Method and system for storing energy based on compressed air through fractures in the formation

Abstract

The present application relates to a kind of method and system based on compressed air through stratum fracture energy storage, it includes the following steps: stratum fracture is constructed;The rock gap and / or microfracture around stratum fracture is blocked;Stratum fracture is judged closed;Compressed air is injected into target wellbore to store energy;Compressed air is discharged to release energy.It not only realizes the accumulation of air compression potential energy, but also can accumulate stress potential energy and rock deformation elastic potential energy, and obtain stratum thermal energy;Most importantly, the method is not restricted by geographical conditions, can carry out underground compressed air energy storage anywhere, greatly expand the application range of compressed air energy storage.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of underground energy storage, and particularly relates to a method and system for storing energy through stratum fractures based on compressed air. BACKGROUND

[0002] Many renewable energy sources such as solar energy and wind energy have daily and seasonal intermittency, and the power output is unstable, which is not suitable for providing basic load power supply. Therefore, it is difficult to directly enter the power grid, and solving the energy storage problem is the key to successfully expanding the production scale of renewable energy. Therefore, how to store excess clean power on a large scale and stably transmit power when sunlight and wind supply are insufficient is still a technical problem that countries are tackling.

[0003] As a mature energy storage technology, compressed air energy storage has the advantages of large energy storage capacity, long service life and strong stability, but it has high requirements for geographical conditions and needs to use abandoned mines or underground caves as gas storage media. Therefore, it can only be used in certain specific areas. Although some people use artificial underground cavities as gas storage media at present, the construction investment cost is extremely high, which greatly limits the commercial prospects of compressed air energy storage.

[0004] Although depleted oil and gas reservoirs can be used as potential application sites for compressed air energy storage, there are still considerable limitations: (1) The rock layers of depleted oil and gas reservoirs are usually subjected to long-term oil and gas exploitation, and may have cracks, faults and high-permeability channels, so air is easy to leak to the surrounding strata or the surface, affecting the gas storage efficiency and environmental safety; (2) Due to long-term exploitation, the strata of the depleted oil and gas reservoirs may have been deformed or compacted, reducing the mechanical integrity of the gas storage, which may cause strata collapse or sliding under high pressure, affecting the safety of the gas storage; (3) There may be a certain amount of oil and gas remaining in the depleted oil and gas reservoirs, which may mix with compressed air and produce unpredictable chemical reactions, which may cause safety hazards (such as explosion or fire); (4) The rock layers of depleted oil and gas reservoirs usually have high permeability, which is helpful for oil and gas exploitation but not conducive to maintaining high pressure during gas storage, resulting in high air leakage rate and reduced long-term operation efficiency; (5) Long-term exploitation of depleted oil and gas reservoirs reduces the rock pore pressure, thereby reducing the strata fracture pressure, making it difficult for the strata to withstand high pressure, limiting the energy storage capacity and efficiency.

[0005] The prior art Chinese patent CN117722132A discloses a method and product for transforming a depleted gas reservoir into a compressed air storage, comprising: selecting a depleted well of a depleted gas reservoir as a base well; drilling a plurality of expansion holes on the wellbore of the selected base well to expand the base well; injecting fracturing fluid into the wellbore to fracture all the expansion holes and further expand the base well; when the power generation capacity is large, using the excess power to compress air into the expanded base well for energy storage; when the power generation capacity is small, releasing the stored air in the base well to drive the generator to generate electricity.

[0006] The limitations of the above prior art are: (1) the cost is high because a branch wellbore needs to be drilled; (2) although the energy storage space is expanded by hydraulic fracturing, the expansion degree of the fracture is constrained by the branch well during fracturing; (3) there is no limit to the fracture pressure during energy storage, which may result in insufficient energy storage pressure and the fracture cannot be opened, or the fracture may expand due to excessive energy storage pressure; (4) the permeability of the depleted gas reservoir is not reduced, and the injected compressed air is easy to diffuse to a distant place through the reservoir pores.

[0007] The Chinese patent application with publication number CN119288417 discloses a method and system for storing and releasing energy through multiple wells, which comprises: selecting at least two wellbores that are hydraulically connected through formation fractures or / and rock pores as energy storage or / and energy release wellbores; during energy storage, injecting high-pressure fluid into one or several of the wellbores to seal the wellbores and maintain the elastic deformation of the formation rock to store energy; during energy release, reducing the wellbore sealing pressure to allow one or several of the wellbores to discharge high-pressure fluid, and generating electricity during the discharge process. However, the limitations of this method are: (1) multiple wells are needed to cooperate, and a single well cannot independently complete the energy storage or other construction processes; (2) the formation fractures extending from different wellbores are interlaced, which may form a complex fracture network, which is not conducive to energy storage efficiency and filtration prevention; (3) during energy storage, the fracture pressure is not limited, and the fracture may expand, which may cause energy storage fluid filtration; (4) using the formation rock pores for energy storage is difficult to control the diffusion of the energy storage fluid.

[0008] Chinese patent application CN114016988A discloses a method and system for storing and releasing energy through a formation, which generates at least one formation fracture by hydraulic fracturing in an energy storage formation without oil and gas, and then injects high-pressure liquid into the obtained formation fracture to make the formation fracture elastically deform to store energy, and then pushes the pre-set hydraulic power generation equipment by backflowing the high-pressure liquid in the fracture to release energy. Chinese patent application CN118654399A discloses a method and system for storing and releasing energy through a formation fracture and obtaining geothermal energy. The system includes a fracturing device for pumping fracturing fluid into a wellbore to generate a formation fracture in an energy storage geothermal layer; an injection pipeline connected with an injection device; a backflow pipeline connected with a hydraulic power generation device, a geothermal power generation device, and a heating system, so that the fluid in the formation fracture can drive the hydraulic power generation device and the geothermal power generation device to generate electricity and heat through the heating system when backflowing through the backflow pipeline; and a plugging device for closing the backflow pipeline to make the fluid in the formation fracture continuously exchange heat with the energy storage geothermal layer to continuously increase the temperature of the fluid. This technology utilizes the elastic deformation of the formation fracture to store and release energy, and utilizes geothermal resources to generate electricity or / and heat. The limitations of the above method are: (1) only the elastic deformation energy of the formation rock is recovered, so it is only suitable for recovering the kinetic energy of incompressible liquid, greatly limiting its output power, and this method is not suitable for compressed air energy storage because the main way of compressed air energy storage is through the compression potential energy of air accumulation; (2) the disclosed method only utilizes the heat exchange between liquid and formation, and does not disclose how to utilize the heat exchange between compressed air and formation to obtain heat energy; (3) there is no processing method for the large change in volume and pressure of compressed air when heated in the formation; (4) based on the huge difference in properties between liquid and compressed air, for example, air has a smaller viscosity than liquid (air has a viscosity 50 times smaller than water), and is more prone to leakage, and the person skilled in the art cannot obtain an indication of the sealing performance of the formation fracture from the disclosed method.

[0009] Therefore, there is an urgent need for a compressed air-based energy storage method that has low investment and construction costs, can complete energy storage and release using a single well, and is not limited by geographical conditions. SUMMARY

[0010] Therefore, the present application provides a compressed air-based energy storage method and system through a formation fracture, which is not limited by geographical conditions and can be carried out anywhere for underground compressed air energy storage, greatly expanding the application range of compressed air energy storage.

[0011] The first aspect of the present application discloses a compressed air-based energy storage method through a formation fracture, comprising the following steps:

[0012] S1, injecting a fracturing fluid into the target wellbore to perform hydraulic fracturing operation to create a formation fracture and / or open an original formation fracture; wherein the target wellbore is configured as an independent wellbore not communicating with another wellbore through the formation fracture, i.e. the target wellbore is not communicated with other wellbore through the formation fracture and can be understood as a single independent wellbore;

[0013] S2, injecting a plugging fluid into the formation fracture to physically and / or chemically plug the rock pore and / or microfracture around the formation fracture;

[0014] S3, performing a wellhead plugging based closure test on the target wellbore to determine the closure of the formation fracture;

[0015] S4, injecting compressed air into the target wellbore and the formation fracture and making the post-air- admission pressure of the formation fracture greater than the fracture closure pressure and less than the fracture propagation pressure to store energy, wherein at least includes the compression potential energy accumulated by the compressed air due to the pressure rise in the overall space formed by the target wellbore and the formation fracture;

[0016] S5, monitoring whether there is a need for energy release, if not, plugging the wellhead; if so, back-flushing the compressed air and obtaining at least the compression potential energy released by the compressed air during the back-flushing;

[0017] Wherein the plugging fluid is configured as a fracturing fluid containing a plugging material, so that step S2 can be integrated in step S1 and completed simultaneously with the implementation of step S1, or the plugging fluid is configured as a fluid containing a plugging material and independent of the fracturing fluid, so that step S2 is implemented independently of step S1.

[0018] According to the method disclosed in the first aspect of the application, the fracturing fluid is added with a thickening agent, and the thickening agent includes at least one natural polymer and its derivative, and / or at least one cellulose and its derivative, and / or at least one synthetic polymer, and / or at least one surfactant type thickening agent, and / or at least one gel type thickening agent, and / or at least one clay.

