Construction of underground heat storage system based on reservoir transformation and evaluation method of heat storage effect

Abstract

The present invention belongs to the technical field of underground energy storage and heat energy extraction, and discloses a method for constructing an underground heat storage system based on reservoir transformation and evaluating the heat storage effect, the method comprising determining a heat storage layer and a transformation layer; determining the permeability of a rock sample of the transformation layer after low-temperature fluid fracturing, injecting proppant and heat-insulating microbeads into the fracturing specimen, and measuring the equivalent thermal conductivity and mechanical parameters of the transformed specimen containing heat-insulating microbeads; changing the fracturing scheme of the low-temperature fracturing fluid, and for the obtained transformed specimen, changing the injection amount of heat-insulating microbeads, and determining the equivalent thermal conductivity and mechanical parameters corresponding to different injection amounts of heat-insulating microbeads; based on the temperature-seepage-stress coupling model, carrying out aquifer heat storage simulation, and determining the injection-production well spacing in the heat storage layer and the reservoir transformation scheme of the transformation layer. The present invention can reduce the heat dissipation rate of the underground heat storage system, and improve the output fluid temperature and the system heat recovery rate.

Description

Technical Field

[0001] The present invention belongs to the technical field of underground energy storage and heat energy extraction, and in particular relates to an underground heat storage system construction based on reservoir transformation and a heat storage effect evaluation method. Background Art

[0002] With the rapid development of the new energy industry, various new energy sources are being utilized, but a considerable amount of energy is still not being used reasonably. Especially for solar energy and wind energy, these two renewable energy sources are unstable, which will result in a large amount of wind and light abandonment, resulting in a large amount of energy loss. On the other hand, energy utilization is unevenly distributed over time, and energy consumption in winter is much greater than in summer. If we can make full use of the waste energy in summer, store it, and use it in winter, this will be of great significance to the realization of energy conservation and emission reduction goals. Aquifer heat storage is a beneficial way to solve the problem of uneven seasonal energy distribution.

[0003] The aquifer thermal energy storage (ATES) system is an open-loop system that temporarily stores sensible heat and sensible cold in underground aquifers through the injection and extraction of groundwater. Usually, in seasons with high heat production rates, excess heat is injected into the aquifer in the form of hot water, and it is extracted when needed to achieve cross-seasonal use of heat. It has the advantages of large reserves, low cost, and cross-season, which can make up for the imbalance in the time / space distribution of energy supply and demand, and can comprehensively utilize various forms of renewable energy to reduce dependence on mineral fuels. Current research focuses on the heat exchange process of aquifer thermal storage, and there is less research on improving the efficiency of aquifer thermal storage. Therefore, the present invention proposes an underground thermal storage system based on reservoir transformation and a thermal storage effect evaluation method, which forms a continuous fracture zone by fracturing in the upper and lower adjacent layers of the thermal storage layer. A mixture of proppant and hollow microspheres is injected during the fracturing process. The function of the proppant is to keep the fractures in the formation at a certain opening, and the function of the hollow microspheres is to have a heat insulation effect. This forms an artificial fracture insulation zone, reduces heat dissipation, and improves the heat storage efficiency of the aquifer.

[0004] The heat storage performance of aquifers is affected by hydrogeological conditions, which is reflected in the evolution of flow and heat transfer performance during the energy storage process. To detect whether it has a good heat storage effect, it is necessary to study the heat transfer characteristics of the aquifer. At present, the evaluation of ATES heat storage effect mostly uses heat extraction efficiency or heat recovery rate as an indicator. This indicator focuses on the evaluation of extraction temperature and heat extraction efficiency, and lacks energy loss analysis of the heat storage process. This method can calculate the heat dissipation rate of the reservoir under different conditions by analyzing the temperature changes in the non-heat storage area, and then combined with the analysis of the extraction temperature, taking into account both the process and the results, which is more comprehensive than previous evaluations.

[0005] Through the above analysis, the problems and defects of the existing technology are as follows: the current method of evaluating the ATES heat storage effect by using the indicators of heat extraction efficiency or heat recovery rate focuses on the evaluation of extraction temperature and heat extraction efficiency, lacks the analysis of energy loss in the heat storage process, and has little analysis on improving the heat storage efficiency of aquifers. Moreover, most of the current research on aquifer heat storage at home and abroad focuses on the influence of reservoir parameters or injection and production parameters on the heat storage effect and the positive significance of aquifer heat storage in reducing carbon emissions, while there are few reports on related research on how to reduce energy loss in the heat storage process and reduce the heat dissipation rate of the heat storage system. Summary of the invention

[0006] In order to overcome the problems existing in the related art, the disclosed embodiment of the present invention provides an underground heat storage system construction and heat storage effect evaluation method based on reservoir transformation, in which artificial cracks are generated in the upper and lower adjacent layers of the heat storage layer by fracturing, and then proppant and heat insulating microbeads are injected to reduce heat dissipation in the heat storage layer and improve heat storage efficiency. The technical solution is as follows:

[0007] The present invention is implemented by constructing an underground heat storage system based on reservoir transformation and evaluating the heat storage effect, comprising:

[0008] S1, measure the physical parameters of each reservoir near the target aquifer and determine the heat storage layer and the reformed layer;

[0009] S2, based on the determined transformation layer, a drilled rock sample specimen containing a center for fracturing and temperature measurement drilling is prepared, and a low-temperature fluid fracturing physical simulation experiment is carried out on the drilled rock sample specimen to obtain the permeability of the specimen after fracturing; proppant and thermal insulation microbeads are injected into the drilled rock sample specimen after fracturing, and the equivalent thermal conductivity coefficient of the drilled rock sample specimen containing proppant and thermal insulation microbeads after transformation is obtained through a thermal conductivity performance experiment; the compressive strength, Young's modulus and Poisson's ratio of the drilled rock sample specimen containing proppant and thermal insulation microbeads after transformation are obtained through uniaxial compression and triaxial compression tests of rocks;

[0010] S3, changing the temperature, flow rate and injection volume of the low-temperature fracturing fluid to obtain the corresponding permeability under different low-temperature fluid fracturing schemes; for different low-temperature fluid fracturing schemes, obtaining reformed borehole rock sample specimens with different permeabilities, changing the injection volume of thermal insulation beads under the condition that the injection volume of the proppant remains unchanged, obtaining the corresponding equivalent thermal conductivity under different injection volumes of thermal insulation beads, and for reformed borehole rock sample specimens under different low-temperature fluid fracturing schemes and different injection volumes of thermal insulation beads, obtaining the corresponding compressive strength, Young's modulus and Poisson's ratio through rock uniaxial compression and triaxial compression tests;

[0011] S4, based on the temperature-seepage-stress coupling model, conduct aquifer heat storage simulation, change the injection-production well spacing in the heat storage layer, the permeability in the reformed layer, and the equivalent thermal conductivity in the reformed layer, use the heat storage system production temperature, heat recovery rate and heat dissipation rate as comprehensive evaluation indicators of heat storage effect, and determine the reservoir reform plan for the injection-production well spacing in the heat storage layer and the reformed layer.

