A method for evaluating the sealing performance of a caprock system and determining the safety sealing pressure

Abstract

The present invention discloses a method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure, and the determination method includes: establishing an inversion model based on the solution of the pressure diffusion wave in the pore medium, considering the thickness of the caprock unit, transferring the critical breakthrough of the caprock unit from the bottom to the top of the caprock, and establishing an inversion model for the critical breakthrough pressure to determine the maximum sealing pressure of the caprock unit; on the basis of the maximum sealing pressure of the caprock unit, determining a quantitative evaluation method for the sealing performance of the caprock system for the site selection evaluation stage, and determining the safe sealing pressure of the caprock system for the engineering design and implementation stage. Taking the critical breakthrough at the top of the caprock unit as the boundary condition, considering the thickness of the caprock unit, allowing compressed air to partially break through the caprock unit under the condition of meeting energy storage and pressure maintenance, thereby tapping the energy storage and sealing capacity of the caprock and providing a basis for calculating the safe sealing pressure of the energy storage project.

Description

Technical Field

[0001] The present invention relates to the field of compressed air pore-type geological energy storage, and particularly to a method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure. Background Art

[0002] The current understanding and evaluation methods of caprock sealing mainly draw on the research basis of caprock sealing in oil and gas reservoirs, natural gas energy storage reservoirs, and CO2 geological sequestration. At present, the characterization of caprock sealing mainly uses parameters such as breakthrough pressure, and qualitative evaluation is carried out by combining multiple parameters. However, the physical characterization of caprock structure sealing factors such as caprock thickness and distribution is still very lacking.

[0003] The existing evaluation characterizations of sealing performance are either too qualitative, difficult to support high-precision calculations, and difficult to make full use of the structural sealing effect of the caprock. Compared with the caprock sealing in the above fields, aquifer compressed air energy storage hopes to maximize the sealing potential of the trap caprock, rather than just judging whether it is sealed. How to fully exploit the sealing potential of the energy storage reservoir to support the design of a more economical energy storage project with a larger capacity requires new evaluation indicators and methods for the caprock sealing. At present, there is no good solution to quantitatively represent this sealing performance and thus fully exploit the sealing potential of the caprock system.

[0004] In the Chinese patent application with the application number: ZL202111412243.2, it involves a quantitative evaluation method for the sealing ability of gypsum-salt rock caprocks. Based on the fuzzy mathematics method, it fully considers the macroscopic and microscopic characteristics of the caprock, introduces the microscopic factor - breakthrough pressure in the analysis of macroscopic factors such as lithology and thickness, and introduces the analytic hierarchy process to determine the weights of various factors in the evaluation system when multiple factors jointly control the caprock sealing ability, so as to reduce the influence of human subjective factors on the evaluation of caprock sealing ability and improve the accuracy and precision of the evaluation; the method is simple and easy to implement, has wide applicability, and has guiding significance for the exploration of deep oil and gas reservoirs under salt. A scoring system is proposed for quantitatively evaluating the caprock sealing ability, but it cannot serve the engineering practice of designing the safe injection pressure of compressed air.

[0005] In the Chinese patent application with the application number: CN202310821132.X, it specifically relates to a method for evaluating the tightness of a carbon dioxide geological storage caprock, which mainly includes the following steps: (1) obtaining the structural parameters of the reservoir and the caprock; (2) obtaining the in-situ stresses of the reservoir and the caprock; (3) obtaining the physical properties, permeability, mechanical parameters and microscopic characteristics of the reservoir and the caprock; (4) obtaining the breakthrough pressure of the caprock; (5) obtaining the parameters for constructing a three-dimensional geological model of seepage-stress coupling; (6) constructing a numerical model of seepage-stress coupling and conducting numerical simulations; (7) determining the evaluation indexes for the tightness of the caprock; (8) on-site monitoring and calibration of the caprock tightness indexes. It has the characteristics of simple process, clear indexes and strong operability, can realize the quantitative evaluation of the tightness of the carbon dioxide geological storage caprock, and at the same time improve the relevant process parameters of the caprock tightness, laying a foundation for the efficient geological utilization and storage of carbon dioxide, and can prevent the failure of the tightness of the carbon dioxide geological storage caprock. By combining indoor experiments with numerical simulation work and using the numerical simulation results to determine the evaluation indexes for the tightness of the caprock, on the one hand, the workload is relatively large, and on the other hand, due to the differences in the geological structures of the studied areas, the obtained evaluation indexes may vary, and it is difficult to popularize the obtained evaluation index system generally.

[0006] In the Chinese patent application with the application number: ZL201910390647.2, a method for predicting the maximum closed hydrocarbon column height of a caprock is disclosed, which includes the following steps: conducting breakthrough pressure tests on the caprock and the reservoir under experimental conditions, correcting the breakthrough pressure of the caprock and the breakthrough pressure of the reservoir to obtain the capillary pressure of the caprock and the capillary pressure of the reservoir, calculating the difference between the two to obtain the capillary pressure difference; obtaining the starting pressure gradient that the formation water flow in the caprock needs to overcome; combining the capillary pressure difference between the hydrocarbon-water interface in the caprock and the reservoir and the starting pressure gradient that the fluid flow in the caprock needs to overcome to obtain the relationship between the hydrocarbon column height and the caprock thickness, and obtaining the predicted height of the maximum closed hydrocarbon column of the caprock. It is applicable to the calculation of the maximum closed hydrocarbon column height of shale caprocks in conventional oil and gas reservoirs, determines the quantitative relationship between the caprock thickness and the maximum closed hydrocarbon column coverage, explains the principle and characteristics of the caprock thickness sealing effect, and provides a feasible method for the prediction of caprock sealing performance and oil and gas reservoir risks. It gives the quantitative relationship between the caprock thickness and the maximum closed hydrocarbon column height, but the hydrocarbon sealing mechanism is not universal and may not be applicable to the field of compressed air pore-type geological energy storage. Summary of the Invention

[0007] In view of the above problems, the present invention is proposed to provide a method for evaluating the tightness of a caprock system and determining the safe sealing pressure to overcome or at least partially solve the above problems.