[0019] According to the method disclosed in the first aspect of the application, the plugging material includes a solid plugging material for physically plugging the rock pore and / or microfracture around the formation fracture.

[0020] In another embodiment, the plugging material comprises a liquid plugging material, which is configured to be able to solidify or gel due to a solidification or gelation reaction between its own components, or due to a solidification or gelation reaction between different liquid plugging materials, or due to a solidification or gelation reaction with the fluid in the rock pore and / or microfracture around the formation fracture, or due to a solidification or gelation reaction caused by a change in salinity or pH or temperature after entering the formation, so as to chemically plug the rock pore and / or microfracture around the formation fracture.

[0021] In a particular embodiment, the plugging material can simultaneously comprise the solid plugging material and the liquid plugging material described above.

[0022] According to the method disclosed in the first aspect of the present application, the plugging material is directly added to the corresponding fluid constituting the plugging fluid. In another embodiment, the plugging material is added to the corresponding fluid constituting the plugging fluid in the form of being wrapped in a degradable capsule.

[0023] According to the method disclosed in the first aspect of the present application, step S3 comprises:

[0024] S301, injecting a high-pressure fluid into the target wellbore, so that the pressure in the formation fracture is greater than the fracture closure pressure and less than the fracture propagation pressure, so that the formation fracture is in an open and unexpanded state;

[0025] S302, sealing the wellhead of the target wellbore;

[0026] S303, determining whether the formation fracture is gradually closed under the condition of wellhead sealing by monitoring the change of the pressure in the wellhead and / or wellbore with time, so as to determine the closure of the formation fracture.

[0027] According to the method disclosed in the first aspect of the present application, in step S3, the high-pressure fluid comprises a liquid high-pressure fluid and a gas high-pressure fluid; steps S301-S303 are performed based on the liquid high-pressure fluid first, and then steps S301-S303 are performed based on the gas high-pressure fluid.

[0028] According to the method disclosed in the first aspect of the present application, in step S301, a test pressure threshold is set, which is configured to be greater than the fracture closure pressure and less than the fracture propagation pressure, so that after the high-pressure fluid is injected into the formation fracture, the maximum value of the pressure in the formation fracture is not less than the test pressure threshold and less than the fracture propagation pressure.

[0029] According to the method disclosed in the first aspect of the present application, in step S303, the determination of the closure of the formation fracture comprises:

[0030] monitoring the temperature in the wellbore during the plugging of the wellhead, and recording the start time of the temperature in the wellbore not changing as time t0 when the temperature in the wellbore is monitored to not change;

[0031] starting from the time t0 or starting from the time t0, judging the closure of the formation fracture according to the change of the pressure in the wellhead and / or the wellbore with time.

[0032] According to the method disclosed in the first aspect of the present application, during the execution of the step S4, the method further comprises the step of monitoring the pressure in the wellhead and / or the wellbore and the flow rate of the compressed air. By monitoring the pressure in the wellhead and / or the wellbore and the flow rate of the compressed air, the fracture pressure can be calculated to ensure that the fracture pressure does not exceed the set value.

[0033] According to the method disclosed in the first aspect of the present application, in the step S5, after the wellhead is plugged without the need for energy release, the method further comprises the step of monitoring the pressure in the wellhead and / or the wellbore to calculate the pressure in the formation fracture;

[0034] When the pressure in the formation fracture approaches or reaches the formation fracture propagation pressure or the set fracture working pressure threshold, the compressed air in the target wellbore is backflowed.

[0035] According to the method disclosed in the first aspect of the present application, the method further comprises the step of enabling the formation fracture to have a residual fracture width after the energy release in the step S5, which comprises:

[0036] setting a critical residual fracture width of the formation fracture;

[0037] determining a critical residual compressed air volume based on the set critical residual fracture width;

[0038] In the step S5, the residual volume of the compressed air in the wellbore and the formation fracture during the energy release is monitored, and when the monitored residual volume approaches or reaches the critical residual compressed air volume, the backflow is ended, so that the formation fracture still has a residual fracture width.

[0039] According to the method disclosed in the first aspect of the present application, in the step S5, a power generation device capable of converting the compressed potential energy of the compressed cavity into electrical energy is provided.

[0040] According to the method disclosed in the first aspect of the present application, in the energy storage of the step S4 and the energy release of the step S5, at least one of the following energies is further included: stress potential energy accumulated by the compressed air overcoming the fracture closure pressure to do work, elastic deformation potential energy accumulated by driving the formation rock to elastically deform, and formation heat energy obtained by the heat exchange between the compressed air in the formation fracture and the formation rock.

[0041] According to the method disclosed by the first aspect of the present application, the method further comprises a step of modifying the fracture tip of the formation, comprising:

[0042] sequentially injecting a first fluid and a second fluid into the fracture of the formation, wherein the first fluid comprises at least one liquid polymer and at least one curing agent, and the second fluid is configured to not have a curing reaction or a polymerization reaction by itself;

[0043] after the second fluid displaces the first fluid to the fracture tip of the fracture of the formation, stopping injecting the second fluid into the fracture of the formation, and waiting for the first fluid to complete the curing reaction from a liquid state substance to a solid state substance.

[0044] By means of the step of fatigue repairing the fracture tip, the risk of fatigue effect of the fracture tip can be effectively reduced, and micro-cracks caused by fatigue effect of the fracture tip due to repeated opening and closing of the fracture of the formation can be avoided, thereby avoiding leakage of compressed air at the fracture tip.

[0045] It should be particularly emphasized that the step of modifying the fracture tip of the formation is performed before energy storage, which aims to prevent cracks caused by fatigue of the fracture tip after energy storage and energy release (because the fracture needs to undergo a very large number of opening and closing cycles), i.e., to achieve the purpose of early prevention.

[0046] Further, the fracture toughness of the solid state substance is greater than the fracture toughness of the formation rock where the fracture of the formation is located, or / and the tensile strength of the solid state substance is greater than the tensile strength of the formation rock where the fracture of the formation is located, or / and the shear strength of the solid state substance is greater than the shear strength of the formation rock where the fracture of the formation is located, or / and the compressive strength of the solid state substance is greater than the compressive strength of the formation rock where the fracture of the formation is located, or / and the fatigue life of the solid state substance is greater than the fatigue life of the formation rock where the fracture of the formation is located, or / and the fatigue limit of the solid state substance is greater than the fatigue limit of the formation rock where the fracture of the formation is located, or / and the fatigue strength of the solid state substance is greater than the fatigue strength of the formation rock where the fracture of the formation is located.

[0047] Further, the ratio M of the viscosity of the first fluid to the viscosity of the second fluid is not more than 10. This can make the second fluid have a better displacement effect on the first fluid in the fracture of the formation and the wellbore, avoid the occurrence of viscous fingering, reduce the residual of the first fluid in the area other than the fracture tip, and avoid mixing of the first fluid and the second fluid in the wellbore.

[0048] Further, the density of the first fluid is not less than 70% of the density of the second fluid. This can effectively prevent the first fluid and the second fluid from generating Rayleigh-Taylor instability phenomenon in the wellbore due to the difference in density, and the first fluid and the second fluid from stratifying in the fracture of the formation.

[0049] In another embodiment, the method further comprises a step of modifying the fracture tip of the formation fracture, which comprises:

[0050] injecting the degradable capsule and the third fluid into the formation fracture, wherein the degradable capsule encapsulates at least one reactant, and the third fluid is configured to not generate a curing reaction or a polymerization reaction by itself;

[0051] after stopping the injection of the degradable capsule, continuing to inject the third fluid to displace the degradable capsule to the fracture tip of the formation fracture;

[0052] after the displacement operation is completed, based on the degradation of the degradable capsule at the fracture tip of the formation fracture, causing the reactants released by the degradable capsule to generate a curing reaction between each other and / or with the third fluid to form a solid-state substance.

[0053] Further, in the step of injecting the degradable capsule and the third fluid into the formation fracture, the density of the degradable capsule is configured to be less than the density of the third fluid, so that the degradable capsule is aggregated at the top edge of the formation fracture; or, in the step of injecting the degradable capsule and the third fluid into the formation fracture, the density of the degradable capsule is configured to be greater than the density of the third fluid, so that the degradable capsule is aggregated at the bottom edge of the formation fracture. This enables the aggregation position of the degradable capsule in the formation fracture to be adjusted by changing the density of the degradable capsule, thereby effectively modifying the specific position of the fracture tip of the formation fracture.

[0054] Further, different reactants can be encapsulated in a single degradable capsule, or different reactants can be encapsulated in different degradable capsules.

