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

Abstract

The invention relates to a method and a system for storing energy through a formation fracture based on compressed air. The method comprises the following steps: constructing the formation fracture; 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

Technical Field

[0001] The present invention belongs to the technical field of underground energy storage, and in particular relates to a method and system for storing energy based on compressed air passing through stratum fractures. Background Art

[0002] Many renewable energy sources, such as solar and wind, have daily and seasonal intermittent output, making them unsuitable for providing baseload power. Consequently, they struggle to directly feed into the grid. Energy storage is crucial for successfully scaling up renewable energy production. Consequently, finding ways to store excess clean electricity on a large scale and reliably transmit it when sunlight and wind are insufficient remains a technological challenge that countries around the world are tackling.

[0003] Compressed air energy storage (CAES) is a mature energy storage technology with advantages such as large storage capacity, long lifespan, and strong stability. However, it is geographically demanding and requires abandoned mines or underground caverns as storage media, limiting its use to certain specific areas. While artificial underground cavities are currently available as gas storage media, the extremely high construction investment costs significantly limit the commercial prospects of CAES.

[0004] Although depleted oil and gas reservoirs can be used as potential application sites for compressed air energy storage, they also have considerable limitations: (1) The rock formations of depleted oil and gas reservoirs are usually subjected to long-term oil and gas production, and may have 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 production, the strata of depleted oil and gas reservoirs may have deformed or compacted, and the mechanical integrity of the gas storage reservoir is reduced. Under high-pressure conditions, the strata may collapse or slip, affecting the safety of the gas storage reservoir; (3) A certain amount of oil and gas may remain in the 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 formations of depleted oil and gas reservoirs usually have high permeability, which is conducive to oil and gas production, but is not conducive to maintaining high pressure during gas storage, resulting in high air leakage rate and reduced long-term operation efficiency; (5) Due to long-term production, the pore pressure of the rock in depleted oil and gas reservoirs is reduced, resulting in low stratum fracture pressure, making it difficult for the stratum to withstand high pressure, limiting energy storage capacity and efficiency.

[0005] Prior art Chinese patent CN117722132A discloses a method and product for transforming a depleted gas reservoir into a compressed air storage, comprising: selecting an abandoned well in the depleted gas reservoir as a foundation well; drilling a plurality of 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, thereby further expanding the foundation well; when the power generation is large, using excess electricity to compress air into the expanded foundation well for energy storage; and when the power generation is small, releasing the air stored in the foundation well to drive a generator to generate electricity.

[0006] The limitations of the above-mentioned existing technologies are: (1) it is necessary to drill branch wellbores, which is costly; (2) although the energy storage space is expanded through hydraulic fracturing, the expansion of the cracks during fracturing is constrained by the branch wells; (3) there is no limit on the crack pressure during energy storage, and the energy storage pressure may be too low and the cracks may not open, or the energy storage pressure may be too high, causing the cracks to expand; (4) the permeability of the depleted gas reservoir is not reduced, and the injected compressed air can easily diffuse to distant places through the reservoir pores.

[0007] The Chinese patent application with publication number CN119288417 discloses a method and system for storing and releasing energy through multiple wells. The method includes: selecting at least two wellbores that are hydraulically connected through formation fractures and / or rock pores as wellbores for storing and / or releasing energy; when storing energy, injecting high-pressure fluid into one or more of the wellbores to seal the wellbores and maintain elastic deformation of the formation rock to store energy; when releasing energy, reducing the wellbore sealing pressure so that one or more of the wellbores can reversely discharge the high-pressure fluid, and obtaining energy and generating electricity during the reverse discharge 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) formation fractures extending from different wellbores are intertwined, easily forming a complex fracture network, which is not conducive to energy storage efficiency and loss prevention; (3) when storing energy, the fracture pressure is not limited, allowing the fractures to expand, which easily causes loss of energy storage fluid; (4) it is difficult to control the diffusion of energy storage fluid when using formation rock pores for energy storage.

[0008] Chinese patent application publication number CN114016988A discloses a method and system for storing and releasing energy through formations. This method involves hydraulically fracturing an energy-storing formation that contains no oil or gas, creating at least one formation fracture. High-pressure liquid is then injected into the fracture, causing elastic deformation to store energy. The high-pressure liquid is then discharged from the fracture to propel a pre-set hydroelectric generator, releasing the energy. Chinese patent application publication number CN118654399A discloses a method and system for storing and releasing energy through formation fractures and obtaining geothermal energy. The system includes a fracturing device for pumping fracturing fluid into the wellbore to create formation fractures in the energy storage geothermal layer; an injection pipeline connected to an injection device; a backflow pipeline connected to hydroelectric power generation equipment, geothermal power generation equipment, and a heating system. This allows the fluid in the formation fractures to flow back through the backflow pipeline, driving the hydroelectric and geothermal power generation equipment to generate electricity and providing heating through the heating system; and a plugging device for closing the backflow pipeline, allowing the fluid in the formation fractures to continuously exchange heat with the energy storage geothermal layer, causing the fluid temperature to continue to rise. This technology utilizes the elastic deformation of formation fractures to store and release energy while utilizing geothermal resources to generate electricity and / or provide heating. The limitations of the above methods are: (1) they only recover the elastic deformation energy of the formation rock, and are therefore only suitable for recovering the kinetic energy of incompressible liquids, which greatly limits their output power. In addition, this method is not suitable for compressed air energy storage, because the main way to store compressed air energy is through the compressed potential energy accumulated by air; (2) the disclosed method only uses liquid and formation to exchange heat energy, and does not disclose how to use compressed air to exchange with the formation to obtain heat energy; (3) it does not take into account 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 difference in properties between liquid and compressed air, for example, because air has a smaller viscosity than liquid (air has a viscosity 50 times smaller than water), it is more prone to leakage. Technical personnel in the relevant field cannot obtain inspiration for judging the sealing properties of formation fractures from the disclosed methods.

[0009] In view of this, there is an urgent need for a compressed air-based energy storage method that has low investment and construction costs, can complete energy storage and release using a single well, and is not restricted by geographical conditions. Summary of the Invention

[0010] In view of this, the present invention provides a method and system for storing energy based on compressed air through stratum cracks. 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.

[0011] The first aspect of the present invention discloses a method for storing energy based on compressed air passing through formation fractures, comprising the following steps:

[0012] S1, injecting fracturing fluid into a target wellbore to perform hydraulic fracturing operations to create formation fractures and / or open existing formation fractures; wherein the target wellbore is constructed as 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;

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

[0014] S3, conducting a sealing test based on wellhead plugging on the target wellbore to determine the sealing performance of the formation fractures;

[0015] S4, injecting compressed air into the target wellbore and its formation fractures, and causing the pressure of the formation fractures after the air is injected to be greater than the fracture closure pressure and less than the fracture expansion pressure, so as to store energy, which at least includes the compression potential energy accumulated by the compressed air due to the pressure increase in the entire space formed by the target wellbore and the formation fractures;

[0016] S5, monitoring whether there is a need to release energy. If not, the wellhead is blocked; if so, compressed air is discharged, and at least the compression potential energy released by the compressed air is obtained during the discharge process;

[0017] The plugging 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 plugging fluid is constructed as a fluid containing a plugging substance and independent of the fracturing fluid, so that step S2 can be implemented separately from step S1.