[0008] According to one aspect of the present invention, a method for evaluating the tightness of a caprock system and determining the safe sealing pressure is provided, and the determination method includes:

[0009] Based on the solution of the pressure diffusion wave in the porous medium, an inversion model is established. Considering the thickness of the caprock unit, the critical breakthrough of the caprock unit is transferred from the bottom to the top of the caprock, and an inversion model for the critical breakthrough pressure is established to determine the maximum sealing pressure of the caprock unit.

[0010] Based on the maximum sealing pressure of the caprock unit, a quantitative evaluation method for the sealing property of the caprock system for the site selection evaluation stage is determined, and the safe sealing pressure of the caprock system for the engineering design and implementation stage is determined.

[0011] Optionally, the determination of the maximum sealing pressure of the caprock unit specifically includes:

[0012] Step 1.1: Establish a single caprock - single reservoir structure model.

[0013] Step 1.2: According to the solution of the pressure diffusion wave propagation in the porous medium, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time, and establish a pressure inversion solution model.

[0014] Step 1.3: Determine the calculation parameters.

[0015] Step 1.4: Determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection into the energy storage reservoir, that is, the injection period.

[0016] Step 1.5: Set the critical condition. When the pressure propagates to the top of the caprock unit after time T and just causes its critical breakthrough; substitute the parameters in Step 1.3 into the pressure inversion solution in Step 1.2 to obtain the maximum sealing pressure of the caprock unit.

[0017] Optionally, the determination of the calculation parameters specifically includes: the thickness of the caprock unit, porosity, the position of the caprock unit, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity, and fluid compressibility.

[0018] Optionally, Step 1.2, according to the solution of the pressure diffusion wave propagation in the porous medium, obtaining the analytical solution of the pressure at any position on the pressure diffusion path at any time and establishing a pressure inversion solution model specifically includes:

[0019] Step 1.2.1: Select or determine the expression of the pressure solution P(x, t)=f(P1) of the pressure propagation model, where P1 is the pressure boundary condition at the bottom of the caprock.

[0020] Step 1.2.2: Set the critical equilibrium condition for gas breakthrough: when the pressure propagates to the top of the direct caprock and just causes the critical breakthrough of the caprock top,

[0021] P(H, T)-P s =P b

[0022] Among them, P(H,T) is the pressure at the top of the caprock, and P s is the formation pressure, and P b is the breakthrough pressure at the top of the caprock;

[0023] Step 1.2.3: Invert the expression of P1 from Step 1.2.2 to obtain the pressure inversion model for the bottom boundary of the caprock.

[0024] Optionally, the method for quantitatively evaluating the sealing performance of the caprock system for the site selection evaluation stage based on the maximum sealing pressure of the caprock unit specifically includes:

[0025] Divide the caprock system into several N caprock units;

[0026] Based on the pressure diffusion solution in the pore medium and considering the thickness of the caprock, determine the minimum value of the maximum sealing pressure P max,n (n = 1, 2,..., N) as the evaluation index C for the sealing performance of the caprock system,

[0027] C = min{P max,1 , P max,2 , …, P max,N}.

[0028] Optionally, the method for quantitatively evaluating the sealing performance of the caprock system for the site selection evaluation stage based on the maximum sealing pressure of the caprock unit specifically includes:

[0029] Step 2.1: According to the site data, perform structural modeling and divide the caprock system into units based on geological parameters to obtain N caprock units;

[0030] Step 2.2: For each caprock unit;

[0031] Step 2.3: Take the minimum value of the maximum sealing pressure of all caprock units, min{P max,1 , P max,2 , …,P max,N};

[0032] As the evaluation index C for the sealing performance of the caprock system, C = min{P max,1 , P max,2 , …, P max,N}.

[0033] Optionally, Step 2.2 for each caprock unit specifically includes:

[0034] Step 2.2.1: According to the pressure diffusion wave propagation model in the pore medium, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model;

[0035] Step 2.2.2, determine the parameters related to the calculation;

[0036] Step 2.2.3, determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection into the energy storage reservoir, i.e., the injection cycle;

[0037] Step 2.2.4, set the critical condition. When the pressure propagates to the top of the caprock unit after time T, it just causes critical breakthrough; substitute the parameters in Step 2.2.2 into the pressure inversion solution in Step 2.2.1 to obtain the maximum sealing pressure P of the caprock unit max,n (n = 1, 2, …, N).

[0038] Optionally, the determination of the safe sealing pressure of the caprock system for the engineering design implementation stage specifically includes: based on the maximum sealing pressure of the caprock unit, considering the engineering reserve coefficient and the fracture pressure constraint condition of the caprock unit, determine the safe sealing pressure of the caprock system.

[0039] Optionally, the determination of the safe sealing pressure of the caprock system based on the maximum sealing pressure of the caprock unit, considering the engineering reserve coefficient and the fracture pressure constraint condition of the caprock unit, specifically includes:

[0040] Step 3.1, according to the site data, carry out structural modeling, and divide the caprock system into units according to the geological parameters to obtain N caprock units;

[0041] Step 3.2, for each caprock unit;

[0042] Step 3.3, for each caprock unit, determine the constraint pressure P of each caprock unit c,n , n = 1, 2, …, N. When the formation structure includes the caprock, take the fracture pressure at the bottom of the caprock unit as the constraint pressure P c ;

[0043] Step 3.4, for each caprock unit, determine the entry pressure P at the bottom of the caprock unit in , n = 1, 2, …, N. The entry pressure P at the bottom of the caprock unit in is the sum of the formation pressure P at the bottom of the caprock unit s,d and the breakthrough pressure P at the bottom of the caprock unit b,d ;

[0044] Step 3.5, take the minimum value of the maximum sealing pressures of all caprock units in Step 3.2, i.e.: min{P max,1 ,P max,2 , …, P max,N}, and find the entry pressure P at the bottom of the corresponding caprock unit A in Step 3.4 in,A, calculate the upper limit value α of the engineering reserve coefficient of the caprock unit A max,A = P max,A / P in,A ;

[0045] Step 3.6, according to the maximum sealing pressure P of the caprock unit A max,A , and considering an engineering reserve coefficient α not exceeding the upper limit value α of the engineering reserve coefficient of the caprock unit A max,A , obtain the sealing pressure P of the caprock unit A with the engineering reserve coefficient A =P α,A / α max,A ; A ;

[0046] Step 3.7, compare the sealing pressure of the caprock unit A with the constraint pressure corresponding to the caprock unit A in Step 3.3. If the sealing pressure P of the caprock unit A α,A is less than or equal to its constraint pressure P c,A , that is, P α,A ≤P c,A , then the pressure is the safe sealing pressure P of the caprock system safe =P α,A ; if not satisfied, adjust the engineering reserve coefficient α in Step 3.6 A until satisfied.

