Method and system for storing energy through stratum artificial fractures based on compressed air

Abstract

The invention relates to a method and system for storing energy through stratum artificial fractures based on compressed air. The method comprises the steps that the stratum fractures are constructed; blocking rock gaps and / or microfractures around the stratum fractures; the sealing performance of the stratum fractures is judged; compressed air is injected into the target shaft for energy storage; compressed air is reversely discharged for releasing energy. Accumulation of air compression potential energy is achieved, stress potential energy and elastic potential energy of rock deformation can be accumulated, and stratum heat energy is obtained. Most importantly, the method is not restrained by geographical conditions, underground compressed air energy storage can be carried out at any place, and the application range of compressed air energy storage is greatly expanded.

Description

[0001] This application is a divisional application of application number 202510513686.2, filed on April 23, 2025, entitled "Method and System for Energy Storage Based on Compressed Air Through Ground Fractures". Technical Field

[0002] This invention belongs to the field of underground energy storage technology, and specifically relates to a method and system for storing energy through artificial fissures in the ground using compressed air. Background Technology

[0003] Many renewable energy sources, such as solar and wind power, are intermittent in daily and seasonal cycles, resulting in unstable power output. They are unsuitable for providing baseload power and therefore difficult to integrate directly into the grid. Solving the energy storage problem is key to successfully scaling up renewable energy production. Thus, how to store excess clean electricity on a large scale and reliably transmit it when sunlight and wind power supplies are insufficient remains a challenging technological problem that countries are tackling.

[0004] Compressed air energy storage, as a mature energy storage technology, has advantages such as large storage capacity, long lifespan, and strong stability. However, it has high requirements for geographical conditions, requiring abandoned mines or underground caverns as the gas storage medium, thus limiting its use to certain specific areas. Although artificial underground cavities are also used as gas storage mediums, the construction investment cost is extremely high, greatly limiting the commercial prospects of compressed air energy storage.

[0005] Although depleted oil and gas reservoirs can serve as potential sites for compressed air energy storage, they also have considerable limitations: (1) The rock strata of depleted oil and gas reservoirs have usually undergone long-term oil and gas extraction, and may contain cracks, faults and high-permeability channels. Air can easily leak into the surrounding strata or the surface, affecting gas storage efficiency and environmental safety; (2) Due to long-term extraction, the strata of depleted oil and gas reservoirs may have deformed or compacted, reducing the mechanical integrity of the gas storage tank. Under high pressure conditions, the strata may collapse or slide, affecting the safety of the gas storage tank; (3) A certain amount of oil and gas may remain in depleted oil and gas reservoirs. These residues may mix with compressed air, producing unpredictable chemical reactions, which may lead to safety hazards (such as explosions or fires); (4) The rock strata of depleted oil and gas reservoirs usually have high permeability. Although this is helpful for oil and gas extraction, it is not conducive to maintaining high pressure during gas storage. The air leakage rate is high, and the long-term operating efficiency is reduced; (5) Due to long-term extraction, the pore pressure of the rock in depleted oil and gas reservoirs is reduced, resulting in low formation fracture pressure. The formation is difficult to withstand high pressure, limiting the energy storage capacity and efficiency.

[0006] Existing Chinese patent CN117722132A discloses a method and product for converting a depleted gas reservoir into a compressed air storage tank, comprising: selecting an abandoned well in the depleted gas reservoir as a foundation well; drilling several expansion holes in the wellbore of the selected foundation well to expand the foundation well; injecting fracturing fluid into the wellbore to fracture all the expansion holes and further expand the foundation well; when the power generation is high, using excess electrical energy to compress air into the expanded foundation well for energy storage; when the power generation is low, releasing the air stored in the foundation well to drive a generator to generate electricity.

[0007] The limitations of the above-mentioned prior art are: (1) it requires drilling branch wells, which is costly; (2) although the energy storage space is expanded by hydraulic fracturing, the degree of fracture expansion is constrained by the branch wells during fracturing; (3) there is no limit to the fracture pressure during energy storage, which may result in the fracture not opening due to insufficient energy storage pressure or the fracture expanding due to excessive energy storage pressure; (4) it does not reduce the permeability of the depleted gas reservoir, and the injected compressed air can easily diffuse to distant places through the reservoir pores.

[0008] Chinese patent application CN119288417 discloses a method and system for energy storage and release through multiple wells. The method includes: selecting at least two wells hydraulically connected through formation fractures and / or rock pores as wells for energy storage and / or energy release; during energy storage, injecting high-pressure fluid into one or more of the wells to seal the wells and maintain elastic deformation of the formation rock to store energy; during energy release, reducing the sealing pressure of the wells to allow one or more of the wells to backflow the high-pressure fluid, and obtaining energy to generate electricity during the backflow process. However, the limitations of this method are: (1) it requires the cooperation of multiple wells, and a single well cannot independently complete energy storage or other construction processes; (2) the formation fractures extending from different wells are intertwined, which can easily form a complex fracture network, which is not conducive to energy storage efficiency and filtration prevention; (3) during energy storage, there is no limit to the fracture pressure, which allows the fractures to expand and easily causes filtration of the energy storage fluid; (4) it is difficult to control the diffusion of the energy storage fluid when using formation rock pores for energy storage.

[0009] Chinese patent application CN114016988A discloses a method and system for storing and releasing energy through geological formations. This method involves hydraulic fracturing in an oil- and gas-free energy storage formation to create at least one formation fracture. High-pressure fluid is then injected into the fracture, causing elastic deformation and storing energy. The energy is then released by backflushing the high-pressure fluid from the fracture to drive a pre-installed hydroelectric power generation device. Chinese patent application CN118654399A discloses a method and system for storing and releasing energy through geological fractures and obtaining geothermal energy. The system includes a fracturing unit for pumping fracturing fluid into the wellbore to create formation fractures in the geothermal energy storage layer; an injection pipe connected to injection equipment; a backflow pipe connected to hydroelectric power generation equipment, geothermal power generation equipment, and a heating system, allowing fluid from the formation fractures to drive the hydroelectric and geothermal power generation equipment to generate electricity and provide heating through the heating system when the fluid is backflowed through the backflow pipe; and a sealing device for closing the backflow pipe, allowing continuous heat exchange between the fluid from the formation fractures and the geothermal energy storage layer, resulting in a continuous increase in fluid temperature. This technology utilizes the elastic deformation of formation fractures for energy storage and release while simultaneously utilizing geothermal resources for power generation and / or heating. The limitations of the above methods are: (1) They only recover the elastic deformation energy of the formation rocks, so they are only suitable for recovering the kinetic energy of incompressible liquids, which greatly limits their output power. Moreover, the method is not suitable for compressed air energy storage because the main way of compressed air energy storage is through the compression potential energy stored in the air; (2) The disclosed method only utilizes the exchange of heat energy between the liquid and the formation, and does not disclose how to use compressed air to exchange with the formation to obtain heat energy; (3) It does not consider the treatment method for the large changes in volume and pressure caused by the heating of compressed air in the formation; (4) Based on the huge differences in properties between liquids and compressed air, for example, because air has a lower viscosity than liquids (air has a viscosity 50 times lower than water), it is easier to leak. Those skilled in the art cannot obtain inspiration from the disclosed methods to judge the sealing of formation fractures.