[0018] According to the method disclosed in the first aspect of the present invention, a thickener is added to the fracturing fluid, and the thickener 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.

[0019] According to the method disclosed in the first aspect of the present invention, the plugging material includes a solid plugging material, which is used to physically seal the rock voids and / or microcracks around the formation fractures.

[0020] In another embodiment, the plugging material includes a liquid plugging material, which is constructed to be able to undergo a solidification or gelation reaction between its own components, or different liquid plugging materials can contact each other to undergo a solidification or gelation reaction, or can contact the fluid in the rock voids and / or microcracks around the formation cracks to undergo a solidification or gelation reaction, or can undergo a solidification or gelation reaction after entering the formation due to changes in salinity, pH or temperature, so as to chemically seal the rock voids and / or microcracks around the formation cracks.

[0021] In a particular embodiment, the plugging material can include both the above-mentioned solid plugging material and liquid plugging material.

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

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

[0024] S301, injecting high-pressure fluid into the target wellbore to make the pressure in the formation fracture greater than the fracture closure pressure and less than the fracture expansion pressure, so that the formation fracture is in an open and non-expanded state;

[0025] S302, plugging the target wellbore;

[0026] S303, by monitoring the change of the pressure at the wellhead and / or in the wellbore over time, it is determined whether the formation fractures are gradually closing under the wellhead plugging condition, thereby determining the sealing performance of the formation fractures.

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

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

[0029] According to the method disclosed in the first aspect of the present invention, in step S303, determining the sealing property of the formation fracture includes:

[0030] Monitor the temperature in the wellbore when the wellhead is plugged. When it is detected that the temperature in the wellbore no longer changes, the start time when the temperature in the wellbore no longer changes is recorded as time t0;

[0031] Starting from time t0 or a moment after time t0, the sealing property of the formation fracture is determined based on the change of the pressure at the wellhead and / or in the wellbore over time.

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

[0033] According to the method disclosed in the first aspect of the present invention, in step S5, after monitoring that there is no energy release demand and the wellhead is blocked, the method further includes the steps of: monitoring the pressure in the wellhead and / or wellbore to calculate the pressure in the formation fracture;

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

[0035] 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 is completed, which includes:

[0036] Setting the critical residual fracture width of formation fractures;

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

[0038] In step S5, the residual volume of compressed air in the wellbore and its formation fractures during the energy release process is monitored. When the monitored residual volume approaches or reaches the critical residual compressed air volume, the backflow is terminated, thereby maintaining the residual fracture width in the formation fractures.

[0039] 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 compression cavity into electrical energy is provided.

[0040] 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 also include at least one of the following energies: stress potential energy accumulated by compressed air doing work to overcome the crack closing pressure, elastic deformation potential energy accumulated by driving the formation rock to undergo elastic deformation, and formation thermal energy obtained by heat exchange between compressed air and formation rock in the formation crack.

[0041] According to the method disclosed in the first aspect of the present invention, the method further includes the step of modifying the formation fracture tip, including:

[0042] Injecting a first fluid and a second fluid into the formation fracture in sequence, wherein the first fluid comprises at least one liquid polymer and at least one curing agent, and the second fluid is a fluid that does not undergo a curing reaction or a polymerization reaction;

[0043] After the second fluid displaces the first fluid to the fracture tip of the formation fracture, injection of the second fluid into the formation fracture is stopped, and the first fluid is left to complete a solidification reaction and change from a liquid state to a solid state.

[0044] By performing fatigue repair on the crack tip, the risk of fatigue effect at the crack tip can be effectively reduced, thereby avoiding the occurrence of microcracks caused by fatigue effect at the crack tip due to repeated opening and closing of formation cracks, thereby avoiding the leakage of compressed air at the crack tip.

[0045] Among them, it should be particularly emphasized that the step of modifying the tip of the formation fracture occurs before energy storage, which is intended to prevent the tip from fatigue after energy storage and release (because the fracture needs to undergo a very large number of opening and closing cycles) and thus cause cracks, that is, to achieve the purpose of early prevention.

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

[0047] Furthermore, the ratio M of the viscosity of the first fluid to the viscosity of the second fluid does not exceed 10. This enables the second fluid to have a better displacement effect on the first fluid in the formation fractures and the wellbore, avoids the occurrence of viscous fingering, reduces the residue of the first fluid in the area outside the fracture tip, and avoids the mixing of the first fluid and the second fluid in the wellbore.

[0048] Furthermore, the density of the first fluid is not less than 70% of the density of the second fluid, which can effectively prevent the first fluid and the second fluid from generating Rayleigh-Taylor instability due to the different densities in the wellbore, and the first fluid and the second fluid from stratifying in the formation fractures.

[0049] In another embodiment, the method further includes the step of modifying the formation fracture tip, comprising:

[0050] injecting a degradable capsule and a third fluid into the formation fracture, wherein the degradable capsule contains at least one reactant, and the third fluid is a fluid that does not undergo a solidification reaction or a polymerization reaction;

[0051] After stopping the injection of the degradable capsules, continuing to inject the third fluid to displace the degradable capsules to the crack tips of the formation cracks;

[0052] After the displacement is completed, the degradable capsules degrade at the fracture tips of the formation fractures, so that reactants released from the degradable capsules undergo solidification reactions with each other and / or with the third fluid to form a solid substance.

[0053] Furthermore, during the step of injecting the degradable capsules and the third fluid into the formation fracture, the density of the degradable capsules is set to be less than the density of the third fluid, so that the degradable capsules accumulate at the top edge of the formation fracture. Alternatively, during the step of injecting the degradable capsules and the third fluid into the formation fracture, the density of the degradable capsules is set to be greater than the density of the third fluid, so that the degradable capsules accumulate at the bottom edge of the formation fracture. This allows the location of the capsule accumulation in the formation fracture to be adjusted by changing the density of the degradable capsules, thereby effectively modifying the specific location of the fracture tip of the formation fracture.

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

[0055] The second aspect of the present invention further discloses a system for implementing the method disclosed according to the first aspect of the present invention, the system comprising:

[0056] A fracturing unit is configured to inject fracturing fluid into a target wellbore to perform hydraulic fracturing operations, thereby creating formation fractures and / or opening existing formation fractures; wherein the target wellbore is configured as an independent wellbore that is not connected to another wellbore through formation fractures;

[0057] A plugging fluid injection unit is configured to inject a plugging fluid into the formation fractures to physically and / or chemically plug the rock voids and / or microcracks surrounding the formation fractures;

[0058] The sealing test unit is configured to perform a sealing test on the target wellbore based on wellhead plugging to determine the sealing performance of the formation fractures. The sealing test unit includes:

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

[0060] A sealing unit configured to seal the target wellbore;

[0061] The pressure monitoring and judgment unit is configured to monitor the wellhead pressure of the target wellbore and judge whether the formation fractures are gradually closing under the wellhead plugging condition by monitoring the change of the wellhead and / or wellbore pressure over time.