[0012] In step S1, the physical parameters of each reservoir near the target aquifer are measured, including reservoir permeability, porosity, thermal conductivity, tensile and compressive strength, Young's modulus and Poisson's ratio.

[0013] Furthermore, the permeability and porosity of the reservoir were measured multiple times and the average values ​​were taken. The abandoned oil reservoir was selected as the target aquifer, and rock samples of each reservoir near the target aquifer were obtained based on coring. The permeability of the core was measured using a core holder, and the porosity of the core was tested using a porosity meter. After multiple measurements, the average values ​​were calculated as the permeability and porosity of each reservoir rock sample. The permeability was 1×10 -10 m 2 Up to 1×10 -12 m 2 The reservoir with a porosity greater than 0.15 is used as the heat storage layer, and the upper and lower adjacent layers of the heat storage layer are used as transformation layers.

[0014] In step S2, the drilled rock sample is in the shape of a cylinder, and a true triaxial core holder is used to perform a low-temperature fluid fracturing physical simulation experiment under loading confining pressure and axial pressure conditions. The low-temperature fluid fracturing uses liquid nitrogen or liquid carbon dioxide as a fracturing fluid. After fracturing transformation, the permeability of the sample after fracturing is measured;

[0015] In the process of measuring the permeability of the test piece after fracturing, the pressure difference between the inlet and outlet of the test piece under the condition of fixed injection flow rate is measured, and its permeability is calculated according to Darcy's formula; after the permeability of the drilled rock sample after fracturing is measured, proppant and thermal insulation beads are injected into the drilled rock sample after fracturing; the drilled rock sample containing proppant and thermal insulation beads after fracturing is taken out from the true triaxial core holder, a heat source is loaded on the wall surface around the drilled rock sample after fracturing and insulation measures are taken on the outside of the heat source, temperature monitoring is performed at the central borehole of the drilled rock sample after fracturing, and the equivalent thermal conductivity of the drilled rock sample after fracturing and injection of thermal insulation beads is calculated based on the cylinder wall thermal conductivity formula;

[0016] The heat conduction formula of the cylindrical wall is:

[0017]

[0018] Where λ is the equivalent thermal conductivity, in units of W / (m·K); r1 is the radius of the drilled rock sample after fracturing, in units of m; r2 is the radius of the borehole of the drilled rock sample after fracturing, in units of m; l is the length of the drilled rock sample after fracturing, in units of m; Δt is the temperature difference between the wall around the specimen and the borehole, in units of ℃.

[0019] In step S2, uniaxial compression and triaxial compression tests are carried out on the drilled rock sample specimens that have been transformed by cryogenic fluid and injected with proppants and thermal insulation beads to obtain the compressive strength, Young's modulus and Poisson's ratio of the transformed proppant and thermal insulation beads specimens.

[0020] In step S3, the temperature, flow rate and injection volume of the low-temperature fracturing fluid are changed to obtain the corresponding permeability under different low-temperature fluid fracturing schemes, including:

[0021] For the reformed specimens with different permeabilities obtained under different low-temperature fluid fracturing schemes, the equivalent thermal conductivity corresponding to different insulation bead injection amounts was obtained by changing the insulation bead injection amount under the condition of unchanged proppant injection amount; for the reformed specimens under different low-temperature fluid fracturing schemes and different insulation bead injection amounts, the corresponding compressive strength, Young's modulus and Poisson's ratio were obtained through rock uniaxial compression and triaxial compression tests.

[0022] In step S4, based on the temperature-seepage-stress coupling model, aquifer heat storage simulation is carried out under different low-temperature fluid fracturing schemes and different insulation microbead injection conditions;

[0023] The temperature-seepage-stress coupling model is composed of a static equilibrium equation considering fluid pressure and temperature changes, a temperature field control equation considering thermal convection and mechanical energy, and a seepage field equation considering stress and temperature effects;

[0024] The static equilibrium equation considering the changes in fluid pressure and temperature is:

[0025]

[0026] Where G is the shear modulus, in Pa. E is the elastic modulus in Pa, v is Poisson's ratio; u i,jj ,u j,ji All of them are tensor representation methods. The subscript u represents the displacement in three directions u, v, w, and the corresponding spatial coordinates are x, y, z. According to Einstein's summation convention, a is the Biot coefficient, p ,i is the pore fluid pressure, in Pa; -K′a T T ,iis the temperature stress term caused by temperature change, K′ is the drainage bulk modulus of the medium, in Pa; a T is the thermal expansion coefficient of rock mass, in K -1 ; T ,i is the reservoir temperature in K, F i is the component of the volume force on the rock in the i direction, where the i direction is divided into the x, y, and z directions in the spatial coordinate system;

[0027] The temperature field control equation considering thermal convection and mechanical energy is:

[0028]

[0029] Where, (ρC) M is the heat capacity of the porous medium containing the fluid, in kJ·m 3 ·K -1 ; ρ S is the density of porous bedrock, in kg / m 3 ; C S ,C l are the porous medium bedrock coefficient and the specific heat coefficient of the fluid, in kJ·kg -1 ·K -1 ; is the porosity of the porous medium bedrock, ρ l is the fluid density, is the mechanical energy term, is the thermal convection term; T is the reservoir temperature, in K; T0 is the reference temperature under zero stress state, in K; ε T is the strain under the action of temperature, in m; is the gradient operator, is the pressure gradient, λ M is the thermal conductivity of the porous medium containing fluid, λ S ,λ l are the thermal conductivity of bedrock and fluid, respectively, in W / (m·K); u is the dynamic viscosity of the fluid, in Pa·s, kg / m 3 ; k is the permeability of the porous medium, in m 2 ;

[0030] The seepage field equation considering stress and temperature is:

[0031]

[0032] In the formula, is the stress term, is the temperature effect term, is the pore pressure effect term, c1 is the stress effect term coefficient; c2 is the temperature effect term coefficient, c3 is the pore pressure effect term coefficient, ε v is the volume strain, in m, g is the gravitational acceleration, in m / s 2 , z is the vertical coordinate, unit is m; K s is the effective bulk modulus of the solid component, in Pa; a l is the volume thermal expansion coefficient of the fluid, in K -1 ; a s is the volume thermal expansion coefficient of the solid component, in K -1 β l is the bulk modulus of the pore fluid, in Pa.

