Frac oil. Application of hydraulic fracturing in oil and gas fields

Hydraulic fracturing consists of three principal operations:

1. creation of artificial cracks in the reservoir (or expansion of natural ones);

2. injection of fluid with fracture filler through the tubing into the near-wellbore zone;

3. forcing fluid with filler into cracks to fix them.

These operations use three liquid categories:

• fracturing fluid,
• sand carrier
• selling liquid.

Work agents must meet the following requirements:

1. Should not reduce the permeability of the CCD. At the same time, depending on the category of the well (producing; injection; producing, converted to water injection), working fluids of different nature are used.

2. The contact of working fluids with the rock of the EZS or with reservoir fluids should not cause any negative physical and chemical reactions, except for the cases of using special working agents with controlled and directed action.

3. Should not contain a significant amount of foreign mechanical impurities (i.e. their content is regulated for each working agent).

4. When using special working agents, for example, an oil-acid emulsion, the products of chemical reactions must be completely soluble in the production of the reservoir and not reduce the permeability of the wellbore zone.

5. The viscosity of the working fluids used must be stable and have a low pour point in winter (otherwise, the hydraulic fracturing process must be carried out using heating).

6. Must be readily available, non-scarce and inexpensive.

Hydraulic fracturing technology :

• Well preparation- a study for inflow or injectivity, which provides data for the evaluation of fracture pressure, fracture fluid volume and other characteristics.
• Well flushing- the well is flushed with a flushing fluid with the addition of certain chemicals to it. If necessary, carry out decompression treatment, torpedoing or acid exposure. In this case, it is recommended to use tubing with a diameter of 3-4 "(pipes of a smaller diameter are undesirable, because friction losses are high).
• Fracture fluid injection- the pressure necessary for rock rupture is created for the formation of new and disclosure of cracks that existed in the CCD. Depending on the properties of the CCD and other parameters, either filterable or slightly filterable liquids are used.

Fracture fluids:

in production wells

degassed oil;

thickened oil, oil-oil mixture;

Hydrophobic oil-acid emulsion;

Hydrophobic water-oil emulsion;

Acid-kerosene emulsion, etc.;

in injection wells

clean water;

Aqueous solutions of hydrochloric acid;

Thickened water (starch, polyacrylamide - PAA, sulfite-alcohol stillage - PRS, carboxymethylcellulose - CMC);

Thickened hydrochloric acid (a mixture of concentrated hydrochloric acid with PRS), etc.

When choosing a fracturing fluid, it is necessary to take into account and prevent clay swelling by introducing chemical reagents into it that stabilize clay particles when wetted (clay hydrophobization).

As already noted, the burst pressure is not a constant value and depends on a number of factors.

Increasing the bottomhole pressure and achieving the fracture pressure value is possible when the injection rate is ahead of the rate of fluid absorption by the formation. In low-permeability formations, fracturing pressure can be achieved by using low-viscosity fluids as the fracturing fluid at a limited injection rate. If the rocks are sufficiently well permeable, then when using low-viscosity injection fluids, a high injection rate is required; with limited pumping rate it is necessary to use higher viscosity fracturing fluids. If the CCD is a high permeability reservoir, then high injection rates and high viscosity fluids should be used. In this case, the thickness of the productive horizon (interlayer), which determines the injectivity of the well, should also be taken into account.

important technological issue is to determine the moment of crack formation and its signs. The moment of formation of a crack in a monolithic reservoir is characterized by a break in the dependence "volumetric flow rate of injection fluid - injection pressure" and a significant decrease in injection pressure. The opening of fractures that already existed in the wellbore zone is characterized by a smooth change in the "flow rate - pressure" dependence, but there is no decrease in injection pressure. In both cases, a sign of fracture opening is an increase in the well injectivity.

• Injection of sand-carrying fluid. Sand or any other material injected into the fracture serves as a fracture filler, being a framework inside it and preventing the fracture from closing after the pressure is removed (reduced). The sand carrier performs transport function. The main requirements for a sand-carrying fluid are high sand-holding capacity and low filterability.