[0055] The second aspect of the present application also discloses a system for implementing the method disclosed in the first aspect of the present application, which comprises:

[0056] a fracturing unit configured to inject a fracturing fluid into a target wellbore to perform hydraulic fracturing to create a formation fracture and / or open an existing formation fracture; wherein the target wellbore is configured as a standalone wellbore that is not in communication with another wellbore through the formation fracture;

[0057] a plugging fluid injection unit configured to inject a plugging fluid into the formation fracture to physically and / or chemically plug the rock pore and / or microfracture around the formation fracture;

[0058] a closed testing unit configured to perform a closed testing on the target wellbore based on a wellhead plugging to determine the closure of the formation fracture; the closed testing unit comprises:

[0059] a high-pressure fluid injection unit configured to inject a high-pressure fluid into the target wellbore so that the pressure in the formation fracture is greater than the fracture closure pressure and less than the fracture propagation pressure, so that the formation fracture is in an open and unexpanded state;

[0060] a plugging unit configured to perform a wellhead plugging on the target wellbore;

[0061] a pressure monitoring and determining unit configured to monitor the wellhead pressure of the target wellbore, and determine whether the formation fracture is gradually closed under the condition of wellhead plugging by monitoring the change of the wellhead pressure and / or the pressure in the wellbore over time,

[0062] so as to determine the closure of the formation fracture;

[0063] a storage working unit configured to inject compressed air into the target wellbore, and make the post-air pressure of the formation fracture greater than the fracture closure pressure and less than the fracture propagation pressure, so as to store energy, wherein at least includes the compression potential energy accumulated by the compressed air due to the pressure rise in the overall space formed by the target wellbore and the formation fracture;

[0064] a release working unit configured to discharge the compressed air, and at least obtain the compression potential energy released by the compressed air during the discharge process;

[0065] wherein the plugging fluid is configured as a fracturing fluid containing a plugging material, so that the plugging fluid injection unit can be integrated into the fracturing unit to complete the work with the fracturing operation, or the plugging fluid is configured as a fluid containing a plugging material and independent of the fracturing fluid, so that the plugging fluid injection unit works independently of the fracturing unit.

[0066] Beneficial effects: In the method and system for storing energy through the formation fracture by compressed air in the present application, the closed artificial formation fracture can be used for storing energy by compressed air. During the energy storage process, not only the air compression potential energy is accumulated, but also the stress potential energy is accumulated by overcoming the fracture closure pressure, and the elastic potential energy is accumulated by driving the elastic deformation of the formation rock. At the same time, the compressed air in the formation fracture exchanges heat with the formation rock to obtain the formation heat energy. Most importantly, this method is not constrained by geographical conditions and can be carried out anywhere to develop underground compressed air energy storage, greatly expanding the application range of compressed air energy storage.

[0067] The method, system and plugging method for storing energy through the closed formation fracture by compressed air in the present application will be disclosed in detail below in combination with the embodiments shown in the drawings and the reference numerals. BRIEF DESCRIPTION OF DRAWINGS

[0068] Figure 1 A flow chart showing the steps of the method of the present invention for storing energy in a closed formation fracture using compressed air.

[0069] Figure 2 A schematic diagram of the present invention for storing energy in a closed formation fracture using compressed air.

[0070] Figure 3 A comparison of conventional compressed air energy storage (using natural or man-made underground cavities) and compressed air energy storage in a closed formation fracture. DETAILED DESCRIPTION

[0071] It is important to note that "fluid" in this document can be, but is not limited to, a liquid, an emulsion, a slurry, and a flow of solid particles with flow characteristics similar to a liquid flow; and "fluid" in this document can be, but is not limited to, a gas, a liquid, an emulsion, a slurry, and a flow of solid particles with flow characteristics similar to a liquid flow.

[0072] "Formation" in this document refers to a subterranean porous and permeable rock formation (e.g., shale formation, sandstone formation, carbonate formation, hot dry rock formation, etc.) that can serve as a storage space for fluids. Typically these fluids can be water, hydrocarbons, or gases.

[0073] "Hydraulic fracturing" or "fracturing" or "fracture" in this document refers to the creation of a fracture in a formation rock under the action of an external force (e.g., a high pressure fluid) and the propagation of the fracture.

[0074] "Formation fracture" or "fracture" in this document refers to a rock opening gap created in a formation after a hydraulic fracturing operation or a naturally occurring gap in a formation. The terms "formation fracture" or "fracture" can be used interchangeably. A "fracture" can refer to a single fracture or a plurality of fractures or a fracture swarm in the same location.

[0075] "wellbore" in this document refers to a hole drilled or inserted into a formation. Typically, a wellbore is cylindrical, and thus the cross-section of a wellbore can be circular. Alternatively, a wellbore can have any other cross-section. A wellbore can be open-hole, i.e., an open-hole wellbore, or it can be lined with a cemented casing to form a cased wellbore. A wellbore can be a vertical wellbore, a horizontal wellbore, or an inclined wellbore.

[0076] The "fracture closure pressure" herein refers to the minimum pressure at which a fracture opens, equal to the in-situ stress normal to the fracture face, usually the minimum principal stress of the formation. For a horizontal fracture, the fracture closure pressure is usually equal to the vertical principal stress of the formation. For a vertical fracture, the fracture closure pressure is usually equal to the horizontal minimum principal stress of the formation.

[0077] The "fracture pressure" or "fracture internal pressure" herein refers to the average pressure within the formation fracture. The fracture pressure can be obtained by monitoring the wellhead pressure (i.e. fracture pressure = wellhead pressure + hydrostatic pressure - frictional resistance) or the pressure within the wellbore, and the dynamic correspondence between the wellhead pressure and the fracture pressure under different injection flow rates and fluid density / temperature conditions can be calculated using a wellbore flow model.

[0078] The "fracture propagation pressure" herein refers to the pressure at which the formation fracture begins to propagate in the length or height direction, usually greater than the "fracture closure pressure". The fracture propagation pressure can be obtained by empirical formula, rock mechanics model equation, logging data, numerical modeling, etc., or by field measurement (overflow test, formation integrity test, small-scale fracturing test, step-by-step pressure test, etc.).

[0079] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application and the above-mentioned explanations of the terms. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0080] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0081] Figure 1 A step flow chart of the method for storing energy based on compressed air through a formation fracture in the present application is shown. In combination with the drawings shown, Figure 1 The present application discloses a method for storing energy based on compressed air through a formation fracture, comprising the following steps:

[0082] S1, injecting a fracturing fluid into the target wellbore to perform hydraulic fracturing construction, creating a formation fracture and / or opening an original formation fracture;

[0083] S2, injecting a plugging fluid into the formation fracture to physically and / or chemically plug the rock interstice and / or microfracture around the formation fracture;

[0084] S3, performing a sealing test on the target wellbore under wellhead plugging conditions to determine the sealing property of the formation fracture;

[0085] S4, injecting compressed air into the target wellbore and making the post-air- intake pressure of the formation fracture greater than the fracture closure pressure and less than the fracture propagation pressure to store energy, at least including the compressed potential energy accumulated by the pressure rise in the overall space formed by the target wellbore and the formation fracture;

[0086] S5, monitoring whether there is a need for energy release, if not, plugging the wellhead; if so, back-bleeding the compressed air and at least obtaining the compressed potential energy released by the compressed air during the back-bleeding.

[0087] In step S1, the formation fracture serves as the main energy storage medium, and the target wellbore can be a new energy storage wellbore or an abandoned wellbore. The wellbore can be a vertical wellbore or a horizontal wellbore. If it is an abandoned oil and gas well, the unwanted perforations need to be permanently plugged. If it is necessary to use the original hydraulic fracture of the abandoned oil and gas well (such as a depleted shale oil and gas well), it is usually necessary to recover the proppant or increase the original fracture area to reduce the impact of the proppant on energy storage (proppant will hinder the closure of the fracture, thereby affecting the energy storage capacity and efficiency). The abandoned well can also be refractured to create new formation fractures that do not contain proppant. Multiple formation fractures can be created at different locations of the wellbore.

[0088] It should be particularly pointed out that in the present application, the target wellbore is configured as a single independent wellbore, which is not in communication with any other wellbore through the formation fracture, otherwise it will seriously affect the propagation and plugging of the formation fracture.

[0089] In addition, the depth of the hydraulic fracturing operation and the size of the formation fracture can be designed according to the required energy storage capacity. For existing formation fractures (such as existing hydraulic fracturing fractures or natural fractures, etc.), their length or height can be increased to meet the design requirements.

[0090] In addition, in the method disclosed in the present embodiment, a thickening agent is added to the fracturing fluid in step S1 to increase the viscosity of the fracturing fluid.

[0091] In the case of "wellhead plugging condition" in step S3 and "plugging the wellhead" in step S5 "monitoring whether there is a need for energy release, if not, plugging the wellhead", the compressed air does not enter or discharge from the target wellbore. At the same time, it should be particularly pointed out that in step S4, the injection of compressed air can be carried out by means of compressed air injection devices (such as compressed air injection pipes) extending into the wellbore from the wellhead, which also involves wellhead plugging, but the wellhead plugging at this time does not limit the injection of compressed air into the target wellbore, that is, the wellhead plugging at this time is to plug the gap between the wellhead and the compressed air injection device at the wellhead of the wellbore, so that the compressed air is fully injected into the wellbore under high pressure; Similarly, it should be particularly emphasized that in step S5, "plugging the wellhead" in "monitoring whether there is a need for energy release, if not, plugging the wellhead" does not limit the operation of discharging the compressed air after step S5 under the condition of "if there is a need for energy release", and the "discharging the compressed air" can be achieved by means of the compressed air discharge device (such as the discharge pipe) passing through the plugging device for plugging the wellhead.