[0010] In view of this, there is an urgent need for a method of energy storage based on compressed air 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 of the Invention

[0011] In view of this, the present invention provides a method and system for energy storage based on compressed air through artificial fissures in the strata. This method and system are not restricted by geographical conditions and can carry out underground compressed air energy storage anywhere, greatly expanding the application scope of compressed air energy storage.

[0012] The first aspect of this invention discloses a method for energy storage based on compressed air passing through artificial fractures in the formation, comprising the following steps:

[0013] S1, inject fracturing fluid into the target wellbore to carry out hydraulic fracturing construction, 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, that is, the target wellbore is not connected to other wellbores through formation fractures, and can be understood as a single independent wellbore.

[0014] S2, inject sealing fluid into the formation fracture to physically and / or chemically seal the rock voids and / or microfractures around the formation fracture;

[0015] S3, Conduct a sealing test on the target wellbore based on wellhead plugging to determine the sealing of formation fractures;

[0016] 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.

[0017] 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.

[0018] The sealing fluid is a fracturing fluid containing a plugging agent, so that step S2 can be integrated into step S1 and completed simultaneously with the implementation of step S1. Alternatively, the sealing fluid is a fluid containing a plugging agent and independent of the fracturing fluid, so that step S2 can be implemented independently of step S1.

[0019] According to the method disclosed in the first aspect of the present invention, a thickener is added to the fracturing fluid, the thickener comprising: 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.

[0020] According to the method disclosed in the first aspect of the present invention, the plugging material includes a solid plugging material for physically sealing rock voids and / or microfractures surrounding formation fractures.

[0021] In another embodiment, 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.

[0022] In a particular embodiment, the plugging material may include both the solid plugging material and the liquid plugging material described above.

[0023] According to the method disclosed in the first aspect of the invention, the plugging material is added directly to the corresponding fluid constituting the sealing fluid. In another embodiment, the plugging material is added to the corresponding fluid constituting the sealing fluid in a manner encapsulated in a biodegradable capsule.

[0024] According to the method disclosed in the first aspect of the present invention, step S3 includes:

[0025] 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.

[0026] S302, wellhead sealing of the target wellbore;

[0027] 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 see if the formation fractures gradually close under wellhead sealing conditions.

[0028] According to the method disclosed in the first aspect of the present invention, in step S3, the high-pressure fluid includes a liquid high-pressure fluid and a gas high-pressure fluid; steps S301-S303 are executed sequentially first based on the liquid high-pressure fluid, and then steps S301-S303 are executed based on the gas high-pressure fluid.

[0029] According to the method disclosed in the first aspect of the present invention, in step S301, a test pressure threshold is set, which is constructed to be greater than the formation fracture closure pressure and less than the formation fracture propagation pressure, such 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.

[0030] According to the method disclosed in the first aspect of the present invention, in step S303, the determination of the sealing property of the formation fracture includes:

[0031] 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.

[0032] 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.

[0033] According to the method disclosed in the first aspect of the present invention, during the execution of step S4, the method further includes monitoring the pressure at the wellhead and / or inside the wellbore, as well as the compressed air flow rate. By monitoring the pressure at the wellhead and / or inside the wellbore, as well as the compressed air flow rate, the fracture pressure can be calculated, thereby ensuring that the fracture pressure does not exceed a set value.

[0034] According to the method disclosed in the first aspect of the present invention, in step S5, after the wellhead is sealed due to the absence of energy release demand, the method further includes the step of monitoring the pressure inside the wellhead and / or wellbore to calculate the pressure inside the formation fracture.

[0035] 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.

[0036] According to the method disclosed in the first aspect of the present invention, the method further includes a step of ensuring that the formation fracture still has a residual fracture width after the energy release in step S5, which includes:

[0037] Define the critical residual fracture width of the formation fracture;

[0038] The critical residual compressed air volume is determined based on the set critical residual crack width.

[0039] 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.

[0040] According to the method disclosed in the first aspect of the present invention, in step S5, a power generation device capable of converting the compression potential energy of the compressed cavity into electrical energy is provided.

[0041] According to the method disclosed in the first aspect of the present invention, the energy storage in step S4 and the energy release in step S5 further include at least one of the following energies: 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 within the formation crack.

[0042] According to the method disclosed in the first aspect of the present invention, the method further includes a step of modifying the tip of the formation fracture, comprising:

[0043] 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.

[0044] 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.

[0045] By using fatigue repair steps at the fracture tip, the risk of fatigue effects at the fracture tip can be effectively reduced, thereby avoiding the formation of microcracks due to fatigue effects at the fracture tip caused by repeated opening and closing of formation fractures, and thus preventing compressed air leakage at the fracture tip.

[0046] It is particularly important to emphasize that the step of modifying the fracture tip in the formation occurs before energy storage. This is intended to prevent tip fatigue (because the fracture needs to undergo a large number of opening and closing cycles) after energy storage and release, thus preventing cracks from forming.

[0047] Furthermore, the fracture toughness of the solid material is greater than the fracture toughness of the stratum rock where the formation fracture is located, or / and the tensile strength of the solid material is greater than the tensile strength of the stratum rock where the formation fracture is located, or / and the shear strength of the solid material is greater than the shear strength of the stratum rock where the formation fracture is located, or / and the compressive strength of the solid material is greater than the compressive strength of the stratum rock where the formation fracture is located, or / and the fatigue life of the solid material is greater than the fatigue life of the stratum rock where the formation fracture is located, or / and the fatigue limit of the solid material is greater than the fatigue limit of the stratum rock where the formation fracture is located, or / and the fatigue strength of the solid material is greater than the fatigue strength of the stratum rock where the formation fracture is located.