These requirements are dictated by the conditions for effective filling of cracks with a filler and the exclusion of possible settling of the filler in individual elements. transport system(mouth, tubing, bottomhole), as well as premature loss of mobility by the filler in the fracture itself. Low filterability prevents filtration of the sand-carrying fluid into the walls of the fracture, maintaining a constant concentration of filler in the fracture and preventing plugging of the crack by the filler at its beginning. Otherwise, the concentration of the filler at the beginning of the crack increases due to the filtration of the sand-carrying fluid into the walls of the crack, and the transfer of the filler in the crack becomes impossible.

As sand-carrying fluids in production wells, viscous fluids or oils are used, preferably with structural properties; oil and oil mixtures; hydrophobic water-oil emulsions; thickened hydrochloric acid, etc. In injection wells, PRS solutions are used as sand-carrying fluids; thickened hydrochloric acid; hydrophilic oil-water emulsions; starch-alkaline solutions; neutralized black contact, etc.

To reduce friction losses during the movement of these fluids with filler along the tubing, special additives (depressants) are used - solutions on soap base; high molecular weight polymers, and the like.

• Displacement fluid injection - squeezing the sand-carrying fluid to the bottom and pushing it into the cracks. In order to prevent the formation of plugs from the filler, the following condition must be observed:

where is the speed of movement of the sand-carrying fluid in the tubing string, m/s;

Viscosity of the sand-carrying liquid, mPa s.

As a rule, liquids with a minimum viscosity are used as displacement fluids. Production wells often use their own degassed oil (if necessary, it is diluted with kerosene or diesel fuel); Injection wells use water, usually commercial.

As a crack filler can be used:

Quartz sorted sand with a grain diameter of 0.5 +1.2 mm, which has a density of about 2600 kg/m3. Since the density of sand is significantly greater than the density of the sand-carrying liquid, the sand can settle, which predetermines high speeds downloads;

Glass balls;

Grains of agglomerated bauxite;

polymer balls;

Special filler - proppant.

Basic requirements for the filler:

High compressive strength (collapse);

Geometrically correct spherical shape.

It is quite obvious that the filler must be inert with respect to the production of the reservoir and not change its properties for a long time. It has been practically established that the concentration of the filler varies from 200 to 300 kg per 1 m3 of the sand-carrying liquid.

• After the filler is injected into the fractures, the well left under pressure. The dwell time should be sufficient for the system (CCD) to pass from an unstable to a stable state, in which the filler will be firmly fixed in the crack. Otherwise, in the process of stimulation of the inflow, development and operation of the well, the filler is carried out of the fractures into the well. If at the same time the well is operated by pumping, the removal of the filler leads to the failure of the submersible installation, not to mention the formation of plugs from the filler at the bottom. The foregoing is an extremely important technological factor, the neglect of which sharply reduces the efficiency of hydraulic fracturing up to a negative result.
• influx call, development of a well and its hydrodynamic study. Conducting a hydrodynamic study is obligatory element technology, because its results serve as a criterion for the technological efficiency of the process.

circuit diagram well equipment for hydraulic fracturing is presented on rice. 5.5. During hydraulic fracturing, the tubing string must be packed and anchored.

Important issues during hydraulic fracturing are determining the location, spatial orientation and size of cracks. Such definitions should be mandatory when fracturing in new regions, because. enable the development of the best process technology. The listed tasks are solved on the basis of the method of observing the change in the intensity of gamma radiation from a crack into which a portion of the filler activated by a radioactive isotope, for example, cobalt, zirconium, and iron, is injected. Essence this method consists in adding a certain portion of the activated filler to the clean filler and conducting gamma-ray logging immediately after the formation of fractures and injection of a portion of the activated filler into the cracks; comparing these results of gamma-ray logging, they judge the number, location, spatial orientation and size of the fractures formed. These studies are carried out by specialized field geophysical organizations.

Rice. 5.5. Schematic diagram of well equipment for hydraulic fracturing:

1 - productive formation; 2 - crack; 3 - shank; 4 - packer; 5 - anchor; 6 - casing string; 7 - tubing string; 8 - wellhead equipment; 9 - fracturing fluid; 10 - sand carrier liquid; 11 - squeezing liquid; 12 - manometer.

Problems of hydraulic fracturing application. ASS where next to the reservoir are layers containing water. It could be aquifers if bottom water. In addition, there may be formations adjacent to the treated formation that are waterflooded.

