Method for analyzing influence of rock mass multi-scale structure on deep geothermal reservoir circulation mining

Abstract

The invention discloses a method for analyzing the influence of a rock mass multi-scale structure on deep geothermal reservoir circular mining, and relates to the technical field of geothermal resource development and rock mechanics, and the method comprises the following steps: establishing a reservoir rock mass mechanical analysis model and a seepage heat transfer analysis model which simultaneously comprise a microscopic microcrack structure and a macroscopic crack structure; based on the reservoir seepage heat transfer analysis model, reservoir temperature field and seepage field analysis is carried out, and water pressure and temperature distribution results are transmitted to the mechanical analysis model; based on the reservoir mechanical analysis model, reservoir stress and deformation analysis under the heat-water-force coupling effect is carried out, and changes of microcosmic microcrack and macroscopic crack structures in the reservoir rock mass are obtained; the rock mass permeability evolution equation and the porosity evolution equation are adopted to calculate the reservoir rock mass permeability tensor and the porosity, and the reservoir rock mass permeability tensor and the porosity are transmitted to the seepage heat transfer analysis model; and solving the reservoir temperature field, the water pressure field and the mechanical response in the geothermal circulation exploitation process by adopting a seepage heat transfer analysis and mechanical analysis staggered iteration mode. By analyzing the reservoir temperature, the water pressure, the deformation evolution response and the reservoir permeability change in the geothermal exploitation process, geothermal exploitation design and protection can be reasonably carried out, and the application prospect is good.

Description

Technical Field

[0001] The present invention relates to the field of intersectional technology between geothermal resource development and rock mechanics, and in particular to a method for analyzing the influence of multi-scale rock mass structure on cyclic exploitation of deep geothermal reservoirs. Background Art

[0002] In recent years, with the transition to a low-carbon energy structure, geothermal energy, a renewable energy source with abundant reserves and high stability, has seen a sharp increase in demand for its use in power generation and heating. Enhanced geothermal systems, in particular, have become a core method for developing deep geothermal resources by activating natural fracture networks in deep, low-permeability rock formations through artificial fracturing.

[0003] Deep geothermal reservoirs are the primary geologic environment for heat transfer and flow in geothermal production. Reservoir rock masses exhibit multiscale structural characteristics, encompassing fracture networks extending from several centimeters to tens of meters in length, as well as microcracks ranging from micrometers to millimeters distributed within the rock blocks cut by these fracture networks. Existing studies often primarily consider the effects of highly permeable and easily deformable fracture structures on reservoir seepage heat transfer, while ignoring the influence of structural variations within the rock blocks. This is true in engineering scenarios where fracture structure dominates reservoir permeability. However, in actual reservoir engineering, where the contribution of microcrack structural variations caused by rock block damage to rock permeability is comparable to that of fracture structure, ignoring the multiscale structural characteristics of the reservoir rock mass makes it difficult to obtain comprehensive and reasonable results in simulations of deep geothermal reservoir cyclic production processes, leading to prediction errors and potential safety hazards in reservoir production design. Existing models struggle to simultaneously account for the impact of multiscale structural variations in the reservoir rock mass on seepage heat transfer under the coupled thermal, hydraulic, and mechanical conditions. Summary of the Invention

[0004] In order to overcome the defects in the above-mentioned prior art, the present invention provides an analysis method for the influence of the multi-scale structure of the rock mass on the cyclic exploitation of deep geothermal reservoirs. According to the microscopic microcrack structure and macroscopic fracture structure characteristics inside the surrounding rock of the deep geothermal reservoir, the damage deformation process induced by the multi-scale structure of the reservoir under thermal-hydraulic-mechanical coupling is coupled with the seepage heat transfer process. By analyzing the reservoir temperature, water pressure and deformation evolution response and reservoir permeability changes during geothermal exploitation, geothermal exploitation design and protection can be rationally carried out, and the application prospect is good.

[0005] To achieve the above object, the present invention adopts the following technical solutions, including:

[0006] The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic production of deep geothermal reservoirs includes the following steps:

[0007] S1. Based on the geological and hydrothermal conditions of the geothermal reservoir, a reservoir mechanics analysis model and a seepage heat transfer analysis model including micro-crack structure and macro-fracture structure are established;

[0008] S2, based on the reservoir seepage heat transfer analysis model, conduct reservoir seepage heat transfer analysis to obtain the reservoir water pressure and temperature field distribution, and transfer the water pressure and temperature calculation results to the mechanical analysis model;

[0009] S3, based on the reservoir mechanics analysis model, conduct reservoir stress and deformation analysis under the action of thermal-hydraulic-mechanical coupling to obtain the changes in the micro-crack structure and macro-fracture structure inside the reservoir;

[0010] S4, establish the permeability evolution equation and porosity evolution equation of the fractured rock mass containing micro-crack structure and macro-crack structure, and calculate the permeability and porosity distribution of the reservoir rock mass based on the changes in the multi-scale structure of the reservoir rock mass. The permeability and porosity calculation results are transferred to the reservoir seepage heat transfer analysis model;

[0011] S5. According to the reservoir cyclic injection and production strategy, in accordance with steps S2 to S4, the seepage heat flow analysis and mechanical analysis are used for staggered iterative solution to analyze the temperature, water pressure and stress deformation of the reservoir, as well as the changes in the multi-scale structure of the reservoir during the geothermal cyclic production process.

[0012] Preferably, step S1 is specifically as follows:

[0013] S11. Based on the geological conditions, tectonic stress characteristics, geothermal gradient, and hydrological conditions of the deep geothermal reservoir project, and in combination with the layout and dimensions of the injection and production wells, an eight-node hexahedron unit is used to establish a finite element calculation model for reservoir rock mechanical analysis and seepage heat transfer analysis. The mechanical analysis calculation model and the seepage heat transfer analysis calculation model have the same unit grid and numbering. The calculation nodes of the mechanical analysis calculation model are the peripheral corner points of the unit, while the calculation nodes of the seepage heat transfer analysis calculation model are the center points of the unit. The Z direction of the entire model is the vertical direction, and the z coordinate represents the actual elevation value.

[0014] S12, determine the spatial distribution characteristics of the reservoir macro-fracture structure;

[0015] S13, determine the distribution characteristics of the reservoir microcrack structure.

[0016] Preferably, step S2 is specifically as follows:

[0017] S21, setting the initial conditions and boundary conditions of the seepage heat transfer analysis model;

[0018] The initial conditions of the seepage heat transfer analysis model are as follows: initially, the internal water pressure of the seepage heat transfer analysis model is distributed in a trapezoidal pattern from top to bottom, and the top water pressure value and water pressure gradient are determined based on the measured water pressure distribution data; initially, the internal temperature of the seepage heat transfer analysis model is distributed in a broken line pattern, and the top temperature value and temperature gradients of different broken line segments are determined based on the measured temperature distribution data;

[0019] The boundaries of the seepage heat transfer analysis model are as follows: the side boundaries of the model are set as adiabatic and impermeable boundaries; the top and bottom temperature and water pressure boundaries are set as Dirichlet boundary conditions;

[0020] S22, using the TOUGH2 open source program to perform seepage heat transfer calculations, and obtain the water pressure and temperature values ​​of different calculation nodes in the seepage heat transfer analysis model;

[0021] S23, the water pressure and temperature calculation results are transferred to the mechanical analysis model through inverse distance weighted interpolation calculation, and the water pressure and temperature calculation values ​​of the calculation nodes, i.e., the unit center points, in the seepage heat transfer analysis model are interpolated to the calculation nodes, i.e., the unit corner points, of the mechanical analysis calculation model.

[0022] Preferably, step S3 is specifically as follows:

[0023] S31, based on thermodynamic principles and multi-scale homogenization technology of rock mass structure, obtains the free enthalpy of reservoir rock mass under thermal-hydraulic-mechanical coupling conditions. First, the Mori-Tanaka method based on Eshelby theory is used to homogenize the rock mass containing microcracks to obtain the free enthalpy of the rock mass containing microcracks. Then, based on the development characteristics of the fracture network and the free enthalpy of a single group of fractures, the volume averaging method is used to homogenize the rock mass containing the fracture network at the macroscale to obtain the free enthalpy of the reservoir rock mass.

