RU2588086C2 - Electric and static fracturing - Google Patents

Abstract

FIELD: mining.

SUBSTANCE: group of inventions can be used for fracturing a geological hydrocarbon reservoir. According to one version, method includes static fracturing (S20) of reservoir by means of hydraulic pressure, and electrical fracturing (S10) of reservoir by generating an electric arc in a well drilled into reservoir. Static fracturing (S20) precedes electrical fracturing (S10).

EFFECT: high efficiency of fracturing.

15 cl, 16 dwg

Description

This invention relates to a device and method for fracturing a geological hydrocarbon reservoir, as well as to a method for producing hydrocarbons.

In hydrocarbon production, the permeability and / or porosity of the formation-forming material affects the production of hydrocarbons, in particular, the speed and, therefore, the profitability of production. This is discussed, for example, in an article by Soeder D.J. “Porosity and permeability of Eastern Devonian Shale gas” published in SPE Formation Evaluation, 1998, Vol.3, No.1, pp.116-124, which describes the study of eight samples of shale gas from the Devonian complex, taken in the Appalachian Mountains. This article explains, in particular, that the production of this shale gas is difficult due to the fact that the formation (more precisely, the material forming the formation) has low permeability.

Therefore, there are various technologies aimed at facilitating the production of hydrocarbons, in particular, from reservoirs with low permeability and low porosity. The essence of technology data is static or dynamic fracturing.

Static fracture is a directed dislocation of a formation by injection at very high pressure of a fluid intended for cracking the rock. Cracks are produced by mechanical “stress” caused by hydraulic pressure, which is created by a fluid pumped under high pressure from a well drilled from the surface. This operation is also called "hydraulic fracturing", or "hydrosilicate fracture" (or "hydraulic fracturing", or, most often, "fracking", or "massive hydraulic fracturing"). The static burst method described above is disclosed in particular in US 2009/044945 A1.

The disadvantage of static fracturing is that fracturing is usually unidirectional. Thus, it will be faster to produce only the hydrocarbon that lies in a section of the formation around a deep, but very clearly localized fracture.

To achieve a wider gap, a method of dynamic, or electric, gap was developed. An electric fracture consists in creating an electric arc in a well drilled in a formation (as a rule, in a production well). An electric arc induces a pressure wave, which causes damage to the formation in all directions around the wave, thereby increasing its permeability.

The electrical break is described in a number of documents. Thus, for example, US Pat. No. 4,074,758 discloses a method consisting in generating an electro-hydraulic shock wave in a fluid in a borehole to increase the rate of oil production. No. 4,164,978 proposes to generate, following a shock wave, an ultrasonic wave. US 5106164 also describes a method for creating a plasma explosion with subsequent rock breaking, but for use in mining in cases with shallow trunks and not for hydrocarbon production. In documents US 4651311 and US 4706228 disclosed a device for creating an electric discharge by electrodes in a chamber containing an electrolyte in which the electrodes are not corroded by the action of the discharge plasma. WO 2009/073475 describes a method for generating an acoustic wave in a fluid located in a well by means of a device comprising two electrodes between an upper packer and a lower packer defining a limited space. According to this document, an acoustic wave is kept in a non-“shock” state in order to cause more severe damage, however, this document does not explain the differences between a “normal” acoustic wave and a “shock” wave.

None of these documents describe a fully satisfactory fracture. Thus, there is a need for an improved method for fracturing a hydrocarbon reservoir.

To this end, a method for fracturing a geological hydrocarbon reservoir is proposed, the method comprising static fracturing by hydraulic pressure and electrical fracturing by creating an electric arc in a well drilled in the reservoir.

According to examples, the method may contain one or more of the following features:

- static break precedes electrical break;

- the well is horizontal;

- electric fracture is repeated in different zones of impact along the well;

- in each impact zone, several arcs are created one after another, said arcs preferably inducing a pressure wave whose rise time decreases;

- in each impact zone create arcs with a frequency equal to the resonant frequency of the material to be fractured in the reservoir;

- create arcs with a frequency of less than 100 Hz, preferably less than 10 Hz, and / or more than 0.001 Hz, preferably more than 0.01 Hz;

- the reservoir has a permeability of less than 10 microdars;

- the reservoir is a shale gas reservoir;

- electric fracture is created by means of a fracturing device, which contains two packers that define between themselves a limited space in the well drilled in the formation; a pump for increasing fluid pressure in a confined space; a device for heating the fluid; at least one pair of two electrodes located in a limited space; and an electric circuit for creating an electric arc between the two electrodes, the circuit comprising at least one voltage source connected to the electrodes and an inductance element between the voltage source and one of the two electrodes;

- the inductance element is a coil with adjustable inductance, preferably from 1 μH to 100 mH, more preferably from 10 μH to 1 mH;

- the distance between the electrodes is adjustable, preferably from 0.2 to 5 cm, more preferably from 1 to 3 cm;

- the voltage source contains a capacitor with an adjustable capacitance;

- the voltage source contains a Marx generator.

