A method and system for analyzing production-induced stress before refracturing of shale gas reservoirs

Abstract

The present invention discloses a method and system for analyzing production-induced stress before refracturing of shale gas reservoirs. By establishing an apparent permeability model to consider the complex gas flow behavior under multi-scale pore structures, and establishing an embedded discrete fracture model to couple the flow in the matrix-fracture and fracture-fracture systems; on this basis, a multi-scale fluid-solid coupling horizontal well seepage model for shale gas reservoirs is established, and the fully implicit finite volume method is used for solving to ensure the solving accuracy and efficiency. Therefore, it conforms to the complex fracture distribution and multi-scale flow mechanism, can more realistically reflect the law of in-situ stress field change before refracturing, is used to simulate the propagation of secondary hydraulic fractures, and is applicable to shale gas reservoirs with developed natural fractures and low porosity and low permeability.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional oil and gas exploitation, and more specifically, to a method and system for analyzing production-induced stress before refracturing of shale gas reservoirs. Background Art

[0002] Horizontal well fracturing is the core key technology for commercial development of shale gas. The concept of "stimulated reservoir volume (SRV)" was first proposed in the United States in 2006, and gradually formed the multi-cluster volume fracturing technology for horizontal wells in shale gas. During the initial exploration and development of shale gas, due to limitations and deviations in the understanding of reservoirs, it may lead to the mismatch between the optimized design and process implementation of fractured wells (or well sections) and reservoir geological conditions, resulting in insufficient reservoir stimulation, poor initial fracturing effects, or rapid decline in post-fracture production. Repeated fracturing practices in the five major shale gas producing areas in North America and repeated fracturing pilot tests in the Fuling and Changning shale gas demonstration areas in China have all shown that secondary fracturing can effectively improve the ultimate recovery rate of reservoirs.

[0003] One of the key technologies for repeated fracturing design lies in clarifying the laws of hydraulic fracture propagation and in-situ stress field changes under secondary fracturing, so as to maximize the reconstruction of fracture networks and expand the stimulated area. During reservoir production, the decrease in formation pore pressure will change the magnitude and direction of the in-situ stress field, thereby affecting the turning and extension of hydraulic fractures during repeated fracturing. At present, the repeated fracturing theory of conventional oil and gas provides the theoretical basis for the spatio-temporal changes of in-situ stress and reservoir pore pressure. However, for shale gas reservoirs with developed natural fractures and low porosity and low permeability, the calculation of production-induced stress before repeated fracturing needs to further consider complex fracture distributions and multi-scale flow mechanisms. Therefore, a production-induced stress calculation method applicable to shale gas reservoirs is needed for studying the laws of repeated fracturing fracture propagation.

[0004] In view of this, the present application is specifically proposed. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that the current repeated fracturing technology for conventional oil and gas does not consider complex fracture distributions and multi-scale flow mechanisms, and is not applicable to shale gas reservoirs with developed natural fractures and low porosity and low permeability. The purpose is to provide a method and system for analyzing production-induced stress before repeated fracturing of shale gas reservoirs, which simulate the flow mode of shale gas in the reservoir, the shale gas flow rate between matrix units and fracture units, and the transmission behavior of shale gas in matrix pores and fracture systems through modeling, analyze the multi-scale flow mechanisms of shale gas in complex fracture distributions and matrix pores, and obtain the analysis results of production-induced stress before repeated fracturing of shale gas reservoirs, so as to form a set of repeated fracturing technologies applicable to shale gas reservoirs with developed natural fractures and low porosity and low permeability.

[0006] The present invention is achieved by the following technical solutions:

[0007] On the one hand, the present invention provides a method for analyzing production-induced stress before refracturing of a shale gas reservoir, comprising the following steps:

[0008] Establish a multi-scale pore seepage apparent permeability model for the shale gas reservoir to characterize the flow pattern of shale gas in the reservoir;

[0009] Establish a fracture-matrix flow coupling model for the shale gas reservoir to characterize the shale gas flow rate between matrix units-fracture units and fracture units-fracture units under different non-adjacent connection forms;

[0010] According to the multi-scale pore seepage apparent permeability model of the shale gas reservoir and the fracture-matrix flow coupling model of the shale gas reservoir, establish a multi-scale fluid-solid coupling horizontal well seepage model for the shale gas reservoir to characterize the mass balance transport behavior of shale gas in the matrix pores and fracture system, as well as the change of the local stress field;

[0011] Initialize the model parameters, and use the model parameters and the multi-scale fluid-solid coupling horizontal well seepage model of the shale gas reservoir to perform numerical processing to obtain the analysis results of the production-induced stress before refracturing of the shale gas reservoir.