[0033] Furthermore, the aquifer heat storage simulation under different low-temperature fluid fracturing schemes and different insulation micro-bead injection conditions includes: establishing a geometric model of the heat reservoir and the transformed layer, assigning values ​​to the heat reservoir physical property parameters based on the obtained physical property parameters of each reservoir near the target aquifer, and assigning values ​​to the transformed layer physical property parameters based on the obtained physical property parameters of the transformed specimens under different low-temperature fluid fracturing schemes and different insulation micro-bead injection amounts;

[0034] Based on the constructed temperature-seepage-stress coupling model, heat storage simulation is carried out to change the injection-production well spacing in the heat reservoir layer, the permeability in the transformed layer, and the equivalent thermal conductivity in the transformed layer. The production temperature, heat recovery rate and heat dissipation rate of the heat storage system are used as comprehensive evaluation indicators of the heat storage effect. Based on the obtained permeability and equivalent thermal conductivity of the transformed specimens under different low-temperature fluid fracturing schemes and different insulation microbead injection amounts, the injection-production well spacing in the heat reservoir layer and the reservoir transformation scheme of the transformed layer are determined; the reservoir transformation scheme of the transformed layer includes a low-temperature fluid fracturing scheme and an insulation microbead injection amount.

[0035] Furthermore, the heat storage system extraction temperature, heat recovery rate and heat dissipation rate are used as comprehensive evaluation indicators of heat storage effect, including:

[0036] The heat recovery rate is calculated as:

[0037]

[0038] Q 采 =∫V out ρ out ·c out ΔT out dt out

[0039] Q 注 =∫V inj ρ inj ·c inj ΔTinj dt inj

[0040] Where R is the heat recovery rate, Q 采 is the total heat extracted, in kJ; Q 注 is the total heat injected, in kJ; V out is the volume flow rate of produced fluid, in m 3 / d;ρ out is the density of produced fluid, in kg / m 3 ;c out is the specific heat capacity of the produced fluid, in J / (kg·K); ΔT out is the difference between the produced fluid temperature and the surface temperature, in K; dt out is the heat collection time, in d; V inj is the volume flow rate of injected fluid, in m 3 / d;ρ inj is the density of the injected fluid, in kg / m 3 ;c inj is the specific heat capacity of the injected fluid, in J / (kg·K); ΔT inj is the difference between the injected fluid temperature and the surface temperature, in K; dt inj is the heat injection time, in d;

[0041] The heat dissipation rate calculation formula is:

[0042]

[0043] Q 耗 =∫c 非 m 非 ΔT 非 dt

[0044] In the formula, H is the heat dissipation rate, Q 耗 is the heat dissipation in the heat storage system, in kJ; c 非 is the equivalent specific heat capacity of the porous medium in the non-heat storage area, in J / (kg·K); c 非S is the specific heat capacity of bedrock in the non-heat storage area, in J / (kg·K); c 非l is the specific heat capacity of pore water in the non-heat storage area, in J / (kg·K); is the rock porosity in the non-heat storage area, ΔT 非 is the temperature change in the non-heat storage area, in K; t is the total operating time of the heat storage system, in d; m 非 is the mass of rock in the non-heat storage area, in kg.

[0045] Combining all the above technical solutions, the advantages and positive effects of the present invention are as follows: the present invention selects a heat storage layer and an adjacent reformed layer, and produces fine and continuous cracks in the reformed layer through low-temperature fluid fracturing. The low-temperature fluid can be liquid nitrogen or liquid carbon dioxide. An artificial insulation belt is formed by injecting proppants and heat-insulating microbeads into the cracks. The role of the proppants is to keep the cracks in the formation at a certain opening, and the role of the hollow microbeads is to play a heat-insulating effect to reduce the heat dissipation in the heat storage layer. Compared with conventional aquifer heat storage schemes, the present invention has better heat extraction effect, less heat dissipation in the heat storage layer, and can effectively reduce the heat dissipation rate of the heat storage system and improve the heat recovery rate. In addition, the present invention comprehensively evaluates the heat storage effect of the aquifer through extraction temperature, heat recovery rate and heat dissipation rate, which can take into account the heat storage process and the heat extraction process, analyze the effect of aquifer heat storage from a more comprehensive perspective, and provide more accurate and scientific guidance for actual production. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description, serve to explain the principles of the present disclosure;

[0047] Figure 1 The method for constructing an underground heat storage system and evaluating the heat storage effect based on reservoir transformation provided in Example 1 of the present invention;

[0048] Figure 2 This is the temperature distribution diagram of the underground heat storage system during the first month of heat injection before reservoir transformation;

[0049] Figure 3 This is the temperature distribution diagram of the underground heat storage system during the heat injection process in the third month before reservoir transformation;

[0050] Figure 4 This is the temperature distribution map of the underground heat storage system during the sixth month of heat injection before reservoir transformation;

[0051] Figure 5 This is the temperature distribution diagram of the underground heat storage system during the heat injection process in the first month after reservoir transformation;

[0052] Figure 6 This is the temperature distribution diagram of the underground heat storage system during the heat injection process in the third month after reservoir transformation;

[0053] Figure 7 This is the temperature distribution diagram of the underground heat storage system during the sixth month of heat injection process after reservoir transformation. DETAILED DESCRIPTION

[0054] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.

[0055] The innovation of the present invention lies in: the present invention determines the heat storage layer and the transformation layer in the underground heat storage system, adopts low-temperature fluid to fracture the transformation layer, injects proppant and thermal insulation beads into the transformation layer, and then forms an artificial thermal insulation belt, which can effectively reduce the heat dissipation rate of the heat storage system and improve the heat recovery rate; provides a test method for the equivalent thermal conductivity of rock sample specimens containing thermal insulation beads after fracturing and transformation; and proposes a method for evaluating the heat storage effect of the underground heat storage system using the production temperature, heat recovery rate and heat dissipation rate as evaluation indicators.

[0056] Embodiment 1, as Figure 1 As shown, the method for constructing an underground heat storage system and evaluating the heat storage effect based on reservoir transformation provided by an embodiment of the present invention includes the following steps:

[0057] S1, measure the physical parameters of each reservoir near the target aquifer and determine the heat storage layer and the reformed layer;

[0058] S2, based on the determined transformation layer, a drilled rock sample specimen containing a center for fracturing and temperature measurement drilling is prepared, and a low-temperature fluid fracturing physical simulation experiment is carried out on the drilled rock sample specimen to obtain the permeability of the specimen after fracturing; proppant and thermal insulation microbeads are injected into the drilled rock sample specimen after fracturing, and the equivalent thermal conductivity coefficient of the drilled rock sample specimen containing proppant and thermal insulation microbeads after transformation is obtained through a thermal conductivity performance experiment; the compressive strength, Young's modulus and Poisson's ratio of the drilled rock sample specimen containing proppant and thermal insulation microbeads after transformation are obtained through uniaxial compression and triaxial compression tests of rocks;

[0059] S3, changing the temperature, flow rate and injection volume of the low-temperature fracturing fluid to obtain the corresponding permeability under different low-temperature fluid fracturing schemes; for different low-temperature fluid fracturing schemes, obtaining reformed borehole rock sample specimens with different permeabilities, changing the injection volume of thermal insulation beads under the condition that the injection volume of the proppant remains unchanged, obtaining the corresponding equivalent thermal conductivity under different injection volumes of thermal insulation beads, and for reformed borehole rock sample specimens under different low-temperature fluid fracturing schemes and different injection volumes of thermal insulation beads, obtaining the corresponding compressive strength, Young's modulus and Poisson's ratio through rock uniaxial compression and triaxial compression tests;

[0060] S4, based on the temperature-seepage-stress coupling model, conduct aquifer heat storage simulation, change the injection-production well spacing in the heat storage layer, the permeability in the reformed layer, and the equivalent thermal conductivity in the reformed layer, use the heat storage system production temperature, heat recovery rate and heat dissipation rate as comprehensive evaluation indicators of heat storage effect, and determine the reservoir reform plan for the injection-production well spacing in the heat storage layer and the reformed layer.