[0062] Thus, the sealing property of formation fractures can be judged;

[0063] An energy storage working unit is configured to inject compressed air into a target wellbore and cause the pressure of the formation fractures after the air is injected to be greater than the fracture closure pressure and less than the fracture expansion pressure, thereby storing energy, which at least includes the compression potential energy accumulated by the compressed air due to the pressure increase in the entire space formed by the target wellbore and the formation fractures;

[0064] An energy-releasing working unit is configured to discharge compressed air and obtain at least compression potential energy released by the compressed air during the discharge process;

[0065] The plugging fluid is constructed as a fracturing fluid containing a plugging substance, so that the plugging fluid injection unit can be integrated into the fracturing unit and complete the work along with the fracturing construction, or the plugging fluid is constructed as a fluid containing a plugging substance and independent of the fracturing fluid, so that the plugging fluid injection unit can work independently of the fracturing unit.

[0066] Beneficial effects: In the method and system for storing energy based on compressed air through formation cracks of the present invention, closed artificial formation cracks can be constructed for compressed air energy storage. During the energy storage process, not only the air compression potential energy is accumulated, but also the stress potential energy is accumulated by overcoming the closing pressure of the cracks, and the formation rock is driven to undergo elastic deformation to accumulate elastic potential energy; at the same time, the compressed air in the formation cracks exchanges heat with the formation rock to obtain formation thermal energy; most importantly, this method is 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.

[0067] The following describes in detail the method, system and plugging method of storing energy by sealing formation fractures based on compressed air with reference to the embodiments and reference numerals shown in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 A flow chart showing the steps of the method for storing energy by sealing formation fractures based on compressed air in the present invention is shown.

[0069] Figure 2 This is a schematic diagram of the present invention for storing compressed air energy by sealing formation fractures.

[0070] Figure 3 The difference between traditional compressed air energy storage (using natural or artificial underground cavities) and compressed air energy storage by sealing formation fractures is shown. DETAILED DESCRIPTION

[0071] It should be noted that the "liquid fluid" in this article can be but is not limited to liquid, emulsion, slurry and solid particle flow with flow characteristics similar to liquid flow; and the "fluid" in this article can be but is not limited to gas, liquid, emulsion, slurry and solid particle flow with flow characteristics similar to liquid flow.

[0072] The term "formation" as used herein refers to a porous and permeable rock formation (e.g., shale formation, sandstone formation, carbonate formation, hot dry rock formation, etc.) underground that can serve as a storage space for fluids. Typically, these fluids can be water, hydrocarbons, or gases.

[0073] The term "hydraulic fracturing" or "fracture" or "cracking" as used herein refers to the generation and expansion of cracks in formation rocks under the action of external forces (such as high-pressure fluid).

[0074] "Formation fractures" or "fractures" in this article are open cracks in the rock created by hydraulic fracturing, or existing natural fractures or faults in the formation. The terms "formation fractures" and "fractures" are used interchangeably. "Fractures" can refer to a single fracture, multiple adjacent fractures at the same location, or a fracture swarm.

[0075] As used herein, "wellbore" refers to a hole drilled or inserted into a formation by conduit. Typically, a wellbore is cylindrical, and therefore may have a circular cross-section. However, a wellbore may have any other cross-section. A wellbore may be open-hole, or cased (cased). A wellbore may be vertical, horizontal, or inclined.

[0076] The term "fracture closure pressure" used in this article refers to the minimum pressure at which a fracture opens. It is equal to the in-situ stress perpendicular to the fracture plane and is typically the minimum principal stress of the formation. For horizontal fractures, the fracture closure pressure is typically equal to the vertical principal stress of the formation. For vertical fractures, the fracture closure pressure is typically equal to the minimum horizontal principal stress of the formation.

[0077] The term "fracture pressure" or "intra-fracture pressure" used in this article refers to the average pressure within formation fractures. Fracture pressure can be determined by monitoring wellhead pressure (fracture pressure = wellhead pressure + hydrostatic pressure - frictional resistance) or wellbore pressure. Using a wellbore flow model, the dynamic relationship between wellhead pressure and fracture pressure can be calculated under varying injection rates and fluid density / temperature conditions.

[0078] The "fracture propagation pressure" in this article refers to the pressure at which a formation fracture begins to expand in length or height, and is typically greater than the "fracture closure pressure." Fracture propagation pressure can be determined using empirical formulas, rock mechanics model equations, well logging data, numerical modeling, and other methods. It can also be obtained through field measurements (such as overflow tests, formation integrity tests, small-scale fracturing tests, and step-by-step pressure tests).

[0079] The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the drawings and the above definitions of terms in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0080] It should be noted that, unless there is any conflict, the embodiments and features in the embodiments of this application can be combined with each other.

[0081] Figure 1 The flowchart of the method for storing energy based on compressed air through formation fractures in the present invention is shown. Figure 1 As shown, the present invention discloses a method for storing energy based on compressed air passing through formation fractures, comprising the following steps:

[0082] S1, injecting fracturing fluid into the target wellbore to perform hydraulic fracturing operations to create formation fractures and / or open existing formation fractures;

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

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

[0085] S4, injecting compressed air into the target wellbore, and causing the pressure of the formation fracture after the air is injected to be greater than the fracture closure pressure and less than the fracture expansion pressure, so as to store energy, which at least includes the compression potential energy accumulated by the compressed air due to the pressure increase in the entire space formed by the target wellbore and the formation fracture;

[0086] S5, monitor whether there is a need to release energy. If not, seal the wellhead; if so, discharge the compressed air, and at least obtain the compression potential energy released by the compressed air during the discharge process.

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

[0088] It should be particularly pointed out that, in the present invention, the target wellbore is constructed as a single independent wellbore, which is not connected to any other wellbore through formation fractures, otherwise it will seriously affect the expansion and blocking of formation fractures.

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

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

[0091] Under the "wellhead plugging condition" in step S3 and the "plugging wellhead" situation in step S5 "monitoring whether there is an energy release demand, if not, plugging the wellhead", compressed air does not enter or exit from the target wellbore. At the same time, it should be pointed out in particular that in step S4, compressed air can be injected into the wellbore from the wellhead with the help of a compressed air injection device (such as a compressed air injection pipe), which will also involve wellhead blocking, but the wellhead blocking at this time does not limit the injection of compressed air into the target wellbore, that is, the wellhead blocking at this time is to block the gap between the wellhead and the compressed air injection device at the wellbore head, so that the compressed air can be fully injected into the wellbore under high pressure; similarly, it should be emphasized that in step S5, "blocking the wellhead" in "monitoring whether there is a need for energy release, and if not, blocking the wellhead" does not limit the operation of back-discharging compressed air after step S5 under the condition of "if there is a need for energy release", and "back-discharging compressed air" can be achieved by a compressed air back-discharge device (such as a back-discharge pipe) passing through the blocking device used to block the wellhead.