A method for fracturing a geological hydrocarbon formation previously subjected to static fracturing by hydraulic pressure is also provided, said method comprising the electrical fracturing described above.

Also proposed is a hydrocarbon production method comprising fracturing a geological hydrocarbon reservoir according to the method described above.

Other features and advantages of the invention are apparent from the following detailed description of embodiments of the invention, which are given solely as examples with reference to the accompanying drawings, in which:

- figure 1-3 shows a diagram illustrating the proposed methods of rupture;

- figure 4-6 shows an example of an electrical break according to the method of breaking shown in any of figures 1-3;

- Figures 7-10 show examples of a specific device for creating an electric arc;

- 11-16 show examples of measurements.

As shown in figure 1, the proposed method of fracturing a geological hydrocarbon reservoir. The method shown in FIG. 1 comprises a static fracture (S20) of the formation by hydraulic pressure. The method shown in FIG. 1 also includes an electrical fracture (S10) of the formation produced before, during, or after the static fracture (S20) (these three possible options are shown in FIG. 1) by creating an electric arc in the well drilled in the formation . The method shown in figure 1, increases the efficiency of fracturing.

The expression "electric arc" refers to the electric current generated in an insulating medium. When creating an electric arc, a “pressure wave” is induced, that is, a mechanical wave, during the passage of which the medium through which the mechanical wave passes undergoes the action of pressure. Creating an electric arc allows you to achieve more extensive / multidirectional damage to the reservoir compared to damage that occurs during a static fracture. Thus, the creation of an electric arc leads to the formation of microcracks in all directions around the location of the electric arc, thereby increasing the permeability of the formation, usually 10-1000 times. In addition, this increase in permeability occurs without the use of means to prevent the closure of microcracks, for example, without proppant injection. Moreover, the electrical break (S10) does not require a large amount of energy or a large amount of water. Therefore, there is no need for a special water recirculation system.

Thus, access to the hydrocarbon contained in the formation, which is difficult to access using static fracturing, can be obtained. Therefore, by combining the static fracture (S20) and the electrical fracture (S10), it is possible to achieve a more effective cumulative fracture.

The electric arc is preferably created in a fluid that is located in a well drilled in the formation. As a result of this, the pressure wave from the electric arc is transmitted with less attenuation. A drilled well contains a fluid, typically water. In other words, when the electrical fracture (S10) follows the drilling operation, the drilled well may be automatically filled with water in the formation. If the drilled well does not automatically fill, you can perform artificial filling.

Static discontinuity (S20) can be any type of static discontinuity known in the art. Typically, a static fracture (S20) may comprise, after possibly drilling a well in a formation, a step of injecting a high pressure fluid into the well. In this case, the static fracture (S20) leads to the formation of one or more unidirectional cracks, usually deeper than the cracks formed during the electrical break (S10).

The fluid may be water, a drilling fluid, or a controlled viscosity technical fluid enriched in solids (sifted sand grains or ceramic beads) that prevent the fracture network from closing when pressure drops.

The static fracture (S20) may comprise a first phase comprising injecting a fracturing fluid into the drilled well that contains thickeners, and a second phase comprising periodically introducing a fixing agent (i.e., a supporting agent) into the fracturing fluid, to feed the fixing agent into the formed gap. As a result, clusters of fixing agent are formed in the gap, preventing the gap from closing and promoting the formation of channels for the flow of hydrocarbons between the clusters. The second phase or its sub-phases provide an additional introduction of reinforcing and / or fixing material, which allows to increase the strength of the clusters of fixing agent formed in the hydraulic fracturing fluid. Thanks to the mentioned static gap (S20), it is possible to obtain discontinuities having, as a rule, a length of 100 to 5000 meters.