[0024] S32, in the framework of thermodynamics, according to the free enthalpy of reservoir rock mass under the conditions of thermal-hydraulic-mechanical coupling, derive the mechanical constitutive equation of rock mass;

[0025] S33, setting the initial conditions and boundary conditions of the reservoir mechanics analysis model;

[0026] The initial conditions of the reservoir mechanics analysis model are as follows: initially, the reservoir in-situ stress is fitted and set according to the results of the on-site in-situ stress distribution test;

[0027] The boundary conditions of the reservoir mechanics analysis model are as follows: zero normal displacement boundary conditions are set on the side boundaries and bottom boundaries of the model, and uniform normal stress boundary conditions are set on the top of the model according to the thickness of the upper formation;

[0028] S34, the changes in microcracks and macrofracture structures within the reservoir rock mass are determined by the evolution of internal variables characterizing the microcrack structure and the internal variables characterizing the fracture structure.

[0029] Preferably, in step S31, the reservoir rock mass is set to contain n c Group microcracks and n fThe free enthalpy of the reservoir rock mass under the combined action of macroscopic stress Σ, water pressure p and temperature T can be expressed as follows:

[0030]

[0031] Among them, W R Indicates containing n c Free enthalpy of rock with microcracks, W J represents the free enthalpy of the Jth group of dominant fractures. The last three terms on the right side of the equation represent the influence of temperature effect on the free enthalpy of rock mass.

[0032]

[0033]

[0034] Where, is the macroscopic strain generated by microcracks, where α represents the macroscopic volumetric strain generated by microcrack opening, γ represents the macroscopic shear internal variable generated by microcrack slip, and n r represents the unit normal direction vector of the rth group of microcracks; The symbol for finding the symmetric component of the vector product of two vectors; is the elastic compliance tensor of the isotropic solid matrix; elastic constant H0 = 3E s / {16[1-(v s ) 2 ]}, H1=H0(1-v s / 2), E s and v s are the elastic modulus and Poisson's ratio of the rock matrix; δ represents the second-order unit tensor; The microcrack density parameter is an internal variable used to represent the microscopic damage of rock blocks, where The microcrack density represents the number of microcracks per unit volume, and a is the average radius of microcracks; and N are the isotropic Biot coefficient and Biot modulus of the rock mass, is the initial porosity of the rock block; : symbol represents the double dot product operation of the tensor;

[0035] E J represents the macroscopic strain generated by the Jth group of cracks, E Jp It is E J The plastic component of S J is the average surface area of ​​the Jth group of fracture surfaces, Ω is the volume of the rock mass characterization unit, represents the average spacing of cracks, m Jrepresents the unit normal direction vector of the Jth group of cracks; g represents the displacement vector of the crack, which includes elastic and plastic displacement vectors, among which the plastic displacement is dominated by the shear slip and dilatation effect of the crack surface. The upper part e and p represent the elastic and plastic parts, T J , and H J are the force vector, Biot coefficient and elastic stiffness matrix of the Jth group of crack surfaces, H N represents the crack normal stiffness and H T represents the fracture tangential stiffness, which characterizes the mechanical response of the fracture normal and shear deformation respectively; T t and T n T J The tangential and normal components along the crack surface are: and T n =m J ·T J ;

[0036] represent the specific heat, thermal strain and equivalent elastic stiffness of the rock mass respectively;

[0037] In step S32, the free enthalpy of the rock mass is differentiated with respect to Σ, and the mechanical constitutive equation of the rock mass containing multi-scale void structure under the thermal-hydraulic-mechanical coupling is obtained as follows:

[0038]

[0039] According to the relationship between strain and stress in the mechanical constitutive equation, the macroscopic stress can be expressed as a function of the macroscopic strain:

[0040]

[0041] Among them, E represents the total strain of the rock mass, which consists of four terms: the elastic strain generated by the rock matrix in the first term on the right side of the rock mass mechanics constitutive equation, the temperature strain in the second term, the strain generated by the change of microcrack structure in the third term, and the strain generated by the change of fracture structure in the fourth term.

[0042] Preferably, in step S34, the changes in the microscopic microcracks and macroscopic fracture structures inside the reservoir rock mass are represented by the internal variables d, β and γ representing the microcrack structure and the internal variable g representing the fracture structure. p The evolution of the internal variables β and γ is determined by their corresponding conjugate thermodynamic forces F β and F γ Sure:

[0043]

[0044]

[0045] Among them, the thermodynamic force F β and F γ Represents the normal and tangential effective stresses on the microcrack wall; when the microcrack opens, F β =0 and F γ =0, then we can directly use F β and F γ The expression is used to calculate β and γ. When the microcrack is in a closed state (F β <0), the associated Mohr-Coulomb criterion is used to determine the changes in β and γ caused by microcrack shear slip, F = |F γ |+F β tanφ c , simulate the slip dilatancy of microcracks, and use the hyperbola model to characterize the β change caused by the normal closure of microcracks, β = -F β β0 / (k0β0-F β ), where φ c , β0 and k0 are the internal friction angle, initial opening and initial normal stiffness of the microcrack, respectively;

[0046] The evolution of the internal variable d is determined by the corresponding conjugate thermodynamic force F d The relevant function criteria determine:

[0047]

[0048]

[0049] In the formula, V(d c ) is the maximum value of microcrack damage extension resistance, d c For critical damage, when the microcrack damage reaches the critical damage, the damage extension resistance takes the maximum value;

[0050] The internal variable g that characterizes the plastic displacement of the crack p The evolution of is determined by the non-associative plastic flow law, where the yield function Y(σ n ,σ τ ) and the plastic potential function Q(σ n ,σ τ ) is as follows:

[0051]

[0052] Q(σ n ,σ τ )=|σ n sinα+σ τ cosα|

[0053]

[0054] Among them, c J and represent the cohesion and friction angle parameters of the Jth group of cracks respectively; α represents the equivalent undulation angle of the rough wall of the crack, α0 is the initial equivalent heave angle, is the wear coefficient of the undulating angle, which is used to simulate the wear and tear effect of the rough wall of the crack surface during the sliding friction process. is the accumulated dissipated plastic shear energy, in Represents the component of the crack plastic displacement along the tangential direction.

[0055] Preferably, in step S4, the calculation of the reservoir rock permeability tensor and porosity is performed using the multi-scale evolution equation of rock permeability characteristics and the porosity evolution equation, including:

[0056] The porosity evolution equation obtained by differentiating the free enthalpy of the rock mass with respect to p is:

[0057]

[0058] The permeability tensor of the reservoir rock mass is calculated using the volume averaging method, while taking into account the contribution of the solid matrix, microcrack structure, and fracture structure within the rock mass to the permeability of the rock mass. The cubic law is used to describe the permeability coefficient of microcracks and fractures. The expression of the permeability tensor of the reservoir rock mass is:

[0059]

[0060] Among them, k s is the permeability of the rock matrix, is the porosity; is the permeability of the microcrack when the initial opening is β0 and the initial damage is d0; χ is the index introduced to characterize the exponential change of microcrack connectivity with damage evolution; e J Indicates the opening of the Jth group of cracks, the initial opening value is ζ is the fracture permeability penalty constant, a parameter introduced to characterize that the water conductivity of a real rough fracture wall is lower than that of a smooth parallel plate.

[0061] Preferably, in step S5, the temperature, water pressure and stress deformation of the reservoir during geothermal cycle mining are analyzed by using an interleaved iterative solution method of seepage heat transfer analysis and mechanical analysis, as shown below:

[0062] S51, according to the reservoir cycle injection and production strategy, set the total calculation time, set internal boundary conditions for the injection well and the production well, the injection well boundary conditions are set according to the mass flow rate and temperature of the injected fluid, and the production well flow boundary conditions are set according to the production flow rate;

[0063] S52, performing a seepage heat transfer calculation based on the reservoir seepage heat transfer analysis model in each time step, and calculating the water pressure and temperature in the reservoir seepage heat transfer analysis model;

[0064] S53, the water pressure and temperature in the reservoir seepage heat transfer analysis model are introduced into the reservoir mechanics calculation model, and the finite element method is used to solve the stress-strain process affected by the changes in water pressure and temperature. The changes in the microcracks and fracture structure of the reservoir rock mass are calculated, and the permeability and porosity of the rock mass are updated according to the changes in the multi-scale structure of the rock mass.