[0047] Optionally, in Step 3.2, for each caprock unit, it specifically includes:

[0048] Step 3.2.1, according to the pressure diffusion wave propagation model in the pore medium, obtain the pressure analytical solution at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model;

[0049] Step 3.2.2, determine the calculation-related parameters, including the caprock unit thickness, porosity, caprock unit position, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity, and fluid compressibility;

[0050] Step 3.2.3, determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection into the energy storage reservoir, and obtain the injection cycle;

[0051] Step 3.2.4, set the critical condition that when the pressure propagates to the top of the caprock unit after time T, it just causes critical breakthrough; substitute the parameters in Step 3.2.2 into the pressure inversion solution in Step 3.2.1 to obtain the maximum sealing pressure P of the caprock unit max,n , n = 1, 2, …, N.

[0052] A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure provided by the present invention. The determination method includes: establishing an inversion model based on the solution of the pressure diffusion wave in the pore medium, considering the thickness of the caprock unit, transferring the critical breakthrough of the caprock unit from the bottom to the top of the caprock, and establishing an inversion model for the critical breakthrough pressure to determine the maximum sealing pressure of the caprock unit; on the basis of the maximum sealing pressure of the caprock unit, determining a quantitative evaluation method for the sealing performance of the caprock system for the site selection evaluation stage and determining the safe sealing pressure of the caprock system for the engineering design and implementation stage. Taking the critical breakthrough at the top of the caprock unit as the boundary condition, considering the thickness of the caprock unit, allowing compressed air to partially break through the caprock unit under the condition of meeting energy storage and pressure maintenance, thereby exploiting the energy storage and sealing capacity of the caprock, and considering the engineering reserve coefficient and the caprock distribution, the engineering safety can be guaranteed, providing a basis for calculating the safe sealing pressure of the energy storage project.

[0053] The above description is only an overview of the technical solution of the present invention. In order to be able to understand the technical means of the present invention more clearly, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features and advantages of the present invention more obvious and understandable, the specific embodiments of the present invention are specifically exemplified below. Brief Description of the Drawings

[0054] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to these drawings.

[0055] Figure 1 It is a flowchart of a method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure provided by an embodiment of the present invention;

[0056] Figure 2 It is the definition of the caprock system and the caprock unit of the present invention;

[0057] In the figure, 1: overlying rock mass; 2-1: first caprock unit; 2-2: second caprock unit; 2-3: third caprock unit; 3: reservoir; 4: surrounding rock mass;

[0058] Figure 3 It is the migration path of compressed air in the caprock unit in the present invention;

[0059] In the figure, 1: overlying rock mass; 2: caprock unit; 3: reservoir; the dotted line is a one-dimensional coordinate system;

[0060] Figure 4 It is a schematic diagram of a single caprock-single reservoir structure model in the process of determining the maximum sealing pressure of the caprock unit in an embodiment of the present invention;

[0061] Figure 5 Schematic diagram of the single-caprock-single-reservoir structure model in the evaluation process of the sealing performance of the caprock system according to the embodiment of the present invention;

[0062] Figure 6 Pore pressure distribution curve in the caprock at the critical breakthrough moment of the caprock in Embodiment 2 of the present invention;

[0063] Figure 7 Structure model of Embodiment 3 of the present invention. Detailed implementation manners

[0064] Hereinafter, exemplary embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be fully conveyed to those skilled in the art.

[0065] The terms "including" and "having" and any variations thereof in the description, embodiments and claims of the present invention are intended to cover non-exclusive inclusion. For example, a series of steps or units are included.

[0066] Hereinafter, the technical solutions of the present invention will be further described in detail with reference to the accompanying drawings and embodiments.

[0067] Embodiment 1

[0068] As Figure 1 shown, a method for determining the maximum sealing pressure of a caprock unit for compressed air energy storage includes the following steps:

[0069] Step 1.1: Establish a single-caprock-single-reservoir structure model, as Figure 4 shown;

[0070] Step 1.2: According to the pressure diffusion wave propagation model in the pore medium, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model;

[0071] Step 1.2.1: Select or determine the pressure solution expression P(x,t)=f(P1) of the pressure propagation model, where P1 is the pressure boundary condition at the bottom of the caprock;

[0072] Step 1.2.2: Set the critical equilibrium condition for gas breakthrough: when the pressure propagates to the top of the direct caprock, it just causes critical breakthrough at that place, that is,

[0073] P(H,T)-P s =P b

[0074] Step 1.2.3. Invert the expression of P1 from Step 1.2.2, and this expression is the pressure inversion model of the bottom boundary of the caprock.

[0075] The definitions of the caprock system and the caprock unit are as Figure 2 shown in the figure. 1: Overlying rock mass; The caprock system includes: the first caprock unit 2-1; the second caprock unit 2-2; the third caprock unit 2-3; 3: Reservoir; 4: Surrounding rock mass.

[0076] For a homogeneous porous medium under one-dimensional conditions, at any time t and any position x, the analytical solution of the pore pressure diffusion wave P is:

[0077] Single-porosity medium: (1)

[0078] Double-porosity medium: (2)

[0079] where P0 is the initial pressure value in the semi-infinite space (Pa); P1 is the pressure boundary condition at the starting boundary position x = 0 (Pa); erf(*) is

[0080] the error function, erf(*) = ;

[0081] x is the one-dimensional coordinate (m); t is the time (s); is the pressure diffusivity (m 2 / s), is the cross-sectional area of pressure diffusion (m 2 ); is the porosity (%); is the fluid compressibility (Pa -1 ); is the fluid viscosity (Pa·s); K is the permeability (mD). P is the pore pressure (Pa), and P is a function of time and coordinates.