Vertical fractures formed during hydraulic fracturing in such cases create a hydrodynamic connection between the well and the aquifer. In most cases, the aquifer has a higher permeability compared to the reservoir where the hydraulic fracturing is carried out. That is why hydraulic fracturing can lead to complete flooding of wells. In old fields, many wells are in disrepair. Hydraulic fracturing under such conditions leads to rupture of the production string. Theoretically, in such wells, a packer is used to protect the string, but due to dents on the string and corrosion, the packer does not fulfill its role in such wells. In addition, due to hydraulic fracturing, cement stone can be destroyed.

During hydraulic fracturing, fractures are created in interlayers with different permeability, but very often it is easier to break a high-permeability interlayer than a low-permeability one. In an interlayer with a higher permeability, the fracture may be longer. With this option, after hydraulic fracturing, the well's oil production rate increases, but the water cut increases if the well was watered. That is why, before and after hydraulic fracturing, it is necessary to analyze the produced water in order to find out where the water came from in the well.

During hydraulic fracturing, as with any stimulation methods, the question always arises of compensating for large production by injection.

Hydraulic fracturing (HF) - technological process increasing the permeability of the bottomhole zone of the productive formation due to the formation of cracks or the expansion and deepening of natural cracks in it. The essence of this process lies in the injection of fluid into the bottomhole zone at high pressure, which exceeds the local rock pressure and the strength properties of the reservoir rock.

Hydraulic fracturing is applied:

To intensify oil production from wells with a heavily contaminated bottomhole zone by creating cracks;

In order to ensure the hydrodynamic connection of the well with the system of natural formation fractures and expansion of the drainage area;

To put into development low-permeability deposits and transfer off-balance oil reserves into commercial ones;

When complex and heterogeneous reservoirs are put into development in order to increase the rate of oil recovery and increase the final oil recovery;

To increase the productivity of oil wells;

To increase the injectivity of injection wells;

In wells with high formation pressure, but with low formation permeability.

It is not recommended to carry out hydraulic fracturing in wells located near water-oil and gas-oil zones, where accelerated coning and water and gas breakthrough into production wells are possible; in depleted reservoirs with low residual reserves, as well as in carbonate reservoirs with chaotic fracturing.

HF is performed in the following order. Tubing is lowered into the well, and a packer and an anchor are installed above the roof of the productive formation in which hydraulic fracturing is planned. The well is washed with water in order to clean the bottomhole from clay and mechanical impurities. If necessary, hydrochloric acid treatment or additional perforation is sometimes carried out before hydraulic fracturing. In such cases, the burst pressure is reduced and its efficiency is increased. Then, a fracturing fluid is injected into the well through the tubing (tubing diameter is at least 89 - 114 mm, it is not advisable to use pipes of a smaller diameter during hydraulic fracturing, since large pressure losses occur in them when pumping fluid) the fracturing fluid is injected in the volumes necessary to create the pressure at the fracturing. To protect the casing from high pressure, a packer is installed above the fractured formation. It completely separates the productive formation zone from the overlying part of the well. In this case, the pressure created by the pumps acts only on the formation and on the lower part of the packer. Install a hydraulic anchor to prevent the packer from moving.

Fracturing fluids fall into three categories: fracturing fluid, sand carrier fluid, and displacement fluid.

Working fluids should not reduce either the absolute or phase permeability of the reservoir rock. In this regard, during hydraulic fracturing in oil wells, hydrocarbon-based fluids are used, and in injection and oil wells, intended for conversion to injection - based on water. However, in wells with carbonate reservoirs, aqueous solutions of hydrochloric acid or other liquids based on it can be used as working fluids.

The fracturing fluid must penetrate well into the formation and naturally existing fractures in it. Fracturing fluids are mainly used:

1. hydrocarbon

2. aqueous solutions

3. emulsions

Working fluids for hydraulic fracturing should not contain mechanical impurities, and in contact with rock and reservoir fluid should not form insoluble precipitates.

The greatest preference for hydraulic fracturing should be given to fluids that are completely soluble in formation fluids. During hydraulic fracturing, the viscosity of working fluids must be stable.

