Hot dry rock terrestrial heat extraction method efficiently adapting to complex heat storage conditions

Abstract

The invention provides a hot dry rock terrestrial heat extraction method capable of efficiently adapting to complex heat storage conditions. The method comprises the steps that a heat-fluid-solid coupling relation in the terrestrial heat exploitation process is established, a dynamic porosity and permeability model is established based on the heat-fluid-solid coupling relation, and a terrestrial heat exploitation model is established based on the porosity and permeability model. According to the method, the heat extraction efficiency and the accumulated heat are remarkably improved, the heat competition effect between the production wells is reduced, the service life of the production wells is prolonged, the mining requirements under the complex geological conditions are met, the prediction precision of the heat-fluid-solid model is improved by dynamically adjusting the reservoir characteristics such as porosity and permeability, and the mining efficiency is improved. The problems of high thermal competition, inaccurate prediction and low mining efficiency in the prior art are solved, and a sustainable optimization scheme with high economical efficiency is provided for geothermal exploitation of the hot dry rock.

Description

Technical Field

[0001] The present invention relates to the technical field of hot dry rock geothermal research, and in particular to a hot dry rock geothermal extraction method that is highly efficient and adaptable to complex heat storage conditions. Background Art

[0002] With the increase in global energy demand and the improvement of environmental protection awareness, how to develop clean and efficient renewable energy has become the focus of attention of all countries. As a clean energy with huge development potential, hot dry rock (HDR) has attracted widespread attention due to its rich reserves, high energy density and low carbon emissions. However, there are certain difficulties in the development of hot dry rock resources: they are often located deep in the earth's crust (generally more than 3,000 meters) and are under complex geological conditions of high temperature (more than 200°C), high pressure and low permeability. Therefore, in order to efficiently and safely develop this underground resource, it is urgently necessary to conduct in-depth research and technological innovation on hot dry rock geothermal mining technology.

[0003] The core of hot dry rock geothermal development is to inject cold water, generate heat exchange through the injection well into the fracture network, and then recover hot fluid through the production well to achieve geothermal extraction. The multi-physical field coupling involved in this process (thermal field-fluid seepage field-rock mechanics field, that is, heat-fluid-solid coupling) is highly complex. Traditional research has focused on single well (single injection and single production) solutions, but its utilization efficiency of the fracture network is low, which is not conducive to achieving high-capacity extraction. In addition, the thermal expansion or thermal stress changes of the rock mass caused by the water injection process in geothermal development cause the porosity and permeability to evolve dynamically, further increasing the difficulty of simulation and design.

[0004] In existing research, some international scholars have proposed a multi-well layout scheme for the Enhanced Geothermal System (EGS) to solve the problems of insufficient heat conduction area and serious thermal interference between wells in the single-well mode. The more typical schemes currently include:

[0005] Single injection and single production well arrangement: This mode is usually used in preliminary experimental research, and heat exchange is achieved through the combination of a single injection well and a single production well. Although the production technology is simple and the cost is low, its heat recovery efficiency is limited, and the thermal breakthrough phenomenon (heat medium is not completely exchanged and lost) is significant, which cannot meet the needs of large-scale commercial development.

[0006] One injection and multiple production scheme: In order to improve the coverage of the fracture network, some studies have proposed the "one injection and two production" or even "one injection and multiple production" scheme, and the layout of injection and production wells has become more diverse. Typical layout patterns include linear distribution (injection and production wells are arranged along a straight line for homogeneous heat storage conditions) and triangular distribution (injection and production wells are arranged in a triangular shape to expand the coverage of the heat conduction area). In contrast, the multi-well arrangement of one injection and two production wells significantly improves the extraction rate of thermal resources, but the thermal competition effect (the superposition of the pressure fields of the two production wells leads to the uneven flow direction of the fluid) has become a new technical problem.

[0007] The existing linear and triangular well layout methods have limitations. The linear well layout has limited coverage under complex heat storage conditions, while the triangular well layout has a wider coverage, but the close distance between production wells leads to significant thermal interference, which cannot effectively avoid the thermal competition effect between wells, thereby reducing the heat extraction efficiency. At the same time, most of the existing well layout methods are designed for uniform or simple heat storage conditions, lacking the flexibility and adaptability to complex geological structures and diverse heat storage characteristics. Therefore, it is very necessary to design a dry hot rock geothermal extraction method that is efficient and adaptable to complex heat storage conditions. Summary of the invention

[0008] In order to overcome the deficiencies of the prior art, the object of the present invention is to provide a method for extracting hot dry rock geothermal energy that is highly efficient and adaptable to complex heat storage conditions.

[0009] To achieve the above object, the present invention provides the following solutions:

[0010] The present invention provides a method for extracting hot dry rock geothermal heat that is highly efficient and adaptable to complex heat storage conditions, comprising:

[0011] Establish the thermal-fluid-solid coupling relationship in geothermal exploitation;

[0012] Establish a dynamic porosity and permeability model based on the thermal-fluid-solid coupling relationship;

[0013] A geothermal extraction model is established based on the porosity and permeability model.