[0012] In the above method for analyzing production-induced stress before refracturing of a shale gas reservoir, the multi-scale pore seepage apparent permeability model of the shale gas reservoir is established on the basis of fully considering the multi-scale pore structure of the shale gas reservoir and the flow behavior characteristics of shale gas in the reservoir. The flow behavior of shale gas in the matrix pores and fracture system is characterized by the apparent permeability model, so as to simulate the complex flow behavior of shale gas in pores of different scales. The fracture-matrix flow coupling model of the shale gas reservoir, on the basis of simulating the flow behavior characteristics of shale gas in pores of different scales, further considers the mutual connection relationship between matrix units and fracture units, and simulates the flow rate of shale gas between fracture units and matrix units under different connection relationships, so as to reflect the flow distribution behavior of gas. The multi-scale fluid-solid coupling horizontal well seepage model of the shale gas reservoir is based on the permeability model and the fluid-solid coupling model, and at the same time considers the influence of mass balance and source-sink terms generated by the flow of shale gas in the reservoir, simulates the mass balance transport behavior of shale gas in the matrix pores and fracture system, and analyzes the production-induced stress before refracturing of the shale gas reservoir through fluid-solid coupling solution. Therefore, the method for analyzing production-induced stress before refracturing of the shale gas reservoir provided by the present invention conforms to the complex fracture distribution and multi-scale flow mechanism, can more realistically reflect the law of in-situ stress field change before refracturing and is used to simulate the propagation of secondary fractures, and is applicable to shale gas reservoirs with developed natural fractures and low porosity and low permeability.

[0013] As a further description of the present invention, the multi-scale pore seepage apparent permeability model of the shale gas reservoir includes:

[0014] A matrix apparent permeability model for characterizing the slip flow behavior, Knudsen diffusion flow behavior, and surface diffusion flow behavior of free gas and adsorbed gas existing in matrix pores;

[0015] A fracture permeability model for characterizing the migration mode of viscous flow of free gas in the fracture system.

[0016] As a further description of the present invention, the non-adjacent connection forms include: a fracture unit piercing through a matrix unit and connecting with the matrix unit, connection between adjacent fracture units in the same fracture, and connection between intersecting fracture units.

[0017] As a further description of the present invention, the method for establishing the multi-scale fluid-solid coupling horizontal well seepage model of the shale gas reservoir is as follows:

[0018] Establish a solid deformation control equation based on elasticity mechanics;

[0019] Establish a continuity equation for matrix pores according to the matrix apparent permeability model;

[0020] Establish a well index model, and establish a continuity equation for the fracture system according to the well index model.

[0021] As a further description of the present invention, the method of numerical processing includes the following steps:

[0022] S1: Divide the flow units. The matrix units are represented by two-dimensional Cartesian grid units, and the fracture units are one-dimensional units;

[0023] S2: Assign physical property parameters; obtain the pressure field of each matrix unit according to the physical property parameters to obtain the pressure field of matrix pores; obtain the pressure field of each fracture unit according to the physical property parameters to obtain the pressure field of the fracture system;

[0024] S3: Solve the solid deformation control equation by using the pressure field of matrix pores and the pressure field of the fracture system;

[0025] S4: According to the solution result of the solid deformation control equation, use the relaxation iteration method to obtain the volume strain of all matrix units and the average effective stress at the current time step;

[0026] S5: Update the physical property parameters by using the average effective stress, and return to S2;

[0027] S6: Iteratively execute S2 to S5 until the iteration time ends to obtain the analysis result of the production-induced stress before refracturing of the shale gas reservoir.

[0028] As a further description of the present invention, before numerical processing, the numerical discretization of the solid deformation control equation, the continuity equation of the matrix pores, and the continuity equation of the fracture system is performed by using the fully implicit finite volume method to obtain the numerical equations of the solid deformation control equation, the numerical equation of the continuity equation of the matrix pores, and the numerical equation of the continuity equation of the fracture system.

[0029] On the other hand, the present invention provides a production-induced stress analysis system before refracturing of a shale gas reservoir, including:

[0030] A permeability model establishment module for establishing a multi-scale pore seepage apparent permeability model of a shale gas reservoir;

[0031] A coupling model establishment module for establishing a fracture-matrix and fracture-fracture coupling model of a shale gas reservoir;

[0032] A seepage model establishment module for establishing a multi-scale fluid-solid coupling horizontal well seepage model of a shale gas reservoir according to the multi-scale pore seepage apparent permeability model of the shale gas reservoir and the flow coupling model of the shale gas reservoir;

[0033] A model parameter module for storing physical property parameters and assigning parameters to each model;

[0034] A numerical processing module for performing numerical processing by using the model parameters and the multi-scale fluid-solid coupling horizontal well seepage model of the shale gas reservoir to obtain the production-induced stress analysis result before refracturing of the shale gas reservoir.

[0035] As a further description of the present invention, the permeability model establishment module includes: a matrix apparent permeability model establishment unit and a fracture permeability model establishment unit; the seepage model establishment module includes: a solid deformation control equation establishment unit, a matrix pore continuity equation establishment unit, and a fracture system continuity equation establishment unit.

[0036] As a further description of the present invention, the numerical processing module includes:

[0037] A grid division unit for discretizing the matrix into two-dimensional grids and discretizing the fractures into one-dimensional elements based on a two-dimensional Cartesian grid;

[0038] A pressure field acquisition unit for obtaining the pressure field of each matrix unit according to the physical property parameters to obtain the pressure field of the matrix pores; obtaining the pressure field of each fracture unit according to the physical property parameters to obtain the pressure field of the fracture system;

[0039] A solid deformation control equation solving unit for solving the solid deformation control equation by using the pressure field of the matrix pores and the pressure field of the fracture system;

[0040] An average effective stress solving unit, which is used to obtain the volumetric strain of all matrix units and the average effective stress at the current time step by using relaxation iteration according to the solution result of the solid deformation control equation;

[0041] A physical property parameter updating unit, which is used to update the physical property parameters by using the average effective stress, and replace the parameters in the model parameter module with the updated physical property parameters;

[0042] An execution control unit, which is used to control the working sequence of each functional unit in the numerical processing module;

[0043] A timing unit, which is used to send an iteration time end signal to the execution control unit.