A sand carrier fluid is a fluid used to drive sand from the surface into resulting fractures. The sand carrier fluid should be non-filterable or rapidly decreasing filterability and should also have high sand holding capacity. The same fluids are used as sand-carrying fluids as for fracturing.

The filler serves to form cracks and prevent them from closing when the pressure is removed. To fix the cracks formed during hydraulic fracturing, quartz sand with a grain size of 0.4 - 1.2 mm is used. Such sand is tested in laboratory conditions for strength and indentation into the surface. rocks, in which a fracture is formed, as well as residual permeability (permeability after sand compression under a press that simulates the effect of rock pressure). Sand for filling cracks during hydraulic fracturing must meet the following requirements: a) have high mechanical strength in order to form reliable sand cushions in cracks and not collapse under the weight of rocks; b) maintain high permeability. This is coarse-grained, well-rolled and uniform in granulometric composition of quartz sand. In cases of high rock pressure or an unstable surface of rocks in which a crack forms, an artificial ceramic or other proppant is used.

During the first hydraulic fracturing, at least 1.5-2 tons of sand should be injected into each fracture.

When pumping large amounts of sand (more than 15-20 tons) into the reservoir in order to penetrate it deeper through the cracks, the first portions of sand (30-40%) are pumped with fine-grained sand of a fine (0.4-0.6 mm) fraction, followed by the transition to injection coarse sand.

Modern hydraulic fracturing design consists of two fundamentally different parts.

In the first part of the design, the goal of hydraulic fracturing is set, wells, formations and interlayers for hydraulic fracturing are determined, and the dimensions (length, width) of the fractures to be formed are calculated. Usually, this part of hydraulic fracturing design is performed by an enterprise or its department (geological, development, enhanced oil recovery), leading the development of fields or some object. By order of an enterprise, design can also be entrusted to a research organization.

The second part of the design is directly related to the selection of hydraulic fracturing parameters that provide such injection rates and volumes of fluids and sand injected into the fractures in the selected wells, which allow creating fractures in the reservoir with the dimensions and throughput designed in the first part. This part of the design consists in calculating the process of formation of a filling crack and fixing it with sand. In the second part of the hydraulic fracturing design, effective fracturing fluids with appropriate properties and sand (proppant) are also selected. The second part of hydraulic fracturing design is performed by a servicing ("service") company, which usually performs the hydraulic fracturing operation.

IN full set equipment for hydraulic fracturing includes pumping and sand mixing units, tank truck, manifold block and wellhead fittings.

The wellhead is equipped with a special head, to which units are connected to inject fracturing fluids into the well. For hydraulic fracturing, the following can be used: pumping units 4AN-700, modernized 5AN-700 or frame AHP-700. The maximum pressure of these units is 70 MPa with a supply of 6 l / s, at a pressure of 20 MPa the supply is 22 l / s. The pumping units are connected to the manifold block using quick-detachable flexible pipe connections, which, in turn, is connected to the wellhead fittings.

In practice, interval hydraulic fracturing is often used. Interval, is used when several reservoirs are developed by a common filter, and the reservoirs are isolated from each other by layers of impermeable rocks.

Directional hydraulic fracturing is also used. In directional hydraulic fracturing using sandblasting, additional perforation is performed in a given interval of the productive formation, in which it is planned to obtain fractures. In this case, both “point” hydro-sandblast perforation and slot-hole perforation are used.

One of the effective new hydraulic fracturing technologies is the technology of proppanate deposition at the end of the fracture (or tip screening of fractures (TSO)), which allows you to purposefully increase the width of the fracture, stopping its growth in length, thereby significantly increasing the conductivity. To intensify the production of reserves from low-permeability layers and reduce the risk of a fracture entering aquifers or gas-bearing formations, selective hydraulic fracturing technology is used.

Installed near residential and industrial premises. In the article, we will consider the purpose, design and classification of hydraulic fracturing. We also give the basic principles for installing points and requirements for their operation.

Deciphering and types of hydraulic fracturing

Gas control point (GRP) is a complex consisting of technological equipment and mechanisms for adjusting gas pressure. The main purpose of the installation is to reduce the inlet pressure of the natural substance and maintain a given level at the outlet, regardless of consumption.