[0065] S54, substituting the updated rock mass permeability and porosity into the reservoir seepage heat transfer analysis model to perform the seepage heat transfer solution, and re-determining the water pressure and temperature in the reservoir seepage heat transfer analysis model;

[0066] S55, repeat the above steps S53 to S54 until the set total calculation time is reached, and obtain the reservoir temperature, water pressure, stress and deformation, rock permeability and internal variables used to describe the multi-scale structural changes of the reservoir rock during the reservoir cyclic mining process.

[0067] Preferably, the method further comprises the following steps:

[0068] S6, respectively analyzes the multi-scale structural combinations of different reservoir rock masses and different coupling effects, and obtains the temperature field, water pressure distribution, permeability, stress and deformation response of the reservoir during the cyclic exploitation of deep geothermal reservoirs under different multi-scale structural distributions of rock masses and different coupling effects, so as to further analyze the influence of micro-crack structure and macro-crack structure on the cyclic exploitation of deep geothermal reservoirs.

[0069] The present invention also provides a computer program product, which includes a computer program / instruction, which, when executed by a processor, implements the analysis method of the influence of the multi-scale structure of the rock mass on the cyclic exploitation of deep geothermal reservoirs.

[0070] The advantages of the present invention are:

[0071] (1) The present invention is a numerical simulation method for the cyclic exploitation process of deep geothermal reservoirs that integrates the dynamic evolution of the multi-scale structure of the rock mass and the thermal-hydraulic-mechanical coupling effect. It is used to accurately predict the changes in reservoir permeability characteristics, temperature field and water pressure during the geothermal exploitation process, and provide technical support for the optimization of deep geothermal reservoir engineering exploitation design.

[0072] (2) The present invention adopts multi-scale homogenization modeling technology to effectively model the multi-scale structure of the reservoir under thermal-hydraulic-mechanical coupling, and combines the TOUGH2 seepage heat transfer calculation program with the finite element damage mechanics program to achieve numerical solution of the deep geothermal circulation mining process through staggered iteration, thus solving the above-mentioned technical bottleneck.

[0073] (3) The present invention uses multi-scale homogenization modeling technology to simulate the microscopic microcrack structure and macroscopic fracture structure inside the reservoir rock mass in detail, reasonably sets the coupling relationship between the reservoir seepage heat transfer process and the rock mass damage and deformation process, constructs a thermal-hydraulic-mechanical coupling analysis method for the reservoir rock mass, and systematically analyzes the influence of multi-field coupling and the multi-scale storage and seepage structure of the reservoir rock mass during the cyclic exploitation of deep geothermal reservoirs. It has good application prospects.

[0074] (4) Based on the microscopic microcrack structure and macroscopic fracture structure characteristics of the surrounding rock of deep geothermal reservoirs, the present invention couples the damage deformation process induced by the multi-scale structure of the reservoir under thermal-hydraulic-mechanical coupling with the seepage heat transfer process. By analyzing the reservoir temperature, water pressure and deformation evolution response and reservoir permeability changes during geothermal mining, geothermal mining design and protection can be rationally carried out, and the application prospect is good.

[0075] (5) When the present invention carries out the disturbance response analysis of the deep geothermal reservoir cyclic mining process, it simultaneously considers the changes in the small-scale microcracks and large-scale fracture structure of the reservoir rock mass on the strain, permeability and porosity of the surrounding rock, and links the macroscopic response of the rock mass with the evolution of its internal macroscopic and microscopic structures. The physical meaning of the model parameters is clear, the theory is more complete, it has innovative significance, and has good application prospects.

[0076] (6) The present invention further analyzes the influence of factors such as the multi-scale structural changes of the rock mass and the changes in the coupling effect on the reservoir temperature field, water pressure field, permeability and stress deformation during the cyclic exploitation of deep geothermal reservoirs, which has innovative significance and broad engineering application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 1 This is a flow chart of the analysis method of the impact of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to the present invention.

[0078] Figure 2 Schematic diagram of the reservoir seepage heat transfer analysis model and mechanical calculation model according to an embodiment of the present invention.

[0079] Figure 3 This is a calculation and solution diagram of the coupling process of the seepage heat transfer analysis and mechanical analysis interleaved iterative calculation method in an embodiment of the present invention.

[0080] Figure 4 This is a temperature evolution diagram of the production well under the conditions of thermal-water coupling and thermal-water-mechanical coupling in an embodiment of the present invention.

[0081] Figure 5 This is a diagram of water pressure evolution in injection wells and production wells under the conditions of thermal-water coupling and thermal-hydraulic-mechanical coupling according to an embodiment of the present invention.

[0082] Figure 6 This is a distribution diagram of vertical deformation of the formation induced by cyclic production of the reservoir in an embodiment of the present invention (unit: cm).

[0083] Figure 7 This is a temperature evolution diagram of the production well during the geothermal cycle mining process under the multi-scale structural distribution of different reservoir rock masses in an embodiment of the present invention.

[0084] Figure 8 This is a diagram of the water pressure evolution of the injection well and the production well during the geothermal cycle production process under the multi-scale structural distribution of different reservoir rock masses in an embodiment of the present invention.

[0085] Figure 9 This is a diagram of the rock permeability evolution at the injection well location under the multi-scale structural distribution of different reservoir rock masses according to an embodiment of the present invention. DETAILED DESCRIPTION

[0086] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0087] like Figure 1 As shown, the method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs of the present invention comprises the following steps:

[0088] S1. Based on the geological and hydrothermal conditions of the geothermal reservoir, a reservoir rock mechanics analysis model and a seepage heat transfer analysis model including micro-crack structure and macro-fracture structure are established;

[0089] S2, based on the reservoir seepage heat transfer analysis model, uses the TOUGH2 open source program to perform reservoir seepage heat transfer analysis, obtains the reservoir water pressure and temperature field distribution, and transfers the water pressure and temperature calculation results to the finite element mechanical analysis model through inverse distance weighted interpolation calculation;

[0090] S3, based on the reservoir mechanics analysis model, uses a finite element calculation program for fractured rock damage mechanics that considers thermal-hydraulic-mechanical coupling to perform reservoir stress-deformation analysis under water pressure and temperature stress, obtaining changes in the micro-crack structure and macro-fracture structure within the reservoir;

[0091] S4. Establish permeability and porosity evolution equations for fractured rock masses containing micro-crack structures and macro-crack structures. Based on the changes in the multi-scale structure of the reservoir rock mass, calculate the permeability and porosity distribution of the reservoir rock mass. Transfer the permeability and porosity calculation results to the seepage heat transfer analysis model through arithmetic averaging.

[0092] S5, according to the reservoir cyclic injection and production strategy, in accordance with steps S2 to S4, using staggered iterative solution of seepage heat flow analysis and mechanical analysis to analyze the temperature, water pressure and stress deformation of the reservoir, as well as the changes in the multi-scale structure of the reservoir during the geothermal cyclic production process;

[0093] S6, respectively analyzes the multi-scale structural combinations and different coupling effects of different reservoir rock masses, and obtains the temperature field, water pressure distribution, permeability, and stress and deformation response of the reservoir during the cyclic exploitation of deep geothermal reservoirs under different multi-scale structural distributions of rock masses and different coupling effects (considering or not considering mechanical coupling effects). Further, the influence of micro-crack structure and macro-fracture structure on the cyclic exploitation of deep geothermal reservoirs is obtained.

[0094] In step S1, the establishment of a reservoir rock mass mechanics analysis model and a seepage heat transfer analysis model including a microscopic microcrack structure and a macroscopic fracture structure specifically includes:

[0095] S11. Based on the geological conditions, tectonic stress characteristics, geothermal gradient and hydrological conditions of the deep geothermal reservoir project, combined with the layout and size of the injection wells and production wells, an eight-node hexahedron unit is used to establish a finite element calculation model for reservoir rock mechanical analysis and seepage heat transfer analysis. Among them, the mechanical analysis calculation model and the seepage heat transfer analysis calculation model have the same unit grid and number. The calculation nodes of the mechanical analysis calculation model are the peripheral corner points of the unit, and the calculation nodes of the seepage heat transfer analysis calculation model are the center points of the unit. The Z direction of the entire model is the vertical direction, and the z coordinate represents the actual elevation value.