[0092] Generally, the lower the viscosity of the fracturing fluid, the easier it is for the fractures to communicate with natural fractures, weak planes of the formation or faults, and form a complex network structure. In conventional hydraulic fracturing operations (such as shale oil and gas, tight oil and gas, and coalbed methane fracturing), in order to form a complex fracture network and improve oil and gas production, the viscosity of the fracturing fluid is usually low. In some cases, in order to further reduce the viscosity of the fracturing fluid and improve the complexity of the fracture network, even supercritical carbon dioxide with ultra-low viscosity is used as the fracturing fluid. However, a complex fracture network is not conducive to the storage of energy in the fractures, because the complex fracture network reduces the pressure-bearing capacity and width of the fractures. In order to reduce the complexity of the fractures and avoid the fractures from generating too many branches, which affects the energy storage efficiency and capacity, a thickening agent is added to the fracturing fluid during the hydraulic fracturing operation to increase the viscosity of the fracturing fluid, so as to reduce the risk of generating complex fractures.

[0093] In the method disclosed in the present embodiment, the thickening agent includes but is not limited to at least one natural polymer and derivative thereof, and / or at least one cellulose and derivative thereof, and / or at least one synthetic polymer, and / or at least one surfactant type thickening agent, and / or at least one gel type thickening agent, and / or at least one clay. Specifically, the thickening agent can be a natural polymer and derivative thereof (such as guanidine gum, carboxymethyl guanidine gum, hydroxypropyl guanidine gum, xanthan gum, safflower gum, etc.), a cellulose and derivative thereof (such as hydroxymethyl cellulose, carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxyethyl cellulose, etc.), a synthetic polymer (such as polyacrylamide, polyethylene oxide, acrylic acid polymer, polyvinyl alcohol, etc.), a surfactant type thickening agent, a gel type thickening agent (such as a gel formed by combining phenolic resin and crosslinking agent), and a clay (such as montmorillonite, bentonite, etc.).

[0094] In the method of the present application, the plugging fluid is configured to contain the plugging material, so that step S2 can be integrated into step S1 and completed simultaneously with the implementation of step S1, or the plugging fluid is configured to contain the plugging material and is independent of the fracturing fluid, so that step S2 is implemented independently of step S1. That is, step S2 can be part of step S1, and step S2 is completed after step S1 is completed, or step S2 is an independent step independent of step S1, i.e., after step S1 is implemented, an additional step S2 is performed.

[0095] In some embodiments, the plugging fluid contains solid plugging material (including particles, fibers, polymers, etc.), which can physically plug the rock pores and / or microfractures around the formation fractures.

[0096] In another embodiment, the plugging fluid contains liquid plugging material, which can chemically plug the rock pores and / or microfractures around the formation fractures. The liquid plugging material can be triggered by chemical reactions between different components of the liquid plugging material, by contact between different liquid plugging materials, by contact between the liquid plugging material and the fluid in the formation pores, or by changes in salinity, pH, or temperature caused by the liquid plugging material entering the formation, thereby changing from a liquid to a solid (including a gel) and plugging the rock pores and / or microfractures. In some embodiments, the fracturing fluid can be configured to contain the liquid plugging material, which allows the plugging of the rock pores and / or microfractures around the fractures during and / or after the fracturing operation, thereby eliminating the need for an additional separate step to set up physical and / or chemical plugging, thereby greatly improving work efficiency and greatly reducing the cost of energy storage based on compressed air.

[0097] After plugging the rock around the fractures, the lower the rock permeability, the better the fracture sealing. Preferably, the rock permeability after plugging is not higher than 0.0001 millidarcy. The change in rock permeability caused by the plugging material can be quantified and evaluated in advance by laboratory experiments, numerical simulation, molecular simulation, etc.

[0098] In addition, those skilled in the art will understand that the physical plugging particles are relatively small, on the order of microns to nanometers, and do not plug the fractures; as for chemical plugging, batch injection of chemical liquid can be used to allow the solidification reaction to occur only in the pores around the fractures, but not in the fractures, thereby achieving the purpose of plugging the surrounding pores while not plugging the formation fractures.

[0099] Since the formation fracture will be repeatedly opened and closed during the energy storage and release process (for example, the energy storage cycle is performed once a day, and there will be 365 openings and closings in a year, while the operating life of an air compression energy storage power station is usually more than 30 years), fatigue effect will occur at the fracture tip, causing microcracks, which will cause compressed air to leak from the fracture tip, affecting the energy storage efficiency. In order to reduce the risk of fracture tip fatigue effect, after step S2, the fracture tip needs to be modified to prevent the occurrence of microcracks at the fracture tip. Or after multiple work cycles of S4 and S5, the fracture tip needs to be modified to repair the damaged fracture tip. The steps of modifying the formation fracture tip include:

[0100] sequentially injecting a first fluid and a second fluid into the formation fracture, wherein the first fluid comprises at least one liquid polymer and at least one curing agent, and the second fluid is a fluid that does not generate solid by itself through a curing reaction or a polymerization reaction;

[0101] After the second fluid displaces the first fluid to the fracture tip of the formation fracture, the injection of the second fluid into the formation fracture is stopped, and the first fluid is allowed to complete the curing reaction from a liquid state to a solid state.

[0102] In some embodiments, the solid state material has a fracture toughness greater than that of the formation rock in which the formation fracture is located, or / and a tensile strength greater than that of the formation rock in which the formation fracture is located, or / and a shear strength greater than that of the formation rock in which the formation fracture is located, or / and a compressive strength greater than that of the formation rock in which the formation fracture is located, or / and a fatigue life greater than that of the formation rock in which the formation fracture is located, or / and a fatigue limit greater than that of the formation rock in which the formation fracture is located, or / and a fatigue strength greater than that of the formation rock in which the formation fracture is located.

[0103] Common "liquid polymers" include, but are not limited to, polyurethane, polyetheramine, liquid natural rubber, liquid polybutadiene, polyimide precursor, polyether ether ketone solution, etc. Common "curing agents" include, but are not limited to, ethylenediamine, triethylenetetramine, m-phenylenediamine, diaminodiphenylmethane, polyamide, modified alicyclic amine, maleic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, phenolic modifier, etc. Common "curing agents" can be divided into amine, anhydride, phenolic, metal salt, imidazole, polyisocyanate, polysulfide. The time of the curing reaction is affected by the type of liquid polymer, the type of curing agent, the mixing ratio of liquid polymer and curing agent, and the temperature.

[0104] In some embodiments, the first fluid has a higher viscosity than the second fluid. In order to achieve a better displacement of the first fluid by the second fluid in the formation fracture and the wellbore, to avoid viscous fingering, to reduce the residual of the first fluid in the area other than the fracture tip, and to avoid the mixing of the first fluid and the second fluid in the wellbore, the ratio M of the viscosity of the first fluid to the viscosity of the second fluid is less than 10.

[0105] In some embodiments, the first fluid has a density different from the second fluid. In order to prevent Rayleigh-Taylor instability of the first fluid and the second fluid in the wellbore, and to prevent the first fluid and the second fluid from segregating in the formation fracture, the density of the first fluid needs to be the same or similar to the density of the second fluid. For example, in some embodiments, the density of the first fluid cannot be less than 70% of the density of the second fluid.

[0106] In some embodiments, the injection rate of the second fluid is less than a threshold rate. In order to prevent the first fluid or the second fluid from turbulent flow in the wellbore and the formation fracture due to a too high injection rate, which causes the mixing of the first fluid and the second fluid, the injection rate of the second fluid needs to be less than a threshold rate, so that the first fluid or / and the second fluid does not experience turbulent flow in the wellbore or the formation fracture. The threshold rate can be calculated using empirical formula, theoretical model or numerical simulation based on the fluid mechanics properties of the first fluid or / and the second fluid, the size of the wellbore and the fracture, the surface roughness of the wellbore and the fracture, etc. For example, the threshold rate can be determined by comparing the calculated Reynolds number with the critical value of turbulent flow.

[0107] In some embodiments, the step of modifying or repairing the fracture tip includes: injecting a degradable capsule and a third fluid into the formation fracture, wherein the degradable capsule contains at least one reactant, and the third fluid is a fluid that does not itself undergo a solidification reaction or a polymerization reaction to form a solid; after stopping the injection of the degradable capsule, continuing to inject the third fluid to displace the degradable capsule to the fracture tip of the formation fracture, i.e., completing the displacement work; after the displacement work is completed, waiting for the degradable capsule to degrade at the fracture tip of the formation fracture, and then the reactants released by the degradable capsule and / or the reactants and the third fluid undergo a solidification reaction to form a solid state material. Wherein, the injection of the degradable capsule and the third fluid can be completed by controlling the injection time and / or the injection amount, and the injection time and the injection amount of each can be set by pre-calculation. This makes the injection of the degradable capsule and the third fluid each be effectively completed, and whether the displacement work is completed can also be judged based on whether the injection of the third fluid is completed, for example, the completion of the injection of the third fluid can be regarded as the completion of the displacement work, or a certain time after the completion of the injection of the third fluid can be regarded as the completion of the displacement work.