[0014] Preferably, the thermal-fluid-solid coupling relationship includes a stress field control equation, a seepage field control equation and a temperature field control equation.

[0015] Preferably, the process of establishing the stress field control equation is:

[0016] Assume that the strain-displacement equation is expressed as:

[0017]

[0018] In the formula, ε ij is the strain tensor component, u i,j is the displacement component, ignoring the inertia force, the equilibrium equation is expressed as:

[0019] σ ij,j +f i =0 (2)

[0020] In the formula, σ ij,j is the stress tensor component, f i is the volume force component;

[0021] Assuming that the thermal strain caused by temperature is only positive strain and the thermal strain is isotropic, the rock strain is expressed as the sum of the thermal strain caused by temperature change and the stress-induced strain, which is:

[0022]

[0023] In the formula, σ KK =σ 11 +σ 22 +σ 33 is the stress invariant, is the Biot coefficient, K is the bulk modulus of the rock mass, K s is the bulk modulus of the rock matrix, G is the shear modulus, α T is the thermal expansion coefficient, P is the fluid pressure, T is the temperature, δ ij is the Kronecker symbol, when i=j,δ ij =1, i≠j when δ ij =0;

[0024] The stress field control equation is obtained by combining the equations:

[0025]

[0026] In the formula, αP ,i It reflects the influence of fluid seepage on rock deformation, α T KT ,i It reflects the effect of temperature change on rock deformation;

[0027] Preferably, the process of establishing the seepage field control equation is:

[0028] The mass balance equation of the fluid is established as:

[0029]

[0030] In the formula, ρ w is the density of the liquid, q w is the Darcy velocity vector, Q s is the source and sink term of the fluid, m is the mass of the fluid in the unit volume of the rock mass, and its expression is m = ρ w φ, where φ is the porosity, which is related to the volumetric strain of the rock mass;

[0031] Assuming that the gravity effect is ignored, Darcy's law can be expressed as:

[0032]

[0033] Where u is the fluid dynamic viscosity, k is the permeability, and the simultaneous equations give:

[0034]

[0035] Preferably, the process of establishing the temperature field control equation is:

[0036] The temperature field control equation of the rock mass skeleton is obtained from the basic laws of thermodynamics:

[0037]

[0038] In the formula, ρ s is the density of the rock mass skeleton, c s is the constant pressure specific heat capacity of the rock mass skeleton, α T is the body thermal expansion coefficient, λ s is the thermal conductivity of the rock mass skeleton, Q Ts is the heat source strength of the rock skeleton. Since the porosity of the rock skeleton is φ<<1, 1-φ≈1. Simplified, we can get:

[0039]

[0040] For the fluid, the corresponding governing equation of the temperature field is:

[0041]

[0042] In the formula, ρ w is the fluid density, c w is the constant pressure specific heat capacity of the fluid, λ w is the thermal conductivity of the fluid, φQ Tw is the heat source intensity of the fluid;

[0043] Assuming that the solid and the fluid are always in thermal equilibrium, the temperature field control equation of the fluid flowing in the thermal reservoir is obtained as follows:

[0044]

[0045] In the formula, the left side of the equal sign is the rate of change of the system's internal energy, the first term on the right side of the equal sign is the rate of change of thermal strain energy, the second term is the convection term, the third term is the heat conduction term, and the fourth term is the heat source term. Among them, Q T =Q Ts +φQ Tw ;

[0046] Preferably, a dynamic porosity and permeability model is established based on the thermal-fluid-solid coupling relationship, specifically:

[0047] From the definition of porosity, we know that:

[0048]

[0049] According to the above formula, we can get:

[0050]

[0051] In the formula, Represents the porosity, V B Represents the rock mass volume, V S is the rock mass skeleton volume, V p is the pore volume, the first term on the right side of the equal sign represents the overall strain increment, and the second term represents the skeleton strain increment;

[0052] The strain increment of the matrix is ​​mainly caused by two aspects: the thermal strain increment caused by thermoelastic expansion The strain increment caused by the compression of the rock skeleton by the fluid pressure change From this we can see that the relationship between the three is:

[0053]

[0054] In the formula,

[0055]

[0056] Combining the above equations, we can get the strain increment of the rock skeleton as:

[0057]

[0058] Where α is the biot coefficient of the rock mass skeleton, K s is the bulk modulus of the rock mass skeleton, K is the bulk modulus of the rock mass skeleton, α T is the bulk thermal expansion coefficient of the rock mass;

[0059] The strain increment of the entire rock mass is:

[0060]

[0061] According to the above formula, we can get:

[0062]

[0063] Integrating it as a whole, we get:

[0064]

[0065] After calculation, we can get:

[0066]

[0067] Rearranging the above formula, we get:

[0068]

[0069] Simplifying it, we get:

[0070]

[0071] Among them, the above formula is the dynamic model of porosity;

[0072] Substituting the dynamic model of porosity into the Kozeny-Kalman model, the dynamic permeability model is obtained:

[0073]

[0074] The Kozeny-Kalman model is:

[0075]

[0076] Preferably, the geothermal exploitation model includes a triangular three-well model and a linear three-well model.