[0048] Furthermore, the viscosity ratio M of the first fluid and the second fluid does not exceed 10. This allows the second fluid to have a better displacement effect on the first fluid in formation fractures and the wellbore, avoids viscous fingering, reduces the residue of the first fluid outside the fracture tip, and prevents mixing of the first and second fluids within the wellbore.

[0049] Furthermore, the density of the first fluid is not less than 70% of the density of the second fluid. This effectively prevents Rayleigh-Taylor instability caused by the density difference between the first and second fluids in the wellbore, as well as stratification of the first and second fluids in formation fractures.

[0050] In another embodiment, the method further includes a step of modifying the tip of the formation fracture, which includes:

[0051] 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.

[0052] 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;

[0053] After the displacement work is completed, based on the degradation of the biodegradable capsules at the fracture tips of the formation fractures, the reactants released by the biodegradable capsules undergo solidification reactions with each other and / or with the third fluid to form solid substances.

[0054] Furthermore, 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 less than the density of the third fluid, so that the biodegradable capsules accumulate at the crest of the formation fracture; or, 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. This allows for adjusting the different locations where the capsules accumulate in the formation fracture by changing the density of the biodegradable capsules, thereby effectively modifying the specific location of the fracture tip.

[0055] Furthermore, different reactants can be encapsulated in a single biodegradable capsule, or different reactants can be encapsulated in different biodegradable capsules.

[0056] A second aspect of the invention also discloses a system for implementing the method disclosed in the first aspect of the invention, the system comprising:

[0057] 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;

[0058] 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.

[0059] 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:

[0060] 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.

[0061] The sealing unit is constructed to seal the wellhead of the target wellbore.

[0062] 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.

[0063] 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.

[0064] 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;

[0065] 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.

[0066] Beneficial effects: In the method and system of the present invention for energy storage based on compressed air through artificial geological fissures, it is possible to use compressed air for energy storage by constructing closed artificial geological fissures. During the energy storage process, not only is the compressed potential energy of the air accumulated, but also the stress potential energy is accumulated by overcoming the pressure of fissure closure and driving the geological rocks to undergo elastic deformation to accumulate elastic potential energy. At the same time, the compressed air in the geological fissures exchanges heat with the geological rocks to obtain geological thermal energy. Most importantly, this method is not restricted by geographical conditions and can carry out underground compressed air energy storage anywhere, which greatly expands the application scope of compressed air energy storage.

[0067] The following describes in detail the method, system, and sealing method of the present invention for energy storage based on compressed air through sealed formation fractures, with reference to the embodiments shown in the accompanying drawings and the reference numerals. Attached Figure Description

[0068] Figure 1 The flowchart illustrates the steps of the method for energy storage based on compressed air through sealed formation fractures in this invention.

[0069] Figure 2 This is a schematic diagram of the present invention for compressed air energy storage through sealing geological fractures.

[0070] Figure 3 This illustrates the difference between traditional compressed air energy storage (using natural or man-made underground cavities) and compressed air energy storage through sealed geological fissures. Detailed Implementation

[0071] It should be noted that the term "liquid fluid" in this article can be, but is not limited to, liquids, emulsions, slurries, and solid particle flows with flow characteristics similar to those of liquids; while the term "fluid" in this article can be, but is not limited to, gases, liquids, emulsions, slurries, and solid particle flows with flow characteristics similar to those of liquids.

[0072] In this article, "strata" refers to porous and permeable rock formations (e.g., shale, sandstone, carbonate, hot dry rock, etc.) that can serve as storage spaces for fluids. These fluids can typically be water, hydrocarbons, or gases.

[0073] In this article, "hydraulic fracturing" or "fracture" refers to the formation and propagation of cracks in rock formations under the influence of external forces (such as high-pressure fluids).

[0074] In this article, "formation fracture" or "fracture" refers to a rock opening or fissure created within the formation after hydraulic fracturing, or a natural fracture or fault fracture that already exists in the formation. The terms "formation fracture" and "fracture" are used interchangeably. "Fracture" can refer to a single fracture, or multiple fractures or fracture swarms located at the same location.

[0075] In this article, "wellbore" refers to a hole formed by drilling or inserting a guide pipe into the formation. Generally, wellbores are cylindrical, and therefore their cross-section may be circular. Alternatively, wellbores may have any other cross-section. Wellbores can be open-hole (open-hole wellbore) or casing wellbores (cased wellbore) with a cemented casing bonded to the inner wall. Wellbores can be vertical, horizontal, or inclined.

[0076] In this article, "fracture closure pressure" refers to the minimum pressure required for a fracture to open, which is equal to the in-situ stress perpendicular to the fracture surface, and is usually the minimum principal stress of the formation. For horizontal fractures, the fracture closure pressure is usually equal to the vertical principal stress of the formation. For vertical fractures, the fracture closure pressure is usually equal to the horizontal minimum principal stress of the formation.

[0077] In this article, "fracture pressure" or "fracture pressure" refers to the average pressure within the formation fractures. Fracture pressure can be obtained by monitoring the wellhead pressure (i.e., fracture pressure = wellhead pressure + hydrostatic pressure - frictional resistance) or the pressure inside the wellbore. Using a wellbore flow model, the dynamic relationship between wellhead pressure and fracture pressure under different injection flow rates and fluid densities / temperatures can be calculated.

[0078] In this article, "fracture propagation pressure" refers to the pressure at which a formation fracture begins to propagate along its length or height, and is typically greater than "fracture closure pressure." Fracture propagation pressure can be obtained through empirical formulas, rock mechanics model equations, well logging data, numerical modeling, or through field measurements (boost tests, formation integrity tests, small-scale fracturing tests, staged pressurization tests, etc.).