[0096] S12, determine the spatial distribution characteristics of the reservoir macro-fracture structure, mainly based on the analysis of engineering geological structures, formation imaging data and drilling core fracture identification results during on-site geological surveys, and use indicators such as the occurrence, aperture, spacing and spatial distribution length of dominant fractures to characterize the distribution of macro-fracture structure.

[0097] S13, determine the distribution characteristics of the reservoir microcrack structure, mainly based on the observation and analysis of engineering rock samples using microscopic observation techniques such as electron microscopy scanning and X-ray diffraction, and use indicators such as the number of microcracks per unit volume in different strike directions and the porosity of microcracks to characterize the distribution of the microcrack structure.

[0098] In step S2, a reservoir seepage heat transfer analysis is performed based on the reservoir seepage heat transfer analysis model, specifically including:

[0099] S21. According to the actual characteristics of geothermal reservoir engineering, set the initial and boundary conditions of the seepage heat transfer analysis model.

[0100] The initial conditions of the seepage heat transfer analysis model are as follows: initially, the internal water pressure of the seepage heat transfer analysis model is distributed in a trapezoidal shape from top to bottom, and the top water pressure value and water pressure gradient are determined according to the measured water pressure distribution data; initially, the internal temperature of the seepage heat transfer analysis model is distributed in a broken line shape, and the top temperature value and the temperature gradients of different broken line segments are determined according to the measured temperature distribution data.

[0101] The boundaries of the seepage heat transfer analysis model are as follows: the side boundaries of the model are set as adiabatic and impermeable boundaries; the top and bottom temperature and water pressure boundaries are set as Dirichlet boundary conditions.

[0102] S22, the TOUGH2 open source program is used to calculate the seepage heat transfer. If the reservoir rock contains only saturated water, the governing equation for the reservoir seepage heat transfer process is expressed as:

[0103]

[0104] Among them, ρ w 、μ w 、h w 、U w are the density, viscosity, specific enthalpy and specific internal energy of saturated water respectively; ρ F represents the density of the rock mass; q w and q h Represents the source and sink terms of the unit volume mass flow of water and the heat source of the unit volume of the cell; c F is the specific heat capacity of the rock mass; p is the water pressure; K is the rock mass permeability tensor; is the porosity; h is the gravity acceleration vector; λ is the thermal conductivity of the rock mass; t represents time; represents the gradient operator.

[0105] The TOUGH2 open source program was used to calculate the seepage heat transfer and obtain the water pressure and temperature values ​​at different calculation nodes in the seepage heat transfer analysis model.

[0106] S23, transfer the water pressure and temperature calculation results obtained by TOUGH2 open source program to the mechanical analysis model. Considering that the calculation node of the seepage heat transfer analysis model is located at the center point of the unit, while the calculation node of the mechanical analysis model is located at the peripheral corner points of the unit, the water pressure and temperature calculation values ​​in the seepage heat transfer analysis model are interpolated to the unit corner points using inverse distance weighted interpolation. If there are n units associated with the unit corner point M, the water pressure value and temperature value of each unit center are p respectively. i and T i , i=1,2,…n, the coordinates of the unit corner point M are (x M ,y M ,z M ), the coordinates of the center point of the associated unit are (xi ,y i ,z i ), i=1,2,…n, then the water pressure and temperature p of the unit corner point M are calculated by inverse distance weighted interpolation. M and T M for:

[0107]

[0108]

[0109] In step S3, based on the reservoir mechanics analysis model, a finite element calculation program for fractured rock damage mechanics that considers thermal-hydraulic-mechanical coupling is used to perform reservoir stress-deformation analysis under water pressure and temperature stress, specifically including:

[0110] S31, based on thermodynamic principles and multi-scale homogenization technology of rock mass structure, obtains the free enthalpy of reservoir rock mass under thermal-hydraulic-mechanical coupling conditions. Reservoir rock mass is a composite medium composed of rock blocks and fracture network structures at the macro scale, in which rock blocks are further composed of matrix and microcrack structures at the micro scale. First, the rock blocks containing microcracks are homogenized using the Mori-Tanaka method based on Eshelby theory to obtain the free enthalpy of the rock blocks containing microcracks; then, according to the development characteristics of the fracture network and the free enthalpy of a single group of fractures, the rock mass containing the fracture network at the macro scale is homogenized using the volume averaging method to obtain the free enthalpy of the reservoir rock mass. Let the reservoir rock mass contain n c Group microcracks and n f The free enthalpy of the reservoir rock mass under the combined action of macroscopic stress Σ, water pressure p and temperature T can be expressed as follows:

[0111]

[0112] Among them, W R Indicates containing n c Free enthalpy of rock with microcracks, W J represents the free enthalpy of the Jth group of dominant fractures, and the last three terms on the right side of the equation represent the influence of temperature effect on the free enthalpy of rock mass.

[0113]

[0114]

[0115] Where: is the macro strain generated by micro cracks, where β represents the macro volume strain generated by micro crack opening, γ represents the macro shear internal variable generated by micro crack slip, and n r represents the unit normal direction vector of the rth group of microcracks; The symbol for finding the symmetric component of the vector product of two vectors; is the elastic compliance tensor of the isotropic solid matrix; elastic constant H0 = 3E s / {16[1-(v s ) 2 ]}, H1=H0(1-v s / 2), E s and v s are the elastic modulus and Poisson's ratio of the rock matrix; δ represents the second-order unit tensor; The microcrack density parameter is an internal variable used to represent the microscopic damage of rock blocks, where N is the microcrack density, which represents the number of microcracks per unit volume, and a is the average radius of microcracks; and N are the isotropic Biot coefficient and Biot modulus of the rock mass, is the initial porosity of the rock block; the symbol represents the double dot product operation of the tensor.

[0116] E J represents the macroscopic strain generated by the Jth group of cracks, E Jp It is E J The plastic component of S J is the average surface area of ​​the Jth group of fracture surfaces, Ω is the volume of the rock mass characterization unit, represents the average spacing of cracks, m J represents the unit normal direction vector of the Jth group of cracks; g represents the displacement vector of the crack, which includes elastic and plastic displacement vectors, among which the plastic displacement is dominated by the shear slip and dilatation effect of the crack surface. The upper part e and p represent the elastic and plastic parts, T J , and H J are the force vector, Biot coefficient and elastic stiffness matrix of the Jth group of crack surfaces, H N represents the crack normal stiffness and H T represents the fracture tangential stiffness, which characterizes the mechanical response of the fracture normal and shear deformation respectively; T t and T n T J The tangential and normal components along the crack surface are: and T n =m J ·T J .

[0117] represent the specific heat, thermal strain and equivalent elastic stiffness of the rock mass, respectively.

[0118] ET =α T (T-T0). T0 is the reference temperature corresponding to the initial temperature. T Represents the thermal expansion tensor of the rock mass. In the isotropic case, α T =α T δ, α T is the thermal expansion coefficient of the rock mass.

[0119] S32, in the thermodynamic framework, the mechanical constitutive equation of the rock mass is derived based on the free enthalpy of the reservoir rock mass under the conditions of thermal-hydraulic-mechanical coupling. By differentiating the free enthalpy of the rock mass with respect to Σ, the mechanical constitutive equation of the rock mass containing multi-scale voids under the conditions of thermal-hydraulic-mechanical coupling is obtained as:

[0120]

[0121] According to the relationship between strain and stress in the mechanical constitutive equation, the macroscopic stress can be expressed as a function of the macroscopic strain:

[0122]

[0123] Among them, E represents the total strain of the rock mass, which consists of four terms: the elastic strain generated by the rock matrix in the first term on the right side of the rock mass mechanics constitutive equation, the temperature strain in the second term, the strain generated by the change of microcrack structure in the third term, and the strain generated by the change of fracture structure in the fourth term.