[0044] As a further description of the present invention, the production-induced stress analysis system before refracturing of a shale gas reservoir further includes: a numerical discretization module, which is used to numerically discretize the solid deformation control equation, the continuity equation of the matrix pores, and the continuity equation of the fracture system to obtain the numerical equations of the solid deformation control equation, the numerical equation of the continuity equation of the matrix pores, and the numerical equation of the continuity equation of the fracture system.

[0045] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0046] 1. A method and system for analyzing production-induced stress before refracturing of a shale gas reservoir provided by an embodiment of the present invention fully consider the complex fracture distribution and multi-scale flow mechanisms, are applicable to the analysis of production-induced stress in shale gas reservoirs with well-developed natural fractures and low porosity and low permeability, and are beneficial to the study of the fracture propagation law of refracturing;

[0047] 2. A method and system for analyzing production-induced stress before refracturing of a shale gas reservoir provided by an embodiment of the present invention can simulate the flow behavior, flow rate change, and transmission behavior of gas in a shale gas reservoir with multi-scale characteristics and complex flow mechanisms;

[0048] 3. A method and system for analyzing production-induced stress before refracturing of a shale gas reservoir provided by an embodiment of the present invention uses the fully implicit finite volume method for solution, ensuring the accuracy and efficiency of model solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0050] Figure 1 Schematic diagram of the distribution of random natural fractures and hydraulic fractures in the shale gas reservoir provided in Embodiment 1 of the present invention;

[0051] Figure 2 Flow chart for solving the seepage model of a multi-scale fluid-solid coupling horizontal well in shale provided in Embodiment 1 of the present invention;

[0052] Figure 3 Distribution map of pore pressure field after 10 years of production in the shale gas reservoir provided in Embodiment 1 of the present invention;

[0053] Figure 4 Distribution map of induced stress in the direction of the minimum horizontal principal stress after 10 years of production in the shale gas reservoir provided in Embodiment 1 of the present invention;

[0054] Figure 5 Distribution map of induced stress in the direction of the maximum horizontal principal stress after 10 years of production in the shale gas reservoir provided in Embodiment 1 of the present invention;

[0055] Figure 6 Distribution map of induced stress in the x direction under different production times of multiple fractures provided in Embodiment 1 of the present invention;

[0056] Figure 7 Distribution map of induced stress in the y direction under different production times of multiple fractures provided in Embodiment 1 of the present invention;

[0057] Figure 8 Schematic diagram of the disturbance of the production-induced stress on the horizontal stress field under an initial horizontal stress difference of 5 MPa provided in Embodiment 1 of the present invention. Detailed implementation manners

[0058] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below in conjunction with embodiments and drawings. The illustrative embodiments and descriptions thereof of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0059] In the following description, a large number of specific details are set forth in order to provide a thorough understanding of the present invention. However, it is obvious to those of ordinary skill in the art that the present invention does not have to employ these specific details.

[0060] Embodiment 1

[0061] Since the current conventional refracturing technology for oil and gas does not consider complex fracture distributions and multi-scale flow mechanisms, it is not applicable to shale gas reservoirs with well-developed natural fractures and low porosity and low permeability. To solve the above technical problems, this embodiment provides a method for analyzing production-induced stress before refracturing of a shale gas reservoir. In this method, it is known that the size of a partial flow region of a typical shale gas reservoir is 500 m × 500 m × 40 m, within which 1,300 natural fractures with lengths ranging from 5 to 15 m are randomly distributed, and there are 5 hydraulic fractures parallel to each other with a spacing of 30 m in the reservoir, as Figure 1 shown. Against this background, the method for analyzing production-induced stress before refracturing of a shale gas reservoir in this embodiment includes the following steps:

[0062] Step 1: Establish a multi-scale pore seepage apparent permeability model for the shale gas reservoir to characterize the flow pattern of shale gas in the reservoir. The multi-scale pore seepage apparent permeability model for the shale gas reservoir includes: a matrix apparent permeability model for characterizing the slip flow behavior, Knudsen diffusion flow behavior, and surface diffusion flow behavior of free gas and adsorbed gas existing in the matrix pores; and a fracture permeability model for characterizing the migration mode of viscous flow of free gas in the fracture system.

[0063] Specifically,

[0064] The pore structure of the shale gas reservoir is mainly composed of micro-nano matrix pores. Considering the random distribution of natural fractures and hydraulic fractures in the reservoir, its pore structure usually has multi-scale characteristics and complex flow mechanisms.