Types of hydraulic fracturing in relation to the installation site of the equipment are:

• GRPSH (cabinet gas control points) - for this type, it is planned to place the corresponding equipment in a special cabinet made of fireproof materials;
• GRU (gas control units) - for this type of equipment, it is mounted on a frame and located at the place where gas is used or elsewhere;
• PGB (gas control block points) - with this placement, the equipment is mounted in container-type buildings, one or more;
• GRP (decoding - stationary gas control points) - with this type of equipment located in specialized buildings or separate rooms, such a device is not accepted as a standard product with full factory readiness.

Classification

Hydraulic fracturing can be classified according to several parameters. For example, if possible, lowering the gas pressure. The breakdown of the hydraulic fracturing is discussed below.

1. Single-stage gas control points. In such systems, the gas pressure from the inlet to the working one is regulated in one stage.
2. Multistage gas control points. In systems with too high pressure, one regulator may not be able to cope with the reduction function. In this case, the adjustment takes place in several steps by setting one or more regulators.

According to the outlet gas pressure, which is provided by hydraulic fracturing (decoding: gas control points), installations are distinguished that provide the same or different pressure.

Also, hydraulic fracturing can be with one or two outlets. The execution of the device is left-handed or right-handed, depending on the place of gas supply.

The entry and exit of a volatile substance can be made from opposite sides of the hydraulic fracturing, on the one hand, be vertical and horizontal.

The gas pressure at the outlet of the point may vary, while hydraulic fracturing is classified:

Hydraulic fracturing reduction lines

The decoding of the hydraulic fracturing has already been given. Items can be dead ends or loops. This scheme is used for the reliability of gas supply. It consists in combining several hydraulic fracturing. It is believed that the more installations are looped, the higher the reliability of the system. A dead-end scheme is considered when it is impractical to use more than one hydraulic fracturing for gas supply to the consumer.

According to the technological schemes of hydraulic fracturing, there are:

1. Single line items. They are equipped with one gas reduction line.
2. Multithreaded. They can be equipped with two or more gas reduction lines connected in parallel. Such a device is used when trying to achieve maximum reliability and performance parameters of hydraulic fracturing.
3. Bypass. Reserve reduction line, which is used during the repair of the main line.

Regulators in multi-line installations can be connected in parallel or in series.

The hydraulic fracturing unit is equipped with the following equipment:

• gas pressure reducer;
• gas filter;
• safety fittings;
• stop valves;
• instrumentation;
• a substance input unit for gas odor;
• gas heaters.

Two locking devices are installed on the reserve line, between which a pressure gauge is mounted.

single thread points

Gas control points (decoding of hydraulic fracturing) with one gas reduction line consist of: process equipment and a frame on which it is placed.

The principle of operation of such devices:

1. The gas passes through the inlet and enters the filter. Here it is cleaned from harmful substances and impurities.
2. Then the gas is supplied to the pressure regulator through a safety shut-off valve, in which the pressure is regulated - lowering to the required parameters, as well as maintaining the values ​​​​at the desired level.

If, when passing through the regulator, the pressure does not decrease to the standard parameters, then the safety relief valve or hydraulic seal is activated.

If the gas is not discharged, then the safety shut-off valve is activated and the gas supply to the RN-GRP is interrupted (decoding: pressure parameter at the beginning of the opening of the slam-shut valve) no more than +0.02 MPa - the normatively set value of the valve actuation (GOST R 53402-2009 8.8.2.7).

In gas control installations, regulators of both direct and indirect action can be used.

When choosing a hydraulic fracturing with one reduction line, they usually rely on the operating parameters of the regulator: throughput, inlet and outlet pressure.

Multi-strand points

Deciphering the abbreviation of hydraulic fracturing - gas control points, this has already been said, there are with one reduction line, with two or more.

Regulators on the gas pressure relief line can be installed either in parallel or in series.

The principle of operation of a multi-thread system:

1. One source is used for gas supply.
2. After entering, the gas is distributed through all hydraulic fracturing lines.
3. At the output, the lines are combined into one collector.

Multi-thread systems are more reliable, because if one reduction line fails, its functions can be performed by the rest. Similar actions are carried out and, if necessary, technical work: regulator replacement, filter cleaning.