[0061] In step S1, the physical property parameters of each reservoir near the target aquifer include: reservoir permeability, porosity, thermal conductivity, tensile and compressive strength, Young's modulus and Poisson's ratio;

[0062] The reservoir permeability and porosity were measured multiple times and the average values ​​were taken. Specifically, abandoned oil reservoirs were selected as target aquifers, and rock samples of various reservoirs near the target aquifers were obtained based on core drilling. The permeability of the cores was measured using a core holder, and the porosity of the cores was tested using a porosity meter. After multiple measurements, the average values ​​were calculated as the permeability and porosity of the rock samples of each reservoir. The permeability was 1×10 -10 m 2 Up to 1×10 -12 m 2 , the reservoir with porosity greater than 0.15 is used as the heat storage layer, and the upper and lower adjacent layers are used as the transformation layer;

[0063] In step S2, for the determined reformed layer, a drilled rock sample specimen containing a center that can be used for fracturing and temperature measurement is prepared. The specimen is cylindrical in shape, and a low-temperature fluid fracturing physical simulation experiment is carried out using a true triaxial fracturing physical simulation experimental device. The low-temperature fluid can be liquid nitrogen or low-temperature liquid carbon dioxide. After fracturing and reformation, the permeability of the drilled rock sample specimen after fracturing is measured. The permeability measurement process of the drilled rock sample specimen after fracturing requires measuring the pressure difference between the specimen inlet and outlet under the condition of a fixed injection flow rate, and then calculating its permeability according to the Darcy formula. After the permeability of the drilled rock sample after fracturing is measured, proppant and thermal insulation beads are injected into the drilled rock sample after fracturing, and the drilled rock sample containing proppant and thermal insulation beads after fracturing is taken out from the true triaxial core holder, and a heat source is loaded on the wall around the sample, and the temperature is monitored from the central borehole of the sample. The equivalent thermal conductivity of the sample after fracturing and injection of thermal insulation beads is calculated based on the thermal conductivity formula of the cylindrical wall; the thermal conductivity formula of the cylindrical wall is:

[0064]

[0065] Where λ is the equivalent thermal conductivity, in units of W / (m·K); r1 is the radius of the drilled rock sample after fracturing, in units of m; r2 is the radius of the borehole of the drilled rock sample after fracturing, in units of m; l is the length of the drilled rock sample after fracturing, in units of m; Δt is the temperature difference between the wall around the specimen and the borehole, in units of ℃.

[0066] The uniaxial and triaxial compression tests of rocks were carried out on cylindrical specimens that had been transformed by cryogenic fluid and injected with proppants and thermal insulation beads, and the compressive strength, Young's modulus and Poisson's ratio of the transformed specimens containing proppants and thermal insulation beads were obtained;

[0067] In step S3, based on the determined transformation layer, a drilled rock sample specimen containing a center for fracturing and temperature measurement is prepared, and a physical simulation experiment of low-temperature fluid fracturing is carried out on the drilled rock sample specimen, and the temperature, flow rate and injection volume of the low-temperature fracturing fluid are changed to obtain the permeability of the drilled rock sample specimen after fracturing under different low-temperature fluid fracturing schemes; based on the obtained permeability of the drilled rock sample specimen under different low-temperature fluid fracturing schemes, the injection volume of the thermal insulation beads is changed without changing the injection volume of the proppant, and the corresponding equivalent thermal conductivity coefficient under different injection volumes of the thermal insulation beads is obtained. For the transformed specimens under different low-temperature fluid fracturing schemes and different injection volumes of the thermal insulation beads, the compressive strength, Young's modulus and Poisson's ratio of the corresponding drilled rock sample specimens are obtained through uniaxial compression and triaxial compression tests of the rock;

[0068] In step S4, the permeability, equivalent thermal conductivity, compressive strength, Young's modulus and Poisson's ratio of the drilled rock sample after fracturing obtained in step S3 are used to change the injection-production well spacing in the heat reservoir layer based on the temperature-seepage-stress coupling model, and perform heat storage simulation of each reservoir near the target aquifer. The heat storage system production temperature, heat recovery rate and heat dissipation rate are used as comprehensive evaluation indicators of the heat storage effect to determine the optimal injection-production well spacing in the heat reservoir layer and the optimal transformation plan for the transformation layer. The temperature-seepage-stress coupling model is composed of a static equilibrium equation considering fluid pressure and temperature changes, a temperature field control equation considering thermal convection and mechanical energy, and a seepage field equation considering stress and temperature.

[0069] The static equilibrium equation considering the changes in fluid pressure and temperature is:

[0070]

[0071] Where G is the shear modulus, in Pa. E is the elastic modulus in Pa, v is Poisson's ratio; u i,jj ,u j,ji All of them are tensor representation methods. The subscript u represents the displacement in three directions u, v, w, and the corresponding spatial coordinates are x, y, z. According to Einstein's summation convention, a is the Biot coefficient, p ,i is the pore fluid pressure, in Pa; -K′a T T ,i is the temperature stress term caused by temperature change, K′ is the drainage bulk modulus of the medium, in Pa; a Tis the thermal expansion coefficient of rock mass, in K -1 ; T ,i is the reservoir temperature in K, F i is the component of the volume force on the rock in the i direction, where the i direction is divided into the x, y, and z directions in the spatial coordinate system;

[0072] The temperature field control equation considering thermal convection and mechanical energy is:

[0073]