Static discontinuity (S20) may precede electrical discontinuity (S10). In this case, the pressure wave generated due to the electric discontinuity (S10) may follow the path of the fluid introduced into the cracks formed during the static discontinuity (S20), thereby increasing the degree of damage. In addition, with this sequence of discontinuities (S20) and (S10), the probability of leaks is rather small. For example, a static break (S20) can be made less than a week before an electrical break (S10).

As shown in figure 2, the proposed method of fracturing a geological hydrocarbon reservoir, previously subjected to static fracture by hydraulic pressure. The method shown in FIG. 2 contains only an electrical fracture (S10) of the formation, carried out in the formation, where one well has already been drilled and has undergone a static fracture. Thanks to the method shown in FIG. 2, damage to formations that are already in operation after a static fracture is possible. In other words, the method shown in FIG. 2 enables the operation of an abandoned, previously operated, formation with the possible reuse of an already drilled well. It should be noted that if the method shown in FIG. 2 is combined with this preliminary static discontinuity, then it corresponds to the method shown in FIG. 1 (where the static discontinuity (S20) corresponds to this preliminary static discontinuity). Thus, preliminary static rupture can be performed using the method shown in figure 1.

As shown in FIG. 3, a method for fracturing a geological hydrocarbon formation, comprising a specific electrical fracture (S10), is proposed. The electrical break (S10) proposed according to the method shown in FIG. 3 can obviously be used in the method shown in FIG. 1 and / or in the method shown in FIG. 2. The method shown in FIG. 3 mainly comprises an electrical fracture (S10) of the formation by creating an electric arc in a fluid located in a well drilled in the formation (and therefore, combined or not combined with a static fracture, for example a static fracture ( S20) according to the method shown in FIG. 1). An electric arc induces a pressure wave whose rise time exceeds 0.1 μs, preferably exceeds 10 μs. The method shown in figure 3, increases the efficiency of fracturing.

The rise time of a pressure wave is the time it takes for the pressure wave to reach a peak pressure, that is, the maximum value of the wave (it is also called "peak pressure"). In this case, a rise time of more than 0.1 μs, preferably more than 10 μs, corresponds to a pressure wave that penetrates deeper into the formation. Such a pressure wave is especially effective (that is, the wave penetrates deeper) when working with low-plastic materials, for example, those that make up shale gas strata. Preferably, the rise time is less than 1 ms, more preferably less than 500 μs.

The pressure wave may have a maximum pressure of up to 10 kbar, preferably more than 100 bar and / or less than 1000 bar. This may correspond to stored energy ranging from 10 JJ to 2 MJ, preferably from 10 kJ to 500 kJ.

The following is a description of various possible options with respect to any of the methods shown in FIG. 1, FIG. 2, or FIG. 3.

The well may be horizontal. For example, the well may be horizontal and have a length preferably in the range of 500 to 5000 m, more preferably 800 to 1200 m, for example, at a depth of 1000 to 10000 m, for example, 3000 to 5000 m.

The electrical fracture (S10) can be re-implemented in different impact zones along the well. In fact, with an electric break (S10), a pressure wave usually penetrates less deeply than with a static break. Thus, when an electric break occurs, as a rule, cracks with a length of less than 100 m, usually less than 50 m and more than 20 m are obtained. If there is a well several hundred meters long, re-performing an electric fracture (S10) along the well allows damage along the entire length of the well and, therefore, a more efficient reservoir operation is possible.

In addition, in each impact zone (or in one impact zone, if it is the only one), several arcs can be created one after another. In this case, the sequence of electric arcs is repeated in a practically fixed position. As a result of this, more severe damage is achieved by repeated exposure to a pressure wave. The created arcs can be either the same or different. For example, in each impact zone, arcs created one after another induce a pressure wave whose rise time decreases. For example, arcs following one after another may have an increasingly steep front, as a result of which a pressure wave with an ever shorter rise time is induced. In this case, the first pulses have slower fronts, penetrating more deeply, while pulses with steeper fronts provide gaps closer to the well and with a higher density. This optimizes the damage process. The first arcs can induce, for example, a pressure wave whose rise time is greater than 10 μs, preferably more than 100 μs. In this case, the last arcs can induce a pressure wave whose rise time is shorter than the rise time in the case of the first arcs, for example, less than 10 μs or 100 μs. The number of first arcs is at least one arc, preferably less than 10000 and even 1000 arcs, and the number of last arcs is at least one arc, preferably less than 10000 and even 1000 arcs.