[0065] For the micro-nano shale matrix pores of shale, since free gas and adsorbed gas coexist in the matrix pores, during the production process, with the decrease of the reservoir pressure, there are slip flow, Knudsen diffusion, and surface diffusion flow behaviors. Therefore, this embodiment first establishes a matrix apparent permeability model to comprehensively characterize the phenomenon of coexistence of multiple flow behaviors. The mathematical expression of the matrix apparent permeability model is:

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073]

[0074] In formulas (1) to (8): k appm represents the apparent permeability of the matrix, with the unit of m 2 ; represents the weight factor of free gas slip flow, which is dimensionless; represents the weight factor of free gas Knudsen diffusion, which is dimensionless; k v represents the permeability considering the real gas slip flow regime, with the unit of m 2 ; K n represents the Knudsen number, which is dimensionless; λ g represents the mean free path of gas molecules, with the unit of m; r m represents the pore radius of the matrix, with the unit of m; r eff represents the effective pore radius of the shale matrix, with the unit of m; D k represents the real gas Knudsen diffusion coefficient, with the unit of m 2 / s; C gm represents the gas compressibility factor in the matrix, with the unit of 1 / Pa; μ gm represents the gas viscosity in the matrix, with the unit of Pa·s; M g represents the mass of gas molecules, with the unit of kg / mol; ρ gm represents the gas density in the matrix, with the unit of kg / m 3 ; D s represents the surface diffusion coefficient, with the unit of m 2 / s; ρ r represents the shale density, with the unit of kg / m 3 ; V std represents the molar volume of gas under standard conditions, with the unit of m 3 / mol; V L represents the Langmuir volume under reservoir conditions, with the unit of m 3 / kg; θ represents the coverage of real gas, which is dimensionless; p m represents the matrix system pressure, with the unit of Pa; Z represents the gas deviation factor, which is dimensionless; p L represents the Langmuir pressure under reservoir conditions, with the unit of Pa.

[0075] Furthermore, relative to the micro-nano scale pores of the matrix system, the fractures in the reservoir are larger in scale and there is no adsorbed gas. Therefore, only considering the migration mode of viscous flow of gas in the fractures, a fracture permeability model is established to characterize the migration mode of viscous flow of free gas in the fracture system. The mathematical expression of the fracture permeability model is:

[0076] In formula (9): kf denotes the fracture permeability, with the unit of m 2 ; w f denotes the fracture width, with the unit of m.

[0077] Step 2: Establish a flow coupling model for shale gas reservoirs to characterize the shale gas flow rates between fracture units - matrix units and between fracture units - fracture units under different non - adjacent connection forms. Among them, the non - adjacent connection forms include: a fracture unit piercing through a matrix unit and connecting with the matrix unit, connection between adjacent fracture units in the same fracture, and connection between intersecting fracture units.

[0078] Specifically,

[0079] For the flow coupling model of shale gas reservoirs, in this embodiment, an embedded discrete fracture model is first established to couple the flow between matrix - fracture units and between fracture units - fracture units. In the embedded discrete fracture model, the matrix unit is a two - dimensional Cartesian grid unit, and the fracture unit is a one - dimensional unit. Then, three non - adjacent connection forms are defined, namely, a fracture unit piercing through a matrix unit and connecting with it, connection between adjacent fracture units in the same fracture, and connection between intersecting fracture units. Based on the two - point flux approximation principle, the flow rate relationship between the above non - adjacent connections is: q = λ l T NNC Δp(10), where in Equation (10): λ l denotes the relative mobility of the fluid, which is dimensionless; T NNC denotes the conductivity between non - adjacent connections, with the unit of kg / (Pa·s); Δp denotes the pressure difference between non - adjacent connections, with the unit of Pa.

[0080] For the conductivity, the conductivities between the corresponding matrix unit and fracture unit under the above three non - adjacent connection forms are considered respectively as follows:

[0081] If a fracture unit pierces through a matrix unit and connects with it, the conductivity between the corresponding matrix unit and fracture unit is:

[0082] Wherein,

[0083]

[0084]

[0085] In Equations (11) - (13): A mf denotes the unilateral area of the fracture segment embedded in the matrix unit, with the unit of m 2 ; k mf denotes the non - adjacent connection permeability, with the unit of m 2 ; k fDenotes the permeability of the fracture element, with the unit of m 2 ; k m Denotes the permeability of the matrix element, with the unit of m 2 ; d fm Denotes the average normal distance from the matrix element to the fracture element, with the unit of m; V denotes the volume of the matrix element, with the unit of m 3 .

[0086] In the same fracture, the conductivity between adjacent fracture elements is:

[0087] Among them,

[0088]

[0089]

[0090] In equations (14) to (16): k f1 Denotes the permeability of fracture element 1, with the unit of m 2 ; k f2 Denotes the permeability of fracture element 2, with the unit of m 2 ; A c Denotes the area of the common surface of adjacent fracture elements, with the unit of m 2 ; d s1 Denotes the distance from the centroid of fracture element 1 to the common surface, with the unit of m; d s2 Denotes the distance from the centroid of fracture element 2 to the common surface, with the unit of m.

[0091] If the intersecting fracture elements are connected, the conductivity between the fracture elements at the intersection is:

[0092]

[0093] Among them,

[0094]

[0095]

[0096]

[0097]

[0098] In equations (17) to (21): L int Denotes the length of the intersecting line, with the unit of m; d f1 and d f2 Are respectively the average normal distances from the centroid of the fracture element to the intersecting line, with the unit of m. S i Denotes the area of fracture segment i, with the unit of m 2 .