[0079] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings and the above-mentioned definitions. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0080] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0081] Figure 1 A flowchart illustrating the steps of the method for energy storage based on compressed air through artificial fractures in the formation, as described in this invention, is shown. Figure 1 As shown, this invention discloses a method for energy storage based on compressed air passing through artificial fractures in the ground, comprising the following steps:

[0082] S1, inject fracturing fluid into the target wellbore to carry out hydraulic fracturing operations, creating formation fractures and / or opening existing formation fractures;

[0083] S2, inject sealing fluid into the formation fracture to physically and / or chemically seal the rock voids and / or microfractures around the formation fracture;

[0084] S3, Conduct a sealing test on the target wellbore under wellhead plugging conditions to determine the sealing performance of formation fractures;

[0085] S4, Inject compressed air into the target wellbore and make the pressure after air intake in the formation fracture greater than the fracture closure pressure and less than the fracture propagation pressure for energy storage, 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 the formation fracture.

[0086] 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.

[0087] In step S1, formation fractures serve as the primary energy storage medium, and the target wellbore can be a new energy storage wellbore or an abandoned wellbore. The wellbore can be a vertical or horizontal well. If it is an abandoned oil and gas well, unnecessary perforations must be permanently sealed. If it is necessary to utilize the existing hydraulic fracturing fractures of abandoned oil and gas wells (such as depleted shale oil and gas wells), it is usually necessary to recover the proppant or increase the area of ​​the existing fractures to reduce the impact of the proppant on energy storage (proppant can hinder fracture closure, thus affecting energy storage capacity and efficiency). Abandoned wells can also be repeatedly fractured to create new formation fractures that do not contain proppant. Multiple formation fractures can be created at different locations within the wellbore.

[0088] It should be noted that in this invention, the target wellbore is constructed as a single, independent wellbore, which is not connected to any other wellbore through formation fractures, otherwise it would seriously affect the propagation and sealing of formation fractures.

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

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

[0091] In the case of "wellhead sealing conditions" in step S3 and "sealing the wellhead" in step S5 "monitoring whether there is an energy release requirement, and sealing the wellhead if not", compressed air does not enter or exit from the target wellbore. Meanwhile, it should be specifically pointed out that in step S4, compressed air can be injected into the wellbore from the wellhead using a compressed air injection device (e.g., a compressed air injection pipe). This also involves wellhead sealing, but the wellhead sealing at this time does not restrict the injection of compressed air into the target wellbore. That is, the wellhead sealing at this time is to seal the gap between the wellhead and the compressed air injection device at the wellhead of the wellbore, so that the compressed air can be fully injected into the wellbore under high pressure. Similarly, it should be emphasized that in step S5, "monitor whether there is a need for energy release, and if not, seal the wellhead" does not restrict the operation of back-venting compressed air after step S5 under the condition of "if there is a need for energy release". "Back-venting compressed air" can be achieved by a compressed air back-venting device (e.g., a back-venting pipe) passing through the sealing device used to seal the wellhead.

[0092] Generally, the lower the viscosity of the fracturing fluid, the easier it is for fractures to connect with natural fractures, weak formation surfaces, or faults, forming a complex network structure. In conventional hydraulic fracturing operations (such as shale oil and gas, tight oil and gas, and coalbed methane fracturing), the fracturing fluid viscosity is typically low to create complex fracture networks and increase oil and gas production. In some cases, to further reduce the fracturing fluid viscosity and increase the complexity of the fracture network, even ultra-low viscosity supercritical carbon dioxide is used as the fracturing fluid. However, complex fracture networks are not conducive to fracture energy storage because they reduce the pressure-bearing capacity and width of the fractures. To reduce fracture complexity and avoid excessive branching that could affect energy storage efficiency and capacity, thickeners are added to the fracturing fluid during hydraulic fracturing operations to increase its viscosity and reduce the risk of developing complex fractures.

[0093] In the method disclosed in this embodiment, the thickener includes, but is not limited to: 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. Specifically, the thickener can be: natural polymers and their derivatives (such as guar gum, carboxymethyl guar gum, hydroxypropyl guar gum, xanthan gum, sage gum, etc.), cellulose and its derivatives (such as hydroxymethyl cellulose, carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxyethyl cellulose, etc.), synthetic polymers (such as polyacrylamide, polyethylene oxide, acrylic polymers, polyvinyl alcohol, etc.), surfactant-type thickeners, gel-type thickeners (such as phenolic resin, guar gum combined with a crosslinking agent to form a gel), and clay (such as montmorillonite, bentonite, etc.).

[0094] Regarding step S2, it should be specifically noted that in the method of energy storage based on compressed air through sealed formation fractures of the present invention, the sealing fluid is constructed as a fracturing fluid containing a plugging material, allowing step S2 to be integrated into step S1 and completed simultaneously with the implementation of step S1. Alternatively, the sealing fluid can be constructed as a fluid containing a plugging material and independent of the fracturing fluid, allowing step S2 to be implemented independently of step S1. In other words, step S2 can be a part of step S1, completed after step S1 is implemented, or step S2 can be an independent step separate from step S1, i.e., an additional step S2 is performed after step S1 is implemented.

[0095] In some embodiments, the sealing fluid contains solid plugging materials (including particles, fibers, polymers, etc.) that can physically seal rock voids and / or microfractures around formation fractures.

[0096] In another embodiment, the sealing fluid contains a liquid plugging material that can chemically seal the rock pores and / or microfractures surrounding formation fractures. The liquid plugging material seals the rock pores and / or microfractures due to chemical reactions of its different components, chemical reactions triggered by contact between different liquid plugging materials, chemical reactions occurring upon contact with fluids within formation pores, or changes in salinity, pH, or temperature upon entering the formation, thus transforming from a liquid to a solid (including a colloid). In some embodiments, the fracturing fluid can be configured to contain a liquid plugging material, allowing for the sealing of rock pores and / or microfractures surrounding fractures during and / or after fracturing operations. This eliminates the need for separate physical and / or chemical sealing steps, significantly improving work efficiency and greatly reducing the cost of compressed air-based energy storage.

[0097] After sealing the rock surrounding the crack, the lower the rock permeability, the better the crack sealing. Preferably, the permeability of the rock after sealing should not exceed 0.0001 millidarcy. The effect of the sealing material on rock permeability can be quantified and evaluated in advance through laboratory experiments, numerical simulations, molecular simulations, and other methods.

[0098] In addition, those skilled in the art will understand that physical sealing particles are relatively small, ranging from micrometers to nanometers, and will not seal the cracks; while chemical sealing can be carried out by injecting chemical liquids in batches, so that the solidification reaction only occurs in the pores around the cracks, and not in the cracks themselves, thereby achieving the purpose of sealing the surrounding voids but not sealing the formation cracks.