[0092] Generally, the lower the viscosity of the fracturing fluid, the easier it is for the cracks to connect with natural fractures, weak surfaces or faults in the formation, forming a complex network structure. In conventional hydraulic fracturing construction (such as shale oil and gas, tight oil and gas, and coalbed methane fracturing), in order to form a complex fracture network and increase oil and gas production, the viscosity of the fracturing fluid is usually low. In some cases, in order to further reduce the viscosity of the fracturing fluid and increase the complexity of the fracture network, ultra-low viscosity supercritical carbon dioxide is even used as a fracturing fluid. However, a complex fracture network is not conducive to fracture energy storage, because a complex fracture network will reduce the pressure bearing capacity and fracture width of the fracture. In order to reduce the complexity of the fracture and avoid excessive branching of the fracture, which affects the energy storage efficiency and capacity, thickeners are added to the fracturing fluid during the hydraulic fracturing construction process to increase the viscosity of the fracturing fluid and reduce the risk of 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, artemisia 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 acid polymer, 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 noted that in the method of storing energy by sealing formation fractures based on compressed air according to the present invention, the plugging 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 plugging fluid is constructed as a fluid containing a plugging substance and independent of the fracturing fluid, so that step S2 can be implemented independently of step S1. In other words, step S2 can be a part of step S1, and after step S1 is implemented, step S2 is also completed, or step S2 is an independent step outside of step S1, that is, after step S1 is implemented, an additional step S2 is performed.

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

[0096] In another embodiment, the plugging fluid contains liquid plugging substances, which can chemically plug the rock voids and / or microcracks around the formation cracks. The liquid plugging substance reacts with the chemical reaction of different components of the liquid plugging substance itself or different liquid plugging substances contact each other to trigger a chemical reaction or the liquid plugging substance contacts the fluid in the formation pores to react chemically or the liquid plugging substance changes from liquid to solid (including colloid) due to the change of salinity, pH value or temperature when entering the formation, thereby plugging the rock pores and / or microcracks. In some embodiments, the fracturing fluid can be constructed to contain liquid plugging substances, which makes it possible to plug the rock voids and / or microcracks around the cracks during and / or after the fracturing construction process, thus eliminating the need for additional physical and / or chemical plugging steps, thereby greatly improving work efficiency and greatly reducing the cost of energy storage based on compressed air.

[0097] After sealing the rock surrounding the fracture, the lower the rock permeability, the better the fracture sealability. Preferably, the rock permeability after sealing is no higher than 0.0001 millidarcy. The effect of plugging materials on rock permeability can be quantified and evaluated in advance through laboratory experiments, numerical simulations, and molecular modeling.

[0098] In addition, those skilled in the art can understand that physical sealing particles are relatively small, ranging from micrometers to nanometers, and will not seal cracks; as for chemical sealing, chemical liquids can be injected in batches so that the solidification reaction only occurs in the pores around the cracks, but not inside the cracks, thereby achieving the purpose of sealing the surrounding gaps but at the same time not sealing the formation cracks.

[0099] Since formation cracks will open and close repeatedly during the energy storage and release process (for example, if an energy storage cycle is performed once a day, there will be 365 openings and closings in a year, and the operating life of an air compression energy storage power station is usually more than 30 years), fatigue effects will occur at the crack tips, resulting in microcracks, causing compressed air to leak from the crack tips, affecting energy storage efficiency. In order to reduce the risk of crack tip fatigue effects, after step S2, the crack tips need to be modified to prevent the generation of microcracks at the crack tips. Or after multiple working cycles of S4 and S5, the crack tips need to be modified to repair the damaged crack tips. The steps for modifying the formation crack tips include:

[0100] Injecting a first fluid and a second fluid into the formation fracture in sequence, wherein the first fluid comprises at least one liquid polymer and at least one curing agent, and the second fluid is a fluid that does not undergo a curing reaction or a polymerization reaction to form a solid;

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

[0102] In some embodiments, the fracture toughness of the solid-state material is greater than the fracture toughness of the formation rock where the formation fracture is located, or / and the tensile strength of the solid-state material is greater than the tensile strength of the formation rock where the formation fracture is located, or / and the shear strength of the solid-state material is greater than the shear strength of the formation rock where the formation fracture is located, or / and the compressive strength of the solid-state material is greater than the compressive strength of the formation rock where the formation fracture is located, or / and the fatigue life of the solid-state material is greater than the fatigue life of the formation rock where the formation fracture is located, or / and the fatigue limit of the solid-state material is greater than the fatigue limit of the formation rock where the formation fracture is located, or / and the fatigue strength of the solid-state material is greater than the fatigue strength of the formation rock where 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 as amines, acid anhydrides, phenolic forms, metal salts, imidazoles, polyisocyanates, and polysulfides. The curing reaction time is affected by the type of liquid polymer, the type of curing agent, the mixing ratio of the liquid polymer to the curing agent, and temperature.

[0104] In other embodiments, the first fluid has a higher viscosity. In order to enable the second fluid to have a better displacement effect on the first fluid in the formation fractures and the 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 to the viscosity of the second fluid does not exceed 10.

[0105] In other embodiments, to prevent Rayleigh-Taylor instabilities due to density differences between the first and second fluids in the wellbore, as well as delamination of the first and second fluids in formation fractures, the density of the first fluid must be equal to or similar to that of the second fluid. For example, in some embodiments, the density of the first fluid must be at least 70% of the density of the second fluid.

[0106] In other embodiments, to prevent turbulence caused by excessive flow of the first or second fluid in the wellbore or formation fractures, thereby causing mixing of the first and second fluids, the flow rate of the second fluid injected needs to be lower than a threshold flow rate, so that the first fluid and / or the second fluid do not cause turbulence in the wellbore or formation fractures. The threshold flow rate can be calculated using empirical formulas, theoretical models, or numerical simulations based on information such as the fluid mechanics properties of the first or / and second fluids, the wellbore and fracture dimensions, and the surface roughness of the wellbore and fractures. For example, the threshold flow rate can be determined by comparing the calculated Reynolds number to see if it meets the critical value for turbulence.

[0107] In other embodiments, the step of modifying or repairing the fracture tip includes: injecting a degradable capsule and a third fluid into the formation fracture, wherein the degradable capsule contains at least one reactant, and the third fluid is a fluid that does not undergo a solidification reaction or polymerization reaction to form a solid; after stopping the injection of the degradable capsule, continuing to inject the third fluid to displace the degradable capsule to the fracture tip of the formation fracture, i.e., completing the displacement process; after the displacement process is completed, waiting for the degradable capsule to degrade at the fracture tip of the formation fracture, so that the reactants released by the degradable capsule undergo a solidification reaction with each other and / or with the third fluid to form a solid substance. The injection of the degradable capsule and the third fluid can be completed by controlling the injection time and / or injection amount, and the respective injection time and injection amount can be set by precalculation. This allows the injection of the degradable capsule and the third fluid to be completed effectively, and the completion of the displacement process can also be determined based on whether the injection of the third fluid is completed. For example, the completion of the displacement process can be determined when the injection of the third fluid is completed, or a certain time after the completion of the injection of the third fluid can be used as a marker for the completion of the displacement process.