In addition, in each impact zone, arcs can be created with a frequency of less than 100 Hz, preferably less than 10 Hz, and / or more than 0.001 Hz, preferably more than 0.01 Hz. Preferably, the frequency of the arcs may be (approximately) equal to the resonant frequency of the material to be fractured in the formation. This ensures a higher degree of damage.

The formation may have a permeability of less than 10 microdars. In particular, the formation may be a shale gas formation. In formations of this type, gas is typically absorbed (up to 85% for the Lewis shale structure) and is poorly retained in the pores. Due to the low permeability of formations of this type, one cannot expect direct production of gases held in such an environment, so that only surface gas (absorbed gas) can be produced. So, when working with a shale gas reservoir, the permeability of which is of the order of one microdars, an electric fracture (S10) made in a radius of 30 m along a horizontal well 1000 m long will allow for a gas recovery coefficient that can exceed 50 MNm ³ (assuming a value of 26 Nm ³ per m ^{3 of} rock, as recommended in the article “Porosity and permeability of Eastern Devonian Shale gas” cited above). Thus, the fracturing method shown in any of FIGS. 1-3 may be included in a method for producing hydrocarbons from a formation, typically a shale gas formation.

By creating an electric arc, a temperature gradient can be induced to generate a pressure wave in the fluid. The electrical break (S10) may comprise pre-injecting an agent into the fluid to increase the ductility of the formation-forming material. This agent may contain a chemical additive. As a chemical additive, a rock break inducing agent can be used. The additive may contain steam, which will further increase the efficiency of the gap.

Next, with reference to FIGS. 4-6, an example of an electrical break (S10) according to a break method shown in any of FIGS. 1-3 is described. In this example, an electric fracture (S10) is performed in the formation 40, in which a horizontal well 43 is drilled. In this case, the electrical fracture (S10) is combined with a static fracture, not shown separately and, if necessary, pre-performed, as a result of which the main cracks 41. Here, when using the proposed fracturing method, hydrocarbon production is carried out through a production pipeline located on the surface at the wellhead 45. In this case, an electric arc is created in the area of the explosive device 47.

In the example shown in FIGS. 4-6, an electric break (S10) induces secondary cracks 42 in the arc zone. According to this example, the secondary cracks 42 are not as long as the main cracks 41, but are more wide. In this example, the electrical fracture (S10) is repeated in different exposure zones along the well. Figure 4 shows, in fact, the initial phase of the electrical fracture (S10) in the bottom of the well. Figure 5 shows the intermediate phase in the middle of the well. Finally, FIG. 6 shows the final phase at the start of the well. Thus, the propagation of secondary cracks 42 during the repetition of electrical discontinuities can be observed here. Thus, the secondary fractures 42 are scattered from all sides of the well 43. In this case, the hydrocarbon surrounding the secondary fractures 42 can be extracted, which is potentially distant from the main fractures 41 and, therefore, is difficult to recover when using a static fracture alone.

In general, an electric arc according to the method shown in any of FIGS. 1-3 or FIGS. 4-6 can be created by any device provided for creating said arc. However, a specific device for creating an arc is described below. Obviously, it is possible to supplement the method shown in any of FIGS. 1-3, and, in particular, the electrical break S10 according to this method, with the various functionalities of a particular device (that is, the various effects that it can have).

A specific device for fracturing a geological hydrocarbon formation contains two packers defining between each other a limited space in a well drilled in the formation (that is, configured to close it at least when a particular device is installed in a well drilled in the formation) and an electrical circuit (configured / adapted / provided) to create an electric arc between two electrodes located in a limited space. The circuit comprises at least one voltage source connected to the electrodes and an inductance element between the voltage source and one of the two electrodes. The device also includes a pump for increasing the pressure of the fluid in a confined space and a device for heating the fluid. A specific device increases the efficiency of fracturing.

Packers may be provided that match in shape with the shape of the borehole wall, usually cylindrical, so that they define a limited space between them. As an alternative or addition, the device may comprise a membrane that separates a limited space. In this case, the membrane is preferably made of a material selected in such a way as to ensure the free passage of pressure waves, which will optimize the electrical break (S10). The word "limited" here means that the limited space is defined in such a way that the pressure and temperature in it can be changed by means of a pump and a heating device, as is known to a person skilled in the technical field. This allows you to optimize the state of the fluid in a limited space in order to accelerate the appearance of an electric arc between the two electrodes, depending on the state of the formation or the nature of the fluid. For example, an increase in temperature at a constant pressure usually contributes to the appearance of an electric arc. Thus, “restriction” can mean, but does not necessarily mean, complete closure, just as tightness can be, but is not necessarily, complete.