[0099] Because formation fractures repeatedly open and close during energy storage and release (for example, one energy storage cycle per day results in 365 opening and closing cycles per year, while the typical operating life of an air compressed air energy storage power station is over 30 years), fatigue effects occur at the fracture tips, leading to micro-cracks. This causes compressed air to leak from the fracture tips, affecting energy storage efficiency. To reduce the risk of fatigue effects at fracture tips, after step S2, the fracture tips need to be modified to prevent the formation of micro-cracks. Alternatively, after multiple working cycles of S4 and S5, the fracture tips need to be modified to repair any damaged fracture tips. The steps for modifying formation fracture tips include:

[0100] 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 a fluid that does not undergo a curing reaction or polymerization reaction to form a solid.

[0101] 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.

[0102] In some embodiments, the fracture toughness of the solid material is greater than the fracture toughness of the rock formation in which the formation fracture is located, and / or the tensile strength of the solid material is greater than the tensile strength of the rock formation in which the formation fracture is located, and / or the shear strength of the solid material is greater than the shear strength of the rock formation in which the formation fracture is located, and / or the compressive strength of the solid material is greater than the compressive strength of the rock formation in which the formation fracture is located, and / or the fatigue life of the solid material is greater than the fatigue life of the rock formation in which the formation fracture is located, and / or the fatigue limit of the solid material is greater than the fatigue limit of the rock formation in which the formation fracture is located, and / or the fatigue strength of the solid material is greater than the fatigue strength of the rock formation 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 precursors, and polyetheretherketone solutions. Common "curing agents" include, but are not limited to: ethylenediamine, triethylenetetramine, m-phenylenediamine, diaminodiphenylmethane, polyamide, modified alicyclic amines, maleic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, and phenolic modifiers. Common curing agents can be categorized into amines, acid anhydrides, phenolic compounds, metal salts, imidazoles, polyisocyanates, and polysulfides. The curing time is affected by the type of liquid polymer, the type of curing agent, the mixing ratio of the liquid polymer and the curing agent, and the temperature.

[0104] In other embodiments, the viscosity of the first fluid is high. In order to enable the second fluid to have a better displacement effect on the first fluid in the formation fracture and wellbore, avoid the occurrence of viscous fingering, reduce the residue of the first fluid in the area outside the fracture tip, and avoid the mixing of the first fluid and the second fluid in the wellbore, the ratio M of the viscosity of the first fluid and the viscosity of the second fluid does not exceed 10.

[0105] In other embodiments, to prevent Rayleigh-Taylor instability due to the density difference between the first and second fluids in the wellbore, and to prevent stratification of the first and second fluids in formation fractures, the density of the first fluid needs to be the same as or close 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 other embodiments, to prevent turbulence caused by excessive flow rates of the first or second fluid in the wellbore and formation fractures, and to ensure mixing of the first and second fluids, the flow rate of the injected second fluid needs to be below a threshold flow rate to prevent turbulence in the first and / or second fluids within the wellbore or formation fractures. The threshold flow rate can be calculated using empirical formulas, theoretical models, or numerical simulations based on the hydrodynamic properties of the first and / or second fluids, the dimensions of the wellbore and fractures, and the surface roughness of the wellbore and fractures. For example, the threshold flow rate can be determined by comparing whether the calculated Reynolds number meets the critical value for turbulence.

[0107] In other embodiments, the steps for modifying or repairing the fracture tip include: injecting a biodegradable capsule and a third fluid into the formation fracture, wherein the biodegradable capsule contains at least one reactant, and the third fluid is a fluid that does not undergo a solidification or polymerization reaction to form a solid; after stopping the injection of the biodegradable capsule, continuing to inject the third fluid to displace the biodegradable capsule to the fracture tip, i.e., completing the displacement work; after the displacement work is completed, waiting for the biodegradable capsule to degrade at the fracture tip, and then the reactants released by the biodegradable capsule undergo a solidification reaction between themselves and / or with the third fluid to form a solid substance. The injection of both the biodegradable capsule and the third fluid can be completed by controlling the injection time and / or injection volume, and their respective injection times and volumes can be set through pre-calculation. This ensures that the injection of both the biodegradable capsule and the third fluid can be effectively completed, and the completion of the displacement work can be determined based on the completion of the third fluid injection. For example, the completion of the third fluid injection can be considered the completion of the displacement work, or a certain point after the completion of the third fluid injection can be used as a marker for the completion of the displacement work.

[0108] In some embodiments, 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.

[0109] 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. After the biodegradable capsules degrade at the top or bottom edge of the formation fracture, the reactants released by the biodegradable capsules undergo a solidification reaction between themselves and / or with the third fluid to form a solid substance.

[0110] In some embodiments, different reactants may be contained in the same biodegradable capsule; in other embodiments, different reactants may be contained in different biodegradable capsules. For example, a polymer in powder form and a curing agent in powder form will not undergo a curing reaction when placed in the same capsule, but after the capsule is injected into a formation fracture, the polymer powder and curing agent powder released during degradation can undergo a curing reaction upon contact with water.

[0111] The reactants in the biodegradable capsules can be in a liquid or solid state (including powder). The biodegradable capsules may also contain other substances to adjust the capsule density.

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

[0113] Furthermore, in this invention, step S3 includes:

[0114] Step 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.

[0115] Step S302: Seal the wellhead of the target wellbore;

[0116] Step S303: By monitoring the change in pressure at the wellhead and / or inside the wellbore over time, it is determined whether the formation fractures gradually close under wellhead plugging conditions, thereby determining the sealing performance of the formation fractures.

[0117] In step S3, the high-pressure fluid injected into the wellbore can be a liquid (such as fracturing fluid) or a gas (such as air, helium, or nitrogen), i.e., the high-pressure fluid includes high-pressure liquid fluid and / or high-pressure gas fluid. For example, the sealing performance of the formation fracture can be judged by the change in the wellhead pressure drop curve over time after the pump is stopped during fracturing (at which point the pressure inside the fracture meets the condition of being 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 (e.g., pressure drop per hour does not exceed 0.2 MPa), it proves that the formation fracture sealing performance is good. The leakage rate of compressed air over a certain period of time can be calculated using the pressure drop data. Alternatively, a daily leakage rate threshold can be given to determine the threshold for pressure drop per unit time. If the wellhead pressure drops rapidly or exceeds the set threshold, it proves that the formation fracture sealing performance is poor, and fluid leakage causes the fracture to begin closing. If the formation fracture sealing performance does not meet the requirements, it is necessary to carry out sealing operations on the rock voids and / or microfractures around the formation fracture again until the formation fracture sealing performance meets the design requirements.