[0108] In some embodiments, during the step of injecting the degradable capsules and the third fluid into the formation fracture, the density of the degradable capsules is set to be less than the density of the third fluid, so that the degradable capsules are accumulated at the fracture top edge of the formation fracture;

[0109] Alternatively, during the step of injecting the degradable capsules and the third fluid into the formation fracture, the density of the degradable capsules is set to be greater than the density of the third fluid, so that the degradable capsules accumulate at the bottom edge of the formation fracture. After the degradable capsules degrade at the top or bottom edge of the formation fracture, reactants released from the degradable capsules undergo a solidification reaction with each other and / or with the third fluid to form a solid substance.

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

[0111] The reactants in the degradable capsule can be in liquid or solid form (including powder). The degradable capsule can also contain other substances for adjusting the density of the capsule.

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

[0113] Furthermore, in the present invention, step S3 includes:

[0114] Step S301: injecting high-pressure fluid into the target wellbore to make the pressure in the formation fracture greater than the fracture closure pressure and less than the fracture expansion pressure, so that the formation fracture is in an open and non-expanded state;

[0115] Step S302, plugging the target wellbore;

[0116] Step S303 , by monitoring the change of the pressure at the wellhead and / or in the wellbore over time, it is determined whether the formation fractures are gradually closing under the wellhead plugging condition, thereby determining the sealing property of the formation fractures.

[0117] In step S3, the high-pressure fluid injected into the wellbore can be a liquid (such as a fracturing fluid) or a gas (such as air, helium or nitrogen), that is, the high-pressure fluid includes a liquid high-pressure fluid and / or a gas high-pressure fluid. For example, the sealing performance of the formation fracture can be judged by the change in the pressure drop curve of the wellhead after the fracturing construction pump is stopped (at this time, the pressure in the crack meets the pressure greater than the formation fracture closing pressure and less than the formation fracture expansion pressure) over time. For example, if the wellhead pressure remains stable or the pressure drop per unit time is less than a threshold value (such as the pressure drop per hour does not exceed 0.2MPa), it is proved that the formation fracture sealing performance is good. Through the pressure drop data, the leakage rate of compressed air in a certain time period can be calculated. The threshold value of the daily leakage rate can also be given to determine the threshold value of the pressure drop per unit time. If the wellhead pressure drops faster or is greater than the set threshold value, it is proved that the formation fracture sealing performance is poor, and fluid leakage causes the fracture to begin to close. If the formation fracture sealing performance does not meet the requirements, it is necessary to again carry out plugging construction on the rock voids and / or microcracks around the formation fracture until the formation fracture sealing performance reaches the design requirements.

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

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

[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 no less than a test pressure threshold, which is greater than the formation fracture closure pressure and less than the formation fracture expansion pressure. The test pressure threshold can be set to the maximum operating pressure during the formation fracture energy storage process. In some cases, as the formation fracture pressure increases, the risk of reopening the rock microcracks surrounding the formation fracture increases. A formation fracture may pass the closure test at a lower pressure (although the formation fracture is still open), but fail the closure test at a higher pressure.

[0121] In another embodiment of the sealing test, due to the high formation temperature, the expansion of compressed air in the formation due to heating will affect the rate of change of pressure at the wellhead and / or in the wellbore. The sealing of the formation fracture can be determined by calculating a modified pressure drop threshold using a physical model (for example, calculating the effect of formation temperature on the density and pressure of compressed air in the formation fracture). Alternatively, the sealing of the formation fracture can be determined by waiting for the compressed air and the formation in the formation fracture to reach heat exchange equilibrium (i.e., the temperature of the compressed air in the formation no longer rises), and then based on the change in pressure over time. The time to reach heat exchange equilibrium can be calculated using a physical model or by directly measuring the temperature in the wellbore (using a downhole thermometer or distributed optical fiber monitoring equipment). Specifically, the wellbore temperature is monitored during wellhead sealing. When the wellbore temperature no longer changes, the time at which the temperature no longer changes is recorded as time t0. Starting from time t0 or a time after time t0, the sealing of the formation fracture can be determined based on the change in pressure at the wellhead and / or in the wellbore over time. In the method disclosed in the embodiment of the present invention, in step S5, a power generation device capable of converting the compression potential energy of compressed air into electrical energy is provided.

[0122] In the method disclosed in this embodiment, the energy storage in step S4 and the energy release in step S5 also include at least one of the following energies: stress potential energy accumulated by compressed air doing work to overcome the crack closing pressure, elastic deformation potential energy accumulated by driving the formation rock to undergo elastic deformation, and formation thermal energy obtained by heat exchange between compressed air and formation rock in the formation crack.

[0123] That is, when energy storage is performed based on step S4, compressed air is injected into the wellbore, causing the pressure within the formation fracture to be greater than the closure pressure of the formation fracture and less than the expansion pressure of the formation fracture. As a result, compressed air continues to enter the formation fracture, widening the formation fracture without expanding it, causing the formation rock to elastically deform and accumulate elastic potential energy. Simultaneously, the pressure within the wellbore and formation fracture increases, accumulating compression potential energy. Alternatively, the compressed air within the formation fracture exchanges heat with the formation rock, generating formation thermal energy. The wellbore is then sealed to maintain pressure within the wellbore and formation fracture, thereby performing energy storage. 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 power generation efficiency. When the formation temperature is high, the compressed air within the formation fractures will also heat up during storage. This significantly reduces the energy required for preheating the compressed air before it is subsequently discharged to the surface and enters the expander for power generation, thereby improving energy storage efficiency. In some embodiments, when the formation temperature is above 100°C, the compressed air does not need to be preheated before entering the expander for power generation, significantly reducing power generation operating costs.

[0125] To prevent formation fractures from expanding during energy storage, which could lead to energy loss and gas leakage, wellhead pressure monitoring is required during the energy storage process to ensure that the pressure within the formation fractures remains below the fracture expansion pressure. It should be noted that during pressure monitoring during energy storage, the wellhead is open. During fracture sealing tests, the wellhead is closed.

[0126] In order to calculate the frictional resistance in the wellbore and the pressure of compressed air in the formation crack more accurately, in the process of energy storage, that is, in the process that described step S4 is performed, it is necessary to monitor pressure and compressed air flow in the wellhead and / or wellbore.Because the frictional resistance and compressed air flow of compressed air in the wellbore and crack are closely related, compressed air flow also determines the flow state (such as laminar flow or turbulent flow) of flow.In certain embodiments, in order to calculate the density of compressed air more accurately, it is also necessary to additionally monitor the temperature in the wellhead and / or wellbore.Temperature in the wellbore can be measured by methods such as installing downhole thermometer or distributed optical fiber.