[0108] In some embodiments, the density of the degradable capsule is set to be less than the density of the third fluid in the step of injecting the degradable capsule and the third fluid into the formation fracture, so that the degradable capsule is gathered at the fracture top of the formation fracture;

[0109] Alternatively, the density of the degradable capsule is set to be greater than the density of the third fluid in the step of injecting the degradable capsule and the third fluid into the formation fracture, so that the degradable capsule is gathered at the fracture bottom of the formation fracture. After the degradable capsule degrades at the fracture top or the fracture bottom of the formation fracture, a solid-state substance is formed by the solidification reaction between the reaction substances released by the degradable capsule and / or the reaction substances and the third fluid.

[0110] In some embodiments, different reaction substances are contained in the same degradable capsule; in some embodiments, different reaction substances are contained in different degradable capsules. For example, the polymer in powder state and the curing agent in powder state are placed in the same capsule and do not have a solidification reaction, but after the capsule is injected into the formation fracture, the polymer powder and the curing agent powder released by degradation can have a solidification reaction when they come into contact with water.

[0111] The reaction substances in the degradable capsule can be in liquid or solid state (including powder). Other substances that adjust the density of the capsule can also be contained in the degradable capsule.

[0112] In other embodiments, only the tip in the height direction of the formation fracture is modified or only the tip in the length direction of the formation fracture is modified. In other embodiments, the geometry of the formation fracture is complex and requires simultaneous modification of the tips in multiple directions.

[0113] Further, in the present application, the step S3 comprises:

[0114] Step S301, injecting a high-pressure fluid into the target wellbore, so that the pressure in the formation fracture is greater than the fracture closure pressure and less than the fracture propagation pressure, so that the formation fracture is in an open and unexpanded state;

[0115] Step S302, sealing the wellhead of the target wellbore;

[0116] Step S303, determining whether the formation fracture is gradually closed under the condition of wellhead sealing by monitoring the change of the pressure in the wellhead and / or the wellbore with time, so as to determine the closure property of the formation fracture.

[0117] In step S3, the high pressure fluid injected into the wellbore can be a liquid (such as a fracturing fluid) or a gas (such as air, helium or nitrogen), i.e. the high pressure fluid includes a liquid high pressure fluid and / or a gas high pressure fluid. The closure of the formation fracture can be determined, for example, by the change of the pressure drop curve of the wellhead over time after the pump down of the fracturing operation (at this time, the pressure in the fracture satisfies greater than the formation fracture closure pressure and less than the formation fracture propagation pressure). For example, if the wellhead pressure remains stable or the pressure drop per unit time is less than a threshold value (such as the pressure drop per hour is not more than 0.2 MPa), it proves that the formation fracture has good closure. Through the pressure drop data, the leakage rate of the compressed air in a certain period of time can be calculated. A threshold value of the daily leakage rate can also be given to determine the threshold value of the pressure drop per unit time. If the wellhead pressure drops faster or is greater than the set threshold value, it proves that the formation fracture has poor closure, and fluid leakage causes the fracture to start to close. If the closure of the formation fracture does not meet the requirements, the rock voids and / or microfractures around the formation fracture need to be re-sealed, until the closure of the formation fracture meets the design requirements.

[0118] The pressure in the wellbore can be measured by installing a downhole pressure gauge or a distributed optical fiber, etc.

[0119] In a particularly preferred embodiment of the closure test of step 3, steps S301-S303 are performed in sequence first based on a liquid high pressure fluid and then based on a gas high pressure fluid. That is, the closure of the formation fracture is tested first with a liquid high pressure fluid (at least once) and then with a gas high pressure fluid (at least once). Specifically, the closure of the formation fracture is tested first with a liquid high pressure fluid. The advantage of this is that if the closure test of the formation fracture fails, the rock voids and / or microfractures around the formation fracture can be re-sealed using existing ground liquid injection equipment and liquid storage facilities (such as ground liquid storage tanks, reservoirs and sealing material mixing equipment). When the closure test of the formation fracture with a liquid is passed, the closure of the formation fracture is tested again with a gas high pressure fluid, because gas molecules are smaller and more likely to leak, and in some cases the sealing effect of certain physical or chemical sealing measures on liquids and gases is quite different.

[0120] In another specific embodiment of the closure test, the high pressure fluid is injected into the target wellbore so that the maximum value of the pressure in the formation fracture is not less than a test pressure threshold, which is greater than the formation fracture closure pressure and less than the formation fracture propagation pressure. The test pressure threshold can be set as the maximum working pressure in the formation fracture energy storage process. Because in some cases, as the pressure in the formation fracture increases, the risk of reopening of the rock microfractures around the formation fracture increases, the formation fracture may pass the closure test at a lower pressure (although the formation fracture is also open), but at a higher pressure, it may not pass the closure test.

[0121] In another embodiment of the closed test, due to the high formation temperature, the temperature rising expansion of the compressed air in the formation will affect the rate of pressure change in the wellhead and / or wellbore. The closure of the formation fracture can be determined by calculating the threshold value of the pressure drop to correct the pressure drop (such as calculating the effect of the formation temperature on the density and pressure of the compressed air in the formation fracture) through a physical model. The closure of the formation fracture can also be determined according to the change of the pressure with time after the compressed air in the formation fracture and the formation reach heat exchange equilibrium (i.e. the temperature of the compressed air in the formation no longer rises). The time to reach the heat exchange equilibrium can be calculated by a physical model or directly measured by the temperature in the wellbore (using downhole temperature meter or distributed optical fiber monitoring equipment). Specifically, the temperature in the wellbore during the plugging of the wellhead is monitored, and when it is monitored that the temperature in the wellbore no longer changes, the starting time of the temperature in the wellbore no longer changing is recorded as time t0; according to the change of the pressure in the wellhead and / or wellbore with time, the closure of the formation fracture is determined. In the method disclosed in the embodiment of the present application, the power generation device capable of converting the compressed potential energy of the compressed air into electrical energy is provided in step S5.

[0122] In the method disclosed in the embodiment, at least one of the following energies is included in the energy storage in step S4 and the energy release in step S5: stress potential energy accumulated by the compressed air overcoming the fracture closure pressure, elastic deformation potential energy accumulated by driving the formation rock to elastically deform, and formation heat energy obtained by heat exchange between the compressed air in the formation fracture and the formation rock.

[0123] That is, when the energy storage is performed based on step S4, the compressed air is injected into the wellbore, so that the pressure in the formation fracture is greater than the formation fracture closure pressure and less than the formation fracture expansion pressure, so that the compressed air continuously enters the formation fracture, increases the width of the formation fracture and the formation fracture does not expand, and drives the formation rock to elastically deform and accumulate elastic potential energy; at the same time, the pressure in the wellbore and the formation fracture is increased, and the air accumulates the compression potential energy; or / and the compressed air in the formation fracture and the formation rock exchange heat, and obtain the formation heat energy; the wellbore is plugged, and the pressure in the wellbore and the formation fracture is maintained to store energy. At the end of the energy storage, the volume of the compressed air contained in the formation fracture is much larger than the volume of the compressed air contained in the wellbore.

[0124] Generally, the compressed air needs to be preheated to a temperature between 100°C and 300°C before entering the expander to generate electricity, in order to improve the efficiency of electricity generation. When the formation temperature is high, the temperature of the compressed air in the formation fracture will also increase during storage, and the energy consumption for preheating before the subsequent backflow to the ground and entering the expander to generate electricity will be greatly reduced, thereby improving the energy storage efficiency. In some embodiments, the formation temperature is higher than 100°C, and the compressed air does not need to be preheated before entering the expander to generate electricity, which greatly reduces the operating cost of electricity generation.

[0125] In order to prevent the formation fracture from expanding during energy storage, thereby causing the risk of energy loss and gas leakage, the wellhead pressure needs to be monitored during the energy storage process, so as to ensure that the pressure in the formation fracture is always less than the fracture propagation pressure. It should be noted that the pressure is monitored during the energy storage process, and the wellhead is in an open state. During the closed test of the formation fracture, the wellhead is in a closed state.

[0126] In order to more accurately calculate the frictional resistance in the wellbore and the pressure of the compressed air in the formation fracture, the wellhead and / or the pressure in the wellbore and the flow rate of the compressed air need to be monitored during the energy storage process, i.e., during the execution of the step S4. Because the frictional resistance of the compressed air in the wellbore and the fracture and the flow rate of the compressed air are closely related, the flow rate of the compressed air also determines the flow state (such as laminar flow or turbulent flow). In some embodiments, in order to more accurately calculate the density of the compressed air, the temperature in the wellbore and / or the wellbore also needs to be monitored. The temperature in the wellbore can be measured by installing a downhole thermometer or a distributed optical fiber, etc.

[0127] The calculation step of the frictional resistance includes: determining the wellbore geometric parameters (length, inner diameter and roughness of the pipe wall) and working condition parameters (such as inlet pressure, temperature and flow rate); looking up a table or calculating the density and viscosity of the air under the working pressure; calculating the Reynolds number and determining the flow state; determining the friction factor according to the flow type and calculating the frictional resistance.