[0124] S33, based on the reservoir mechanics analysis model, setting the initial and boundary conditions of the reservoir mechanics analysis model.

[0125] The initial conditions of the reservoir mechanics analysis model are as follows: initially, the reservoir in-situ stress is fitted and set according to the on-site in-situ stress distribution test results.

[0126] The boundary conditions of the reservoir mechanics analysis model are as follows: zero normal displacement boundary conditions are set on the side boundaries and bottom boundaries of the model, and uniform normal stress boundary conditions are set on the top of the model according to the thickness of the upper formation.

[0127] The specific method of fitting the on-site geostress distribution test results is: based on the on-site geostress test data, the geostress distribution in different directions is fitted into a function related to the burial depth, and then the geostress at different burial depths of the reservoir is set according to the function:

[0128] σ x0 =A1+B1z

[0129] σ y0 =A2+B2zσ z0 =A3+B3z

[0130] Among them, σ z0 is the initial stress in the vertical direction, σx0 is the initial stress on the horizontal surface along the x direction, σ y0 is the initial stress along the y direction of the horizontal plane; A1 and B1 are fitted to the ground stress σ x0 The coefficients of distribution, A2 and B2 respectively fit the ground stress σ y0 The coefficients of distribution, A3 and B3 respectively fit the ground stress σ z0 The coefficient of the distribution.

[0131] S34, the changes in micro-cracks and macro-fracture structures inside the reservoir rock mass are determined by the internal variables d, β and γ that characterize the micro-crack structure and the internal variable g that characterizes the fracture structure. p The changes of internal variables β and γ are determined by their corresponding conjugate thermodynamic forces F β and F γ Sure:

[0132]

[0133]

[0134] Among them, the thermodynamic force F β and F γ Essentially, it represents the normal and tangential effective stress on the microcrack wall. When the microcrack opens, F β =0 and F γ =0, then we can directly use F β and F γ The expression is used to calculate β and γ. When the microcrack is in a closed state (F β <0), the associated Mohr-Coulomb criterion is used to determine the changes in β and γ caused by microcrack shear slip, F = |F γ |+F β tanφ c , simulate the slip dilatancy of microcracks, and use the hyperbola model to characterize the β change caused by the normal closure of microcracks, β = -F β β0 / (k0β0-F β ), where φ c , β0 and k0 are the internal friction angle, initial opening and initial normal stiffness of the microcrack, respectively.

[0135] The evolution of the internal variable d is determined by the corresponding conjugate thermodynamic force F d The relevant function criteria determine:

[0136]

[0137]

[0138] In the formula, V(d c) is the maximum value of microcrack damage extension resistance, d c It is critical damage. When the microcrack damage reaches the critical damage, the damage extension resistance takes the maximum value.

[0139] Crack plastic displacement g p The evolution of internal variables is determined by the non-associated plastic flow law, where the yield function Y(σ n ,σ τ ) and the plastic potential function Q(σ n ,σ τ ) is as follows:

[0140]

[0141] Q(σ n ,σ τ )=|σ n sinα+σ τ cosα|

[0142]

[0143] Among them, c J and Represent the cohesion and friction parameters of the Jth group of cracks respectively. α represents the equivalent undulation angle of the rough wall of the crack, α0 is the initial equivalent heave angle, is the wear coefficient of the undulating angle, which is used to simulate the wear and tear effect of the rough wall of the crack surface during the sliding friction process. is the accumulated dissipated plastic shear energy, in Represents the component of the crack plastic displacement along the tangential direction.

[0144] In step S4, the permeability tensor and porosity of the reservoir rock mass are calculated using the multi-scale evolution equation of rock mass permeability characteristics and the porosity evolution equation, including:

[0145] The porosity of the reservoir rock mass is calculated based on the porosity evolution equation. In the thermodynamic framework, the porosity evolution equation is derived by differentiating the rock mass free enthalpy with respect to p:

[0146]

[0147] The permeability tensor of the reservoir rock mass is calculated using the volume averaging method, while taking into account the contribution of the solid matrix, microcrack structure, and fracture structure within the rock mass to the permeability of the rock mass. The cubic law is used to describe the permeability coefficient of microcracks and fractures. The expression of the permeability tensor of the reservoir rock mass is:

[0148]

[0149] Among them, k s is the permeability of the rock matrix, is the porosity; is the permeability of the microcrack when the initial opening is β0 and the initial damage is d0; X is the index introduced to characterize the exponential change of microcrack connectivity with damage evolution; e J Indicates the opening of the Jth group of cracks, the initial opening value is ζ (0<ζ<1 / 12) is the fracture permeability penalty constant, which is a parameter introduced to characterize that the water conductivity of a real rough fracture wall is lower than that of a smooth parallel plate.

[0150] In step S5, the temperature, water pressure, and stress deformation of the reservoir during geothermal cycle exploitation are analyzed using an interleaved iterative solution method of seepage heat transfer analysis and mechanical analysis. Specifically, the following steps are included:

[0151] S51: Set the total calculation time based on the reservoir cycle injection-production strategy. Set internal boundary conditions for the injection well and production well. The injection well boundary conditions are set based on the mass flow rate and temperature of the injected fluid, while the production well boundary conditions are set based on the production flow rate.

[0152] S52, in each time step, based on the reservoir seepage heat transfer analysis model, TOUGH2 open source program is used to perform seepage heat transfer calculations to calculate the water pressure and temperature in the reservoir seepage heat transfer analysis model;

[0153] S53, the water pressure and temperature in the reservoir seepage heat transfer analysis model are introduced into the reservoir mechanics calculation model, and the finite element method is used to solve the stress-strain process affected by the changes in water pressure and temperature. The changes in the microcracks and fracture structure of the reservoir rock mass are calculated, and the permeability and porosity of the rock mass are updated according to the changes in the multi-scale structure of the rock mass.

[0154] S54, substituting the updated rock mass permeability and porosity into the reservoir seepage heat transfer analysis model to perform the seepage heat transfer solution, and re-determining the water pressure and temperature in the reservoir seepage heat transfer analysis model;

[0155] S55, repeat the above steps S53 to S54 until the set total calculation time is reached, and the temperature, water pressure, stress and deformation of the reservoir during the reservoir cyclic exploitation process, as well as the rock permeability and the internal variables d, β, γ and g used to describe the multi-scale structural changes of the reservoir rock mass are obtained. p .

[0156] Example 1

[0157] This example uses the Rittershoffen Enhanced Geothermal Project, located in deep granite formations with a target depth of -2.2 to -2.4 km. Using a self-developed thermal-hydraulic-mechanical coupled numerical model that considers the multi-scale structure of the reservoir rock mass, a thermal-hydraulic-mechanical coupled response simulation of the geothermal reservoir extraction process is carried out.

[0158] like Figure 1 As shown in the figure, the analysis method of the influence of the multi-scale structure of the rock mass on the cyclic production of deep geothermal reservoirs in this example is as follows:

[0159] S1. According to the reservoir environment, a reservoir rock mechanics analysis model and a seepage heat transfer analysis model are established.

[0160] According to the geological and hydrogeological conditions of the Rittershoffen enhanced geothermal project and the characteristics of the well pattern layout, a reservoir rock mechanics calculation model and a seepage heat flow analysis model were established, such as Figure 2 As shown, the three-dimensional model covers an area of ​​2.0 km × 2.0 km × 2.7 km and considers four material zones: sedimentary caprock, granite formation, high-permeability formation, and stimulation zone. The reservoir rock mechanics calculation model and the seepage heat transfer analysis model use the same cell grid, with a total of 18,125 cells. The mechanical model calculates 19,860 nodes at the corners of each cell, while the seepage heat transfer model calculates 18,125 nodes at the center of each cell. The model's Z direction is vertical. Based on project site data, the stimulation zone contains randomly distributed microcracks and two groups of dominant fractures, one horizontally and one vertically. A coupled thermal-hydraulic-mechanical model that considers the reservoir's multiscale structure is used to represent the zone. Other strata are represented using a matrix-only model.