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

[0128] Furthermore, to further reduce the risk of formation fracture expansion, a threshold is set for formation fracture pressure, ensuring that it never exceeds the threshold during energy storage. The threshold can be set based on the risk of fracture expansion. For example, if the risk of fracture expansion is high, the threshold might be set at -1 MPa. If the risk of fracture expansion is low, the threshold might be set at -0.5 MPa.

[0129] After completing S4 and injecting compressed air 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 cracks and store energy; if there is a need to release energy, the compressed air can be discharged from the wellbore.

[0130] In a particularly preferred embodiment, in step S5, after monitoring that there is no need to release energy and sealing the wellhead, the step also includes: monitoring the pressure at the wellhead and / or wellbore to calculate the pressure in the formation fractures; when the pressure in the formation fractures approaches or reaches the formation fracture expansion pressure or the set fracture working pressure threshold, compressed air is discharged from the target wellbore.

[0131] That is, during the wellbore plugging process, it is necessary to monitor the wellhead and / or wellbore pressure and calculate the formation fracture pressure to ensure that the formation fracture pressure does not exceed the formation fracture expansion pressure or a set threshold. When the formation fracture pressure approaches the formation fracture expansion pressure or reaches the set threshold, compressed air needs to be discharged from the wellbore (regardless of whether there is a need to release energy) to prevent the formation fracture from spontaneously expanding. This is because, unlike using liquid energy storage, the increase in temperature of compressed air in the formation fracture will cause volume expansion and pressure increase, while the volume of liquid (such as water) is very little affected by temperature. For example, using the ideal gas state equation, it can be calculated that when the air storage volume remains unchanged, the pressure will increase by 12.7% when the temperature increases 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). In some embodiments, when the location of the pressure measured in the wellbore is very close to the location of the formation fracture, the measured pressure value can be approximately equivalent to the formation fracture pressure value during calculation.

[0132] The flow rate range for compressed air energy storage (CAES) is typically 1,000 to 4,000 cubic meters per minute, far exceeding the flow range for liquid energy storage in formation fractures (typically only 5 to 10 cubic meters per minute). Therefore, compared to liquid energy storage, the efficiency of CAES is more significantly affected by fracture width. Excessively narrow fracture widths increase flow resistance, significantly reducing energy storage efficiency. At the same flow rate, lower fracture pressure and smaller fracture widths increase flow velocity, leading to greater energy loss due to flow resistance.

[0133] In another embodiment, in order to improve energy storage efficiency and reduce the flow resistance of the injected and reversed compressed air in the formation fractures, after the energy release in step S5 is completed, a portion of residual compressed air is still retained in the formation fractures, so that the pressure in the formation fractures is higher than the formation fracture closure pressure and 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 is completed includes: setting a critical residual fracture width of the formation fractures; determining a critical residual compressed air volume of the compressed air based on the set critical residual fracture width; in step S5, monitoring the residual volume of the compressed air in the wellbore and its formation fractures during the energy release process, and ending the reverse flow when the monitored residual volume approaches or reaches the critical residual compressed air volume, thereby maintaining the residual fracture width of the formation fractures.

[0134] A critical residual crack width (corresponding to a critical residual compressed air volume) can be calculated by physical models or numerical simulations, and a balance between energy storage efficiency and energy storage working capacity (capacity that can be used for reverse discharge power generation) can be sought, so that the average width of the formation cracks after the energy release is completed is greater than the critical residual crack width. The larger the critical residual crack width, the more residual compressed air is still retained in the formation cracks, the smaller the crack flow resistance loss is, the greater the energy storage efficiency is, and the smaller the energy storage working capacity is. In some embodiments, after the energy release in step S5 is completed, 30% of the residual compressed air of the maximum energy storage capacity is still retained in the formation cracks. After completing S5 to reversely discharge the compressed air from the wellbore, the wellbore can be sealed to maintain the pressure in the wellbore and the formation cracks and the width of the formation cracks.

[0135] In one embodiment, during energy storage, the heat generated by air compression is transferred through a heat exchanger to a thermal energy storage medium (such as water, molten salt, or ceramic particles). When the air needs to be released for power generation, the stored heat is used to preheat the compressed air entering the expander, reducing the need for an external heat source. This stored heat can also be used for industrial production, heating, domestic hot water, and other purposes, enabling thermal energy reuse.

[0136] During the energy release process of step S5, the wellbore sealing pressure is reduced to reverse the compressed air, and in the process of reverse discharge, at least one of 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 heat energy carried by the compressed air is obtained.

[0137] The system utilizes pre-defined power generation equipment to convert at least one of the following: the kinetic energy of compressed air converted from elastic potential energy stored in the formation rock, the compression potential energy released by the compressed air, and the thermal energy carried by the compressed air into electrical energy. This power generation equipment includes, but is not limited to, turbines, turbine engines, 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 compression 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 by the expander during operation for other purposes, such as preheating the air entering the expander or providing heat energy for other processes.

[0140] In one embodiment of the method of the present invention, 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, the compressed air is heated in the formation fractures due to the high formation temperature, and therefore does not need to be reheated when the air expands to generate electricity.

[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 there is no need to recover the heat energy generated by the compressed air on the ground.

[0144] Figure 2 This is a schematic diagram of the present invention for compressed air energy storage by sealing formation cracks. Figure 2 As shown, a formation fracture 2 is constructed in the target wellbore 1, a plugging device 3 is provided at the wellhead of the target wellbore 1, and a compressor 4 for supplying compressed air into the target wellbore 1 and a power generation device 5 for generating electricity by means of the compression potential energy accumulated in the compressed air, the kinetic energy of the compressed air converted from the elastic potential energy accumulated in the formation rock, and the geothermal energy carried by the compressed air when the compressed air is discharged are provided outside the target wellbore 1 and above the ground surface 6. As mentioned above, the power generation device 5 includes but is not limited to: one or more of a turbine, a turbine, an expander, a heat exchanger, and an organic Rankine cycle power generation equipment.

[0145] Combine Figure 2 As shown, compressed air can be injected and discharged through a single target wellbore, significantly reducing investment costs. A single energy storage project can have multiple wellbores, but there is no hydraulic connectivity between them (not through fractures or rock pores). Each closed fracture connected to a wellbore is independent of any closed fractures connected to other wellbores. The energy storage and release of each wellbore is unaffected by the energy of other wellbores.

[0146] Figure 3 The figure shows the difference between traditional compressed air energy storage (using natural or artificial underground cavities) and compressed air energy storage by closing formation cracks, where the upper dotted line in the figure represents the formation crack expansion pressure, and the lower dotted line represents the formation crack closure pressure, and curve A represents the relationship between the compressed air injection volume and pressure in the compressed air energy storage process based on the closed formation cracks of the present invention, while curve B represents the relationship between the compressed cavity injection volume and pressure in the traditional compressed cavity energy storage process using natural or artificial underground cavities. It can be seen from the figure that compared with the traditional method of using natural or artificial underground cavities for compressed air energy storage, under the same injection volume, the present invention has a higher energy storage pressure and thus a greater energy storage density by closing formation cracks for compressed air energy storage.