[0118] The pressure inside the wellbore can be measured by installing downhole pressure gauges or using distributed optical fibers.

[0119] In a particularly preferred embodiment for the sealing test in step 3, steps S301-S303 are performed sequentially, first based on a high-pressure liquid fluid, and then based on a high-pressure gas fluid. That is, the formation fracture is sealed using a high-pressure liquid fluid (at least once) followed by a high-pressure gas fluid (at least once). Specifically, the formation fracture is sealed first using a high-pressure liquid fluid. The advantage of this is that if the formation fracture sealing test fails, existing surface liquid injection equipment and liquid storage facilities (such as surface storage tanks, reservoirs, and sealing material mixing equipment) can be used to re-seal the rock voids and / or microfractures around the formation fracture. After the liquid sealing test is passed, the high-pressure gas fluid is then used for the sealing test, because gas molecules are smaller and more prone to leakage, and in some cases, specific physical or chemical sealing measures have significantly different sealing effects on liquids and gases.

[0120] In another specific embodiment of the closure test, high-pressure fluid is injected into the target wellbore so that the maximum pressure within the formation fracture is not less than a test pressure threshold. This test pressure threshold is greater than the formation fracture closure pressure but less than the formation fracture propagation pressure. This test pressure threshold can be set as the maximum operating pressure during the formation fracture energy storage process. This is because, in some cases, as the formation fracture pressure increases, the risk of reopening of microfractures in the surrounding rock increases. The formation fracture may pass the closure test at lower pressures (although the fracture is already open), but fail at higher pressures.

[0121] In another embodiment of the sealing test, due to the high formation temperature, the expansion of compressed air in the formation due to temperature rise will affect the rate of pressure change at the wellhead and / or inside the wellbore. The sealing performance of the formation fractures can be determined by calculating a threshold for pressure drop using a physical model (e.g., calculating the effect of formation temperature on the density and pressure of compressed air within the formation fractures). Alternatively, the sealing performance can be determined by waiting for the compressed air and formation to reach heat exchange equilibrium (i.e., the temperature of the compressed air within the formation no longer rises), and then based on the pressure change over time. The time to reach heat exchange equilibrium can be calculated using a physical model or by directly measuring the temperature inside the wellbore (using downhole thermometers or distributed fiber optic monitoring equipment). Specifically, the temperature inside the wellbore is monitored during wellhead plugging. When the temperature inside the wellbore stops changing, the start time of this cessation is recorded as time t0. The sealing performance of the formation fractures is then determined based on the pressure change at the wellhead and / or inside the wellbore over time, starting from time t0 or a time after time t0. In the method disclosed in the embodiments of the present invention, in step S5, a power generation device is provided that can convert the compression potential energy of compressed air into electrical energy.

[0122] In the method disclosed in this embodiment, the energy storage in step S4 and the energy release in step S5 further include at least one of the following energies: 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.

[0123] That is, during energy storage based on step S4, compressed air is injected into the wellbore, making the pressure within the formation fracture greater than the fracture closure pressure but less than the fracture propagation pressure. This allows compressed air to continuously enter the fracture, increasing its width without propagating it, thus driving elastic deformation of the formation rock and accumulating elastic potential energy. Simultaneously, the pressure within the wellbore and formation fracture increases, and the air accumulates compressive potential energy; or / and the compressed air within the fracture exchanges heat with the formation rock, gaining formation thermal energy. The wellbore is then sealed to maintain pressure within the wellbore and formation fracture, thereby storing energy. At the end of energy storage, the volume of compressed air contained in the formation fracture is significantly greater than the volume of compressed air contained in the wellbore.

[0124] Typically, compressed air needs to be preheated to between 100°C and 300°C before entering the expander for power generation to improve efficiency. When the formation temperature is high, the temperature of the compressed air stored within the formation fissures also increases, significantly reducing the energy consumed in preheating before it is subsequently discharged to the surface and enters the expander for power generation, thus improving energy storage efficiency. In some embodiments, where the formation temperature is above 100°C, preheating of the compressed air before entering the expander for power generation is not required, greatly reducing the operating costs of power generation.

[0125] To prevent formation fractures from expanding during energy storage, thus avoiding energy loss and gas leakage risks, wellhead pressure needs to be monitored throughout the storage process to ensure that the pressure within the formation fractures remains below the fracture propagation pressure. It's important to note that the wellhead is open during pressure monitoring during energy storage, but closed during formation fracture sealing tests.

[0126] To more accurately calculate the frictional resistance within the wellbore and the pressure of compressed air within the formation fractures, during energy storage, specifically during step S4, it is necessary to monitor the pressure at the wellhead and / or within the wellbore, as well as the compressed air flow rate. This is because the frictional resistance of compressed air within the wellbore and fractures is closely related to the compressed air flow rate, which also determines the flow regime (e.g., laminar or turbulent). In some embodiments, to more accurately calculate the compressed air density, it is also necessary to additionally monitor the temperature at the wellhead and / or within the wellbore. The temperature within the wellbore can be measured using methods such as installing downhole thermometers or distributed optical fibers.

[0127] The steps for calculating frictional resistance include: determining the wellbore's geometric parameters (length, inner diameter, and wall roughness) and operating parameters (such as inlet pressure, temperature, and flow rate); looking up tables or calculating the density and viscosity of air at the operating pressure; calculating the Reynolds number and determining the flow state; and determining the friction coefficient and calculating the frictional resistance based on the flow type.

[0128] Furthermore, to further reduce the risk of formation fracture propagation, a threshold is set for the formation fracture pressure, ensuring that the pressure does not exceed this threshold during energy storage. The threshold value can be set based on the risk of fracture propagation. For example, if the risk of fracture propagation is high, the threshold can be set to: fracture propagation pressure value - 1 MPa; if the risk of fracture propagation is low, the threshold can be set to: fracture propagation pressure value - 0.5 MPa.

[0129] After S4 is completed and compressed air is injected into the wellbore, if there is no need to release energy, the wellbore can be sealed to maintain the pressure in the wellbore and formation fractures and store energy; if there is a need to release energy, compressed air can be discharged from the wellbore.