[0099] In the actual production simulation process, the source-sink term is usually set in the fracture element. Based on the Peaceman model, the well index is expressed as:

[0100]

[0101] Among them,

[0102]

[0103] In equations (22) to (23): w f represents the width of the fracture element, with the unit of m; k f represents the permeability of the fracture element, with the unit of m 2 ; r e represents the equivalent radius of the wellbore, with the unit of m; r w represents the radius of the wellbore, with the unit of m; d xf represents the length of the fracture element, with the unit of m; h f represents the height (reservoir thickness) of the fracture element, with the unit of m.

[0104] Step 3: According to the multi-scale pore seepage apparent permeability model of the shale gas reservoir and the flow coupling model of the shale gas reservoir, establish a multi-scale fluid-solid coupling horizontal well seepage model for the shale gas reservoir to characterize the mass balance transport behavior and local stress field changes of the shale gas in the matrix pores and fracture system.

[0105] The method for establishing the multi-scale fluid-solid coupling horizontal well seepage model for the shale gas reservoir includes the following steps:

[0106] S1: Establish a solid deformation control equation based on elasticity theory;

[0107] S2: Establish a continuity equation for the matrix pores according to the matrix apparent permeability model;

[0108] S3: Establish a well index model, and establish a continuity equation for the fracture system according to the well index model.

[0109] Specifically,

[0110] Assume that the shale is an elastic medium with small deformation, and the reservoir and fluid temperatures are constant. Based on the elasticity theory, establish a solid deformation control equation:

[0111]

[0112] Among them,

[0113]

[0114]

[0115]

[0116]

[0117] In equations (24) to (28): G represents the shear modulus with the unit of Pa; λ represents the first Lame constant with the unit of Pa; ε v represents the volume strain, which is dimensionless; ε s represents the desorption-induced volume strain, which is dimensionless; ε L represents the maximum desorption volume strain, which is dimensionless; p L represents the Langmuir pressure with the unit of Pa; p m represents the matrix pressure with the unit of Pa; u represents the displacement vector with the unit of m; E represents the Young's modulus with the unit of Pa; ν represents the Poisson's ratio, which is a dimensionless quantity.

[0118] Under the original conditions, the formation is in equilibrium and there is no disturbance from production wells. Therefore, it can be assumed that the displacement of the shale boundary under the initial conditions is zero; due to the sufficiently large solution domain and the extremely low permeability of the shale gas reservoir, it is assumed that there is no displacement at the boundary.

[0119] It is assumed that the gas in the shale gas reservoir is single-component methane gas, the reservoir temperature is constant, and the gas adsorption and desorption satisfy the Langmuir isothermal adsorption law. Under the condition of well-developed natural fractures, considering the storage form of coexistence of free gas and adsorbed gas in the pores of organic matter shale, and at the same time the matrix pores supply gas to the fracture system, based on the apparent permeability model of the shale matrix, its continuity equation is:

[0120]

[0121] The mass flow exchange term between the matrix system and the fracture system is:

[0122]

[0123] The gas adsorption amount per unit shale volume is:

[0124] q μ =(1 - φ m )M g c μ (31)

[0125] In equations (29) to (31): ρ gm represents the density of the gas in the matrix system with the unit of kg / m 3 ; μ gm represents the viscosity of the gas in the matrix system with the unit of Pa·s; k appm represents the apparent permeability of the gas in the matrix system with the unit of m 2 ; φ mDenotes the effective porosity in the matrix system, which is dimensionless; q μ Denotes the adsorbed gas volume per unit shale volume, with the unit of kg / m 3 ; W mf Denotes the mass flow exchange term between the matrix system and the fracture system, with the unit of kg / s; p m Denotes the pore pressure in the matrix system, with the unit of Pa.

[0126] Since there is only viscous flow in the fracture system. Considering the mass exchange term between the matrix pores and the fractures and the presence of the production well, the continuity equation for the fracture system is:

[0127]

[0128] Based on the well index model established in Step 2, the production term for the fracture unit points with production wells is:

[0129]

[0130] In Eqs. (32) - (33): q well Denotes the production term, with the unit of kg / s; p f Denotes the pressure within the hydraulic fracture unit, with the unit of Pa; p wf Denotes the bottom-hole flowing pressure, with the unit of Pa; ρ gf Denotes the density of the gas in the fracture system, with the unit of kg / m 3 ; μ gf Denotes the viscosity of the gas in the matrix system, with the unit of Pa·s.

[0131] Step 4: Numerically discretize the solid deformation control equation, the continuity equation of the matrix pores, and the continuity equation of the fracture system using the fully implicit finite volume method to obtain the numerical equations of the solid deformation control equation, the numerical equation of the continuity equation of the matrix pores, and the numerical equation of the continuity equation of the fracture system.

[0132] Specifically,

[0133] Both the reservoir apparent permeability and the adsorption - desorption function are non - linear functions of pressure, and the shale multi - scale flow - solid coupling horizontal well seepage model has strong non - linearity. To improve the computational efficiency, stability, and accuracy of the model, the fully implicit finite volume method is used to numerically discretize the shale gas reservoir flow equation and the solid deformation equation.