[0082] Step 1.3. Determine the parameters related to the calculation, including the thickness of the caprock unit, porosity, caprock position, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity, and fluid compressibility;

[0083] Step 1.4. Determine the critical breakthrough time T, which is generally taken as the time from the start to the end of gas injection into the energy storage reservoir, that is, the injection cycle. In Example 1, T = 24 hours is taken;

[0084] Step 1.5. Set the critical condition that when the pressure propagates to the top of the caprock unit after time T, it just causes critical breakthrough (equilibrium); Substitute the parameters in Step 1.3 into the pressure inversion solution in Step 1.2 to obtain the maximum sealing pressure of the caprock unit.

[0085] The critical breakthrough at the top of the caprock unit means that the pressure P(H,T) at the top of the caprock unit is exactly equal to the formation pressure P s,u at this location, and the difference is exactly equal to the breakthrough pressure P b,u at the top of the caprock unit, that is:

[0086] (3)

[0087] Substitute the critical condition (formula (3)) into the pressure propagation solution (formulas (1) and (2)) in the porous medium to obtain the pressure P1 at the bottom of the caprock unit:

[0088] Single-porosity medium: (4)

[0089] Dual-porosity medium: ,

[0090] (5)

[0091] where H is the thickness of the caprock unit (m); P s,u is the formation pressure at the top of the caprock unit (Pa); P b,u is the breakthrough pressure at the top of the caprock unit (Pa); T is the critical time for pressure propagation (s), and T is equal to the engineering injection cycle.

[0092] The pressure P1 at the bottom of the caprock unit is the maximum sealing pressure P max of the caprock unit, that is, P max = P1.

[0093] A method for evaluating the sealing performance of a caprock system derived from a method for determining the maximum sealing pressure of a caprock unit for compressed air energy storage, the specific steps are as follows:

[0094] Step 2.1, according to the site data, perform structural modeling, and the specific structural model is as shown in Figure 5 and divide the caprock system into units to obtain N caprock units;

[0095] Step 2.2, for each caprock unit;

[0096] Step 2.2.1, according to the pressure diffusion wave propagation model in the porous medium, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model;

[0097] Step 2.2.2, determine the calculation-related parameters, including the thickness of the caprock unit, porosity, caprock position, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity, and fluid compressibility;

[0098] Step 2.2.3: Determine the critical breakthrough time T, which is generally taken as the time from the start to the end of gas injection into the energy storage reservoir, i.e., the injection cycle.

[0099] Step 2.2.4: Set the critical condition that when the pressure propagates to the top of the caprock unit after time T, it just causes the critical breakthrough (equilibrium); substitute the parameters in Step 2.2.2 into the pressure inversion solution in Step 2.2.1 to obtain the maximum sealing pressure P of the caprock unit max,n (n = 1, 2, …, N);

[0100] Step 2.3: Take the minimum value of the maximum sealing pressures of all caprock units as the sealing evaluation index C of the caprock system, i.e., C = min{P max,1 , P max,2 , …, P max,N}.

[0101] As Figure 3 shown, the migration path of compressed air in the caprock unit. In the figure, 1: overlying rock mass; 2: caprock unit; 3: reservoir; the dashed line is the one-dimensional coordinate system.

[0102] A method for determining the safety sealing pressure of a caprock system derived based on a method for determining the maximum sealing pressure of a caprock unit for compressed air energy storage, the specific steps:

[0103] Step 3.1: According to the site data, conduct structural modeling and divide the caprock system into elements to obtain N caprock units;

[0104] Step 3.2: For each caprock unit;

[0105] Step 3.2.1: According to the pressure diffusion wave propagation model in the porous medium, obtain the pressure analytical solution at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model;

[0106] Step 3.2.2: Determine the calculation-related parameters, including but not limited to the caprock unit thickness, porosity, caprock position, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity, fluid compressibility;

[0107] Step 3.2.3: Determine the critical breakthrough time T, which is generally taken as the time from the start to the end of gas injection into the energy storage reservoir, i.e., the injection cycle;

[0108] Step 3.2.4: Set the critical condition that when the pressure propagates to the top of the caprock unit after time T, it just causes the critical breakthrough (equilibrium); substitute the parameters in Step 3.2.2 into the pressure inversion solution in Step 3.2.1 to obtain the maximum sealing pressure P of the caprock unit max,n (n = 1, 2, …, N);

[0109] Step 3.3: For each caprock unit, determine the confinement pressure P c,n (n = 1, 2, …, N). When the formation structure includes a caprock, use the fracture pressure at the bottom of the caprock unit as the confinement pressure;

[0110] Step 3.4: For each caprock unit, determine the entry pressure P in,n (n = 1, 2, …, N). The entry pressure P in at the bottom of the caprock unit is the sum of the formation pressure P s,d at the bottom of the caprock unit and the breakthrough pressure P b,d at the bottom of the caprock unit, i.e., P in = P s,d + P b,d ;

[0111] Step 3.5: Take the minimum value of the maximum sealing pressures of all caprock units in Step 3.2, i.e., min{P max,1 , P max,2 , …, P max,N}, and find the entry pressure P in,A at the bottom of the caprock unit corresponding to the caprock unit A in Step 3.4. Calculate the upper limit value α max,A of the engineering reserve coefficient of the caprock unit A. The upper limit value of the engineering reserve coefficient is the ratio of the maximum sealing pressure of the caprock unit to the entry pressure at the bottom of the caprock unit, i.e., α max,A = P max,A / P in,A ;

[0112] Step 3.6: Based on the maximum sealing pressure P max,A of the caprock unit A, and considering an engineering reserve coefficient α max,A that does not exceed the upper limit value α A of the caprock unit A, obtain the sealing pressure P α,A of the caprock unit A with the engineering reserve coefficient, i.e., P max,A = P A / α

[0113] Step 3.7: Compare the sealing pressure of the caprock unit A with the confinement pressure corresponding to the caprock unit A in Step 3.3. If the sealing pressure P α,A of the caprock unit A is less than or equal to its confinement pressure P c,A , i.e., P α,A ≤ P c,A , then this pressure is the safety sealing pressure P safe = P α,A ; if not satisfied, adjust the engineering reserve coefficient α in Step 3.6A , until satisfied.