[0077] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0078] The present invention provides a hot dry rock geothermal extraction method that is highly adaptable to complex heat storage conditions, the method comprising establishing a thermal-fluid-solid coupling relationship in the geothermal exploitation process, establishing a dynamic porosity and permeability model based on the thermal-fluid-solid coupling relationship, and establishing a geothermal exploitation model based on the porosity and permeability model. The present invention significantly improves the heat extraction efficiency and cumulative heat, reduces the thermal competition effect between production wells, prolongs the life of production wells, and adapts to the exploitation needs under complex geological conditions. By dynamically adjusting reservoir characteristics such as porosity and permeability, the prediction accuracy of the thermal-fluid-solid model is improved, and the problems of strong thermal competition, inaccurate prediction and low exploitation efficiency in the prior art are solved, providing an economical and sustainable optimization solution for hot dry rock geothermal development. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0080] Figure 1 It is a schematic diagram of the coupling relationship between the three field equations of heat, fluid and solid;

[0081] Figure 2 It is a schematic diagram of a typical one-dimensional coupling numerical example;

[0082] Figure 3 Schematic diagram of pore pressure change at point P;

[0083] Figure 4 Schematic diagram of temperature change at point P;

[0084] Figure 5 Schematic diagram of the geometric model of the triangular three-well model and the linear three-well model (a, triangular three-well model; b, linear three-well model);

[0085] Figure 6 It is the distribution cloud diagram of temperature in the thermal reservoir (linear three-well model);

[0086] Figure 7 It is the distribution cloud diagram of thermal reservoir temperature (triangular three-well model);

[0087] Figure 8 Schematic diagram of geothermal extraction variation of triangular three-well model and linear three-well model;

[0088] Fig. 9 It is a schematic diagram of the variation of permeability ratio at different monitoring points;

[0089] Fig.10 Schematic diagram of the changing law of geothermal extraction rate of the triangular three-well model and the linear three-well model;

[0090] Fig.11 It is a schematic diagram of the changing law of geothermal extraction rate under different mining conditions of the triangular three-well model;

[0091] Fig.12 It is a schematic diagram of the variation of the cumulative extracted heat of the triangular three-well model and the linear three-well model;

[0092] Fig.13 A flow chart of a method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0093] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0094] The purpose of the present invention is to provide a hot dry rock geothermal extraction method that is highly efficient and adaptable to complex heat storage conditions, significantly improves heat extraction efficiency and accumulated heat, reduces the thermal competition effect between production wells, prolongs the life of production wells, and adapts to mining needs under complex geological conditions. By dynamically adjusting reservoir characteristics such as porosity and permeability, the prediction accuracy of the thermal fluid-solid model is improved, and the problems of strong thermal competition, inaccurate prediction and low mining efficiency in the prior art are solved, providing an economical and sustainable optimization solution for hot dry rock geothermal development.

[0095] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0096] FIG* is a flow chart of a method provided by an embodiment of the present invention. As shown in FIG*, the present invention provides a method for extracting hot dry rock geothermal energy that is highly efficient and adaptable to complex heat storage conditions, including:

[0097] Step 100: Establishing the thermal-fluid-solid coupling relationship in the geothermal exploitation process;

[0098] Step 200: Establishing a dynamic porosity and permeability model based on the thermal-fluid-solid coupling relationship;

[0099] Step 300: Establish a geothermal production model based on the porosity and permeability model.

[0100] In step 100, the thermal-fluid-solid coupling relationship in the geothermal exploitation process is established, specifically:

[0101] The thermal-fluid-solid coupling model of hot dry rock is strongly coupled, and the various factors interact and influence each other, which makes the relationship between the stress field complicated and difficult to accurately describe the entire physical process. Therefore, a clear understanding of the relationship between thermal-fluid-solid coupling is crucial for model establishment; the thermal-fluid-solid coupling relationship in geothermal reservoirs can be roughly expressed as Figure 1 illustrate, Figure 1 The coupling relationship between the various fields is specifically manifested as follows: ① The influence of thermal strain caused by temperature change on the strain field of the rock skeleton; ② The influence of heat generated by the internal dissipation of the deformation energy of the rock skeleton on the temperature field; ③ The influence of fluid thermal convection on the distribution of the temperature field; ④ The change of fluid seepage caused by temperature change; ⑤ The change of fluid pressure changes the effective stress and affects the deformation of the rock skeleton; ⑧ The deformation of the rock skeleton changes the porosity and permeability, thereby affecting the fluid;

[0102] In order to have an overall understanding of the THM coupling effect in the geothermal mining process, it is necessary to first establish a group of partial differential equations for the coupling process. Since the THM coupling effect in geothermal mining is an extremely complex physical effect, a series of reasonable assumptions can be introduced in the process of establishing the group of partial differential equations to simplify the numerical calculation. The present invention analyzes and summarizes the reasonable assumptions and basic laws used in the previous research process and introduces the following assumptions:

[0103] 1. The thermal reservoir rock mass is a homogeneous and isotropic porous medium, and its deformation is small;