[0074] Where, (ρc) M is the heat capacity of the porous medium containing the fluid, in kJ·m 3 ·K -1 ; ρ s is the density of porous bedrock, in kg / m 3 ; C s ,C l are the porous medium bedrock coefficient and the specific heat coefficient of the fluid, in kJ·kg -1 ·K -1 ; is the porosity of the porous medium bedrock, ρ l is the fluid density, is the mechanical energy term, is the thermal convection term; T is the reservoir temperature, in K; T0 is the reference temperature under zero stress state, in K; ε T is the strain under the action of temperature, in m; is the gradient operator, is the pressure gradient, λ M is the thermal conductivity of the porous medium containing fluid, λ S ,λ l are the thermal conductivity of bedrock and fluid, respectively, in W / (m·K); u is the dynamic viscosity of the fluid, in Pa·s, kg / m 3 ; k is the permeability of the porous medium, in m 2 ;

[0075] The seepage field equation considering stress and temperature is:

[0076]

[0077] In the formula, is the stress term, is the temperature effect term, is the pore pressure effect term, c1 is the stress effect term coefficient; c2 is the temperature effect term coefficient, C3 is the pore pressure effect term coefficient, ε v is the volume strain, in m, g is the gravitational acceleration, in m / s 2 , z is the vertical coordinate, unit is m; K S is the effective bulk modulus of the solid component, in Pa; a l is the volume thermal expansion coefficient of the fluid, in K -1 ; a S is the volume thermal expansion coefficient of the solid component, in K -1 β l is the bulk modulus of the pore fluid, in Pa.

[0078] In the embodiment of the present invention, the aquifer heat storage simulation needs to establish the geometric model of the heat storage layer and the transformed layer, assign the heat storage layer physical property parameters based on the obtained physical property parameters of each reservoir near the target aquifer, and assign the transformed layer physical property parameters based on the obtained physical property parameters of the transformed borehole rock sample specimens under different low-temperature fluid fracturing schemes and different insulation micro-bead injection amounts. Based on the constructed temperature-seepage-stress coupling model, the heat storage simulation is carried out, the injection-production well spacing in the heat storage layer, the permeability in the transformed layer, and the equivalent thermal conductivity in the transformed layer are changed, and the heat storage system production temperature, heat recovery rate and heat dissipation rate are used as comprehensive evaluation indicators of the heat storage effect. Based on the obtained different low-temperature fluid fracturing schemes and the permeability and equivalent thermal conductivity of the transformed specimens under different insulation micro-bead injection amounts, the reservoir transformation scheme of the injection-production well spacing and the transformed layer in the heat storage layer is determined. The reservoir transformation scheme of the transformed layer includes the low-temperature fluid fracturing scheme and the insulation micro-bead injection amount.

[0079] The heat storage system's output temperature, heat recovery rate and heat dissipation rate are used as comprehensive evaluation indicators of heat storage effect.

[0080] The heat recovery rate is calculated as:

[0081]

[0082] Q 采 =∫V out ·ρ out ·c out ΔT out dt out

[0083] Q 注 =∫V inj ·ρ inj ·c inj ΔT inj dt inj

[0084] Where R is the heat recovery rate, Q 采 is the total heat extracted, in kJ; Q 注 is the total heat injected, in kJ; V out is the volume flow rate of produced fluid, in m 3 / d;ρ out is the density of produced fluid, in kg / m 3 ;c out is the specific heat capacity of the produced fluid, in J / (kg·K); ΔT out is the difference between the produced fluid temperature and the surface temperature, in K; dt out is the heat collection time, in d; V inj is the volume flow rate of injected fluid, in m 3 / d;ρ inj is the density of the injected fluid, in kg / m 3 ;c inj is the specific heat capacity of the injected fluid, in J / (kg·K); ΔT inj is the difference between the injected fluid temperature and the surface temperature, in K; dt inj is the heat injection time, in d;

[0085] The heat dissipation rate calculation formula is:

[0086]

[0087] Q 耗 =∫c 非 m 非 ΔT 非 dt

[0088] In the formula, H is the heat dissipation rate, Q 耗 is the heat dissipation in the heat storage system, in kJ; c 非 is the equivalent specific heat capacity of the porous medium in the non-heat storage area, in J / (kg·K); c 非S is the specific heat capacity of bedrock in the non-heat storage area, in J / (kg·K); c 非l is the specific heat capacity of pore water in the non-heat storage area, in J / (kg·K); is the rock porosity in the non-heat storage area, ΔT 非 is the temperature change in the non-heat storage area, in K; t is the total operating time of the heat storage system, in d; m 非 is the mass of rock in the non-heat storage area, in kg.

[0089] Figures 2 to 4 This is the temperature distribution diagram of the aquifer during heat storage before transformation. Figures 5 to 7This is the temperature distribution diagram of the aquifer heat storage process after fracturing and injection of thermal insulation beads. After comparing before and after the transformation, it can be seen that after the reservoir transformation, the heat is effectively stored in the heat storage layer, reducing the heat dissipation.

[0090] Example 2, as another implementation of the present invention, the method for evaluating the heat storage effect of an underground heat storage system under different reservoir transformation schemes provided in the embodiment of the present invention specifically includes the following steps:

[0091] Step 1: Determine the physical parameters of each reservoir near the target aquifer;

[0092] Physical properties of reservoirs near the target aquifer, including reservoir permeability, porosity, thermal conductivity, tensile and compressive strength, Young's modulus and Poisson's ratio;

[0093] The reservoir permeability and porosity were measured multiple times and the average values ​​were taken. Specifically, abandoned oil reservoirs were selected as target aquifers, and rock samples of various reservoirs near the target aquifers were obtained based on core drilling. The permeability of the cores was measured using a core holder, and the porosity of the cores was tested using a porosity meter. After multiple measurements, the average values ​​were calculated as the permeability and porosity of the rock samples of each reservoir. The permeability was 1×10 -10 m 2 Up to 1×10 -12 m 2 , the reservoir with porosity greater than 0.15 is used as the heat storage layer, and the upper and lower adjacent layers are used as the transformation layer;

[0094] Step 2, for the transformed layer determined in step 1, prepare a borehole rock sample specimen containing a center that can be used for fracturing and temperature measurement. The specimen is in the shape of a cylinder, and a low-temperature fluid fracturing physical simulation experiment is carried out using a true triaxial fracturing physical simulation experimental device. The low-temperature fluid can be liquid nitrogen or low-temperature liquid carbon dioxide. After fracturing and transformation, the permeability of the borehole rock sample specimen after fracturing is measured. The permeability measurement process of the borehole rock sample specimen after fracturing requires measuring the pressure difference between the inlet and outlet of the specimen under the condition of a fixed injection flow rate, and then calculating its permeability according to the Darcy formula. After the permeability of the borehole rock sample specimen after fracturing is measured, proppant and thermal insulation beads are injected into the borehole rock sample specimen after fracturing, and the borehole rock sample specimen containing proppant and thermal insulation beads after fracturing is taken out from the true triaxial core holder, and a heat source is loaded on the wall around the specimen. The temperature is monitored from the central borehole of the specimen, and the equivalent thermal conductivity of the specimen after fracturing and injection of thermal insulation beads is calculated based on the cylindrical wall thermal conductivity formula;

[0095] The heat conduction formula of the cylindrical wall is:

[0096]

[0097] Where λ is the equivalent thermal conductivity, in units of W / (m·K); r1 is the radius of the drilled rock sample after fracturing, in units of m; r2 is the radius of the borehole of the drilled rock sample after fracturing, in units of m; l is the length of the drilled rock sample after fracturing, in units of m; Δt is the temperature difference between the wall around the specimen and the borehole, in units of ℃.