The schemes are mainly used at high pressure points, for example, for supplying industrial consumers. Multi-thread systems are more expensive than single-thread counterparts, they have large dimensions.

Fracturing with bypass line

The above describes how hydraulic fracturing is deciphered and what types it happens. In this paragraph, the last option for organizing a gas control point will be presented - with a bypass.

A bypass is called a bypass, another name is a reserve, natural gas reduction line. It is used at the time of repair of the main.

Multi-thread or single-thread circuits are endowed with a bypass line. It is equipped with the same equipment as the worker, but does not participate in the gas supply process if the main line is working.

For hydraulic fracturing, first of all, wells with low productivity are chosen, due to the natural low permeability of the rocks, or wells, the filtration capacity of the bottomhole zone of which has deteriorated when the reservoir is opened. It is also necessary that the formation pressure be sufficient to ensure the flow of oil into the well. Before rupture of the rocks, the well is examined for inflow and its absorption capacity and absorption pressure are determined. The results of the inflow study and data on the absorption capacity of the well before and after the fracture make it possible to judge the results of the operation, help to roughly assess the fracture pressure, correctly select the appropriate properties and amount of fluid for fracturing, judge changes in the permeability of the rocks of the bottomhole zone after the fracture. Before starting work, the well is cleaned of dirt by drainage and flushed to improve the filtration properties of the bottomhole zone. Good results fracturing can be obtained by pre-treatment of the well with hydrochloric or clay acid (a mixture of hydrochloric and hydrofluoric), since when opening the reservoir, the permeability of the rocks deteriorates in those intervals where the filtrate and clay solution penetrate the most. Such proppasts are the most permeable sections of the section, which, after opening the formation when drilling in clay mud, sometimes become slightly permeable to the fracturing fluid. After preliminary acid treatment, the filtration properties of such formations are improved and favorable conditions to form cracks.

Pump pipes with a diameter of 76 or 102 mm are lowered into the washed, cleaned well, through which the fracturing fluid is fed to the bottomhole (Figure 1.3). When lowering pipes of smaller diameter, due to significant pressure losses, the rupture process becomes more difficult. To protect the casing from high pressure, a packer is installed above the formation. So that it does not move along the column when the pressure on the pipes increases, it is recommended to install a hydraulic anchor (Figure 1.4). The greater the pressure in the pipes and inside the anchor, the more force the anchor pistons extend and press against the casing. The annular edges at the end of the pistons, crashing into the column, have the greater the braking effect, the higher the pressure. There are anchors and other types.

Figure 1.3 - Scheme of well equipment for hydraulic fracturing: 1 - packer; 2 - hydraulic anchor; 3 - tubing; 4 - filling head

Figure 1.4 - Scheme of the hydraulic anchor device

The wellhead is equipped with a special head, to which liquid injection units are connected. The general scheme of piping and location of equipment near wells is shown in Figure 1.5.

Figure 1.5 - Equipment piping scheme for hydraulic fracturing: 1 - oil tank; 2, 4 - high pressure units; 3 - well; 5 - auxiliary unit; 6 - sand mixer; 7 - tank trucks

The fracturing of the formation is carried out by injecting the fracturing fluid into the pipes until the formation is stratified, which is marked by a significant increase in the well injectivity factor. If a poorly filterable fluid is used for fracturing, and also if the permeability of rocks in the near-bottom zone is noticeably worsened due to clogging with clay mud, a decrease in injection pressure is sometimes observed at the time of fracturing.

The first fracturing fluids were oil-based, but since the late 50s. began to use water-based liquids, 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. Oil-based gels are used in 5% of cases, foams with compressed gas (usually CO 2 and N 2) are 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.

Modern materials used to fix cracks in the open state are proppants. They are classified as follows: quartz sands and medium to high strength synthetic proppants. 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 main and most widely used material for fixing cracks is sand. Its density is 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 2 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 2 . 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/cm 2 . Synthetic resin-coated proppants are also produced and used.

Strength is the main criterion in the selection of proppants for specific reservoir conditions in order to ensure long-term fracture conductivity at the reservoir depth. Therefore, the following types of proppants are used for different depths: quartz sands - up to 2500 m; medium strength proppants - up to 3500 m; high strength proppants - over 3500 m.