[0104] 2. The flow of fluid in the reservoir is saturated single-phase flow and follows Darcy's law;

[0105] 3. The inertial force of fluid seepage movement and rock deformation movement, and the volume force of fluid are negligible;

[0106] 4. Ignore the effect of temperature on fluid density, dynamic viscosity, and heat transfer parameters of rock and fluid;

[0107] 5. Solids and fluids are always in thermal equilibrium;

[0108] 6. The transfer of heat in rock mass only considers heat conduction, not heat radiation;

[0109] In summary, the thermal-fluid-solid coupling relationship includes the stress field control equation, the seepage field control equation and the temperature field control equation, which are introduced in detail:

[0110] 1. The process of establishing the stress field control equation is as follows:

[0111] Assume that the strain-displacement equation is expressed as:

[0112]

[0113] In the formula, ε ij is the strain tensor component, u i,j is the displacement component, ignoring the inertia force, the equilibrium equation is expressed as:

[0114] σ ij,j +f i =0 (2)

[0115] In the formula, σ ij,j is the stress tensor component, f i is the volume force component;

[0116] Assuming that the thermal strain caused by temperature is only positive strain and the thermal strain is isotropic, the rock strain is expressed as the sum of the thermal strain caused by temperature change and the stress-induced strain, which is:

[0117]

[0118] In the formula, σ KK =σ 11 +σ 22 +σ 33 is the stress invariant, is the Biot coefficient, K is the bulk modulus of the rock mass, K s is the bulk modulus of the rock matrix, G is the shear modulus, α T is the thermal expansion coefficient, P is the fluid pressure, T is the temperature, δ ij is the Kronecker symbol, when i=j,δ ij =1, i≠j when δ ij =0;

[0119] Combining formulas (1) to (3), we get the stress field control equation:

[0120]

[0121] Formula (4) is the governing equation for high temperature rock mass deformation, αP ,i It reflects the influence of fluid seepage on rock deformation, α T KT ,i It reflects the effect of temperature change on rock deformation;

[0122] 2. The process of establishing the seepage field control equation is as follows:

[0123] The mass balance equation of the fluid is established as:

[0124]

[0125] In the formula, ρ w is the density of the liquid, q w is the Darcy velocity vector, Q s is the source and sink term of the fluid, m is the mass of the fluid in the unit volume of the rock mass, and its expression is m = ρ w φ, where φ is the porosity, which is related to the volumetric strain of the rock mass;

[0126] Assuming that the gravity effect is ignored, Darcy's law can be expressed as:

[0127]

[0128] Where u is the fluid dynamic viscosity, k is the permeability, and the simultaneous equations give:

[0129]

[0130] 3. The process of establishing the temperature field control equation is:

[0131] The temperature field control equation of the rock mass skeleton is obtained from the basic laws of thermodynamics:

[0132]

[0133] In the formula, ρ s is the density of the rock mass skeleton, c s is the constant pressure specific heat capacity of the rock mass skeleton, α T is the body thermal expansion coefficient, λ s is the thermal conductivity of the rock mass skeleton, Q Ts is the heat source strength of the rock skeleton. Since the porosity of the rock skeleton is φ<<1, 1-φ≈1. Therefore, simplifying formula (5) yields:

[0134]

[0135] For the fluid, the corresponding governing equation of the temperature field is:

[0136]

[0137] In the formula, ρ w is the fluid density, c w is the constant pressure specific heat capacity of the fluid, λ w is the thermal conductivity of the fluid, φQ Tw is the heat source intensity of the fluid;

[0138] Assuming that the solid and the fluid are always in thermal equilibrium, the temperature field control equation of the fluid flowing in the thermal reservoir is obtained by superimposing formula (6) and formula (7):

[0139]

[0140] In formula (8), the left side of the equal sign is the rate of change of the system internal energy, the first term on the right side of the equal sign is the rate of change of thermal strain energy, the second term is the convection term, the third term is the heat conduction term, and the fourth term is the heat source term. T =Q Ts +φQ Tw ;

[0141] In step 200, a dynamic porosity and permeability model is established based on the thermal-fluid-solid coupling relationship, specifically:

[0142] Porosity plays an important role in the mechanical parameters of porous media. Classical seepage mechanics focuses on the study of seepage characteristics of porous media. It is generally believed that the skeleton of porous media does not deform, so the traditional three-field coupling theory regards porosity as a constant. However, this assumption is obviously not in line with reality, because the compression deformation caused by changes in geostress and fluid pressure in geothermal reservoirs and the thermal expansion deformation caused by changes in reservoir temperature will cause the skeleton of porous media to deform to a certain extent. As the depth of the reservoir increases, the geostress, fluid pressure and temperature will change to varying degrees, resulting in corresponding changes in porosity.