[0130] In a particularly preferred embodiment, step S5, after sealing the wellhead due to the absence of energy release demand, further includes the following steps: monitoring the pressure inside the wellhead and / or wellbore to calculate the pressure inside the formation fracture; 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.

[0131] That is, during the wellbore plugging process, it is necessary to monitor the pressure at the wellhead and / or inside the wellbore, and calculate the formation fracture pressure 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, compressed air needs to be backflushed from the wellbore (regardless of whether there is an energy release requirement) to prevent spontaneous propagation of the formation fracture. This is because, unlike using liquids for energy storage, the temperature increase of compressed air in the formation fracture leads to volume expansion and pressure increase, while the volume of liquids (such as water) is minimally affected by temperature. For example, using the ideal gas law, it can be calculated that when the air storage volume remains constant, an increase in temperature from 40 degrees Celsius (such as the temperature of the compressed air injected into the wellhead) to 80 degrees Celsius (such as the formation temperature where the formation fracture is located) will increase the pressure by 12.7%. In some embodiments, when the pressure measured inside the wellbore is very close to the location of the formation fracture, the measured pressure value can be approximated as the formation fracture pressure value during calculation.

[0132] The flow rate range for compressed air energy storage is typically 1000 to 4000 cubic meters per minute, far exceeding the flow rate range of liquid energy storage in formation fractures (usually only 5 to 10 cubic meters per minute). Therefore, compared to liquid energy storage, the efficiency of compressed air energy storage is more significantly affected by fracture width. If the formation fracture is too narrow, it increases flow resistance and drastically reduces energy storage efficiency. At the same flow rate, the lower the formation fracture pressure, the narrower the fracture, and the higher the flow velocity, the greater the energy loss due to flow resistance.

[0133] In another embodiment, to improve energy storage efficiency and reduce the flow resistance of injected and discharged compressed air in formation fractures, after the energy release in step S5, a portion of residual compressed air remains in the formation fractures, ensuring that the pressure within the formation fractures is higher than the formation fracture closure pressure and that there is a certain residual fracture width. Specifically, the step of ensuring that the formation fractures still have a residual fracture width after the energy release in step S5 includes: setting a critical residual fracture width for the formation fractures; determining a critical residual compressed air volume based on the set critical residual fracture width; and in step S5, monitoring the remaining volume of compressed air in the wellbore and its formation fractures during the energy release process, and ending the discharge when the monitored remaining volume approaches or reaches the critical residual compressed air volume, thereby maintaining a residual fracture width in the formation fractures.

[0134] A critical residual fracture width (corresponding to a critical residual compressed air volume) can be calculated using physical models or numerical simulations to seek a balance between energy storage efficiency and energy storage capacity (capacity usable for reverse discharge power generation), such that the average width of the formation fracture after energy release is greater than the critical residual fracture width. A larger critical residual fracture width results in more residual compressed air remaining in the formation fracture, lower fracture flow resistance losses, higher energy storage efficiency, and lower energy storage capacity. In some embodiments, after energy release in step S5, 30% of the maximum energy storage capacity of residual compressed air remains in the formation fracture. After reverse discharge of compressed air from the wellbore in step S5, the wellbore can be sealed to maintain the pressure in the wellbore and formation fracture, as well as the width of the formation fracture.

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

[0136] During the energy release process in step S5, the wellbore sealing pressure is reduced to backflow the compressed air, and during the backflow process, at least one of the following is obtained: the kinetic energy of the compressed air converted from the elastic potential energy accumulated in the formation rock, the compression potential energy released by the compressed air, and the thermal energy carried by the compressed air.

[0137] The system utilizes pre-designed power generation equipment to convert at least one of the following into electrical energy: the elastic potential energy stored in the earth's rock layers, the kinetic energy of compressed air, the compressive potential energy released by the compressed air, and the thermal energy carried by the compressed air. The power generation equipment includes, but is not limited to: turbines, turbomachinery, expanders, heat exchangers, and organic Rankine cycle power generation equipment.

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

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

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

[0141] In one embodiment of the method of the present invention, the compressed air needs to be heated before entering the expander to generate electricity.

[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 methods utilizing 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] Traditional compressed air energy storage is limited by the strength constraints of underground cavities, and the storage pressure cannot exceed the formation fracture pressure. For example, the operating pressure of underground salt cavern energy storage systems is approximately 7 MPa to 12 MPa. However, the maximum energy storage pressure of formation fractures is far greater than that of underground salt caverns, significantly increasing the energy storage density. For instance, the minimum principal stress of a formation fracture at a depth of 3000 meters is 50 MPa, and the fracture propagation pressure is 55 MPa; therefore, the operating pressure for underground fracture energy storage is 50 MPa to 55 MPa. Generally, the deeper the formation fracture, the greater the fracture closure pressure, and thus the greater the energy storage pressure.

[0148] In addition to storing the compressive potential energy of air, traditional compressed air energy storage also includes the rock elastic potential energy accumulated by increasing the fracture width and the stress potential energy accumulated by overcoming the fracture closure pressure, further increasing the energy storage density. Among these, the stress potential energy accumulated by overcoming the fracture closure pressure can significantly reduce the fracture pressure drop per unit volume of air discharged.

[0149] This invention also discloses a system for implementing the method disclosed in the first aspect of the invention, comprising a fracturing unit, a sealing fluid injection unit, a sealing test unit, an energy storage unit, and an energy release unit; wherein the fracturing unit is configured to inject 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 not connected to another wellbore through formation fractures; the sealing fluid injection unit is configured to inject sealing fluid into the formation fractures to treat the rock voids and / or microfractures surrounding the formation fractures. Physical and / or chemical sealing; the sealing test unit is constructed to perform a sealing test on the target wellbore based on wellhead sealing to determine the sealing performance of formation fractures; the energy storage working unit is constructed to inject compressed air into the target wellbore, ensuring that the pressure after air intake in the formation fractures is greater than the fracture closure pressure and less than the fracture propagation pressure, in order to store energy, including at least the compressive potential energy accumulated by the compressed air due to the increased pressure within the overall space formed by the target wellbore and formation fractures; the energy release working unit is constructed to backflush the compressed air, and during the backflush process, at least the compressive potential energy released by the compressed air is acquired;

[0150] The sealing test unit includes a high-pressure fluid injection unit, a sealing unit, and a pressure monitoring and judgment unit. The high-pressure fluid injection unit is designed to inject high-pressure fluid into the target wellbore, so that the pressure inside the formation fracture is greater than the fracture closure pressure but less than the fracture propagation pressure, thus keeping the formation fracture in an open but not propagating state. The sealing unit is designed to seal the wellhead of the target wellbore. The pressure monitoring and judgment unit is designed to monitor the wellhead pressure of the target wellbore and determine whether the formation fracture gradually closes under the wellhead sealing condition by monitoring the change of pressure at the wellhead and / or inside the wellbore over time, thereby judging the sealing performance of the formation fracture.

