined by the arithmetic difference between the actual wells with hydraulic fracturing and the calculated booty without hydraulic fracturing (basic booty).

When counting oil production over the past period, the main task is only to correct definition basic oil production.

One of the methods is a variant calculation of technological indicators of development, based on physically meaningful mathematical models. In this case, a sufficiently reliable adaptation of the calculated indicators to the actual ones is possible if there are initial physical parameters and a long history exploitation. With reliable adaptation, the method allows you to determine changes prey by groups of wells, deposits and is especially attractive by the possibility of quantifying the mutual influence (interference) of wells. The accuracy of the results depends both on the reliability and completeness of the initial information and the capabilities of the mathematical model.

As for the calculation methods of evaluation, then, based on specific situation, the following should be noted. Wells with hydraulic fracturing are dispersed throughout almost the entire territory of a large field. The creation of a calculation model of objects, even for individual areas, is associated with a huge amount of work and the use of powerful computer science. In addition, to date, there is very scarce geological-physical and geological-field information on wells, some of which is subject to changes in the process exploitation wells, in time. As a result, the adaptation of the calculation model and obtaining reliable predictive technological development indicators is greatly hampered. At the same time, it seems that the results are the most acceptable or suffer from the least error for relative estimates of the mutual influence of wells, i.e. their interference.

In conclusion, it can be noted that hydraulic fracturing allows solving the following tasks:

1) increase in productivity (injectivity) of the well in the presence of contamination of the bottomhole zone or low reservoir permeability;

2) expansion of the inflow (absorption) interval with a multilayer structure of the object;

3) stimulation of inflow oil, for example, using granular magnesium; water inflow isolation; injectivity profile control, etc.

A group of researchers concluded that fracking may have an effect on the low birth weight of a child born within three kilometers of the zone of its use.

What is fracking?

If you are aware of the most discussed apocalyptic scenarios that are based on the anthropogenic factor, then you are probably aware of the possible depletion of the resources of our planet and the plunging of humanity into the chaos of anarchy. Despite the rather distant prospects for such a development of events, the limited resources necessary for a comfortable life, and this word must be emphasized, really exist. However, in addition to a dozen directions for finding a comprehensive solution to this problem, from the invention of a perpetual motion machine to the development of projects for the extraction of resources on other planets, there are a couple of simplified solutions: find new sources or give old ones a good shake.

If the first option, in principle, can be accompanied by the construction of infrastructure around a new facility containing minerals, then the second one really causes concern. One of the methods that is especially popular today in the fuel and energy industry is fracking.

Fracking, or hydraulic fracturing, implies, as the name implies, a tough, but the most efficient (from an economic point of view) way to develop an already depleted field. Fracking technologies are based on the use of a whole range of chemical reagents, which, when interacting, cause the formation of highly conductive fractures to pump out the last oil and gas residues located in hard-to-reach earth layers.

Data collection

This barbaric technique has already fallen into disrepute, but the laws of certain countries, including the United States, allow its use. While individual states are trying to ban the use of fracking in their territories in order to stop money-hungry companies, an undeniable body of evidence of its negative impact on the environment and public health is required.

In particular, a study published in Science Advance contributes to this fight. A team of researchers from Princeton, Cambridge and other US universities found that fracking has a direct impact on the health of pregnant women. Their work showed that children born within three kilometers of a fracturing resource extraction zone were 25% more at risk of being born with a low birth weight.

The study examined the birth records of more than 1 million children from 2004 to 2013. Moreover, for the purity of the study, the marital status of each mother, her place of residence, race and education were additionally studied.

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