[0147] Traditional compressed air energy storage is limited by the strength of the underground cavity, limiting the storage pressure to levels above the formation's fracture pressure. For example, the operating pressure of underground salt cavern energy storage systems is approximately 7 MPa to 12 MPa. However, the maximum storage pressure in formation fractures is far greater than this, significantly increasing the energy storage density. For example, the minimum principal stress of a formation fracture at 3,000 meters underground is 50 MPa, and the fracture expansion pressure is 55 MPa. Therefore, the operating pressure of underground fracture energy storage is 50 MPa to 55 MPa. Generally, the deeper the formation fracture, the greater the fracture closure pressure and the greater the storage pressure.

[0148] In addition to the traditional compressed air energy storage capacity, which stores the compressed potential energy of air, compressed air energy storage through closed formation fractures also includes the elastic potential energy of the rock accumulated by increasing the fracture width and the stress potential energy accumulated by overcoming the fracture closure pressure, further increasing the energy storage density. 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] The present invention also discloses a system for implementing the method disclosed in the first aspect of the present invention, comprising a fracturing unit, a plugging fluid injection unit, a sealing test unit, an energy storage working unit and an energy release working unit; wherein the fracturing unit is configured to inject fracturing fluid into a target wellbore to perform hydraulic fracturing construction to create formation cracks and / or open existing formation cracks; wherein the target wellbore is configured as an independent wellbore that is not connected to another wellbore through formation cracks; the plugging fluid injection unit is configured to inject plugging fluid into the formation cracks to open the rock voids and / or microcracks around the formation cracks. Physical and / or chemical plugging; the sealing test unit is configured to perform a sealing test on the target wellbore based on wellhead plugging to determine the sealing property of the formation fracture; the energy storage working unit is configured to inject compressed air into the target wellbore and make the pressure of the formation fracture after air intake greater than the fracture closure pressure and less than the fracture expansion pressure to store energy, which at least includes the compression 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 configured to discharge the compressed air and at least obtain the compression potential energy released by the compressed air during the discharge process;

[0150] Among them, the sealing test unit includes a high-pressure fluid injection unit, a sealing unit and a pressure monitoring and judgment unit, wherein the high-pressure fluid injection unit is constructed to inject high-pressure fluid into the target wellbore, so that the pressure in the formation fracture is greater than the fracture closing pressure and less than the fracture expansion pressure, so that the formation fracture is in an open and non-expanded state; the sealing unit is constructed to perform wellhead sealing on the target wellbore; the pressure monitoring and judgment unit is constructed to monitor the wellhead pressure of the target wellbore, and judge whether the formation fracture is gradually closed under the wellhead sealing condition by monitoring the changes in the wellhead and / or wellbore pressure over time, thereby judging the sealing of the formation fracture.

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

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

[0153] In another embodiment of the system, it also includes a control unit and a calculation unit, wherein the calculation unit is capable of calculating the pressure in the formation fracture based on the wellhead and / or wellbore pressure monitored by the pressure monitoring and judgment unit; the control unit is communicated with the calculation unit, and when the pressure in the formation fracture approaches or reaches the formation fracture expansion pressure or the set fracture working pressure threshold, the control unit controls the reverse discharge device to reverse the compressed air from the target wellbore, regardless of whether there is a demand for power generation, thereby avoiding spontaneous expansion of the formation fracture.

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

[0155] Another embodiment of the system further includes a power generation device capable of converting at least one of the following: the kinetic energy of compressed air converted from 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 into electrical energy. The power generation device includes, but is not limited to, a turbine, a turbine, an expander, a heat exchanger, and an organic Rankine cycle power generation device. The turbine converts the kinetic energy of the compressed air into electrical energy. In one embodiment, the expander converts the compression potential energy of the 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 by the expander during operation and use it for other purposes, such as preheating the air entering the expander or providing heat energy for other processes.

[0157] In another embodiment of the system, 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 compressed air increases in temperature within the formation fractures due to the higher formation temperature, and thus does not require reheating 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 energy generated by the compressed air can be stored in the formation fractures, and there is no need to recover the heat energy generated by the compressed air on the ground.

[0161] It should be noted that the plugging fluid injection unit is configured to be integrated into the fracturing unit or independent of the fracturing unit. The term "integrated" means that the plugging fluid injection unit is no longer required as an additional unit, and that its functions and features are achieved through the fracturing unit. That is, the functions of the plugging fluid injection unit can be directly implemented based on the fracturing unit, so that the fracturing unit incorporates the functions to be achieved by the plugging fluid injection unit, allowing the fracturing unit to function as both a fracturing and plugging fluid injection unit. In another embodiment, the plugging fluid injection unit can exist as an independent unit, thereby implementing chemical plugging on its own.

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

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

[0164] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" 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 and claims of the present application and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can, for example, be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0166] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall 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 limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for storing energy based on compressed air through formation fractures, characterized in that: The following steps are involved: S1, injecting fracturing fluid into a target wellbore to perform hydraulic fracturing to create formation fractures and / or open existing formation fractures; wherein the target wellbore is constructed as an independent wellbore that is not connected to other wellbores through formation fractures; S2, injecting a plugging fluid into the formation fracture to physically and / or chemically plug the rock voids and / or microcracks around the formation fracture; S3, conducting a sealing test based on wellhead plugging on the target wellbore to determine the sealing performance of the formation fractures; S4, injecting compressed air into the target wellbore and its formation fractures, and causing the pressure of the formation fractures after the air is injected to be greater than the fracture closure pressure and less than the fracture expansion pressure, so as to store energy, which at least includes the compression potential energy accumulated by the compressed air due to the pressure increase in the entire space formed by the target wellbore and the formation fractures; S5, monitoring whether there is a need to release energy. If not, the wellhead is blocked; if so, compressed air is discharged, and at least the compression potential energy released by the compressed air is obtained during the discharge process; The plugging 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 plugging fluid is constructed as a fluid containing a plugging substance and independent of the fracturing fluid, so that step S2 can be implemented separately from step S1.

2. The method according to claim 1, characterized in that A thickener is added to the fracturing fluid, and the thickener 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 a solid plugging material, which is used to physically plug the rock voids and / or microcracks around the formation cracks; And / or, the plugging material includes a liquid plugging material, which is constructed to be able to undergo a solidification or gelation reaction between its own components, or different liquid plugging materials can contact each other to undergo a solidification or gelation reaction, or can contact the fluid in the rock voids and / or microcracks around the formation cracks to undergo a solidification or gelation reaction, or can undergo a solidification or gelation reaction after entering the formation due to changes in salinity, pH or temperature, so as to chemically seal the rock voids and / or microcracks around the formation cracks.