[0128] In addition, in order to further reduce the risk of fracture expansion, a threshold value can be set for the pressure of the formation fracture, so that the pressure of the formation fracture does not exceed the threshold value during energy storage. The size of the threshold value can be set according to the risk of fracture expansion, such as: if the risk of fracture expansion is high, the threshold value can be set to: fracture propagation pressure value-1MPa; such as: if the risk of fracture expansion is small, the threshold value can be set to: fracture propagation pressure value-0.5MPa.

[0129] After the injection of the compressed air into the wellbore in S4 is completed, if there is no energy release demand, the wellbore can be plugged to maintain the pressure in the wellbore and the formation fracture, thereby storing energy; if there is an energy release demand, the compressed air can be backflowed from the wellbore.

[0130] In a particularly preferred embodiment, step S5 further comprises the step of monitoring the wellhead and / or the wellbore pressure to calculate the formation fracture pressure, and when the formation fracture pressure approaches or reaches the formation fracture propagation pressure or a set fracture operating pressure threshold, the compressed air is vented from the target wellbore.

[0131] That is, during the plugging of the wellbore, the wellhead and / or the wellbore pressure is monitored and the formation fracture pressure is calculated to ensure that the formation fracture pressure does not exceed the formation fracture propagation pressure or a set threshold. When the formation fracture pressure approaches the formation fracture propagation pressure or reaches the set threshold, the compressed air is vented from the wellbore (whether or not there is a need to release energy) to prevent spontaneous propagation of the formation fracture. Because, unlike the use of a liquid energy store, the temperature increase of the compressed air in the formation fracture results in a volume expansion and pressure increase, whereas the volume of a liquid (such as water) is very little affected by temperature. For example, using the ideal gas law, it can be calculated that the pressure will increase by 12.7% when the temperature of the compressed air increases from 40 degrees Celsius (for example, the temperature of the compressed air at the wellhead) to 80 degrees Celsius (for example, the temperature of the formation in which the formation fracture is located) if the volume of the air storage is constant. In some embodiments, when the measured pressure in the wellbore is very close to the location of the formation fracture, the measured pressure can be approximated as being equivalent to the formation fracture pressure when calculating the formation fracture pressure.

[0132] The flow rate of the compressed air energy store is typically in the range of 1000 to 4000 cubic meters per minute, which is much greater than the flow rate of a liquid energy store in a formation fracture (typically only 5 to 10 cubic meters per minute), and therefore the efficiency of the compressed air energy store is more affected by the fracture width than the efficiency of a liquid energy store. If the width of the formation fracture is too small, the flow resistance is increased and the efficiency of the energy store is greatly reduced. At the same flow rate, the smaller the formation fracture pressure, the smaller the width of the formation fracture, and the higher the flow velocity, the greater the energy loss due to flow resistance.

[0133] In further embodiments, to improve the efficiency of the energy store and reduce the flow resistance of the injected and vented compressed air in the formation fracture, a portion of the compressed air is left in the formation fracture after the energy release of step S5 is complete, so that the pressure in the formation fracture is higher than the closure pressure of the formation fracture and there is a residual fracture width. Specifically, the step of leaving a residual fracture width in the formation fracture after the energy release of step S5 is complete comprises: setting a critical residual fracture width of the formation fracture; determining a critical residual compressed air volume based on the set critical residual fracture width; and monitoring the remaining volume of the compressed air in the wellbore and the formation fracture of the formation fracture during the energy release of step S5, and when the monitored remaining volume approaches or reaches the critical residual compressed air volume, the venting is stopped, so that the formation fracture still has a residual fracture width.

[0134] A critical residual fracture width (corresponding to a critical residual compressed air volume) can be calculated by physical modeling or numerical simulation, etc. to seek a balance between the energy storage efficiency and the energy storage capacity (the capacity that can be used for reverse discharge power generation) so that the average width of the formation fracture after the energy release is greater than the critical residual fracture width. The greater the critical residual fracture width, the more residual compressed air remains in the formation fracture, the smaller the fracture flow resistance loss, the greater the energy storage efficiency, and the smaller the energy storage capacity. In some embodiments, after the energy release in step S5 is completed, the formation fracture still retains 30% of the maximum energy storage capacity of residual compressed air. After the compressed air is discharged from the wellbore in S5, the wellbore can be plugged to maintain the pressure in the wellbore and the formation fracture and the width of the formation fracture.

[0135] In an embodiment, during energy storage, the heat generated during compression of the air is transferred to a thermal energy storage medium (such as water, molten salt, ceramic particles, etc.) through a heat exchanger for storage. When it is necessary to release the air for power generation, the stored heat is used to preheat the compressed air about to enter the expander, thereby reducing the need for external heat sources. The stored heat can also be used for industrial production, heating, domestic hot water, etc. for the reuse of thermal energy.

[0136] During the energy release process in step S5, the compressed air is discharged by reducing the plugging pressure of the wellbore, and at least one of the kinetic energy of the compressed air converted from the elastic potential energy stored in the formation rock, the compressed potential energy released by the compressed air, and the thermal energy carried by the compressed air is obtained.

[0137] At least one of the kinetic energy of the compressed air converted from the elastic potential energy stored in the formation rock, the compressed potential energy released by the compressed air, and the thermal energy carried by the compressed air is converted into electrical energy by a predetermined power generation device. The power generation device includes but is not limited to a turbine, a turbine, an expander, a heat exchanger, and an organic Rankine cycle power generation device.

[0138] In an embodiment of the method of the present application, a turbine is used to convert the kinetic energy of the compressed air into electrical energy. In an embodiment, an expander is used to convert the compressed potential energy of the compressed air into electrical energy.

[0139] In an embodiment of the method of the present application, the expander is equipped with a waste heat recovery device, and the heat generated during the operation of the expander can be recovered for other purposes, such as preheating the air entering the expander or providing thermal energy for other processes.

[0140] In an embodiment of the method of the present application, the waste heat generated during the expansion of the compressed air can be used to drive an organic Rankine cycle expander to convert the thermal energy into electrical energy.

[0141] In an embodiment of the method of the present application, the compressed air needs to be heated before entering the expander for power generation.

[0142] In one embodiment of the method of the present invention, since the ground temperature is high, the temperature of the compressed air rises in the ground fissures, so no further heating is required when generating electricity by air expansion.

[0143] In one embodiment of the method of the present invention, due to the low thermal conductivity of the formation, the heat energy generated by the compressed air can be stored in the formation fractures, and it is not necessary to recover the heat energy generated by the compressed air at the ground.

[0144] Figure 2 This is a schematic diagram illustrating the energy storage of compressed air through sealing geological fractures according to the present invention. Figure 2 As shown, a formation fracture 2 is constructed in the target wellbore 1. A sealing device 3 is installed at the wellhead of the target wellbore 1. A compressor 4 is installed outside the target wellbore 1 and above the ground surface 6 for supplying compressed air into the target wellbore 1. A power generation device 5 is installed to generate electricity by using the compression potential energy stored in the compressed air, the elastic potential energy stored in the formation rocks to convert into the kinetic energy of the compressed air, and the geothermal energy carried by the compressed air when the compressed air is discharged. As mentioned above, the power generation device 5 includes, but is not limited to, one or more of the following: turbine, turbine, expander, heat exchanger, organic Rankine cycle power generation equipment.

[0145] Combination Figure 2 As shown, compressed air can be injected and vented through a single target well, significantly reducing investment costs. An energy storage project can have multiple wells, but there is no hydraulic connection between them (not through fractures or rock pores). Each sealed fracture connected to a well is not connected to sealed fractures connecting other wells. The energy storage and release of each well are unaffected by the energy stored in or released by other wells.

[0146] Figure 3 This diagram illustrates the difference between traditional compressed air energy storage (utilizing natural or artificial underground cavities) and compressed air energy storage through sealed formation fractures. The upper dashed line represents the fracture propagation pressure, while the lower dashed line represents the fracture closure pressure. Curve A represents the relationship between the injected volume and pressure of compressed air during compressed air energy storage using the sealed formation fractures of this invention, while curve B represents the relationship between the injected volume and pressure of compressed air during compressed air energy storage using traditional natural or artificial underground cavities. As can be seen from the diagram, compared to traditional compressed air energy storage using natural or artificial underground cavities, this invention achieves higher energy storage pressure and thus greater energy storage density for the same injected volume through sealed formation fractures.

[0147] The traditional compressed air energy storage is limited by the strength constraint of the underground cavity, and the energy storage pressure is limited to be lower than the formation fracture pressure. For example, the working pressure of the underground salt cavern energy storage system is about 7 MPa to 12 MPa. The maximum energy storage pressure of the formation fracture is much greater than that of the underground salt cavern, which greatly increases the energy storage density. For example, the minimum principal stress of the underground 3000-meter formation fracture is 50 MPa, and the fracture propagation pressure is 55 MPa, so the working pressure of the underground fracture energy storage is 50 MPa to 55 MPa. Generally, the deeper the formation fracture, the greater the fracture closure pressure, and the greater the energy storage pressure.