[0143] From the definition of porosity, we know that:

[0144]

[0145] According to the above formula, we can get:

[0146]

[0147] In the formula, Represents the porosity, V B Represents the rock mass volume, V S is the volume of rock mass skeleton, V p is the pore volume. In formula (13), the first term on the right side of the equal sign represents the overall strain increment, and the second term represents the strain increment of the skeleton;

[0148] The strain increment of the matrix is ​​mainly caused by two aspects: the thermal strain increment caused by thermoelastic expansion The strain increment caused by the compression of the rock skeleton by the fluid pressure change From this we can see that the relationship between the three is:

[0149]

[0150] In the formula,

[0151]

[0152] Combining formulas (14)-(16), we get the strain increment of the rock mass skeleton:

[0153]

[0154] Where α is the biot coefficient of the rock mass skeleton, K s is the bulk modulus of the rock mass skeleton, K is the bulk modulus of the rock mass skeleton, α T is the bulk thermal expansion coefficient of the rock mass;

[0155] The strain increment of the entire rock mass is:

[0156]

[0157] Substituting formula (17) and (18) into formula (13), we can obtain:

[0158]

[0159] By integrating (19), we obtain:

[0160]

[0161] After calculation, we can get:

[0162]

[0163] After rearranging (21), we obtain:

[0164]

[0165] According to the small deformation assumption in the basic assumptions of the theory, equation (22) can be simplified as follows:

[0166]

[0167] Among them, formula (23) is the dynamic model of porosity. From formula (23), it can be seen that the porosity of rock is not only related to the initial porosity It is related to the water pressure in the rock, the rock temperature, and the rock mass strain;

[0168] Substituting the dynamic model of porosity into the Kozeny-Kalman model, the dynamic permeability model is obtained:

[0169]

[0170] From formula (25), it can be seen that rock permeability is not only related to initial permeability, but also to initial porosity, water pressure in rock, rock temperature, and rock body strain;

[0171] Among them, the Kozeny-Carman model is the most widely used permeability model. Based on this and the dynamic porosity model, the dynamic permeability model can be obtained. The Kozeny-Carman model is:

[0172]

[0173] In step 300, the geothermal production model includes a triangular three-well model and a linear three-well model.

[0174] The present invention provides a numerical solution example:

[0175] Based on the control equations of thermal-hydraulic-mechanical full coupling of porous media, Bai Bing analytically solved the partial differential equations of the three-field coupling model in one-dimensional case through finite Fourier transform and its inverse transform, and obtained the one-dimensional expressions of temperature and pressure as follows:

[0176]

[0177] The temperature and pressure at any point in one-dimensional condition can be obtained according to the above two equations. Different from the model established in the present invention, the model in the literature fails to consider the dynamic change of permeability, which is inconsistent with the actual situation.

[0178] like Figure 2 As shown, it is a schematic diagram of the three-field coupling calculation example in one-dimensional case. In the model, the upper boundary is a uniform load of 0.1MPa, the temperature is 60℃, the lower boundary is a fixed boundary, the initial pore pressure is 0.1MPa, and the initial temperature is 10℃. The main calculation parameters are shown in Table 1;

[0179] Table 1 Simulation physical parameters

[0180]

[0181] The model constructed by the present invention is used to simulate and solve the example, and the results obtained by the simulation are compared with the results obtained by calculating by equations (26) and (27). For the convenience of explanation, Figure 2 The pressure and temperature changes at point P in the middle are taken as the research objects;

[0182] like Figure 3 and Figure 4 As shown in Figure 2, they are the variation curves of pore pressure and temperature at point P. Figure 3 It is not difficult to see that the simulated value of pore pressure change is slightly greater than the theoretical value. This is because during the numerical simulation, the temperature of the column has been rising, the porous medium has expanded due to heat, the porosity has decreased, and the permeability has decreased. Since the thermal conductivity coefficient has not changed during the calculation process, Figure 4 As shown in the figure, the numerical simulation results of temperature change are in good agreement with the analytical solution calculation results.

[0183] like Figure 5The following are two different well layouts of the three-well model in the simulation. In the triangular three-well model, the production wells are all located on the upper side of the injection wells; in the linear three-well model, the production wells are located on the upper and lower sides of the injection wells. In the triangular three-well model, the injection well coordinates are (30, 30), and the production well coordinates are (25, 39) and (35, 39), respectively. In the linear three-well model, the injection well coordinates are (30, 40), and the production well coordinates are (30, 20) and (30, 60), respectively. In order to ensure the comparability of the model, the distance between the production and injection wells in each well layout is 20m, and the model size is 60mx80m.

[0184] When geothermal extraction begins, the injection well pressure is 29MPa, the injection temperature is 20℃, the production well pressure is 21MPa, and the production well temperature is calculated. The initial pressure of the geothermal reservoir is 24MPa, and the initial temperature is 200℃. The remaining boundaries are no flow boundaries and no heat conduction boundaries. There is a uniform load of 65MPa at the top of the model, the left and right boundaries are fixed x-direction displacement boundary conditions, the bottom boundary is a fixed y-direction displacement boundary condition, and the wellhead is a fixed displacement boundary.

[0185] The focus of the simulation is to analyze the impact of two different well layout methods on geothermal exploitation. By analyzing the variation of extraction temperature, reservoir permeability, geothermal extraction rate and cumulative extracted heat in different models, a reasonable well layout method is finally obtained. The corresponding simulation parameters are shown in Table 2.