The circuit contains at least one inductance element between the voltage source and the electrode to which it is connected. The inductance element can be any component that provides a delay in current over time relative to voltage. The inductance value is expressed in Henry. The inductance element may be a coil, optionally wound on a core of ferromagnetic material or on ferrites. For the inductance element, the terms “inductor”, “solenoid”, when it comes to the coil, or “own inductance” are also used. The inductance element weakens the current front in the circuit. This allows you to achieve a slower increase in the pressure wave, and therefore, to obtain a pressure wave, which penetrates deeper into the reservoir. As a result, formation damage is deeper. In particular, the inductance value may be more than 1 μH or more than 10 μH and / or less than 100 mH or less than 1 mH.

The device can be made with the possibility of movement along the well and can be fixed before creating an electric arc. For example, the device may comprise means of movement, for example, by remote control. Due to this, the device can be adapted, in particular, for the method of rupture shown in Fig.4-6, with all the ensuing advantages. In this case, the device can receive power from a high-voltage voltage source located on the surface and connected to the device with electric cables laid along the well. (In fact, in the example shown in FIGS. 4-6, the mobility of the bursting device 47, which may be a specific device, allows fracturing along the entire length of the well. In this example, the device 47 is powered by a high voltage voltage source 44 located on surface and connected to the device 47 by cables 46.) In this case, the device may also include a trip system. Due to this, you can leave the device in the well in case of blocking. In this case, it is possible to restore the well and / or drill string.

The device may have a generally elongated shape, which facilitates its movement in the well. The device may also contain several pairs of electrodes located at the same distance. Electrodes can receive power from several storage capacitors. This allows you to break faster. In fact, it is possible to simultaneously create several electric arcs between each pair of electrodes and simultaneously perform several damage operations.

The device may include a system for injecting a chemical additive containing a storage tank for storing the additive and a pump for pumping the additive into a limited volume during operation of the device. The heating device may comprise a source of hot fluid and a supply pipe, the pipe having an opening adjacent to the electrodes so that during the operation of the device it is possible to supply hot fluid from the source to the electrodes to create a temperature gradient between them. The supply pipe may pass through one or both electrodes. Thanks to these various features, optimization of the conditions conducive to the formation of an electric arc is possible.

The following is a description of other potential features of a particular fracturing device for a geological hydrocarbon formation with reference to FIGS. 7-10, which shows a device 100 representing an example of the above specific fracturing device for a geological hydrocarbon formation.

The device 100 shown in FIG. 7 contains two packers 102 and 103 defining a limited space 104 between them. In this case, the limited space 104 is further separated by a membrane 108. The device 100 also contains two electrodes 106 located in a limited space 104. In this As an example, two electrodes 106 are connected, respectively, to a voltage source via an input element 109 and to the mass 103 of the circuit (in this case, it is aligned with the packer 103), which allows an electric arc to be formed between the two electrodes 106. The radius of the electrodes may be from 0.1 mm to 50 mm, preferably from 1 mm to 30 mm.

A pump for increasing the pressure of a fluid in a confined space and a device for heating a fluid are not shown in FIG. An electric circuit for creating an electric arc between two electrodes 106, a voltage source for it and an inductance element are also not shown, but they can be made as shown in Figs. 8-10, where exemplary embodiments of the device 100 are schematically shown.

The device 100 shown in FIG. 8 comprises an inductor 110. The voltage source comprises a capacitor 112. As shown in the diagram of FIG. 8, when a capacitor 112 is discharged, an electric arc may appear between the electrodes 106. Capacitor 112 has a capacitance of more than 1 μF, preferably more than 10 μF. Thanks to this capacity, you can get the energy that causes the appearance of a subsonic arc.

An electric arc is called “subsonic” or “supersonic” depending on its speed. As a rule, a “subsonic” arc is associated with thermal processes: the arc propagates through gas bubbles formed as a result of heating water. In such cases, they speak of a “slow” propagation of the electric discharge, usually of the order of 10 m / s. The main characteristics of a subsonic discharge are associated with the significant energies involved (usually more than several hundred joules), with thermal processes due to the long time of application of voltage, and with low voltage levels (weak electric field). In this discharge mode, the pressure wave propagates in a large volume of gas before it begins to propagate in the fluid. A “supersonic” arc is usually associated with electronic processes. The discharge spreads in water without the participation of a thermal process in a filiform form. In such cases, they speak of a "fast" propagation of an electric discharge, of the order of 10 km / s. The characteristics of a supersonic discharge are associated with insignificant energies involved, with high voltages, which are associated with a short application time, and with strong electric fields (MV / cm). In this discharge mode, the thermal effects are negligible. Considering that a discharge cannot develop directly in the liquid phase, one can use the concept of microbubbles to explain the development of this discharge mode. The gas volume involved here is less than in the case of subsonic discharges.