[0148] In addition, the traditional compressed air energy storage can store the compressed potential energy of air, and the compressed air energy storage by sealing the formation fracture further includes the rock elastic potential energy accumulated by the increase of the fracture width and the stress potential energy accumulated by the work against the fracture closure pressure, which further increases the energy storage density. Among them, the stress potential energy accumulated by the work against the fracture closure pressure can greatly reduce the fracture pressure drop per unit volume of air.

[0149] The present application also discloses a system for implementing the method disclosed in the first aspect of the present application, comprising a fracturing unit, a plugging fluid injection unit, a sealing test unit, an energy storage working unit and an energy release working unit; wherein the fracturing unit is configured to inject fracturing fluid into the target wellbore, perform hydraulic fracturing construction, create formation fractures and / or open existing formation fractures; wherein the target wellbore is configured as a standalone wellbore that is not connected to another wellbore through the formation fractures; the plugging fluid injection unit is configured to inject plugging fluid into the formation fractures to physically and chemically plug the rock interstices and / or microfractures around the formation fractures; the sealing test unit is configured to test the sealing of the target wellbore based on the wellhead sealing to determine the sealing of the formation fractures; the energy storage working unit is configured to inject compressed air into the target wellbore, and the post-gas pressure of the formation fractures is greater than the fracture closure pressure and less than the fracture propagation pressure, to store energy, wherein at least includes the compressed potential energy of the compressed air accumulated due to the pressure rise in the overall space formed by the target wellbore and the formation fractures; the energy release working unit is configured to discharge the compressed air, and at least obtain the compressed potential energy released by the compressed air during the discharge process;

[0150] The closed test unit comprises a high-pressure fluid injection unit, a closed unit and a pressure monitoring and judging unit. The high-pressure fluid injection unit is configured to inject high-pressure fluid into the target wellbore, so that the pressure in the formation fracture is greater than the fracture closure pressure and less than the fracture propagation pressure, so that the formation fracture is in an open and unexpanded state. The closed unit is configured to seal the wellhead of the target wellbore. The pressure monitoring and judging unit is configured to monitor the wellhead pressure of the target wellbore, and to judge whether the formation fracture is gradually closed under the condition of wellhead sealing by monitoring the change of the wellhead and / or wellbore pressure with time, so as to judge the closure of the formation fracture.

[0151] In the system of the present application, the plugging fluid is configured as a fracturing fluid containing plugging substances, so that the plugging fluid injection unit can be integrated into the fracturing unit and work with the completion of the fracturing operation, or the plugging fluid is configured as a fluid containing plugging substances and independent of the fracturing fluid, so that the plugging fluid injection unit works independently of the fracturing unit.

[0152] In another embodiment of the system, a compressed air flow monitoring unit is further included, which is configured to monitor the flow rate of compressed air during the energy storage and / or release process. This is because the frictional resistance of compressed air in the wellbore and the fracture is closely related to the flow rate of compressed air, and the flow rate of compressed air also determines the flow regime (such as laminar flow or turbulent flow). In another embodiment, in order to more accurately calculate the density of compressed air, it is also necessary to monitor the temperature in the wellhead and / or wellbore, which can be achieved by setting a temperature monitoring device, for example, the temperature in the wellbore can be measured by installing a downhole thermometer or a distributed optical fiber.

[0153] In another embodiment of the system, a control unit and a calculation unit are further included, wherein the calculation unit can calculate the pressure in the formation fracture according to the wellhead and / or wellbore pressure monitored by the pressure monitoring and judging unit; the control unit is in communication connection with the calculation unit, and when the pressure in the formation fracture approaches or reaches the formation fracture propagation pressure or the set fracture working pressure threshold, the control unit controls the backflow device to backflow the compressed air from the target wellbore, regardless of whether there is a power generation demand, so as to avoid spontaneous propagation of the formation fracture.

[0154] In another embodiment of the system, a critical residual fracture width setting unit is further included, which is configured to set a critical residual fracture width of the formation fracture. The aforementioned computing unit or the control unit is further configured to determine the critical residual compressed air volume based on the set critical residual fracture width. The compressed air volume monitoring unit is further configured to monitor the residual volume of the compressed air in the wellbore and the formation fracture during the energy releasing process. When the monitored residual volume approaches or reaches the critical residual compressed air volume, the control unit controls the backflow device to end the backflow, so as to keep the formation fracture still having the residual fracture width.

[0155] In another embodiment of the system, a power generation device is further included, by which at least one of the kinetic energy of the compressed air, the compressed potential energy released by the compressed air, and the heat energy carried by the compressed air, which are converted from the elastic potential energy accumulated by the formation rock, can be converted into electric energy. The power generation device includes but is not limited to a turbine, a turbine, an expander, a heat exchanger, and an organic Rankine cycle power generation device. Among them, the turbine is used to convert the kinetic energy of the compressed air into electric energy. In an embodiment, the expander is used to convert the compressed potential energy of the compressed air into electric energy.

[0156] In another embodiment of the system, the expander is provided with a waste heat recovery device, and the heat generated during the operation of the expander can be recovered for other purposes, such as preheating the air entering the expander or providing heat energy for other processes.

[0157] In another embodiment of the system, the waste heat generated during the expansion of the compressed air can be used to drive an organic Rankine cycle expander, and then the heat energy is converted into electric energy.

[0158] In an embodiment of the system, the compressed air needs to be heated before entering the expander for power generation.

[0159] In an embodiment of the system, due to the high temperature of the formation, the temperature of the compressed air increases in the formation fracture, so that the air does not need to be heated again when the air expands for power generation.

[0160] In an embodiment of the system, due to the low thermal conductivity of the formation, the heat energy generated by the compressed air can be stored in the formation fracture, and the heat energy generated by the compressed air does not need to be recovered on the ground.

[0161] It should be noted that the plugging fluid injection unit is configured to be integrated in the fracturing unit or independent of the fracturing unit. The so-called "integration" means that the plugging fluid injection unit is no longer additionally arranged, but the functions and features of the plugging fluid injection unit are realized by the fracturing unit, i.e., the functions of the plugging fluid injection unit can be directly realized based on the fracturing unit, so that the fracturing unit includes the functions to be realized by the plugging fluid injection unit, and the fracturing unit exists as the fracturing and plugging fluid injection unit. In another embodiment, the plugging fluid injection unit can exist as a unit with independent functions, so as to implement chemical plugging based on itself.

[0162] The related functions and technical features described in the method disclosed in the first aspect are also applicable to the system described in the fifth embodiment of the application, and will not be described here again.

[0163] For the convenience of description, spatial relative terms such as "above", "upper", "top", "top surface", and the like can be used herein to describe the spatial relationship of one device or feature to another device or feature as shown in the drawings. It should be understood that the spatial relative terms are intended to include different orientations of the device in use or operation in addition to the orientation of the device described in the drawings. For example, if the device in the drawings is inverted, the device described as "above" or "on" other devices or structures will be positioned "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below" orientations. The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatial relative descriptions used herein are interpreted accordingly.

[0164] It should be noted that the terms used herein are only for the purpose of describing specific embodiments, and are not intended to limit the exemplary embodiments according to the present application. As used herein, the singular form is intended to include the plural form unless the context clearly indicates otherwise, and it should also be understood that when the terms "comprise" and / or "include" are used in the specification, there is a feature, step, operation, device, component and / or combination thereof.

[0165] It is to be understood that the terms "first", "second", and the like, used in the description and the claims of the present application as well as the preceding description of the drawings merely refer to structure that is different, and not necessarily to an order or sequence. It is to be understood that the use of the term "or" in the description and the claims of the present application has the same meaning as "and / or" unless stated otherwise. Similarly, it is to be understood that terms such as "comprising", "including", and "having" are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements or steps.

[0166] The preferred embodiments of the application are described above in detail with reference to the accompanying drawings, and although the application is described with reference to these preferred embodiments, it will be understood by those skilled in the art that various changes and modifications can be applied to the application and its preferred embodiments without departing from the spirit and scope of the application as described in the claims.

[0167] Finally, it should be noted that the above-mentioned embodiments are merely used to illustrate the technical solutions of the present application, but are not intended to limit the present application. Although the present application is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application, and all such modifications, replacements and improvements should be included in the protection scope of the claims of the present application.

Claims

1. A method for energy storage based on compressed air passing through formation fractures, characterized in that, Includes the following steps: S1, inject fracturing fluid into the target wellbore to perform hydraulic fracturing to create formation fractures and / or open existing formation fractures; wherein, the target wellbore is an independent wellbore that is not connected to other wellbores through formation fractures; S2, inject sealing fluid into the formation fracture to physically and / or chemically seal the rock voids and / or microfractures around the formation fracture; S3, Conduct a sealing test on the target wellbore based on wellhead plugging to determine the sealing of formation fractures; S4, Inject compressed air into the target wellbore and its formation fractures, and make the pressure of the formation fractures after air intake greater than the fracture closure pressure and less than the fracture propagation pressure, so as to store energy, including at least the compressive potential energy stored by the compressed air due to the pressure increase in the overall space formed by the target wellbore and formation fractures. S5, monitor whether there is a need for energy release. If not, seal the wellhead; if so, backflush the compressed air and obtain at least the compressed potential energy released by the compressed air during the backflush process. The sealing fluid is constructed as a fracturing fluid containing a plugging substance, so that step S2 can be integrated into step S1 and completed simultaneously with the implementation of step S1; or, the sealing fluid is constructed as a fluid containing a plugging substance and independent of the fracturing fluid, so that step S2 is implemented independently of step S1. Step S3 includes: S301, inject high-pressure fluid into the target wellbore so that the pressure inside the formation fracture is greater than the fracture closure pressure and less than the fracture propagation pressure, so that the formation fracture is in an open and non-propagating state. S302, wellhead sealing of the target wellbore; S303 determines the sealing performance of formation fractures by monitoring the change in pressure at the wellhead and / or inside the wellbore over time to determine whether formation fractures gradually close under wellhead plugging conditions. The high-pressure fluid includes a liquid high-pressure fluid and a gas high-pressure fluid; steps S301-S303 are executed first based on the liquid high-pressure fluid, and then steps S301-S303 are executed based on the gas high-pressure fluid.