[0186] Table 2 Numerical simulation parameters

[0187]

[0188] Analysis of the simulation results of the model:

[0189] Reservoir temperature distribution: Figure 6-7 These are the temperature distribution cloud diagrams in the thermal reservoir of the triangular three-well model and the linear three-well model. It is not difficult to see from the figure that as geothermal extraction proceeds, the low-temperature area in the reservoir gradually spreads from the injection well to the surrounding area, and due to the effect of the seepage field, the diffusion direction of the low-temperature area tends to the direction of the production well. Taking the triangular three-well model as an example, in the 20th year, the production well has been affected by exploitation, and the reservoir temperature distribution presents an "olive" shape.

[0190] Due to the symmetry of the production wells in the triangular three-well model, only the production wells on the left are taken for research. The production well on the upper side of the linear three-well model is called P1 well, and the production well on the lower side is called P2 well. Figure 8The figure shows the changing trend of the geothermal extraction temperature in the production wells in the triangular three-well model and the linear three-well model. Since the geothermal gradient is taken into account, the initial temperature distribution in the reservoir increases with the increase of reservoir depth. Therefore, the initial geothermal extraction temperature will have obvious differences with the change of the buried depth of the production well. Figure 5 It can be seen that the burial depth of P2 well in the linear three-well model is greater, while the burial depth of P1 well is shallower, so the initial extraction temperature of P2 well in the linear three-well model is the highest, about 199.70℃, and the initial extraction temperature of P1 well is the lowest, about 198.80℃. In addition, the geothermal extraction temperature of the production wells in the triangular three-well model and the P1 production well in the linear three-well model shows a change pattern of first rising and then falling after 6 years of exploitation. The geothermal extraction temperature of the P2 production well in the linear three-well model is always decreasing.

[0191] This phenomenon is caused by the fact that the production wells in the triangular three-well model and the P1 production well in the linear three-well model are located above the injection well relative to the injection well. The temperature of the rock mass around the injection well in the thermal reservoir is higher than that around the production well. Therefore, in the initial geothermal extraction, the heat extraction fluid will carry a large amount of heat from the bottom to heat the reservoir near the production well, making the extraction temperature higher than the initial temperature. The geothermal extraction temperature of the production well in the triangular three-well model reaches a maximum of about 199.20℃ when the geothermal system has been running for about 3 years. The geothermal extraction temperature of P1 in the linear three-well model reaches a maximum of about 198.80℃ when the geothermal system has been running for about 2.25 years. The P2 production well in the linear three-well model is located below the injection well. The initial temperature around the P2 well in the reservoir is higher than the initial temperature around the injection well. Therefore, the initial geothermal extraction temperature of the P2 production well is relatively high, but it cannot be compensated by heat from the injection well.

[0192] After 5 years, affected by the heat exchange of the heat extraction medium, the geothermal extraction temperature in the production wells began to drop significantly. Among them, the extraction temperature of the production well P2 in the linear three-well model dropped the most. After 30 years of operation, the geothermal extraction temperature was 142.78℃, which was the lowest among the three production wells. The production well in the three-well model 1 was followed by the geothermal extraction temperature of 143.18℃ after 30 years of operation. The extraction temperature of the production well P1 in the linear three-well model was 149.84℃ in the 30th year. This is because gravity is considered in the vertical direction. Taking the linear three-well model as an example, since the production well P1 is above the injection well and the production well P2 is below the injection well, the seepage velocity of the heat extraction medium to the upper production well is relatively low, and the lower production well is more obviously affected by the seepage, so its temperature drops faster.

[0193] Permeability change law: Fig. 9It is the permeability ratio variation law of the midpoint between the production well and the injection well in the triangular three-well model and the linear three-well model. For the convenience of description, the midpoint between the production well and the injection well in the triangular three-well model is named point A, the midpoint between the production well and the injection well P1 in the linear three-well model is named point B, and the midpoint between the production well and the injection well P2 in the linear three-well model is named point C. It can be seen from the figure that the permeability variation law of the three points is consistent: the permeability remains stable at the beginning of exploitation, and then gradually increases. In the 30th year of production, the permeability of point A increased by 30.92% compared with the initial stage, point B increased by 29.52% compared with the initial stage, and point C increased by 30.59% compared with the initial stage. The reason for this change is that in the early stage of geothermal extraction, the temperature of the three monitoring points was not affected by geothermal extraction, and the permeability at these three locations remained stable; with the progress of geothermal exploitation, the temperature at the three monitoring points in the reservoir continued to decrease, causing the rock matrix to be restricted and contracted, resulting in an increase in porosity, thereby increasing the permeability. It can be seen that temperature change is the main factor causing permeability change.