4. The method according to claim 3, characterized in that The plugging material is directly added to the corresponding fluid constituting the plugging fluid; Alternatively, the plugging substance in the fluid is added to the corresponding fluid constituting the plugging fluid in a manner of being at least partially encapsulated in a degradable capsule.

5. The method according to claim 1, wherein The step S3 comprises: S301, injecting high-pressure fluid into the target wellbore to make the pressure in the formation fracture greater than the fracture closure pressure and less than the fracture expansion pressure, so that the formation fracture is in an open and non-expanded state; S302, plugging the target wellbore; S303, by monitoring the change of the pressure at the wellhead and / or in the wellbore over time, it is determined whether the formation fractures are gradually closing under the wellhead plugging condition, thereby determining the sealing performance of the formation fractures.

6. The method according to claim 5, characterized in that In step S3, the high-pressure fluid includes liquid high-pressure fluid and gas high-pressure fluid; steps S301-S303 are first executed 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 expansion pressure, so that after the high-pressure fluid is injected into the formation fracture, the maximum pressure in the formation fracture is not less than the test pressure threshold and is less than the formation fracture expansion pressure.

8. The method according to claim 6, characterized in that In step S303, the determination of the sealing property of the formation fractures includes: Monitor the temperature in the wellbore when the wellhead is plugged. When it is detected that the temperature in the wellbore no longer changes, the start time when the temperature in the wellbore no longer changes is recorded as time t0; Starting from time t0 or a moment after time t0, the sealing property of the formation fracture is determined based on the change of the pressure at the wellhead and / or in the wellbore over time.

9. The method according to claim 1, characterized in that During the execution of step S4, the step of monitoring the pressure at the wellhead and / or in the wellbore and the flow rate of compressed air is also included.

10. The method according to claim 1, characterized in that In the step S5, after the wellhead is plugged after monitoring that there is no energy release demand, the step further includes: monitoring the pressure in the wellhead and / or wellbore to calculate the pressure in the formation fracture; When the pressure in the formation fracture approaches or reaches the formation fracture expansion pressure or the set fracture working pressure threshold, compressed air is discharged from the target wellbore.

11. The method according to claim 1, wherein 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 is completed, which comprises: Setting the critical residual fracture width of formation fractures; determining a critical residual compressed air volume of the compressed air based on a set critical residual crack width; In step S5, the residual volume of compressed air in the wellbore and its formation fractures during the energy release process is monitored. When the monitored residual volume approaches or reaches the critical residual compressed air volume, the backflow is terminated, thereby maintaining the residual fracture width in the formation fractures.

12. The method according to claim 1, characterized in that In step S5 , a power generation device capable of converting the compression potential energy of the compression cavity into electrical energy is provided.

13. The method according to claim 1, wherein The energy storage in step S4 and the energy release in step S5 also include at least one of the following energies: stress potential energy accumulated by compressed air doing work to overcome the crack closing pressure, elastic deformation potential energy accumulated by driving the formation rock to undergo elastic deformation, and formation thermal energy obtained by heat exchange between compressed air and formation rock in the formation cracks.

14. The method according to claim 1, wherein The method further includes the step of modifying the formation fracture tip, comprising: Injecting a first fluid and a second fluid into the formation fracture in sequence, wherein the first fluid comprises at least one liquid polymer and at least one curing agent, and the second fluid is a fluid that does not undergo a curing reaction or a polymerization reaction; After the second fluid displaces the first fluid to the fracture tip of the formation fracture, injection of the second fluid into the formation fracture is stopped, and the first fluid is left to complete a solidification reaction and change from a liquid state to a solid state.

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

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

17. The method according to claim 14, characterized in that The density of the first fluid is not less than 70% of the density of the second fluid.

18. The method according to claim 1, wherein The method further includes the step of modifying the formation fracture tip, which comprises: injecting a degradable capsule and a third fluid into the formation fracture, wherein the degradable capsule contains at least one reactant, and the third fluid is a fluid that does not undergo a solidification reaction or a polymerization reaction; After stopping the injection of the degradable capsules, continuing to inject the third fluid to displace the degradable capsules to the crack tips of the formation cracks; After the displacement is completed, the degradable capsules degrade at the fracture tips of the formation fractures, so that reactants released from the degradable capsules undergo solidification reactions with each other and / or with the third fluid to form a solid substance.

19. The method according to claim 18, characterized in that In the step of injecting the degradable capsules and the third fluid into the formation fracture, the density of the degradable capsules is set to be less than the density of the third fluid, so that the degradable capsules are gathered at the top edge of the formation fracture; Alternatively, in the step of injecting the degradable capsules and the third fluid into the formation fracture, the density of the degradable capsules is set to be greater than the density of the third fluid, so that the degradable capsules are gathered at the fracture bottom edge of the formation fracture.

20. The method according to claim 18, wherein Different reactants can be encapsulated in a single degradable capsule, or different reactants can be encapsulated in different degradable capsules.

21. A system for implementing the method according to any one of claims 1 to 20, characterized in that: include: A fracturing unit is configured to inject fracturing fluid into a target wellbore to perform hydraulic fracturing operations, thereby creating formation fractures and / or opening existing formation fractures; wherein the target wellbore is configured as an independent wellbore that is not connected to another wellbore through formation fractures; A plugging fluid injection unit is configured to inject a plugging fluid into the formation fractures to physically and / or chemically plug the rock voids and / or microcracks surrounding the formation fractures; The sealing test unit is configured to perform a sealing test on the target wellbore based on wellhead plugging to determine the sealing performance of the formation fractures. The sealing test unit includes: a high-pressure fluid injection unit configured to inject high-pressure fluid into a target wellbore so that the pressure in the formation fracture is greater than the fracture closure pressure and less than the fracture expansion pressure, so that the formation fracture is in an open and non-expanded state; A sealing unit configured to seal the target wellbore; a pressure monitoring and judgment unit configured to monitor the wellhead pressure of a target wellbore and to judge whether formation fractures are gradually closing under the wellhead plugging condition by monitoring the change in pressure at the wellhead and / or in the wellbore over time, thereby judging the sealing property of the formation fractures; An energy storage working unit is configured to inject compressed air into a target wellbore and cause the pressure of the formation fractures after the air is injected to be greater than the fracture closure pressure and less than the fracture expansion pressure, thereby storing energy, which at least includes the compression potential energy accumulated by the compressed air due to the pressure increase in the entire space formed by the target wellbore and the formation fractures; An energy-releasing working unit is configured to discharge compressed air and obtain at least compression potential energy released by the compressed air during the discharge process; The plugging fluid is constructed as a fracturing fluid containing a plugging substance, so that the plugging fluid injection unit can be integrated into the fracturing unit and complete the work along with the fracturing construction, or the plugging fluid is constructed as a fluid containing a plugging substance and independent of the fracturing fluid, so that the plugging fluid injection unit can work independently of the fracturing unit.

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