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 80s. during the hydraulic fracturing, mainly domestic or Romanian equipment was used, in some cases - American.

Now there are wide potential opportunities 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) territories, Saratov (2000 m) , Orenburg (3000-4000 m) and Astrakhan (Karachaganak field (4000-5000 m)) regions.

Choice technological scheme and processing efficiency largely depend on the capacity of the equipment. Determined that best results obtained with high pressures injection and great performance equipment, which is apparently due to the significant opening of cracks at high pressures and their filling with sand. Domestic industry produces units 2AN-500 and 4AN-700, designed for hydraulic fracturing. Unit AN-500 can create working pressure up to 50 MN/m 2 . The use of 3-4 units simultaneously makes it possible to inject fracturing fluid into the well at a rate of 10-15 dm 3 /sec at a pressure of up to 50 MN/m 2 . The process of mixing sand with liquid is mechanized with the help of special sand mixing units. The P-100 sand mixing unit designed by Hydroneftemash is capable of creating sand content in the sand carrier up to 1000 kg/m 3 with a dry sand capacity of up to 100 t/h. A mobile laboratory has been designed to continuously monitor the parameters of fracturing fluids and the technology of the process.

In addition to the described hydraulic fracturing scheme, other technological schemes are used depending on the process conditions and its purpose.

In shallow wells, fracturing can be performed without running tubing, or with tubing but no packer. In the first case, the liquid is injected directly through the casing pipes, and in the second case, both through the pipes and through the annulus. With this technology, it is possible to significantly reduce the pressure loss in the well when injecting a very viscous fluid. Multiple fracturing can also be used to improve inflow conditions. Its essence lies in the fact that several fractures are created in the reservoir at different depths and, thus, the permeability of the rocks of the bottomhole zone in the wells is significantly increased.

Multiple fracturing of the formation can be carried out in the following ways:

1. Perform hydraulic fracturing using conventional technology, and then, together with the fluid, inject substances into the well that temporarily plug the fracture or close the perforations against the fracture interval. This makes it possible to increase the pressure again and break the reservoir in another place. Granular naphthalene, elastic plastic balls, etc. were used as a plugging material. During well development, naphthalene dissolves in oil and is removed from the fracture, and the balls are brought to the surface by the flow.

2. The zone intended for the formation of fractures can be separated each time by two packeramns or hydraulic gates and the formation can be fractured using conventional technology.

3. Perform multiple fracturing with isolation of the underlying layers of the productive formation with a sand plug.

In sections with a large number of clay interlayers, i.e. with low vertical permeability, it is highly desirable to create vertical fractures connecting productive interlayers. To form vertical cracks, non-filterable fracturing fluids are used. Vertical fractures can also be formed during the injection of filterable fracturing fluids with a rapid increase in fluid flow and bottomhole pressure.

To facilitate the fracturing of the formations in a pre-selected place, it is possible to preliminarily carry out sandblasting or torpedoing of the string: the same section is separated (sealed) by packers.

More than 2,500 hydraulic fracturing operations were carried out annually in the fields of the USSR. The efficiency of hydraulic fracturing is approximately 70%.

Hydraulic fracturing technology is rapidly improving. Workers of research institutes and crafts have proposed a large number of various options interval fracturing, methods of protecting the cement sheath from destruction or rupture, and various technological methods that improve fracturing results.

Very important issue during hydraulic fracturing, one that requires special attention is determining the location and nature of the resulting fractures. This problem is successfully solved by methods of radioactive logging, carried out after the introduction of a mixture of ordinary and radioactive sand into the fracture. Sand is activated by adsorption and fixation of radioactive substances on its surface. The adsorbed active ingredient can be fixed by coating the grains of sand with water- and oil-insoluble adhesives. There are clear radioactivity anomalies in the gamma ray curves in the fracture interval.

In the modern oil production industry, hydraulic fracturing (HF) is a effective method impact on the bottomhole area of ​​the 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 to develop 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 the 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. At correct application the efficiency of oil production in the 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, a multi-stage fracturing is carried out, in which different layers receive 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.