[0134] First, numerically discretize and organize Eq. (24) based on the finite volume method to obtain the numerical equation of the solid deformation equation:

[0135]

[0136] Among them,

[0137]

[0138]

[0139]

[0140]

[0141] Then, substitute and simplify equations (29) to (31), and based on the finite volume method for implicit discretization, the numerical equation of the gas seepage equation in the matrix system can be obtained:

[0142]

[0143] In equation (39), Γ gm is the conductivity between adjacent matrix units, and its essence is the harmonic mean of the mobilities between adjacent matrix units. Based on the two-point flow similarity principle, the conductivity between adjacent fracture surfaces is:

[0144]

[0145]

[0146] In equations (39) to (41): A y = Δxh f ; A x = Δyh f ; V i,j = ΔxΔyh f ; Δx is the size of the matrix unit in the x direction, with the unit of m; Δy is the size of the matrix unit in the y direction, with the unit of m; h f is the reservoir thickness (fracture height), with the unit of m.

[0147] The conductivity T ij,k between the matrix unit (i, j) and the fracture unit k is:

[0148]

[0149] Next, substitute and simplify equations (32) and (33), and based on the finite volume method for implicit discretization, the numerical equation of the gas seepage equation in the fracture system can be obtained:

[0150]

[0151] Among them,

[0152]

[0153]

[0154]

[0155]

[0156] Step 5: Initialize the model parameters, and perform numerical processing using the model parameters and the multi-scale fluid-solid coupling horizontal well seepage model of the shale gas reservoir to obtain the production-induced stress analysis results before refracturing of the shale gas reservoir. Among them, the numerical processing method can be realized through the following steps:

[0157] S1: Divide the flow units. The matrix units are represented by two-dimensional Cartesian grid units, and the fractures are discretized into one-dimensional units;

[0158] S2: Assign physical property parameters; obtain the pressure field of each matrix unit according to the physical property parameters to obtain the pressure field of the matrix pores; obtain the pressure field of each fracture unit according to the physical property parameters to obtain the pressure field of the fracture system;

[0159] S3: Solve the solid deformation control equation using the pressure field of the matrix pores and the pressure field of the fracture system;

[0160] S4: According to the solution results of the solid deformation control equation, use the relaxation iteration method to obtain the volume strain of all matrix units and the average effective stress at the current time step;

[0161] S5: Update the physical property parameters using the average effective stress, and return to S2;

[0162] S6: Iteratively execute S2 to S5 until the iteration time ends to obtain the production-induced stress analysis results before refracturing of the shale gas reservoir.

[0163] The numerical processing flow of the multi-scale fluid-solid coupling horizontal well seepage model of shale can refer to Figure 2 .

[0164] Specifically,

[0165] The detailed input parameters of the model are shown in Table 1.

[0166] Table 1 Model input parameters of the embodiment

[0167]

[0168]

[0169] Substitute the reservoir physical property parameters in Table 1 into the numerical model in Step 4, and the distributions of the pressure field and stress field before refracturing of the shale gas reservoir can be obtained through programming calculations. Figure 3 Figure 50 shows the pore pressure field distribution after 10 years of production in the shale gas reservoir of the embodiment; Figure 4Induced stress distribution in the direction of the minimum horizontal principal stress after 10 years of production in the shale reservoir of the first embodiment; Figure 5 Induced stress distribution in the direction of the maximum horizontal principal stress after 10 years of production in the shale gas reservoir of the first embodiment.

[0170] In the above step four,

[0171] To investigate the influence of production time on the stress field before refracturing in the shale reservoir, under the condition of not considering the influence of natural fractures, the induced stress distributions of 5 hydraulic fractures in the x-direction and y-direction at different production times are respectively as Figure 6 and Figure 7 shown. It can be seen from the simulation results that at a fracture spacing of 30m, when the production time increases from 1 year to 10 years, the maximum induced stress in the x-direction increases from -0.68MPa to -2.2MPa; while the maximum induced stress in the y-direction increases from -1.5MPa to -3.8MPa. Generally speaking, the longer the production time, the greater the amplitude of the induced stress caused by the decrease in pore pressure in the horizontal direction.

[0172] To investigate the influence of production time on the stress field before refracturing in the shale reservoir, under the condition of not considering the influence of natural fractures, it can be seen from the simulation results that as the production time increases, the induced stresses in the x-direction and y-direction increase synchronously. Therefore, the initial horizontal stress difference determines the disturbance of the production-induced stress on the stress field. Figure 8 The disturbance result of the production-induced stress on the horizontal stress field under an initial horizontal stress difference of 5MPa. Taking the reservoir production of 10 years as an example, under the condition of a horizontal stress difference of 5MPa, the decrease in pore pressure can only reduce the horizontal stress difference to an extreme value of 3.4MPa, and the production-induced stress can only reduce the horizontal stress difference. Therefore, it is difficult to make the fracture turn and extend in a large range to communicate the area not communicated by the primary hydraulic fracture during refracturing, and auxiliary processes such as forced fracture turning should be considered.