[0194] Taking the linear three-well model as an example, the permeability change trend of points B and C is analyzed. Point B is located above the injection well, and point C is below the injection well. Due to the influence of the gravity gradient, the heat recovery medium tends to flow to the production well below, resulting in the reservoir temperature at point C being lower than that at point B in the same period of time, so the permeability of point C is higher than that of point B. It can be seen that in the same model, the change in temperature is the main factor affecting the change in permeability. The initial reservoir temperature at point A was 199.20℃, and the reservoir temperature after 30 years was 42.52℃, with a temperature difference of 156.68℃. The initial temperature of point C was 198.75℃, and the temperature after 30 years was 44.55℃, with a temperature difference of 154.20℃, which is lower than the temperature difference at point A, so the permeability of point A was higher than that of point C during the same period.

[0195] Geothermal output: Fig.10 The figure is the change curve of the geothermal extraction rate of the triangular three-well model and the linear three-well model. It can be seen from the figure that the change law of the geothermal extraction rate of the two models during the geothermal extraction process is consistent: both show an ups and downs change law of first increasing and then decreasing. In the same period, the geothermal extraction rate of the triangular three-well model is always lower than that of the linear three-well model.

[0196] Taking the linear three-well model as an example, the previous analysis shows that the extraction temperature of the triangular three-well model remains stable in the early stage of mining, while the mass flow rate always gradually increases under constant pressure mining. Therefore, in the early stage of mining, the geothermal extraction rate of the linear three-well model increases rapidly, and around the 9th year, the geothermal extraction rate reaches a maximum of about 10.93kW. From then until the 30th year, the extraction temperature of the two production wells in the triangular three-well model began to drop significantly. The slow increase in mass flow rate could not make up for the impact of the rapid drop in extraction temperature, and the geothermal extraction rate began to decline. In the 30th year, the geothermal extraction rate was about 7.59kW, which was 30.56% lower than the peak period.

[0197] Fig.11 It is the changing law of the geothermal extraction rate of each production well when one production well is closed and two production wells are producing at the same time in the triangular three-well model. It can be clearly seen from the figure that when only one production well is opened, the geothermal extraction rate of each production well is significantly higher than the geothermal extraction rate when the two production wells are opened at the same time. This shows that when the two production wells are working at the same time, the working efficiency of each production well is reduced, that is, the two production wells have a mutual influence, which reduces the geothermal extraction rate. It is precisely because of the interaction between the production wells that the geothermal extraction rate of the triangular three-well model is lower than that of the linear three-well model. It can be seen that the distance between the production wells is too close, which will reduce the geothermal extraction rate of each well, and placing the production wells on both sides of the injection well can effectively avoid this situation.

[0198] Fig.12 The cumulative heat extraction of the triangle three-well model and the linear three-well model changes. Taking 30 years of production as an example, the cumulative heat extraction of the triangle three-well model is 218.74MW·h, and the cumulative heat extraction of the linear three-well model is 243.787MW·h. The latter extracts 11.49% more heat than the former. This shows that the layout of production wells has a significant impact on the cumulative heat extraction.

[0199] From the perspective of reservoir permeability, it can be seen that: in the geothermal extraction process, the permeability increase of the triangular three-well model is the largest. The two production wells in the triangular three-well model should have obtained a larger flow rate at the same time, so that the geothermal extraction rate of each production well is higher than that of the P1 production well and the P2 production well in the linear three-well model. However, due to the small distance between the production wells in the triangular three-well model, the geothermal extraction rate of each production well in the triangular three-well model has dropped significantly due to the mutual inhibition between the production wells, which ultimately leads to the cumulative extraction of heat in the triangular three-well model being lower than that of the linear three-well model in the same period. It can be seen that there are two factors that affect geothermal production capacity: one is the permeability of the reservoir. The increase in reservoir permeability will directly increase the geothermal extraction rate; the other is the well layout method. From the simulation of the present invention, it can be seen that if the distance between production wells is too close, they will inhibit each other's geothermal extraction rate.

[0200] In summary, the results show that during the 30-year exploitation period, the linear and triangular well layouts show obvious differences: the cumulative geothermal heat extraction of the linear three-well model reaches 243.787MW·h, which is 11.49% more than that of the triangular model, showing significant superiority. When the production well is located above the injection well, the heat extraction fluid can carry the heat of the bottom high-temperature rock mass to heat the upper rock mass, so that the initial extraction temperature of production shows a trend of first increasing and then decreasing, which prolongs the exploitation stability period and improves the life of the production well; when the production well is located below the injection well, the initial extraction temperature is high, but due to the influence of gravity, the later extraction temperature drops rapidly. In addition, during geothermal exploitation, the decrease in reservoir temperature causes matrix shrinkage and increased porosity, which improves the overall permeability. Among them, in terms of permeability ratio, the permeability of the monitoring point in the triangular model increased by 30.92% after 30 years, slightly higher than that of the monitoring point in the linear model (increased by 30.59%). In terms of geothermal extraction rate, the peak rate of the linear three-well model can reach 10.93kW, which is significantly higher than that of the triangular model. However, the triangular well layout is limited by the competition between production wells, and its cumulative production capacity is damaged and production efficiency is reduced. Reasonable well layout (such as arranging production wells on both sides of injection wells to increase the distance) can effectively reduce competition between wells and improve heat extraction efficiency. The decrease in reservoir temperature leads to matrix shrinkage and increased porosity, which in turn increases permeability. The permeability of the upper well layout increases slower than that of the lower well layout.