Capacitor 112 may have a capacitance of less than 1000 microfarads, preferably less than 200 microfarads.

The capacitor 112 is separated from the inductance element by a surge arrester 114, which can be activated from the pulse generator 116. This allows you to control the discharges of the capacitor 112 and, thereby, the pressure waves generated by the electric arc. In particular, the pulse generator 116 may be configured to provide wave re-formation, as described above.

The voltage source (i.e., capacitor 112) is charged by a high voltage charger 120 located in the auxiliary circuit 122 to a voltage U of 1 to 500 kV, preferably 50 to 200 kV. The auxiliary circuit is preferably located on the surface, therefore, is configured to separate from the device.

The device 100 shown in FIG. 9 differs from the example shown in FIG. 8 in that the capacitor 112 and the arrester 114 + pulse generator 116 assembly are replaced by the Marx generator 118. Thanks to the Marx generator 118, during its discharge it is possible to create a supersonic electron arc by applying a higher voltage than when using the capacitor 112.

In the device 100 shown in FIG. 10, the voltage source comprises a capacitor 112 shown in FIG. 8 and a Marx generator 118 shown in FIG. 9. However, the pulse generator 116 triggers the first spark gap 117 of the Marx generator 118. The device 100 further comprises ferrites 119 forming a saturation inductor on a path leading from the capacitor directly to the inductance element. Ferrites 119 are configured to saturate after the discharge of the Marx generator. After the saturation of the ferrites 119, only the capacitor 112 is discharged. This ensures the temporary isolation of the capacitor 112, and therefore the transition (i.e., switching) from a supersonic arc to a subsonic arc. Thus, the device provides a connection between supersonic and subsonic discharge. Thanks to this combination of two modes, supersonic and subsonic, it is possible to achieve higher electro-acoustic efficiency and, consequently, more effective damage at a lower cost of electrical energy. As for the subsonic discharge created by the capacitor 112, it occurs after a delay, which corresponds to the breakdown time of the Marx generator 118. Switching can be completed in less than 1 s. The duration of the discharge created by the Marx generator 118 is usually very short, less than 1 microsecond, and the amplitude exceeds 100 kV.

In accordance with the three examples shown in FIGS. 8-10, and, as shown in the drawings, the various components of the device 100 have adjustable characteristics, that is, their characteristics can be changed before use depending on the formation or during use depending on the reaction or advancing the gap process. Thus, the coil 110 may have an adjustable inductance. The characteristics of the Marx generator 118 (the capacity of each capacitor connected in parallel, the number of working capacitors) can also be adjustable. You can also adjust the distance between the electrodes 106, preferably from 0.2 to 5 cm, more preferably from 1 to 3 cm. You can also adjust the capacitance of the capacitor 112. This allows you to get the device 100, suitable for fracturing any type of formation. In fact, there is no need to replace the device 100 when changing the layer to be fractured (and when the material turns out to be different), since it is enough to change one or more adjustable parameters. It also allows you to optimize the degree of damage by changing as an option remotely the parameters during use.

The above explanations are shown using theoretical constructions and tests described with reference to FIGS. 11-16 and, in particular, in connection with the device 100 shown in FIGS. 8-10.

As shown in FIG. 11, where the normalized voltage amplitude at the terminals of the capacitor 112 is shown, the generation of the pressure wave can be decomposed into two phases: a pre-discharge state phase S100 and a post-discharge state phase S110, which are separated by the appearance of an arc S105.

In the pre-discharge state phase S100, the voltage drops. This voltage drop corresponds to the discharge of the equivalent capacity of the energy bank or Marx generator in the equivalent resistance of the device 100. The higher the equivalent resistance, the better the energy is stored in the pre-breakdown phase. Thus, in each case (subsonic or supersonic), the configuration of the electrodes can provide the lowest energy loss. This corresponds to optimization of water heating in one case and electric field in another case.

In the discharge phase S110, the circuitry can be modeled by an oscillating resistive-inductive-capacitive circuit (RLC circuit).