[0161] The calculation parameters of each reservoir formation material are as follows: The elastic modulus of the sedimentary rock cap layer is E s =15GPa, Poisson's ratio is v s =0.23, matrix permeability k s =1.1×10 -15 m 2 , thermal conductivity is 1.4W / (m·K), specific heat capacity is 800J / (kg·K); elastic modulus of granite formation is E s =25GPa, Poisson's ratio is v s =0.25, matrix permeability k s =1.1×10 -15 m 2 , thermal conductivity is 3.1W / (m·K), specific heat capacity is 800J / (kg·K); the elastic modulus of high permeability formation is E s =20GPa, Poisson's ratio is v s=0.25, matrix permeability k s =1.6×10 -14 m 2 , thermal conductivity is 3.1W / (m·K), specific heat capacity is 800J / (kg·K); elastic modulus of rock matrix in the stimulation area is E s =20GPa, matrix Poisson's ratio v s =0.25, matrix permeability k s =1.1×10 -15 m 2 , matrix thermal conductivity is 3.1W / (m·K), matrix specific heat capacity is 800J / (kg·K), initial microcrack opening is 0.01, initial damage density is 0.1, critical damage density is 1.0, maximum damage extension resistance is 0.12MPa, initial permeability The initial normal stiffness is 3000 MPa, the friction angle is 35°, the initial crack opening is 0.1 mm, the average spacing is 0.4 m, the normal stiffness is 100 MPa / mm, the tangential stiffness is 100 MPa / mm, the permeability penalty constant is 0.008, the friction angle is 30°, and the equivalent relief angle is 10°; the Biot coefficient of each layer material is 0.6, and the thermal expansion coefficient is 1.4×10 -5 K -1 .

[0162] S2, based on the reservoir seepage heat transfer analysis model, sets the initial and boundary conditions of the seepage heat transfer analysis model, uses the TOUGH2 open source program to perform seepage heat transfer calculations, and transfers the water pressure and temperature calculation results to the mechanical analysis model.

[0163] The initial conditions of the seepage heat transfer analysis model are as follows: initially, the internal water pressure of the seepage heat transfer analysis model is distributed in a trapezoidal shape from top to bottom, with the top water pressure value being 11.76 MPa and the water pressure gradient varying with depth being 9 MPa / m; initially, the internal temperature of the seepage heat transfer analysis model is distributed in a broken line shape, with the top temperature value being 120°C, the temperature gradient varying with depth being 40°C / km from -1.2 km to -2.2 km, and the temperature gradient varying with depth being 18°C / km from -2.2 km to -3.9 km.

[0164] The boundaries of the seepage heat transfer analysis model are as follows: the side boundaries of the model are set as adiabatic and impermeable boundaries; the top and bottom temperature and water pressure boundaries are set as Dirichlet boundary conditions.

[0165] In step S2, the seepage heat transfer analysis model uses the TOUGH2 open source program to calculate the water pressure and temperature of each calculation node. The calculation is located at the center point of the unit, which does not correspond to the calculation node in the mechanical analysis model. Inverse distance weighted interpolation is used to interpolate the water pressure and temperature calculated values ​​in the seepage heat transfer analysis model to the unit corner points.

[0166] S3, based on the reservoir mechanics analysis model, set the initial and boundary conditions of the mechanical analysis model, carry out reservoir stress and deformation analysis under the action of water pressure and temperature stress, and obtain the changes in the micro-cracks and macro-crack structures inside the reservoir rock mass.

[0167] In step S3, the initial conditions of the reservoir mechanics analysis model are as follows: Initially, the reservoir stress function is: vertical stress σ z =30+25.5z kPa, the stress in the x direction is σ x =1.78+14.09z kPa, the stress in the y direction is σ y =1.3+24.9z kPa, where z represents the vertical coordinate in meters. The boundary conditions for the reservoir mechanics analysis model are as follows: zero normal displacement boundary conditions are set on the side and bottom boundaries of the model, and a uniformly distributed normal stress of 30.57 MPa is set on the top boundary of the model.

[0168] S4, according to the changes in the multi-scale structure of the rock mass, the multi-scale evolution equation of rock permeability characteristics and the porosity evolution equation that consider the influence of multi-scale structural changes are used to calculate the permeability tensor and porosity of the reservoir rock mass, and pass them to the seepage heat transfer analysis model.

[0169] S5 uses an interleaved iterative solution method of seepage heat transfer analysis and mechanical analysis to solve the temperature, water pressure and stress deformation of the reservoir during geothermal cycle mining.

[0170] In step S5, according to the reservoir cyclic injection and production strategy, the total calculation time is set to 40 years. The internal boundary conditions of injection flow rate of 70 kg / s and injection temperature of 70°C are set for the injection well, and the internal boundary conditions of production flow rate of 70 kg / s are set for the production well.

[0171] The coupling process is solved by interleaving iterative calculation of seepage heat transfer analysis and mechanical analysis, which mainly includes the seepage heat transfer calculation module and the finite element mechanics calculation module, such as Figure 3 As shown, specifically including:

[0172] S51. In each time step, based on the reservoir seepage heat transfer analysis model, the TOUGH2 open source program is used to perform seepage heat transfer calculations to calculate the water pressure and temperature in the reservoir seepage heat transfer analysis model.

[0173] S52, brings the water pressure and temperature in the reservoir seepage heat transfer analysis model into the reservoir mechanics calculation model, performs finite element solution of the stress-strain process affected by water pressure and temperature changes, calculates the changes in the microcracks and fracture structure of the reservoir rock mass, and updates the permeability and porosity of the rock mass according to the changes in the multi-scale structure of the rock mass.

[0174] S53, substituting the updated rock mass permeability and porosity into the reservoir seepage heat transfer analysis model and then solving the seepage heat transfer, thereby re-determining the water pressure and temperature in the reservoir seepage heat transfer analysis model.

[0175] S54, repeat the above steps S52 to S53 until the set total calculation time is reached, and the temperature, water pressure, stress and deformation of the reservoir during the reservoir cyclic exploitation process, as well as the rock permeability and the internal variables d, β, γ and g used to describe the multi-scale structural changes of the reservoir rock mass are obtained. p Based on this, the response of deep geothermal reservoir cyclic production is evaluated and analyzed to provide support for engineering design and operation.

[0176] S6. Considering the multi-scale structural changes of the reservoir rock mass and the effects of different coupling effects, a simulation analysis was carried out to obtain the temperature field and water pressure distribution during the reservoir exploitation process under different coupling effects and the multi-scale structural distribution of the rock mass. The influence of the micro-crack structure and the macro-crack structure on the cyclic exploitation of deep geothermal reservoirs was further obtained.

[0177] The numerical simulation analysis results of step S6 are as follows:

[0178] The temperature and water pressure changes of the injection well and the production well induced by reservoir cyclic production under the consideration of thermal-hydraulic-mechanical coupling and thermal-hydraulic coupling are shown in the figure. Figure 4 and Figure 5 As shown. Figure 4 and Figure 5 It can be seen that when only thermal-hydraulic coupling is considered, the water pressure of the injection well increases continuously, while the temperature of the production well decreases slowly, and the final production temperature is 130.2°C. When thermal-hydraulic-mechanical coupling is considered, the water pressure of the injection well increases first, then decreases slightly, and then changes slowly. The temperature of the production well decreases relatively quickly with the production time, and the final production temperature is 123.9°C. After the reservoir is cyclically injected and produced, the rock mass above the injection well will undergo settlement deformation, and the rock mass below will undergo uplift deformation, such as Figure 6 It can be seen that the consideration of mechanical coupling has a significant impact on the disturbance response of reservoir cyclic production, and it is very important to consider the thermal-hydraulic-mechanical coupling in actual analysis.

[0179] The thermal-hydraulic-mechanical coupling characteristics of reservoir rock masses are influenced by the distribution of multi-scale structures within them. Influenced by long-term geological processes and enhanced geothermal reservoir transformation, the rock mass has a complex internal structure. This example simulates and analyzes the impact of different structural distributions within the rock mass in the stimulation zone on the reservoir's response to cyclic production disturbances. Four scenarios are considered: rock masses containing only microcracks, microcracks with fractures spaced 0.4m apart, microcracks with fractures spaced 0.1m apart, and microcracks with fractures spaced 4.0m apart, taking into account the thermal-hydraulic-mechanical coupling.