[0098] The uniaxial and triaxial compression tests of rocks were carried out on cylindrical specimens that had been transformed by cryogenic fluid and injected with proppants and thermal insulation beads, and the compressive strength, Young's modulus and Poisson's ratio of the transformed specimens containing proppants and thermal insulation beads were obtained;

[0099] S3, change the temperature, flow rate, and injection volume of the low-temperature fracturing fluid to obtain the corresponding permeability under different low-temperature fluid fracturing schemes. For the modified specimens with different permeabilities obtained by different low-temperature fluid fracturing schemes, change the injection volume of thermal insulation beads to obtain the corresponding equivalent thermal conductivity under different injection volumes of thermal insulation beads. For the modified specimens under different low-temperature fluid fracturing schemes and different injection volumes of thermal insulation beads, obtain the corresponding compressive strength, Young's modulus, and Poisson's ratio through rock uniaxial compression and triaxial compression tests;

[0100] Step 4, based on the temperature-seepage-stress coupling model, conduct aquifer heat storage simulation under different low-temperature fluid fracturing schemes and different insulation microbead injection conditions. The temperature-seepage-stress coupling model is composed of a static equilibrium equation considering fluid pressure and temperature changes, a temperature field control equation considering thermal convection and mechanical energy, and a seepage field equation considering stress and temperature effects.

[0101] The static equilibrium equation considering the changes in fluid pressure and temperature is:

[0102]

[0103] Where G is the shear modulus, in Pa. E is the elastic modulus in Pa, v is Poisson's ratio; u i,jj ,u j,ji All of them are tensor representation methods. The subscript u represents the displacement in three directions u, v, w, and the corresponding spatial coordinates are x, y, z. According to Einstein's summation convention, a is the Biot coefficient, p ,i is the pore fluid pressure, in Pa; -K′a T T ,i is the temperature stress term caused by temperature change, K′ is the drainage bulk modulus of the medium, in Pa; a T is the thermal expansion coefficient of rock mass, in K -1 ; T ,i is the reservoir temperature in K, F i is the component of the volume force on the rock in the i direction, where the i direction is divided into the x, y, and z directions in the spatial coordinate system;

[0104] The temperature field control equation considering thermal convection and mechanical energy is:

[0105]

[0106] Where, (ρC) M is the heat capacity of the porous medium containing the fluid, in kJ·m 3 ·K -1 ; ρ S is the density of porous bedrock, in kg / m 3 ; C S ,C l are the porous medium bedrock coefficient and the specific heat coefficient of the fluid, in kJ·kg -1 ·K -1 ρ l is the fluid density, is the mechanical energy term, is the thermal convection term; T is the reservoir temperature, in K; T0 is the reference temperature under zero stress state, in K; ε T is the strain under the action of temperature, in m; is the gradient operator, is the pressure gradient, λ M is the thermal conductivity of the porous medium containing the fluid, is the porosity of the porous medium bedrock, λ S ,λ l are the thermal conductivity of bedrock and fluid, respectively, in W / (m·K); u is the dynamic viscosity of the fluid, in Pa·s, kg / m 3 ; k is the permeability of the porous medium, in m 2 ;

[0107] The seepage field equation considering stress and temperature is:

[0108]

[0109] In the formula, is the stress term, is the temperature effect term, is the pore pressure term, c1 is the stress term coefficient; c2 is the temperature term coefficient, c3 is the pore pressure as volume strain, in m, g is the gravitational acceleration, in m / s 2 , z is the vertical coordinate, unit is m; K Sis the effective bulk modulus of the solid component, in Pa; a l is the volume thermal expansion coefficient of the fluid, in K -1 ; a S is the volume thermal expansion coefficient of the solid component, in K -1 β l is the bulk modulus of the pore fluid, in Pa.

[0110] Based on the temperature-seepage-stress coupling model, aquifer heat storage simulation was carried out under different low-temperature fluid fracturing schemes and different insulation microbead injection conditions.

[0111] The aquifer heat storage simulation requires the establishment of a geometric model of the heat storage layer and the reformed layer, assigning the heat storage layer physical property parameters based on the obtained physical property parameters of each reservoir near the target aquifer, and assigning the reformed layer physical property parameters based on the obtained physical property parameters of the reformed borehole rock sample specimens under different low-temperature fluid fracturing schemes and different insulation microbead injection amounts. Based on the constructed temperature-seepage-stress coupling model, a heat storage simulation is carried out to change the injection-production well spacing in the heat storage layer, the permeability in the reformed layer, and the equivalent thermal conductivity in the reformed layer. The heat storage system production temperature, heat recovery rate, and heat dissipation rate are used as comprehensive evaluation indicators of the heat storage effect. Based on the obtained different low-temperature fluid fracturing schemes and the reformed specimen permeability and equivalent thermal conductivity under different insulation microbead injection amounts, the reservoir reform scheme of the reformed layer is determined. The reservoir reform scheme of the reformed layer includes the low-temperature fluid fracturing scheme and the insulation microbead injection amount.

[0112] The heat storage system's output temperature, heat recovery rate and heat dissipation rate are used as comprehensive evaluation indicators of heat storage effect.

[0113] The heat recovery rate is calculated as:

[0114]

[0115] Q 采 =∫V out ·ρ out ·c out ΔT out dt ouu ,Q 注 =∫V inj ·ρ inj ·c inj ΔT inj t inj

[0116] Where R is the heat recovery rate, Q 采 is the total heat extracted, in kJ; Q 注 is the total heat injected, in kJ; V outis the volume flow rate of produced fluid, in m 3 / d;ρ out is the density of produced fluid, in kg / m 3 ; C out is the specific heat capacity of the produced fluid, in J / (kg·K); ΔT out is the difference between the produced fluid temperature and the surface temperature, in K; dt out is the heat collection time, in d; V inj is the volume flow rate of injected fluid, in m 3 / d;ρ inj is the density of the injected fluid, in kg / m 3 ; C inj is the specific heat capacity of the injected fluid, in J / (kg·K); ΔT inj is the difference between the injected fluid temperature and the surface temperature, in K; dt inj is the heat injection time, in d;

[0117] The heat dissipation rate calculation formula is:

[0118]

[0119] Q 耗 =∫c 非 m 非 ΔT 非 dt

[0120] In the formula, H is the heat dissipation rate, Q 耗 is the heat dissipation in the heat storage system, in kJ; c 非 is the equivalent specific heat capacity of the porous medium in the non-heat storage area, in J / (kg·K); c 非s is the specific heat capacity of bedrock in the non-heat storage area, in J / (kg·K); c 非l is the specific heat capacity of pore water in the non-heat storage area, in J / (kg·K); is the rock porosity in the non-heat storage area, ΔT 非 is the temperature change in the non-heat storage area, in K; t is the total operating time of the heat storage system, in d; m 非 is the mass of rock in the non-heat storage area, in kg.