[0114] A method for evaluating the sealing performance and determining the safe sealing pressure of a caprock system for compressed air energy storage provided by the present invention has the following characteristics: (1) According to the pore medium pressure diffusion wave model, considering the thickness of the caprock unit, allowing compressed air to partially break through the caprock unit under the condition of meeting the energy storage and pressure retention, is expected to increase the maximum sealing pressure of the caprock unit, exert the energy storage sealing potential of the caprock, and increase the energy storage capacity; (2) Different from the previous qualitative evaluation methods for the sealing performance of the caprock system, this method is based on the mathematical model of pore medium pressure diffusion, has strict mathematical and physical meanings, and is a quantitative evaluation method; (3) Different from the method of using the gas pressure at which compressed air breaks through the bottom of the caprock as the sealing pressure of the caprock system, this method takes the critical breakthrough at the top of the caprock unit as the boundary condition, considers the thickness of the caprock unit, allows compressed air to partially break through the caprock unit under the condition of ensuring the storage pressure of the energy storage reservoir, can effectively exert the energy storage sealing potential of the caprock, and considers the engineering reserve coefficient and the caprock distribution, which can ensure the engineering safety and provide a basis for determining the safe sealing pressure of the energy storage project.

[0115] To better explain the feasibility of the present invention, the following embodiments are provided:

[0116] Determination of the maximum sealing pressure of the caprock unit

[0117] Step 1.1, according to the data (Table 1), establish a corresponding single caprock - single reservoir structure model, as Figure 4 shown, C1 is the caprock, R1 is the reservoir, both are single - porosity media;

[0118] Step 1.2, according to the publicly available solution of pressure diffusion wave propagation in pore media, obtain the pressure solution at any position on the pressure diffusion path at any time;

[0119]

[0120] And establish its pressure inversion solution model.

[0121] Step 1.3, determine the calculation - related parameters - the geological parameters are shown in Table 1, and also include the fluid compressibility of 1×10 -9 Pa -1 and the fluid viscosity of 1×10 -3 Pa·s;

[0122] Table 1: Formation data for Example 1

[0123]

[0124] Step 1.4, determine the critical breakthrough time T. The engineering injection cycle is 24 hours, i.e., T = 24 h;

[0125] Step 1.5, set the critical conditions. When the pressure propagates to the top of the caprock unit after time T, it just causes critical breakthrough (equilibrium); substitute the parameters in Step 1.3 into the pressure inversion solution in Step 1.2, that is, in formula (4), to obtain the maximum sealing pressure P of the caprock unit max (at the bottom of C1 layer and the top of R1 layer) is 19.31 MPa.

[0126] Sealing evaluation of the caprock system

[0127] Consider four caprock systems at the field scale, which are single caprock - single reservoir structures. Now, evaluate the sealing of each caprock system.

[0128] Step 2.1, according to the field data, perform structural modeling and divide the caprock system into units according to geological parameters. Since all four caprock systems are single caprock - single reservoir structures, they are all divided into only one caprock unit, namely caprock units 1, 2, 3, and 4, corresponding to caprocks C1, C2, C3, and C4, and R1, R2, R3, and R4 are the reservoirs in the four fields respectively; all strata are homogeneous single - porosity media;

[0129] Step 2.2, for caprock units 1, 2, 3, 4;

[0130] Step 2.2.1, according to the pressure diffusion wave propagation model in the porous medium, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time,

[0131] (7)

[0132] And establish its pressure inversion solution model.

[0133] Step 2.2.2, determine the calculation - related parameters - the geological parameters of the caprock units in each field are shown in Table 2, the fluid compressibility is 1×10 -9 Pa -1 and the fluid viscosity is 1×10 -3 Pa·s;

[0134] Table 2: Stratum data for Example 2

[0135]

[0136] Step 2.2.3, determine the critical breakthrough time T, generally taken as the time from the start to the end of gas injection into the energy storage reservoir, that is, the injection cycle. In Example 2, it is taken as T = 24 hours;

[0137] Step 2.2.4: Set the critical condition that when the pressure propagates to the top of the caprock unit through time T, it just causes critical breakthrough (equilibrium); substitute the parameters in Step 2.2.2 into the pressure inversion solution in Step 2.2.1, that is, in formula (4), to obtain the maximum sealing pressure P of caprock unit 1 max,1 is 19.31 MPa, and the maximum sealing pressure P of caprock unit 2 max,2 is 19.98 MPa, and the maximum sealing pressure P of caprock unit 3 max,3 is 21.69 MPa, and the maximum sealing pressure P of caprock unit 4 max,4 is 23.85 MPa. In addition, at the critical breakthrough moment of each caprock unit, the pressure distribution at any position within the caprock unit is as shown in Figure 6 shown.

[0138] Step 2.3: Take the minimum value of the maximum sealing pressures of all caprock units in the same caprock system, that is: min{P max,1 , P max,2 , …, P max,N}, as the sealing evaluation index C of the caprock system. Since there is only caprock unit 1 in caprock system 1, the sealing evaluation index C1 of caprock system 1 = P max,1 = 19.31 MPa; since there is only caprock unit 2 in caprock system 2, the sealing evaluation index C2 of caprock system 2 = P max,2 = 19.98 MPa; since there is only caprock unit 3 in caprock system 3, the sealing evaluation index C3 of caprock system 3 = P max,3 = 21.69 MPa; since there is only caprock unit 4 in caprock system 4, the sealing evaluation index C4 of caprock system 4 = P max,4 = 23.85 MPa. Conduct a comprehensive evaluation of the sealing of the four caprock systems. The sealing evaluation index of caprock system 4 is the highest, and the sealing evaluation index of caprock system 1 is the lowest.