[0180] The response of temperature and water pressure changes of injection wells and production wells induced by geothermal circulation mining under the internal structure distribution of rock mass in different reservoir transformation stimulation areas is as follows: Figure 7 and Figure 8 As shown. Figure 7 and Figure 8 It can be seen that when the rock mass within the reservoir stimulation zone contains only microcracks, the injection well water pressure increases the most over time, while the production well temperature changes the slowest, with the final production temperature reaching 126.7°C. When the rock mass contains both microcracks and macrocracks, the injection well water pressure increases less over time, while the production well temperature changes more rapidly. Both the injection well water pressure and the production well temperature increase with increasing average fracture spacing. When the average fracture spacing within the fracture network is 0.4 m, 0.1 m, and 4.0 m, the final production temperatures are 123.9°C, 123.1°C, and 125.9°C, respectively. Therefore, the thermal recovery response of EGS is influenced by the multiscale structural characteristics within the fractured rock mass, and this influence increases with the size of the stimulation zone.

[0181] The change in reservoir permeability that affects the disturbance response of geothermal reservoir mining is also controlled by the multi-scale structural characteristics of the rock mass. Under different internal structural distributions of reservoir rock masses, the permeability K of the reservoir rock mass along the y-axis at the injection well location is y As the mining process evolves Figure 9 As shown. Figure 9 It can be seen that when the rock mass contains only microcracks, the increase in reservoir rock permeability is the smallest. After 40 years of geothermal exploitation, the rock permeability at the injection well of the reservoir increases from the initial 2.4×10 -14 m 2 Increased to 4.8×10 - 14 m 2 , the increase is 1 times. When the rock mass contains both microcrack structure and macrocrack structure, the increase in reservoir rock permeability increases significantly, and as the average spacing of the crack structure decreases, the increase in rock permeability gradually increases. When the average spacing of the cracks in the fracture network structure is 0.4m, 0.1m and 4.0m respectively, the rock permeability at the reservoir injection well after 40 years of geothermal mining is 1.3×10 -13m 2 , 2.3×10 -13 m 2 , 6.0×10 -14 m 2 It can be seen that the different distribution of microcracks and fracture structures in the reservoir rock mass significantly affects the water pressure and temperature disturbance response and rock permeability changes caused by cyclic exploitation of the geothermal reservoir. Accurate characterization of the multi-scale structural characteristics inside the reservoir rock mass is very important for simulating the disturbance response of deep geothermal reservoir exploitation.

[0182] Therefore, the present invention analyzes the impact of the multi-scale structure of the reservoir rock mass and the thermal-hydraulic-mechanical coupling on the cyclic mining disturbance of the deep geothermal reservoir, so as to rationally design the deep geothermal reservoir mining plan and operate and control it, and has good application prospects.

[0183] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An analysis method for the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs, characterized by: The following steps are involved: S1. Based on the geological and hydrothermal conditions of the geothermal reservoir, a reservoir mechanics analysis model and a seepage heat transfer analysis model including micro-crack structure and macro-fracture structure are established; S2, based on the reservoir seepage heat transfer analysis model, conduct reservoir seepage heat transfer analysis to obtain the reservoir water pressure and temperature field distribution, and transfer the water pressure and temperature calculation results to the mechanical analysis model; S3, based on the reservoir mechanics analysis model, conduct reservoir stress and deformation analysis under the action of thermal-hydraulic-mechanical coupling to obtain the changes in the micro-crack structure and macro-fracture structure inside the reservoir; S4, establish the permeability evolution equation and porosity evolution equation of the fractured rock mass containing micro-crack structure and macro-crack structure, and calculate the permeability and porosity distribution of the reservoir rock mass based on the changes in the multi-scale structure of the reservoir rock mass. The permeability and porosity calculation results are transferred to the reservoir seepage heat transfer analysis model; S5. According to the reservoir cyclic injection and production strategy, in accordance with steps S2 to S4, the seepage heat flow analysis and mechanical analysis are used for staggered iterative solution to analyze the temperature, water pressure and stress deformation of the reservoir, as well as the changes in the multi-scale structure of the reservoir during the geothermal cyclic production process.

2. The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to claim 1, characterized in that: Step S1 is specifically as follows: S11. Based on the geological conditions, tectonic stress characteristics, geothermal gradient, and hydrological conditions of the deep geothermal reservoir project, and in combination with the layout and dimensions of the injection and production wells, an eight-node hexahedron unit is used to establish a finite element calculation model for reservoir rock mechanical analysis and seepage heat transfer analysis. The mechanical analysis calculation model and the seepage heat transfer analysis calculation model have the same unit grid and numbering. The calculation nodes of the mechanical analysis calculation model are the peripheral corner points of the unit, while the calculation nodes of the seepage heat transfer analysis calculation model are the center points of the unit. The Z direction of the entire model is the vertical direction, and the z coordinate represents the actual elevation value. S12, determine the spatial distribution characteristics of the reservoir macro-fracture structure; S13, determine the distribution characteristics of the reservoir microcrack structure.

3. The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to claim 2, characterized in that: Step S2 is specifically as follows: S21, setting the initial conditions and boundary conditions of the seepage heat transfer analysis model; The initial conditions of the seepage heat transfer analysis model are as follows: initially, the internal water pressure of the seepage heat transfer analysis model is distributed in a trapezoidal pattern from top to bottom, and the top water pressure value and water pressure gradient are determined based on the measured water pressure distribution data; initially, the internal temperature of the seepage heat transfer analysis model is distributed in a broken line pattern, and the top temperature value and temperature gradients of different broken line segments are determined based on the measured temperature distribution data; The boundaries of the seepage heat transfer analysis model are as follows: the side boundaries of the model are set as adiabatic and impermeable boundaries; the top and bottom temperature and water pressure boundaries are set as Dirichlet boundary conditions; S22, using the TOUGH2 open source program to perform seepage heat transfer calculations, and obtain the water pressure and temperature values ​​of different calculation nodes in the seepage heat transfer analysis model; S23, the water pressure and temperature calculation results are transferred to the mechanical analysis model through inverse distance weighted interpolation calculation, and the water pressure and temperature calculation values ​​of the calculation nodes, i.e., the unit center points, in the seepage heat transfer analysis model are interpolated to the calculation nodes, i.e., the unit corner points, of the mechanical analysis calculation model.

4. The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to claim 1, characterized in that: Step S3 is specifically as follows: S31, based on thermodynamic principles and multi-scale homogenization technology of rock mass structure, obtains the free enthalpy of reservoir rock mass under thermal-hydraulic-mechanical coupling conditions. First, the Mori-Tanaka method based on Eshelby theory is used to homogenize the rock mass containing microcracks to obtain the free enthalpy of the rock mass containing microcracks. Then, based on the development characteristics of the fracture network and the free enthalpy of a single group of fractures, the volume averaging method is used to homogenize the rock mass containing the fracture network at the macroscale to obtain the free enthalpy of the reservoir rock mass. S32, in the framework of thermodynamics, according to the free enthalpy of reservoir rock mass under the conditions of thermal-hydraulic-mechanical coupling, derive the mechanical constitutive equation of rock mass; S33, setting the initial conditions and boundary conditions of the reservoir mechanics analysis model; The initial conditions of the reservoir mechanics analysis model are as follows: initially, the reservoir in-situ stress is fitted and set according to the results of the on-site in-situ stress distribution test; The boundary conditions of the reservoir mechanics analysis model are as follows: zero normal displacement boundary conditions are set on the side boundaries and bottom boundaries of the model, and uniform normal stress boundary conditions are set on the top of the model according to the thickness of the upper formation; S34, the changes in microcracks and macrofracture structures within the reservoir rock mass are determined by the evolution of internal variables characterizing the microcrack structure and the internal variables characterizing the fracture structure.