The following is the equation for the change in current in a serial RLC circuit:

Where

Here U _B is the voltage at the time of the dielectric breakdown of water, and the parameters L, C and R are the inductance, capacitance and resistance of the circuit, respectively.

This current i (t) is a function of the breakdown voltage U _V (dielectric breakdown of the medium) of the capacitor, inductance and circuit resistance.

The experiments showed the linear nature of the peak pressure as a function of the maximum current at the time of the dielectric breakdown of water in two modes of breakdown. An example of the results is shown in FIGS. 12 and 13, where the dependence of the measured peak pressures on the maximum current in the breakdown phase S110 is shown, as well as the linear regression of the measurements, respectively, in subsonic and supersonic modes. It should be noted that the pressure at the same peak current values will be greater in the case of a “supersonic” type discharge. This can be partially explained by the processes that contribute to the creation of an electric arc in water, and in the volume of gas between the electric arc and the liquid in the available space between the electrodes.

Additional experiments showed the influence of the interelectrode distance on the peak value of the pressure wave generated in both modes of dielectric breakdown. It turned out that the length of the electric arc directly affects the pressure. The larger the interelectrode distance, the greater the peak pressure value, as shown in the graph depicted in Fig. 14.

In the experiments, the influence of the geometry of the electrodes on the pressure wave was investigated. The results are shown in FIG. They allow us to conclude that the shape of the electrodes used to create the pressure wave does not affect the peak pressure value. However, it can help minimize electrical losses before the formation of an electric arc.

In addition, a pressure sensor was used to visualize the shapes of the generated pressure waves depending on the frequency spectrum. The fact is that this frequency spectrum can be changed by using the dielectric breakdown mode, by means of the parameters of the electric circuit, by the volume of gas, and also by the nature of the liquid used. Two examples of the frequency spectrum with a discharge in subsonic and supersonic modes were tested. As it turned out, the more low-frequency spectrum, the less extensive damage.

The results of various experiments performed demonstrate the linear nature of the ratio dP _max / dt _p depending on the current front di _max / dt _i , as shown in Fig. 16. The current front affects the pressure front. The slower the current front, the lower the frequency of the pressure.

In addition, the studies clearly demonstrated the effect of damage accumulation depending on the number of strokes. Therefore, the concept of pulse repetition rate is, apparently, a criterion that affects the degree of damage.

Reduction of the above principles

Calculation of peak current indicated by i _max

To calculate the current i _max the following conditions are accepted:

If we use equations (1) and (2), then

In the case where the value of w is approximated (the value of R is extremely small),

Energy ratio

where E _b - energy and U _b - voltage at the time of action of the electric arc.

If we substitute equation (8) in (3), we obtain:

The peak current i _{max is} controlled by the energy available at the time of the arc and indicated by the symbol E _b and the inductance L of the circuit, and these two parameters must be influenced by the user. The resistance R is considered extremely small, and the capacitance C is a function of energy E _b .

The relationship between peak pressure and maximum current

From the results presented in Figs. 12, 13 and 15, the following expression can be derived:

where k ₁ is the function of the interelectrode distance and the breakdown mode.

The greater the interelectrode distance, the greater the coefficient k ₁ .

This implies:

If we substitute equation (11) in (9), we obtain:

Thus, the generated peak pressure is controlled by the current i _max (parameters E _b and L) and the coefficient k ₁ (which is a function of the interelectrode distance and the dielectric breakdown mode of water). Thus, to obtain the desired pressure, it is possible to act on E _b , L and k ₁ .

The ratio between dP _max / dt _p depending on di _max / dt _i

From Fig.16 we can derive the following expression:

where k ₂ is the function of the interelectrode distance and the breakdown mode.

Coefficient k ₂ corresponds to electro-acoustic physical connection.

If we use equations (11) and (15), then

Thus, the front of the pressure wave is controlled by the coefficients k ₁ , k ₂ and the values of L, C (parameters of the electric circuit).

Thus, if we summarize these studies, we can note that:

- in both modes of breakdown, the maximum pressure wave that occurs during the dielectric breakdown of water depends mainly on the value of the maximum current, which has the designation I _max ;

- This peak current value is a function of the breakdown voltage and the impedances of the electrical circuit. When a circuit configuration is specified, one way to optimize the current is to increase the breakdown voltage of the distance. This leads to a maximum increase in electrical energy switched to the environment;

- when the circuit is not fixed, but the switched electric energy is kept constant, the amplitude of the pressure wave is optimized by reducing the impedance of the circuit;

- the form of current injection, the mode of dielectric breakdown, and the nature of the liquid affect the dynamics of the pressure wave. This dynamics together with the acoustic efficiency of the device can also be changed by injection of artificial bubbles and by using the “double pulse” method (subsonic and supersonic);

- when a constant current is injected, the peak pressure value is higher in a supersonic mode than in a subsonic mode;

- when a constant current is injected, the peak pressure value is higher, the greater the interelectrode distance.