[0173] In summary, the analysis method of production-induced stress before refracturing in the shale gas reservoir provided in this embodiment overcomes the current situation of imperfect stress field analysis before refracturing in the existing shale gas reservoir. By establishing an apparent permeability model to consider the complex gas flow behavior under the multi-scale pore structure, and establishing an embedded discrete fracture model to couple the matrix-fracture and fracture-fracture flows; on this basis, a multi-scale fluid-solid coupling horizontal well seepage model for the shale gas reservoir is established, and the full implicit finite volume method is used for solving to ensure the solving accuracy and efficiency. This embodiment establishes an analysis method for the in-situ stress before refracturing in the shale gas reservoir, which is of great significance for understanding the fracture propagation and parameter optimization design of refracturing in the shale gas reservoir.

[0174] Embodiment 2

[0175] This embodiment provides a system corresponding to the method for analyzing production-induced stress before refracturing of a shale gas reservoir described in Embodiment 1, including:

[0176] A permeability model establishment module, configured to establish a multi-scale pore seepage apparent permeability model for the shale gas reservoir;

[0177] A coupling model establishment module, configured to establish a fracture-matrix and fracture-fracture flow coupling model for the shale gas reservoir;

[0178] A seepage model establishment module, configured to establish a multi-scale fluid-solid coupling horizontal well seepage model for the shale gas reservoir according to the multi-scale pore seepage apparent permeability model of the shale gas reservoir and the flow coupling model of the shale gas reservoir;

[0179] A model parameter module, configured to store physical property parameters and assign parameters to each model;

[0180] A numerical processing module, configured to perform numerical processing by using the model parameters and the multi-scale fluid-solid coupling horizontal well seepage model of the shale gas reservoir to obtain the analysis result of production-induced stress before refracturing of the shale gas reservoir.

[0181] By using the above system, the flow behavior, flow rate change and transmission behavior of gas in a shale gas reservoir with multi-scale characteristics and complex flow mechanisms can be simulated, so as to obtain the analysis result of production-induced stress before refracturing of the shale gas reservoir, and this analysis result can be used to understand the fracture propagation and parameter optimization design of the shale gas reservoir during refracturing.

[0182] Among them,

[0183] The permeability model establishment module includes: a matrix apparent permeability model establishment unit and a fracture permeability model establishment unit; the seepage model establishment module includes: a solid deformation control equation establishment unit, a matrix pore continuity equation establishment unit and a fracture system continuity equation establishment unit.

[0184] The numerical processing module includes:

[0185] A grid division unit, configured to discretize the matrix into two-dimensional grids and discretize the fractures into one-dimensional elements based on a two-dimensional Cartesian grid;

[0186] A pressure field acquisition unit, configured to obtain the pressure field of each matrix unit according to the physical property parameters to obtain the pressure field of the matrix pores; obtain the pressure field of each fracture unit according to the physical property parameters to obtain the pressure field of the fracture system;

[0187] A solid deformation control equation solving unit, configured to solve the solid deformation control equation by using the pressure field of the matrix pores and the pressure field of the fracture system;

[0188] An average effective stress solving unit, which is used to obtain the volumetric strain of all matrix units and the average effective stress at the current time step by means of relaxation iteration according to the solution result of the solid deformation control equation;

[0189] A physical property parameter updating unit, which is used to update the physical property parameters by using the average effective stress, and replace the parameters in the model parameter module with the updated physical property parameters;

[0190] An execution control unit, which is used to control the working sequence of each functional unit in the numerical processing module;

[0191] A timing unit, which is used to send an iteration time end signal to the execution control unit.

[0192] In addition,

[0193] The shale gas reservoir repeated fracturing pre-production induced stress analysis system further includes: a numerical discretization module, which is used to numerically discretize the solid deformation control equation, the continuity equation of the matrix pores, and the continuity equation of the fracture system to obtain the numerical equations of the solid deformation control equation, the numerical equations of the continuity equation of the matrix pores, and the numerical equations of the continuity equation of the fracture system.

[0194] The model solving accuracy and efficiency can be further improved by the fully implicit numerical discretization module.

[0195] The above specific embodiments have further elaborated on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for analyzing production-induced stress before refracturing of a shale gas reservoir, characterized in that Including the following steps: Establish a multi-scale pore seepage apparent permeability model for shale gas reservoirs to characterize the flow pattern of shale gas in the reservoir. Among them, the multi-scale pore seepage apparent permeability model for shale gas reservoirs includes: a matrix apparent permeability model used to characterize the slip flow behavior, Knudsen diffusion flow behavior, and surface diffusion flow behavior of free gas and adsorbed gas in matrix pores; a fracture permeability model used to characterize the migration pattern of viscous flow of free gas in the fracture system. Establish a coupled model of fracture and matrix flow in shale gas reservoirs to characterize the shale gas flow rate between matrix units - fracture units and fracture units - fracture units under different non-adjacent connection forms. Based on the multi-scale pore seepage apparent permeability model of the shale gas reservoir and the coupled model of fracture and matrix flow in the shale gas reservoir, establish a multi-scale fluid-solid coupling horizontal well seepage model for the shale gas reservoir to characterize the mass balance transport behavior of shale gas in matrix pores and the fracture system, as well as the change of the local stress field. Among them, the method for establishing the multi-scale fluid-solid coupling horizontal well seepage model of the shale gas reservoir is: establish a solid deformation control equation based on elasticity mechanics; establish a continuity equation for matrix pores according to the matrix apparent permeability model; establish a well index model, and establish a continuity equation for the fracture system according to the well index model. Initialize the model parameters, and use the model parameters and the multi-scale fluid-solid coupling horizontal well seepage model of the shale gas reservoir for numerical processing to obtain the analysis results of production-induced stress before refracturing of the shale gas reservoir. Among them, the method of the numerical processing includes the following steps: S1: Divide the flow units. The matrix units are represented by two-dimensional Cartesian grid units, and the fracture units are one-dimensional units. S2: Assign physical property parameters; obtain the pressure field of each matrix unit according to the physical property parameters to obtain the pressure field of matrix pores; obtain the pressure field of each fracture unit according to the physical property parameters to obtain the pressure field of the fracture system. S3: Solve the solid deformation control equation using the pressure field of matrix pores and the pressure field of the fracture system. S4: According to the solution results of the solid deformation control equation, use the relaxation iteration method to obtain the volume strain of all matrix units and the average effective stress at the current time step. S5: Update the physical property parameters using the average effective stress, and return to S2. S6: Iteratively execute S2 to S5 until the iteration time ends to obtain the analysis results of production-induced stress before refracturing of the shale gas reservoir.