[0201] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0202] The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core ideas of the present invention. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for extracting hot dry rock geothermal heat efficiently and adapting to complex heat storage conditions, characterized in that: include: Establish the thermal-fluid-solid coupling relationship in geothermal exploitation; Establish a dynamic porosity and permeability model based on the thermal-fluid-solid coupling relationship; A geothermal extraction model is established based on the porosity and permeability model.

2. The method according to claim 1, characterized in that The thermal-fluid-solid coupling relationship includes a stress field control equation, a seepage field control equation and a temperature field control equation.

3. The method according to claim 2, characterized in that The process of establishing the stress field control equation is: Assume that the strain-displacement equation is expressed as: In the formula, ε ij is the strain tensor component, u i,j is the displacement component, ignoring the inertia force, the equilibrium equation is expressed as: s ij,j +f i =0 (2) In the formula, σ ij,j is the stress tensor component, f i is the volume force component; Assuming that the thermal strain caused by temperature is only positive strain and the thermal strain is isotropic, the rock strain is expressed as the sum of the thermal strain caused by temperature change and the stress-induced strain, which is: In the formula, σ KK =σ 11 +σ 22 +σ 33 is the stress invariant, is the Biot coefficient, K is the bulk modulus of the rock mass, K s is the bulk modulus of the rock matrix, G is the shear modulus, α T is the thermal expansion coefficient, P is the fluid pressure, T is the temperature, δ ij is the Kronecker symbol, when i=j, δ ij =1, i≠j when δ ij =0; The stress field control equation is obtained by combining the equations: In the formula, αP ,i It reflects the influence of fluid seepage on rock deformation, α T KT ,i It reflects the influence of temperature change on rock deformation.

4. The method according to claim 2, characterized in that: The process of establishing the seepage field control equation is as follows: The mass balance equation of the fluid is established as: In the formula, ρ w is the density of the liquid, q w is the Darcy velocity vector, Q s is the source and sink term of the fluid, m is the mass of the fluid in the unit volume of the rock mass, and its expression is m = ρ w φ, where φ is the porosity, which is related to the volumetric strain of the rock mass; Assuming that the gravity effect is ignored, Darcy's law can be expressed as: Where u is the fluid dynamic viscosity, k is the permeability, and the simultaneous equations give:

5. The method according to claim 2, characterized in that: The process of establishing the temperature field control equation is: The temperature field control equation of the rock mass skeleton is obtained from the basic laws of thermodynamics: In the formula, ρ s is the density of the rock mass skeleton, c s is the constant pressure specific heat capacity of the rock mass skeleton, α T is the body thermal expansion coefficient, λ s is the thermal conductivity of the rock mass skeleton, Q Ts is the heat source strength of the rock skeleton. Since the porosity of the rock skeleton is φ<<1, 1-φ≈1. Simplified, we can get: For the fluid, the corresponding governing equation of the temperature field is: In the formula, ρ w is the fluid density, c w is the constant pressure specific heat capacity of the fluid, λ w is the thermal conductivity of the fluid, φQ Tw is the heat source intensity of the fluid; Assuming that the solid and the fluid are always in thermal equilibrium, the temperature field control equation of the fluid flowing in the thermal reservoir is obtained as follows: In the formula, the left side of the equal sign is the rate of change of the system's internal energy, the first term on the right side of the equal sign is the rate of change of thermal strain energy, the second term is the convection term, the third term is the heat conduction term, and the fourth term is the heat source term. Among them, Q T =Q Ts +φQ Tw。 6. The method according to claim 3, characterized in that A dynamic porosity and permeability model is established based on the thermal-fluid-solid coupling relationship, specifically: From the definition of porosity, we know that: According to the above formula, we can get: In the formula, Represents the porosity, V B Represents the rock mass volume, V S is the volume of rock mass skeleton, V p is the pore volume, the first term on the right side of the equal sign represents the overall strain increment, and the second term represents the strain increment of the skeleton; The strain increment of the matrix is ​​mainly caused by two aspects: the thermal strain increment caused by thermoelastic expansion The strain increment caused by the compression of the rock skeleton by the fluid pressure change From this we can see that the relationship between the three is: In the formula, Combining the above equations, we can get the strain increment of the rock skeleton as: Where α is the biot coefficient of the rock mass skeleton, K s is the bulk modulus of the rock mass skeleton, K is the bulk modulus of the rock mass skeleton, α T is the bulk thermal expansion coefficient of the rock mass; The strain increment of the entire rock mass is: According to the above formula, we can get: Integrating it as a whole, we get: After calculation, we can get: Rearranging the above formula, we get: Simplifying it, we get: Among them, the above formula is the dynamic model of porosity; Substituting the dynamic model of porosity into the Kozeny-Kalman model, the dynamic permeability model is obtained: The Kozeny-Kalman model is:

7. The method according to claim 1, characterized in that The geothermal exploitation model includes a triangular three-well model and a linear three-well model.

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