The geometry of the electrodes during injection of a constant current does not affect the peak pressure generated, but can play a role in reducing electric losses in the phase of the pre-discharge state.

In conclusion, it is worth noting that the above studies confirm the usefulness of introducing an inductance element between the voltage source and one of the two electrodes for influencing the pressure wave created in the end. Studies also confirm the advantage of having adjustable parameters, for example, inductance, capacitor capacitance, and characteristics of a Marx generator. In fact, since the pressure wave depends on these parameters, the possibility of their regulation will provide control of the pressure wave.

Of course, this invention is not limited to the examples described above and shown in the drawings, but may have many variations that are obvious to a person skilled in the art. For example, the principles set forth above can be applied to the production of seismic data. This is possible due to the fact that, in accordance with one of the alternative options, when creating an electric arc, it is possible to induce a pressure wave with characteristics slightly worse than those necessary for fracturing. This can be realized, for example, by adapting the charging voltage of the rupture device and the charging voltage, as well as by changing the inductance value. In this case, this method of obtaining seismic data may include receiving a reflected pressure wave, and the reflected wave then, as a rule, is modulated as a result of its passage through the material forming the reservoir. The method for obtaining seismic data may also include analysis of the reflected wave to determine the characteristics of the reservoir. Based on the information received, a seismic survey can then be performed.

Claims (15)

1. A method of fracturing a geological hydrocarbon reservoir, the method comprising:
- static fracture (S20) of the formation by hydraulic pressure, and
- electrical fracture (S10) of the formation by creating an electric arc in the well drilled in the formation, and the static fracture (S20) precedes the electrical fracture (S10). 2. The method according to claim 1, in which the well (43) is horizontal. 3. The method according to any one of claims 1 or 2, in which the electrical fracture (S10) is repeated in different zones of exposure along the well (43). 4. The method according to claim 3, in which in each zone of influence several arcs are created one after another, said arcs preferably inducing a pressure wave whose rise time decreases. 5. The method according to claim 4, in which in each zone of influence create arcs with a frequency equal to the resonant frequency of the material to be fractured in the reservoir. 6. The method according to claim 5, in which create arcs with a frequency of less than 100 Hz, preferably less than 10 Hz, and / or more than 0.001 Hz, preferably more than 0.01 Hz. 7. The method according to any one of claims 1 to 2 or 4-6, in which the formation (40) has a permeability of less than 10 microdars. 8. The method according to any one of claims 1, 2 or 4-6, in which the reservoir (40) is a shale gas reservoir. 9. A method of fracturing a geological hydrocarbon formation previously subjected to static fracturing by hydraulic pressure, the method comprising an electrical fracture (S10) of the formation according to any one of claims 1 to 8. 10. The method of rupture according to any one of claims 1, 2, 4-6 or 9, in which an electrical rupture (S10) is created by a tensile device (100), which contains:
- two packers (102, 103), which define a limited space (104) between themselves in a well drilled in the formation;
- a pump for increasing the pressure of the fluid in the specified limited space;
- a device for heating the fluid;
- at least one pair of two electrodes (106) located in the specified limited space (104); and
- an electric circuit for creating an electric arc between two electrodes (106), said circuit comprising at least one voltage source (112, 118) connected to the electrodes (106), and an inductance element (110) between the voltage source and one of two electrodes. 11. The method according to claim 10, in which the inductance element is a coil (110) with adjustable inductance, preferably from 1 μH to 100 mH, more preferably from 10 μH to 1 mH. 12. The method according to claim 10, in which the distance between the electrodes is adjustable, preferably from 0.2 to 5 cm, more preferably from 1 to 3 cm. 13. The method according to claim 10, in which the voltage source contains a capacitor (112) with an adjustable capacitance. 14. The method of claim 10, wherein the voltage source comprises a Marx generator (118). 15. A method of producing hydrocarbons comprising fracturing a geological hydrocarbon reservoir according to the method of any one of claims 1-14.

Images