[0139] Determination of the safe sealing pressure of the caprock system

[0140] Step 3.1: According to the site data, conduct structural modeling and divide the caprock system into units to obtain 3 caprock units, as shown in Figure 7 shown, corresponding to caprocks C1, C2, C3 respectively, and R1, R2, R3 are the corresponding reservoirs;

[0141] Step 3.2: For caprock units 1, 2, and 3, they are all homogeneous single-porosity media;

[0142] Step 3.2.1: According to the pressure diffusion wave propagation model in the porous medium, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time

[0143]

[0144] And establish a pressure inversion solution model.

[0145] Step 3.2.2: Determine the calculation-related parameters - The geological parameters of each caprock unit are shown in Table 3, the fluid compressibility is 1×10 -9 Pa -1 , and the fluid viscosity is 1×10 -3 Pa·s;

[0146] Table 3: Formation structure parameters of Example 3

[0147]

[0148] Step 3.2.3, determine the critical breakthrough time T, which is generally taken as the time from the start to the end of gas injection into the energy storage reservoir, that is, the injection cycle. In Example 3, take T = 24 hours;

[0149] Step 3.2.4, set the critical condition. When the pressure propagates to the top of the caprock unit after time T and just causes its critical breakthrough (equilibrium); substitute the parameters in Step 3.2.2 into the pressure inversion solution in Step 3.2.1, that is, in formula (4), to obtain the maximum sealing pressure P max,1 of caprock unit 1 is 18.65 MPa, the maximum sealing pressure P max,2 of caprock unit 2 is 18.92 MPa, and the maximum sealing pressure P max,3 of caprock unit 3 is 19.10 MPa;

[0150] Step 3.3, for caprock units 1, 2, and 3, determine the confinement pressure P c,n (n = 1, 2, 3). When the formation structure includes a caprock, take the fracture pressure at the bottom of the caprock unit as the confinement pressure; according to the data in Table 3, the confinement pressures of each caprock unit are: P c,1 = 23.50 MPa, P c,2 = 28.30 MPa, P c,3 = 25.65 MPa;

[0151] Step 3.4, for caprock units 1, 2, and 3, determine the entry pressure P in,n (n = 1, 2, 3) at the bottom of the caprock unit; the entry pressure at the bottom of the caprock unit is the sum of the formation pressure and the breakthrough pressure at the bottom of the caprock unit; after calculation, the entry pressures at the bottom of each caprock unit are respectively: P in,1 = 17.0 MPa, P in,2 = 18.4 MPa, Pin,3 = 18.3 MPa;

[0152] Step 3.5. For caprock units 1, 2, and 3, according to the maximum sealing pressure of each caprock unit obtained in Step 3.2, take the minimum value min{P max,1 , P max,2 , P max,3} = P max,1 = 18.65 MPa. The corresponding caprock unit is caprock unit 1, and the bottom entry pressure of the caprock unit is P in,1 = 17.0 MPa. Calculate the upper limit value α of the engineering reserve coefficient of caprock unit 1 max,1 = P max,1 / P in,1 = 1.097;

[0153] Step 3.6. According to the maximum sealing pressure P max,1 of caprock unit 1, and considering an engineering reserve coefficient α1 = 1.008 that does not exceed the upper limit value α max,1 of caprock unit 1, obtain the sealing pressure P α,1 of caprock unit 1 with the engineering reserve coefficient, where P max,1 = P α,1 / α1 = 18.50 MPa;

[0154] Step 3.7. Compare the sealing pressure P c,1 of caprock unit 1 with the constraint pressure P α,1 corresponding to caprock unit 1 in Step 3.3. There is P c,1 = 18.50 MPa < 23.50 MPa = P safe = P α,1 = 18.50 MPa.

[0155] Example 2

[0156] A method for determining the maximum sealing pressure of a caprock unit for compressed air energy storage. According to the pressure diffusion wave solution in the pore medium, an inversion model is established. Considering the thickness of the caprock unit, the critical breakthrough of the caprock unit is transferred from the bottom to the top of the caprock, and a critical breakthrough pressure inversion model is established to determine the maximum sealing pressure of the caprock unit. The method includes the following steps:

[0157] Step 1.1. Establish a single caprock - single reservoir structure model;

[0158] Step 1.2: According to the propagation solution of the pressure diffusion wave in the porous medium, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model;

[0159] Step 1.2.1: Select or determine the expression of the pressure solution P(x, t) = f(P1) of the pressure propagation model, where P1 is the pressure boundary condition at the bottom of the caprock;

[0160] Step 1.2.2: Set the critical equilibrium condition for gas breakthrough: When the pressure propagates to the top of the direct caprock, it just causes critical breakthrough at that place, that is,

[0161] P(H, T) - P s = P b

[0162] Step 1.2.3: Inversely solve the expression of P1 from Step 1.2.2, and this expression is the pressure inversion model of the bottom boundary of the caprock.

[0163] Step 1.3: Determine the calculation parameters, including but not limited to the thickness of the caprock unit, porosity, position of the caprock unit, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity, and fluid compressibility;

[0164] Step 1.4: Determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection into the energy storage reservoir, that is, the injection cycle;

[0165] Step 1.5: Set the critical condition that when the pressure propagates to the top of the caprock unit after time T, it just causes critical breakthrough (equilibrium); Substitute the parameters in Step 1.3 into the pressure inversion solution in Step 1.2 to obtain the maximum sealing pressure of the caprock unit.

[0166] Among them, a method for evaluating the sealing performance of a caprock system is characterized in that the caprock system is divided into several (N) caprock units, and the minimum value of the maximum sealing pressure P max,n (n = 1, 2,..., N) of the caprock unit determined according to the pressure diffusion solution in the porous medium and considering the caprock thickness is used as the evaluation index C of the sealing performance of the caprock system, that is, C = min{P max,1 , P max,2 , …, P max,N}. It includes the following steps:

[0167] Step 2.1: According to the site data, perform structural modeling and divide the caprock system into units according to geological parameters to obtain N caprock units;

[0168] Step 2.2: For each caprock unit;

[0169] Step 2.2.1: According to the pressure diffusion wave propagation model in the porous medium, obtain the pressure analytical solution at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model.

[0170] Step 2.2.2: Determine the calculation-related parameters, including but not limited to the thickness of the caprock unit, porosity, position of the caprock unit, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity, and fluid compressibility.

[0171] Step 2.2.3: Determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection into the energy storage reservoir, i.e., the injection cycle.