[0121] Step 5: Combine the test data obtained from the experiment and assign values ​​to the heat storage layer and the reformed layer in the simulation model. Through simulation analysis, it can be found that when a fracturing scheme corresponding to a more complex fracturing crack morphology is adopted, the heat dissipation rate can be reduced by 5.34%, which can further improve the energy utilization efficiency. After simulation analysis, the reformed layer involved in the present invention can effectively improve the heat recovery rate and reduce heat dissipation when the following indicators are achieved:

[0122] (1) The transformed layer after fracturing can form a continuous fracture network;

[0123] (2) The thermal conductivity of the modified layer filled with thermal insulation beads is 2 orders of magnitude lower than that of the reservoir before modification.

[0124] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0125] It can be seen from the above embodiments that the present invention transforms the aquifer heat storage system through fracturing, and forms a heat insulation area in the transformed layer of the heat storage system, which can reduce the heat dissipation rate by 8.73%.

[0126] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered within the protection scope of the present invention.

Claims

1. Construction of underground heat storage system based on reservoir transformation and heat storage effect evaluation method, characterized in that: The method includes: S1, measure the physical parameters of each reservoir near the target aquifer and determine the heat storage layer and the reformed layer; S2, based on the determined transformation layer, a drilled rock sample specimen containing a center for fracturing and temperature measurement drilling is prepared, and a low-temperature fluid fracturing physical simulation experiment is carried out on the drilled rock sample specimen to obtain the permeability of the specimen after fracturing; proppant and thermal insulation microbeads are injected into the drilled rock sample specimen after fracturing, and the equivalent thermal conductivity coefficient of the drilled rock sample specimen containing proppant and thermal insulation microbeads after transformation is obtained through a thermal conductivity performance experiment; the compressive strength, Young's modulus and Poisson's ratio of the drilled rock sample specimen containing proppant and thermal insulation microbeads after transformation are obtained through uniaxial compression and triaxial compression tests of rocks; S3, changing the temperature, flow rate and injection volume of the low-temperature fracturing fluid to obtain the corresponding permeability under different low-temperature fluid fracturing schemes; for different low-temperature fluid fracturing schemes, obtaining reformed borehole rock sample specimens with different permeabilities, changing the injection volume of thermal insulation beads under the condition that the injection volume of the proppant remains unchanged, obtaining the corresponding equivalent thermal conductivity under different injection volumes of thermal insulation beads, and for reformed borehole rock sample specimens under different low-temperature fluid fracturing schemes and different injection volumes of thermal insulation beads, obtaining the corresponding compressive strength, Young's modulus and Poisson's ratio through rock uniaxial compression and triaxial compression tests; S4, based on the temperature-seepage-stress coupling model, conduct aquifer heat storage simulation, change the injection-production well spacing in the heat storage layer, the permeability in the reformed layer, and the equivalent thermal conductivity in the reformed layer, use the heat storage system production temperature, heat recovery rate and heat dissipation rate as comprehensive evaluation indicators of heat storage effect, and determine the reservoir reform plan for the injection-production well spacing in the heat storage layer and the reformed layer.

2. The method for constructing an underground heat storage system and evaluating heat storage effect based on reservoir transformation according to claim 1, characterized in that: In step S1, the physical parameters of each reservoir near the target aquifer are measured, including reservoir permeability, porosity, thermal conductivity, tensile and compressive strength, Young's modulus and Poisson's ratio.

3. The method for constructing an underground heat storage system and evaluating heat storage effect based on reservoir transformation according to claim 2, characterized in that: The permeability and porosity of the reservoir were measured multiple times and the average values ​​were taken. Abandoned oil reservoirs were selected as the target aquifers. Rock samples of various reservoirs near the target aquifers were obtained based on core drilling. The permeability of the cores was measured using a core holder, and the porosity of the cores was tested using a porosity meter. After multiple measurements, the average values ​​were calculated as the permeability and porosity of the rock samples of each reservoir. The permeability was 1×10 -10 m 2 Up to 1×10 -12 m 2 , the reservoir with a porosity greater than 0.15 is used as the heat storage layer, and the upper and lower adjacent layers of the heat storage layer are used as transformation layers.

4. The method for constructing an underground heat storage system and evaluating heat storage effect based on reservoir transformation according to claim 1, characterized in that: In step S2, the shape of the drilled rock sample is cylindrical, and a low-temperature fluid fracturing physical simulation experiment is carried out under the conditions of loading confining pressure and axial pressure using a true triaxial core holder. The low-temperature fluid fracturing physical simulation experiment uses liquid nitrogen or liquid carbon dioxide as a fracturing fluid. After fracturing transformation, the permeability of the sample after fracturing is measured; In the process of measuring the permeability of the test piece after fracturing, the pressure difference between the inlet and outlet of the test piece under the condition of fixed injection flow rate is measured, and its permeability is calculated according to Darcy's formula; after the permeability of the drilled rock sample after fracturing is measured, proppant and thermal insulation beads are injected into the drilled rock sample after fracturing; the drilled rock sample containing proppant and thermal insulation beads after fracturing is taken out from the true triaxial core holder, a heat source is loaded on the wall surface around the drilled rock sample after fracturing and insulation measures are taken on the outside of the heat source, temperature monitoring is performed at the central borehole of the drilled rock sample after fracturing, and the equivalent thermal conductivity of the drilled rock sample after fracturing and injection of thermal insulation beads is calculated based on the cylinder wall thermal conductivity formula; The heat conduction formula of the cylindrical wall is: Where λ is the equivalent thermal conductivity, in units of W / (m·K); r1 is the radius of the drilled rock sample after fracturing, in units of m; r2 is the radius of the borehole of the drilled rock sample after fracturing, in units of m; l is the length of the drilled rock sample after fracturing, in units of m; Δt is the temperature difference between the wall around the specimen and the borehole, in units of ℃.

5. The method for constructing an underground heat storage system and evaluating heat storage effect based on reservoir transformation according to claim 4 is characterized in that: In step S2, uniaxial compression and triaxial compression tests are carried out on the drilled rock sample specimens that have been transformed by cryogenic fluid and injected with proppants and thermal insulation beads to obtain the compressive strength, Young's modulus and Poisson's ratio of the transformed proppant and thermal insulation beads specimens.