2. The method according to claim 1, characterized in that, The fracturing fluid contains a thickener, which includes: at least one natural polymer and its derivatives, and / or at least one cellulose and its derivatives, and / or at least one synthetic polymer, and / or at least one surfactant-type thickener, and / or at least one gel-type thickener, and / or at least one clay.

3. The method according to claim 1, characterized in that, The plugging material includes solid plugging material, used to physically seal the rock voids and / or micro-fractures around the formation fractures; And / or, the plugging material includes a liquid plugging material, which is configured to undergo a solidification or gelation reaction due to the interaction of its components, or to undergo a solidification or gelation reaction due to contact between different liquid plugging materials, or to undergo a solidification or gelation reaction due to contact with fluids in the rock pores and / or microfractures surrounding the formation fractures, or to undergo a solidification or gelation reaction due to changes in salinity, pH or temperature after entering the formation, so as to chemically seal the rock pores and / or microfractures surrounding the formation fractures.

4. The method according to claim 3, characterized in that, The plugging material is added directly to the corresponding fluid that constitutes the sealing fluid; Alternatively, the plugging material in the fluid may be added to the corresponding fluid constituting the plugging fluid in such a way that it is at least partially encapsulated in a biodegradable capsule.

5. The method according to claim 1, characterized in that, In step S301, a test pressure threshold is set. The test pressure threshold is constructed to be greater than the formation fracture closure pressure and less than the formation fracture propagation pressure, so that after the high-pressure fluid is injected into the formation fracture, the maximum value of the pressure inside the formation fracture is not lower than the test pressure threshold and less than the formation fracture propagation pressure.

6. The method according to claim 1, characterized in that, In step S303, the determination of the sealing performance of formation fractures includes: Monitor the temperature inside the wellbore during wellhead plugging. When the temperature inside the wellbore stops changing, record the start time when the temperature inside the wellbore stops changing as time t0. The sealing of formation fractures is determined based on the change in pressure at the wellhead and / or inside the wellbore over time, starting from time t0 or a time after t0.

7. The method according to claim 1, characterized in that, During the execution of step S4, the process also includes monitoring the pressure at the wellhead and / or inside the wellbore, as well as the compressed air flow rate.

8. The method according to claim 1, characterized in that, In step S5, after the wellhead is sealed due to the absence of energy release requirements, the method further includes: monitoring the pressure at the wellhead and / or inside the wellbore to calculate the pressure within the formation fractures. When the pressure inside the formation fracture approaches or reaches the formation fracture propagation pressure or the set fracture working pressure threshold, compressed air is back-vented from the target wellbore.

9. The method according to claim 1, characterized in that, The method further includes a step that ensures the formation fracture retains a residual fracture width after the energy release in step S5, which includes: Define the critical residual fracture width of the formation fracture; The critical residual compressed air volume is determined based on the set critical residual crack width. In step S5, the remaining volume of compressed air in the wellbore and its formation fractures during the energy release process is monitored. When the monitored remaining volume approaches or reaches the critical residual compressed air volume, the reverse discharge is terminated, thereby maintaining the formation fractures with residual fracture width.

10. The method according to claim 1, characterized in that, In step S5, a power generation device is provided that can convert the compression potential energy of compressed air into electrical energy.

11. The method according to claim 1, characterized in that, The energy storage in step S4 and the energy release in step S5 also include at least one of the following: stress potential energy accumulated by compressed air overcoming the crack closure pressure, elastic deformation potential energy accumulated by driving the formation rock to undergo elastic deformation, and formation thermal energy obtained by the heat exchange between compressed air and formation rock in the formation crack.

12. The method according to claim 1, characterized in that, The method also includes a step of modifying the tip of the formation fracture, including: A first fluid and a second fluid are sequentially injected into the formation fractures. The first fluid contains at least one liquid polymer and at least one curing agent, and the second fluid is configured to be a fluid that does not undergo curing or polymerization reactions itself. After the second fluid displaces the first fluid to the tip of the formation fracture, the injection of the second fluid into the formation fracture is stopped, and the first fluid is allowed to complete its solidification reaction and change from a liquid to a solid state.

13. The method according to claim 12, characterized in that, The solid material has a fracture toughness greater than that of the rock formation containing the formation fracture, or / and the solid material has a tensile strength greater than that of the rock formation containing the formation fracture, or / and the solid material has a shear strength greater than that of the rock formation containing the formation fracture, or / and the solid material has a compressive strength greater than that of the rock formation containing the formation fracture, or / and the solid material has a fatigue life greater than that of the rock formation containing the formation fracture, or / and the solid material has a fatigue limit greater than that of the rock formation containing the formation fracture, or / and the solid material has a fatigue strength greater than that of the rock formation containing the formation fracture.

14. The method according to claim 12, characterized in that, The ratio M of the viscosity of the first fluid and the viscosity of the second fluid does not exceed 10.

15. The method according to claim 12, characterized in that, The density of the first fluid is not less than 70% of the density of the second fluid.

16. The method according to claim 1, characterized in that, The method also includes a step of modifying the tip of the formation fracture, which includes: A biodegradable capsule and a third fluid are injected into a formation fracture, wherein the biodegradable capsule contains at least one reactant, and the third fluid is configured to be a fluid that does not undergo a solidification or polymerization reaction itself. After stopping the injection of the biodegradable capsule, continue to inject the third fluid to displace the biodegradable capsule to the tip of the formation fracture; After the displacement work is completed, the biodegradable capsules degrade at the fracture tips of the formation fractures, causing the reactants released from the biodegradable capsules to undergo solidification reactions with each other and / or with the third fluid to form solid substances.

17. The method according to claim 16, characterized in that, In the step of injecting biodegradable capsules and a third fluid into formation fractures, the density of the biodegradable capsules is set to be less than the density of the third fluid, so that the biodegradable capsules accumulate at the top edge of the formation fractures. Alternatively, in the step of injecting biodegradable capsules and a third fluid into the formation fracture, the density of the biodegradable capsules is set to be greater than the density of the third fluid, so that the biodegradable capsules accumulate at the bottom edge of the formation fracture.

18. The method according to claim 16, characterized in that, A single biodegradable capsule can encapsulate different reactants, or different reactants can be encapsulated in different biodegradable capsules.

19. A system for implementing the method according to any one of claims 1-18, characterized in that, include: The fracturing unit is constructed by injecting fracturing fluid into the target wellbore to perform hydraulic fracturing operations, creating formation fractures and / or opening existing formation fractures; wherein the target wellbore is an independent wellbore that is not connected to other wellbores through formation fractures. The sealing fluid injection unit is constructed to inject sealing fluid into formation fractures to physically and / or chemically seal the rock voids and / or microfractures surrounding the formation fractures. The sealing test unit is constructed to perform a sealing test on the target wellbore based on wellhead plugging to determine the sealing performance of formation fractures; the sealing test unit includes: The high-pressure fluid injection unit is constructed to inject high-pressure fluid into the target wellbore, so that the pressure inside the formation fracture is greater than the fracture closure pressure and less than the fracture propagation pressure, so that the formation fracture is in an open and non-propagating state. The sealing unit is constructed to seal the wellhead of the target wellbore. The pressure monitoring and judgment unit is constructed to monitor the wellhead pressure of the target wellbore and to determine whether the formation fractures are gradually closing under wellhead sealing conditions by monitoring the changes in wellhead and / or wellbore pressure over time, thereby judging the sealing performance of the formation fractures. The energy storage working unit is constructed by injecting compressed air into the target wellbore and making the pressure after air intake in the formation fracture greater than the fracture closure pressure and less than the fracture propagation pressure, so as to store energy, including at least the compressive potential energy accumulated by the compressed air due to the pressure increase in the overall space formed by the target wellbore and the formation fracture. The energy release working unit is constructed to back-vent compressed air and, during the back-venting process, at least acquire the compression potential energy released by the compressed air; The sealing fluid is a fracturing fluid containing a plugging substance, which allows the sealing fluid injection unit to be integrated into the fracturing unit and complete its work during fracturing operations. Alternatively, the sealing fluid is a fluid containing a plugging substance and independent of the fracturing fluid, which allows the sealing fluid injection unit to operate independently of the fracturing unit.

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