[0172] Step 2.2.4: Set the critical condition that when the pressure propagates to the top of the caprock unit after time T, it just causes critical breakthrough (equilibrium); substitute the parameters in Step 2.2.2 into the pressure inversion solution in Step 2.2.1 to obtain the maximum sealing pressure P of the caprock unit max,n (n = 1, 2, …, N);

[0173] Step 2.3: Take the minimum value of the maximum sealing pressures of all caprock units, i.e.: min{P max,1 , P max,2 , …, P max,N}, as the sealing evaluation index C of the caprock system, i.e., C = min{P max,1 , P max,2 , …, P max,N}.

[0174] Among them, a method for determining the safe sealing pressure of a caprock system is characterized in that, based on the maximum sealing pressure of the caprock unit, considering the engineering reserve coefficient and the fracture pressure constraint condition of the caprock unit, the safe sealing pressure of the caprock system is determined. It includes the following steps:

[0175] Step 3.1: According to the site data, perform structural modeling, and divide the caprock system into units according to geological parameters to obtain N caprock units.

[0176] Step 3.2: For each caprock unit;

[0177] Step 3.2.1: According to the pressure diffusion wave propagation model in the porous medium, obtain the pressure analytical solution at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model.

[0178] Step 3.2.2: Determine the calculation-related parameters, including but not limited to the thickness of the caprock unit, porosity, position of the caprock unit, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity, and fluid compressibility.

[0179] Step 3.2.3: Determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection into the energy storage reservoir, i.e., the injection cycle;

[0180] Step 3.2.4: Set the critical condition that when the pressure propagates to the top of the caprock unit after time T, it exactly causes critical breakthrough (equilibrium); Substitute the parameters in Step 3.2.2 into the pressure inversion solution in Step 3.2.1 to obtain the maximum sealing pressure P of the caprock unit max,n (n = 1, 2, …, N);

[0181] Step 3.3: For each caprock unit, determine the confinement pressure P of each caprock unit c,n (n = 1, 2, …, N). When the formation structure includes a caprock, the fracture pressure at the bottom of the caprock unit is taken as the confinement pressure P c ;

[0182] Step 3.4: For each caprock unit, determine the bottom entry pressure P of the caprock unit in (n = 1, 2, …, N). The bottom entry pressure P of the caprock unit in is the sum of the formation pressure P at the bottom of the caprock unit s,d and the breakthrough pressure P at the bottom of the caprock unit b,d ;

[0183] Step 3.5: Take the minimum value of the maximum sealing pressures of all caprock units in Step 3.2, i.e.: min{P max,1 , P max,2 , …, P max,N}, and find the bottom entry pressure P of the corresponding caprock unit A in Step 3.4 in,A , and calculate the upper limit value α of the engineering reserve coefficient of caprock unit A max,A = P max,A / P in,A ;

[0184] Step 3.6: According to the maximum sealing pressure P of caprock unit A max,A , and considering an engineering reserve coefficient α max,A that does not exceed the upper limit value α of the engineering reserve coefficient of caprock unit A A , obtain the sealing pressure P of caprock unit A with the engineering reserve coefficient α,A = P max,A / α A ;

[0185] Step 3.7: Compare the sealing pressure of caprock unit A with the corresponding confinement pressure of caprock unit A in Step 3.3. If the sealing pressure P of caprock unit A α,A is less than or equal to its confinement pressure P c,A , i.e., Pα,A ≤P c,A If so, this pressure is the safety sealing pressure P of the caprock system safe = P α,A ; if not, adjust the engineering reserve coefficient α in step 3.6 A until satisfied.

[0186] Advantages: (1) Considering the caprock thickness, it allows part of the compressed air to break through the caprock under the condition of meeting the energy storage and pressure preservation conditions, thus tapping the energy storage and sealing potential of the caprock, improving the maximum sealing capacity of the caprock, defining a larger compressed air energy storage capacity than the traditional index, and fully tapping the sealing performance of the caprock and improving the economy of the compressed air energy storage project; this method essentially gives a new definition and specific calculation method for the sealing of the caprock structure.

[0187] (2) It can enrich the means of obtaining the maximum sealing pressure of the caprock unit, that is, the analytical means, which complements and verifies the indoor test;

[0188] (3) The present invention can enrich the quantitative evaluation method for the sealing of the caprock system and assist in the siting and evaluation of energy storage projects;

[0189] (4) The safety sealing pressure of the caprock system determined by the present invention can provide guidance for setting safety constraints for related projects such as compressed air energy storage and CO2 geological sequestration.

[0190] (5) The same method index can be applied to both the siting stage and the subsequent design and operation stages, with unity, rather than almost completely different indexes being adopted in different stages as in the traditional method.

[0191] The above specific implementation manners further elaborate the purpose, technical solution and beneficial effects of the present invention. It should be understood that the above are only the specific implementation manners of the present invention and are not used to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure, characterized in that: The determination method comprises: According to the pressure diffusion wave solution in the porous medium, an inversion model is established. Considering the thickness of the cap layer unit, the critical breakthrough of the cap layer unit is transferred from the bottom to the top of the cap layer, and a critical breakthrough pressure inversion model is established to determine the maximum sealing pressure of the cap layer unit, including: Step 1.1, establish a single cap layer-single reservoir structure model; Step 1.2, according to the pressure diffusion wave propagation solution in the porous medium, the analytical solution of the pressure at any position on the pressure diffusion path at any time is obtained, and the pressure inversion solution model is established; Step 1.2.1: Select or determine the pressure solution P(x,t)=f(P1) expression for the pressure propagation model; Single-porous media: (1) Dual porosity media: (2) Where, P0: initial pressure value in semi-infinite space; P1: pressure boundary condition at the initial boundary position x = 0; erf(*): error function, erf(*) = ; Step 1.2.2: Set the critical equilibrium condition for gas breakthrough: When the pressure propagates to the top of the immediate cap layer, a critical breakthrough will occur at the top of the cap layer. P(x,t)-P s =P b Where P(x,t) is the pressure on the top of the caprock, P s is the formation pressure, P b is the rupture pressure at the top of the caprock; Step 1.2.3: Inversely solve the expression of P1 from step 1.2.2, which is the pressure inversion model of the bottom boundary of the caprock; Step 1.3, determine the calculation parameters; Step 1.4, determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection in the energy storage reservoir, that is, the injection cycle; Step 1.5, set the critical condition, when the pressure propagates to the top of the cap layer unit after time T, it just causes a critical breakthrough; substitute the parameters in step 1.3 into the pressure inversion solution in step 1.2 to obtain the maximum sealing pressure of the cap layer unit; Based on the maximum sealing pressure of the cap rock unit, a quantitative evaluation method for the sealing of the cap rock system in the site selection evaluation stage is determined, as is the safe sealing pressure of the cap rock system in the engineering design implementation stage.