[0151] In the system of the present invention, the sealing fluid is constructed as a fracturing fluid containing a plugging substance, so that the sealing fluid injection unit can be integrated into the fracturing unit and work together with the fracturing operation; or, the sealing fluid is constructed as a fluid containing a plugging substance and independent of the fracturing fluid, so that the sealing fluid injection unit works independently of the fracturing unit.

[0152] In another embodiment of the system, a compressed air flow monitoring unit is also included, configured to monitor the compressed air flow rate during energy storage and / or release. This is because the frictional resistance of compressed air within the wellbore and fractures is closely related to the compressed air flow rate, which also determines the flow regime (e.g., laminar or turbulent). In another embodiment, to more accurately calculate the compressed air density, it is also necessary to additionally monitor the temperature at the wellhead and / or within the wellbore. This can be achieved by installing temperature monitoring devices, such as downhole thermometers or distributed optical fibers, to measure the temperature within the wellbore.

[0153] In another embodiment of the system, a control unit and a computing unit are also included. The computing unit is capable of calculating the pressure inside the formation fracture based on the pressure at the wellhead and / or inside the wellbore monitored by the pressure monitoring and judgment unit. The control unit is communicatively connected to the computing unit. When the pressure inside 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 compressed air from the target wellbore, regardless of whether there is a power generation demand, thereby preventing the formation fracture from spontaneously propagating.

[0154] In another embodiment of the system, a critical residual fracture width setting unit is also included, configured to set a critical residual fracture width for the formation fractures. The aforementioned calculation unit or control unit is further configured to determine a critical residual compressed air volume based on the set critical residual fracture width. The compressed air cavity flow monitoring unit is further configured to monitor the remaining volume of compressed air in the wellbore and its formation fractures during the energy release process; when the monitored remaining volume approaches or reaches the critical residual compressed air volume, the control unit controls the backflow device to terminate backflow, thereby maintaining a residual fracture width in the formation fractures.

[0155] In another embodiment of the system, a power generation device is also included, capable of converting at least one of the following into electrical energy: the kinetic energy of compressed air converted from the elastic potential energy stored in the geological rock, the compressive potential energy released by the compressed air, and the thermal energy carried by the compressed air. The power generation device includes, but is not limited to: turbines, turbines, expanders, heat exchangers, and organic Rankine cycle power generation equipment. In this embodiment, a turbine is used to convert the kinetic energy of compressed air into electrical energy. In one embodiment, an expander is used to convert the compressive potential energy of compressed air into electrical energy.

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

[0157] In another embodiment of the system, the waste heat generated during the expansion of compressed air can be used to drive an organic Rankine cycle expander, thereby converting thermal energy into electrical energy.

[0158] In one embodiment of the system, the compressed air needs to be heated before entering the expander to generate electricity.

[0159] In one embodiment of the system, the temperature of the compressed air rises in the formation fissures due to the high ground temperature, so no further heating is required when the air expands to generate electricity.

[0160] In one embodiment of the system, due to the low thermal conductivity of the formation, the heat generated by the compressed air can be stored in the formation fissures, eliminating the need to recover the heat generated by the compressed air at the ground.

[0161] It should be noted that the sealing fluid injection unit can be integrated into the fracturing unit or independent of the fracturing unit. "Integration" means that a separate sealing fluid injection unit is not required; instead, the fracturing unit performs the functions and characteristics of the sealing fluid injection unit. In other words, the function of the sealing fluid injection unit can be directly implemented based on the fracturing unit, thus allowing the fracturing unit to include the functions required by the sealing fluid injection unit, making the fracturing unit exist as both a fracturing and sealing fluid injection unit. In another embodiment, the sealing fluid injection unit can exist as an independent unit, thereby performing chemical sealing on its own.

[0162] The related functions and technical features described in the method disclosed in the first aspect above are also applicable to the system described in Embodiment 5 of the present invention, and will not be repeated here.

[0163] For ease of description, spatial relative terms such as "above," "on top of," "on the upper surface of," "above," etc., are used herein to describe the spatial positional relationship of a device or feature as shown in the figures to other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation beyond the orientation of the device as described in the figures. For example, if the device in the figures were inverted, a device described as "above" or "on top of" other devices or structures would subsequently be positioned as "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below." The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatial relative descriptions used herein will be interpreted accordingly.

[0164] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0165] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0166] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0167] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for energy storage based on compressed air passing through artificial fissures in the ground, 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. 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.

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 2 or 3, characterized in that, 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 see if the formation fractures gradually close under wellhead sealing conditions.

6. The method according to claim 5, characterized in that, In step S3, 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.

7. The method according to claim 6, 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.

8. The method according to claim 6, 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.

9. 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.

10. 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.

11. 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 the compressed cavity into electrical energy.

12. 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.

13. 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.

14. The method according to claim 13, characterized in that, The fracture toughness of the solid material is greater than that of the rock formation in which the formation fracture is located, or / and the tensile strength of the solid material is greater than that of the rock formation in which the formation fracture is located, or / and the shear strength of the solid material is greater than that of the rock formation in which the formation fracture is located, or / and the compressive strength of the solid material is greater than that of the rock formation in which the formation fracture is located, or / and the fatigue life of the solid material is greater than that of the rock formation in which the formation fracture is located, or / and the fatigue limit of the solid material is greater than that of the rock formation in which the formation fracture is located, or / and the fatigue strength of the solid material is greater than that of the rock formation in which the formation fracture is located.

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

16. The method according to claim 13, characterized in that, The density of the first fluid is not less than 70% of the density of the second fluid.

17. 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.

18. The method according to claim 17, 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.

19. The method according to claim 17, characterized in that, A single biodegradable capsule can encapsulate different reactants, or different reactants can be encapsulated in different biodegradable capsules.

20. A system for implementing the method according to any one of claims 1-19, 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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