2. The method for analyzing production-induced stress before refracturing of a shale gas reservoir according to claim 1, characterized in that Before numerical processing, use the fully implicit finite volume method to numerically discretize the solid deformation control equation, the continuity equation of matrix pores, and the continuity equation of the fracture system to obtain the numerical equations of the solid deformation control equation, the numerical equations of the continuity equation of matrix pores, and the numerical equations of the continuity equation of the fracture system.

3. The method for analyzing production-induced stress before refracturing of a shale gas reservoir according to claim 1, characterized in that The non-adjacent connection forms include: a fracture unit piercing through a matrix unit and connecting with the matrix unit, connection between adjacent fracture units in the same fracture, and connection between intersecting fracture units.

4. A system for analyzing production-induced stress before refracturing of a shale gas reservoir, characterized in that Including: The permeability model establishment module is used to establish the multi-scale pore seepage apparent permeability model of shale gas reservoir; wherein, the permeability model establishment module includes: a matrix apparent permeability model establishment unit and a fracture permeability model establishment unit; the matrix apparent permeability model establishment unit is used to construct a matrix apparent permeability model characterizing the slip flow behavior, Knudsen diffusion flow behavior and surface diffusion flow behavior of free gas and adsorbed gas existing in matrix pores; the fracture permeability model establishment unit is used to construct a fracture permeability model characterizing the migration mode of viscous flow of free gas in the fracture system; The coupling model establishment module is used to establish the fracture-matrix and fracture-fracture flow coupling model of shale gas reservoir; The seepage model establishment module is used to establish the multi-scale fluid-solid coupling horizontal well seepage model of shale gas reservoir according to the multi-scale pore seepage apparent permeability model of shale gas reservoir and the fracture-matrix and fracture-fracture flow coupling model of shale gas reservoir; the seepage model establishment module includes: a solid deformation control equation establishment unit, a matrix pore continuity equation establishment unit and a fracture system continuity equation establishment unit; Wherein, the method for establishing the multi-scale fluid-solid coupling horizontal well seepage model of shale gas reservoir is: establishing a solid deformation control equation based on elasticity mechanics; establishing a continuity equation of matrix pores according to the matrix apparent permeability model; establishing a well index model and establishing a continuity equation of the fracture system according to the well index model; The model parameter module is used to store physical property parameters and assign parameters to each model; The numerical processing module is used to perform numerical processing by using the model parameters and the multi-scale fluid-solid coupling horizontal well seepage model of shale gas reservoir to obtain the analysis result of production-induced stress before refracturing of shale gas reservoir; Wherein, the numerical processing module includes: The grid division unit is used to discretize the matrix into two-dimensional grids and the fractures into one-dimensional units based on two-dimensional Cartesian grids; The pressure field acquisition unit is used to acquire the pressure field of each matrix unit according to the physical property parameters to obtain the pressure field of matrix pores; and acquire the pressure field of each fracture unit according to the physical property parameters to obtain the pressure field of the fracture system; The solid deformation control equation solving unit is used to solve the solid deformation control equation by using the pressure field of matrix pores and the pressure field of the fracture system; The average effective stress solving unit is used to obtain the volume strain of all matrix units and the average effective stress at the current time step by using relaxation iteration according to the solution result of the solid deformation control equation; The physical property parameter updating unit is used to update the physical property parameters by using the average effective stress and replace the parameters in the model parameter module with the updated physical property parameters; The execution control unit is used to control the working sequence of each functional unit in the numerical processing module; The timing unit is used to send an iteration time end signal to the execution control unit.

5. The system for analyzing production-induced stress before refracturing of a shale gas reservoir according to claim 4, characterized in that Including: A numerical discretization module, configured to numerically discretize the solid deformation control equation, the continuity equation of the matrix pores, and the continuity equation of the fracture system, to obtain the numerical equations of the solid deformation control equation, the numerical equation of the continuity equation of the matrix pores, and the numerical equation of the continuity equation of the fracture system.

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