2. A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure according to claim 1, characterized in that: The determined calculation parameters specifically include: cap layer unit thickness, porosity, cap layer unit position, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity and fluid compressibility.

3. A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure according to claim 1, characterized in that: The method of quantitatively evaluating the sealing performance of the cap rock system in the site selection evaluation stage is determined based on the maximum sealing pressure of the cap rock unit, specifically including: The cap rock system is divided into a number of N cap rock units; The maximum sealing pressure P of the cap layer unit is determined based on the pressure diffusion solution in the porous medium and the thickness of the cap layer. max,n The minimum value of (n = 1, 2, …, N) is used as the evaluation index C of the sealing performance of the caprock system. C = min{P max,1 , P max,2 , …, P max,N }。 4. A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure according to claim 1, characterized in that: The method of quantitatively evaluating the sealing performance of the cap rock system in the site selection evaluation stage is determined based on the maximum sealing pressure of the cap rock unit, specifically including: Step 2.1, structural modeling is performed based on site data, and the cap rock system is divided into units based on geological parameters to obtain N cap rock units; Step 2.2, for each cap layer unit; Step 2.3, take the minimum value of the maximum sealing pressure of all cap layer units, min{P max,1 , P max,2 , …, P max,N }; As the evaluation index of the sealing performance of the caprock system, C = min{P max,1 , P max,2 , …, P max,N }.

5. A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure according to claim 4, characterized in that: The step 2.2 specifically includes, for each cap layer unit: Step 2.2.1, based on the pressure diffusion wave propagation model in porous media, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model; Step 2.2.2, determine the calculation related parameters; Step 2.2.3, determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection in the energy storage reservoir, that is, the injection cycle; Step 2.2.4, set the critical condition, when the pressure propagates to the top of the cap layer unit after time T, it just causes a critical breakthrough; substitute the parameters in step 2.2.2 into the pressure inversion solution in step 2.2.1 to obtain the maximum sealing pressure P of the cap layer unit max,n (n = 1, 2, …, N).

6. A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure according to claim 1, characterized in that: The determination of the safe sealing pressure of the cap rock system for the engineering design implementation stage specifically includes: taking the maximum sealing pressure of the cap rock unit as a basis, considering the engineering reserve coefficient, and the cap rock unit rupture pressure constraint condition, and determining the safe sealing pressure of the cap rock system.

7. A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure according to claim 6, characterized in that: The method of determining the safe sealing pressure of the cap rock system based on the maximum sealing pressure of the cap rock unit, taking into account the engineering reserve coefficient and the constraint condition of the fracture pressure of the cap rock unit, specifically includes: Step 3.1, structural modeling is performed based on site data, and the cap rock system is divided into units based on geological parameters to obtain N cap rock units; Step 3.2, for each cap layer unit; Step 3.3: For each cap layer unit, determine the constraint pressure P of each cap layer unit. c,n ,n = 1, 2, …, N,when the stratum structure contains a cap layer, the fracture pressure at the bottom of the cap layer unit is used as the constraint pressure P c ; Step 3.4: For each cap layer unit, determine the bottom entry pressure P of the cap layer unit. in , n = 1, 2, …, N, the bottom entry pressure of the cap layer unit P in is the formation pressure at the bottom of the caprock unit P s,d Breakthrough pressure P b,d sum; Step 3.5, take the minimum value of the maximum sealing pressure of all cap layer units in step 3.2, that is: min{P max,1 , P max,2 ,…, P max,N }, and find out the corresponding cap layer unit A in step 3.

4. in,A , calculate the upper limit value α of the engineering reserve coefficient of cap layer unit A max,A =P max,A / P in,A ; Step 3.6, according to the maximum closing pressure P of the cover unit A max,A , and consider a value not exceeding the upper limit of the engineering reserve coefficient of the caprock unit A max,A Engineering reserve coefficient α A , the sealing pressure P of caprock unit A including engineering reserve coefficient is obtained α,A =P max,A / α A ; Step 3.7, compare the closing pressure of cap layer unit A with the constraint pressure corresponding to cap layer unit A in step 3.

3. If the closing pressure of cap layer unit A P α,A Less than or equal to its restraining pressure P c,A , that is, P α,A ≤P c,A , then the pressure is the safe sealing pressure P of the caprock system safe =P α,A If not, adjust the engineering reserve coefficient α in step 3.6 A , until satisfied.

8. A method for evaluating the sealing performance of a caprock system and determining the safe sealing pressure according to claim 7, characterized in that: The step 3.2 specifically includes, for each cap layer unit: Step 3.2.1, based on the pressure diffusion wave propagation model in porous media, obtain the analytical solution of the pressure at any position on the pressure diffusion path at any time, and establish its pressure inversion solution model; Step 3.2.2, determine the calculation-related parameters, including caprock unit thickness, porosity, caprock unit position, formation pressure, breakthrough pressure, permeability, fracture pressure, fluid viscosity and fluid compressibility; Step 3.2.3, determine the critical breakthrough time T, which is taken as the time from the start to the end of gas injection in the energy storage reservoir, and obtain the injection cycle; Step 3.2.4, set the critical condition, when the pressure propagates to the top of the cap layer unit after time T, the critical breakthrough occurs; substitute the parameters in step 3.2.2 into the pressure inversion solution in step 3.2.1 to obtain the maximum sealing pressure P of the cap layer unit max,n , n = 1, 2, …, N.

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