5. The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to claim 4, characterized in that: In step S31, the reservoir rock mass is set to contain n c Group microcracks and n f The free enthalpy of the reservoir rock mass under the combined action of macroscopic stress Σ, water pressure p and temperature T can be expressed as follows: Among them, W R Indicates containing n c Free enthalpy of rock with microcracks, W J represents the free enthalpy of the Jth group of dominant fractures. The last three terms on the right side of the equation represent the influence of temperature effect on the free enthalpy of rock mass. Where, is the macro strain generated by micro cracks, where β represents the macro volume strain generated by micro crack opening, γ represents the macro shear internal variable generated by micro crack slip, and n r represents the unit normal direction vector of the rth group of microcracks; The symbol for finding the symmetric component of the vector product of two vectors; is the elastic compliance tensor of the isotropic solid matrix; elastic constant H0 = 3E s / {16[1-(v s ) 2 ]}, H1=H0(1-v s / 2), E s and v s are the elastic modulus and Poisson's ratio of the rock matrix; δ represents the second-order unit tensor; The microcrack density parameter is an internal variable used to represent the microscopic damage of rock blocks, where The microcrack density represents the number of microcracks per unit volume, and a is the average radius of microcracks; and N are the isotropic Biot coefficient and Biot modulus of the rock mass, is the initial porosity of the rock block; : symbol represents the double dot product operation of the tensor; E J represents the macroscopic strain generated by the Jth group of cracks, E Jp It is E J The plastic component of S J is the average surface area of ​​the Jth group of fracture surfaces, Ω is the volume of the rock mass characterization unit, represents the average spacing of cracks, m J represents the unit normal direction vector of the Jth group of cracks; g represents the displacement vector of the crack, which includes elastic and plastic displacement vectors, among which the plastic displacement is dominated by the shear slip and dilatancy effect of the crack surface, g = g e +g p , The upper part e and p represent the elastic and plastic parts, T J , and H J are the force vector, Biot coefficient and elastic stiffness matrix of the Jth group of crack surfaces, T J =Σ·m J , H N represents the crack normal stiffness and H T represents the fracture tangential stiffness, which characterizes the mechanical response of the fracture normal and shear deformation respectively; T t and T n T J The tangential and normal components along the crack surface are: and T n =m J ·T J ; c F ,E T , represent the specific heat, thermal strain and equivalent elastic stiffness of the rock mass respectively; In step S32, the free enthalpy of the rock mass is differentiated with respect to Σ, and the mechanical constitutive equation of the rock mass containing multi-scale void structure under the thermal-hydraulic-mechanical coupling is obtained as follows: According to the relationship between strain and stress in the mechanical constitutive equation, the macroscopic stress can be expressed as a function of the macroscopic strain: Among them, E represents the total strain of the rock mass, which consists of four terms: the elastic strain generated by the rock matrix in the first term on the right side of the rock mass mechanics constitutive equation, the temperature strain in the second term, the strain generated by the change of microcrack structure in the third term, and the strain generated by the change of fracture structure in the fourth term.

6. The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to claim 5, characterized in that: In step S34, the changes in microscopic microcracks and macroscopic fracture structures inside the reservoir rock mass are represented by the internal variables d, β and γ that characterize the microcrack structure and the internal variable g that characterizes the fracture structure. p The evolution of the internal variables β and γ is determined by their corresponding conjugate thermodynamic forces F β and F γ Sure: Among them, the thermodynamic force F β and F γ Represents the normal and tangential effective stresses on the microcrack wall; when the microcrack opens, F β =0 and F γ =0, then we can directly use F β and F γ The expression is used to calculate β and γ. When the microcrack is in a closed state (F β <0), the associated Mohr-Coulomb criterion is used to determine the changes in β and γ caused by microcrack shear slip, F = |F γ |+F β tanφ c , simulate the slip dilatancy of microcracks, and use the hyperbola model to characterize the β change caused by the normal closure of microcracks, β = -F β β0 / (k0β0-F β ), where φ c , β0 and k0 are the internal friction angle, initial opening and initial normal stiffness of the microcrack, respectively; The evolution of the internal variable d is determined by the corresponding conjugate thermodynamic force F d The relevant function criteria determine: In the formula, V(d c ) is the maximum value of microcrack damage extension resistance, d c For critical damage, when the microcrack damage reaches the critical damage, the damage extension resistance takes the maximum value; The internal variable g that characterizes the plastic displacement of the crack p The evolution of is determined by the non-associative plastic flow law, where the yield function Y(σ n ,σ τ ) and the plastic potential function Q(σ n ,σ τ ) is as follows: Q(s n ,s τ )=|σ n sinα+s τ thing| Among them, c J and represent the cohesion and friction angle parameters of the Jth group of cracks respectively; α represents the equivalent undulation angle of the rough wall of the crack, α0 is the initial equivalent heave angle, is the wear coefficient of the undulating angle, which is used to simulate the wear and tear effect of the rough wall of the crack surface during the sliding friction process. is the accumulated dissipated plastic shear energy, in Represents the component of the crack plastic displacement along the tangential direction.

7. The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to claim 4, characterized in that: In step S4, the permeability tensor and porosity of the reservoir rock mass are calculated using the multi-scale evolution equation of rock mass permeability characteristics and the porosity evolution equation, including: The porosity evolution equation obtained by differentiating the free enthalpy of the rock mass with respect to p is: The permeability tensor of the reservoir rock mass is calculated using the volume averaging method, while taking into account the contribution of the solid matrix, microcrack structure, and fracture structure within the rock mass to the permeability of the rock mass. The cubic law is used to describe the permeability coefficient of microcracks and fractures. The expression of the permeability tensor of the reservoir rock mass is: Among them, k s is the permeability of the rock matrix, is the porosity; is the permeability of the microcrack when the initial opening is β0 and the initial damage is d0; X is the index introduced to characterize the exponential change of microcrack connectivity with damage evolution; e J Indicates the opening of the Jth group of cracks, the initial opening value is ζ is the fracture permeability penalty constant, a parameter introduced to characterize that the water conductivity of a real rough fracture wall is lower than that of a smooth parallel plate.

8. The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to claim 1, characterized in that: In step S5, the temperature, water pressure, and stress deformation of the reservoir during geothermal cycle mining are analyzed using an interleaved iterative solution method of seepage heat transfer analysis and mechanical analysis, as shown below: S51, according to the reservoir cycle injection and production strategy, set the total calculation time, set internal boundary conditions for the injection well and the production well, the injection well boundary conditions are set according to the mass flow rate and temperature of the injected fluid, and the production well flow boundary conditions are set according to the production flow rate; S52, performing a seepage heat transfer calculation based on the reservoir seepage heat transfer analysis model in each time step, and calculating the water pressure and temperature in the reservoir seepage heat transfer analysis model; S53, the water pressure and temperature in the reservoir seepage heat transfer analysis model are introduced into the reservoir mechanics calculation model, and the finite element method is used to solve the stress-strain process affected by the changes in water pressure and temperature. The changes in the microcracks and fracture structure of the reservoir rock mass are calculated, and the permeability and porosity of the rock mass are updated according to the changes in the multi-scale structure of the rock mass. S54, substituting the updated rock mass permeability and porosity into the reservoir seepage heat transfer analysis model to perform the seepage heat transfer solution, and re-determining the water pressure and temperature in the reservoir seepage heat transfer analysis model; S55, repeat the above steps S53 to S54 until the set total calculation time is reached, and obtain the reservoir temperature, water pressure, stress and deformation, rock permeability and internal variables used to describe the multi-scale structural changes of the reservoir rock during the reservoir cyclic mining process.

9. The method for analyzing the influence of the multi-scale structure of rock mass on the cyclic exploitation of deep geothermal reservoirs according to claim 1, characterized in that: The following steps are also included: S6, respectively analyzes the multi-scale structural combinations of different reservoir rock masses and different coupling effects, and obtains the temperature field, water pressure distribution, permeability, stress and deformation response of the reservoir during the cyclic exploitation of deep geothermal reservoirs under different multi-scale structural distributions of rock masses and different coupling effects, so as to further analyze the influence of micro-crack structure and macro-crack structure on the cyclic exploitation of deep geothermal reservoirs.

10. A computer program product, characterized in that It includes a computer program / instruction, which, when executed by a processor, implements the analysis method of the influence of the multi-scale structure of the rock mass on the cyclic exploitation of deep geothermal reservoirs as described in any one of claims 1-9.

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