6. The method for constructing an underground heat storage system and evaluating heat storage effect based on reservoir transformation according to claim 1, characterized in that: In step S3, the temperature, flow rate and injection volume of the low-temperature fracturing fluid are changed to obtain the corresponding permeability under different low-temperature fluid fracturing schemes, including: For the reformed specimens with different permeabilities obtained under different low-temperature fluid fracturing schemes, the equivalent thermal conductivity corresponding to different insulation bead injection amounts was obtained by changing the insulation bead injection amount under the condition of unchanged proppant injection amount; for the reformed specimens under different low-temperature fluid fracturing schemes and different insulation bead injection amounts, the corresponding compressive strength, Young's modulus and Poisson's ratio were obtained through rock uniaxial compression and triaxial compression tests.

7. The method for constructing an underground heat storage system and evaluating heat storage effect based on reservoir transformation according to claim 1, characterized in that: In step S4, based on the temperature-seepage-stress coupling model, aquifer heat storage simulation is carried out under different low-temperature fluid fracturing schemes and different insulation microbead injection conditions; The temperature-seepage-stress coupling model is composed of a static equilibrium equation considering fluid pressure and temperature changes, a temperature field control equation considering thermal convection and mechanical energy, and a seepage field equation considering stress and temperature effects; The static equilibrium equation considering the changes in fluid pressure and temperature is: Where G is the shear modulus, in Pa. E is the elastic modulus in Pa, v is Poisson's ratio; u i，jj ,u j，ji All of them are tensor representation methods. The subscript u represents the displacement in three directions, u, v, w, and the corresponding spatial coordinates are x, y, z. According to Einstein's summation convention, a is the Biot coefficient, pi is the pore fluid pressure, the unit is Pa; -K′a T T ，i is the temperature stress term caused by temperature change, K′ is the drainage bulk modulus of the medium, in Pa; a T is the thermal expansion coefficient of rock mass, in K -1 ; T ，i is the reservoir temperature in K, F i is the component of the volume force on the rock in the i direction, where the i direction is divided into the x, y, and z directions in the spatial coordinate system; The temperature field control equation considering thermal convection and mechanical energy is: Where, (ρC) M is the heat capacity of the porous medium containing the fluid, in kJ·m 3 ·K -1 ; ρ S is the density of porous medium bedrock, in kg / m 3 ; C S , C l are the porous medium bedrock coefficient and the specific heat coefficient of the fluid, in kJ·kg -1 ·K -1 ; is the porosity of the porous medium bedrock, ρ l is the fluid density, is the mechanical energy term, is the heat convection effect; T is the reservoir temperature, in K; T0 is the reference temperature under zero stress state, in K; ε T is the strain under the action of temperature, in m; is the gradient operator, is the pressure gradient, λ M is the thermal conductivity of the porous medium containing fluid, λ S ,λ l are the thermal conductivity of bedrock and fluid, respectively, in W / (m·K); u is the dynamic viscosity of the fluid, in Pa·s, kg / m 3 ; k is the permeability of the porous medium, in m 2 ; The seepage field equation considering stress and temperature is: In the formula, is the stress term, is the temperature effect term, is the pore pressure effect term, c1 is the stress effect term coefficient; c2 is the temperature effect term coefficient, c3 is the pore pressure effect term coefficient, ε v is the volume strain, in m, g is the gravitational acceleration, in m / s 2 , z is the vertical coordinate, unit is m; K S is the effective bulk modulus of the solid component, in Pa; a l is the volume thermal expansion coefficient of the fluid, in K -1 ; a S is the volume thermal expansion coefficient of the solid component, in K -1 ; β l is the bulk modulus of the pore fluid, in Pa.

8. The method for constructing an underground heat storage system and evaluating heat storage effect based on reservoir transformation according to claim 7, characterized in that: The aquifer heat storage simulation under different low-temperature fluid fracturing schemes and different insulation micro-bead injection conditions includes: establishing a heat reservoir and a transformed layer geometric model, assigning values ​​to the heat reservoir physical property parameters based on the obtained physical property parameters of each reservoir near the target aquifer, and assigning values ​​to the transformed layer physical property parameters based on the obtained physical property parameters of the transformed specimens under different low-temperature fluid fracturing schemes and different insulation micro-bead injection amounts; Based on the constructed temperature-seepage-stress coupling model, heat storage simulation is carried out to change the injection-production well spacing in the heat reservoir layer, the permeability in the transformed layer, and the equivalent thermal conductivity in the transformed layer. The production temperature, heat recovery rate and heat dissipation rate of the heat storage system are used as comprehensive evaluation indicators of the heat storage effect. Based on the obtained permeability and equivalent thermal conductivity of the transformed specimens under different low-temperature fluid fracturing schemes and different insulation microbead injection amounts, the injection-production well spacing in the heat reservoir layer and the reservoir transformation scheme of the transformed layer are determined; the reservoir transformation scheme of the transformed layer includes a low-temperature fluid fracturing scheme and an insulation microbead injection amount.

9. The method for constructing an underground heat storage system and evaluating heat storage effect based on reservoir transformation according to claim 8, characterized in that: The heat storage system extraction temperature, heat recovery rate and heat dissipation rate are used as comprehensive evaluation indicators of heat storage effect, including: The heat recovery rate is calculated as: Q 采 =∫V out ·ρ out ·c out ·ΔT out dt out Q 注 =∫V inj ·ρ inj ·c inj ·ΔT inj dt inj Where R is the heat recovery rate, Q 采 is the total heat extracted, in kJ; Q 注 is the total heat injected, in kJ; V out is the volume flow rate of produced fluid, in m 3 / d;ρ out is the density of produced fluid, in kg / m 3 ;c out is the specific heat capacity of the produced fluid, in J / (kg·K); ΔT out is the difference between the produced fluid temperature and the surface temperature, in K; dt out is the heat collection time, in d; V inj is the volume flow rate of injected fluid, in m 3 / d;ρ inj is the density of the injected fluid, in kg / m 3 ;c inj is the specific heat capacity of the injected fluid, in J / (kg·K); ΔT inj is the difference between the injected fluid temperature and the surface temperature, in K; dt inj is the heat injection time, in d; The heat dissipation rate calculation formula is: Q 耗 =∫c 非 m 非 ΔT 非 dt In the formula, H is the heat dissipation rate, Q 耗 is the heat dissipation in the heat storage system, in kJ; c 非 is the equivalent specific heat capacity of the porous medium in the non-heat storage area, in J / (kg·K); c 非s is the specific heat capacity of bedrock in the non-heat storage area, in J / (kg·K); c 非l is the specific heat capacity of pore water in the non-heat storage area, in J / (kg·K); is the rock porosity in the non-heat storage area, ΔT 非 is the temperature change in the non-heat storage area, in K; t is the total operating time of the heat storage system, in d; m 非 is the mass of rock in the non-heat storage area, in kg.

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