Solution method for matrix-natural fracture dual medium seepage model

Abstract

The present invention provides a method for solving a matrix-natural fracture dual medium seepage model, which relates to the technical field of medium seepage model, and the specific steps include: S1. Collecting the porosity of the matrix system and the fracture system, the density of the fracturing return fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and time data, according to the collected porosity of the matrix system and the fracture system, the density of the fracturing return fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and time data. The present invention makes the groundwater seepage channel more uniform and homogeneous through the seepage differential equation, the initial conditions of the seepage phenomenon at the initial moment, the inner boundary conditions and outer boundary conditions of the dual medium, and the function of the crack width changing with the spatial position, thereby improving the integrity of the seepage model, as well as the prediction accuracy of the flow direction, flow velocity and water level change of groundwater, and avoiding large deviations between the simulation results and the actual situation.

Description

Technical Field

[0001] The invention relates to the technical field of medium seepage models, in particular to a method for solving a matrix-natural fracture dual medium seepage model. Background Art

[0002] In the prior art, the publication number CN113642272A discloses a method for establishing a shale oil and gas reservoir seepage-horizontal wellbore flow coupling model, including the following steps: setting a shale oil and gas reservoir horizontal well water fracturing to form m fractures; establishing a matrix-natural fracture seepage model; establishing a natural fracture-artificial fracture seepage model; establishing an artificial fracture-wellbore seepage model; establishing a horizontal wellbore flow model; and establishing a shale oil and gas reservoir seepage-horizontal wellbore flow coupling model based on the continuity of pressure and the mass conservation theorem. The present invention finally achieves the establishment of a shale oil and gas reservoir seepage-horizontal wellbore flow model by establishing three seepage models and one flow model, so that the shale oil and gas reservoir situation can be calculated in real time according to the change of time in practice.

[0003] However, there are still the following shortcomings: the above scheme does not take into account the impact of cracks and faults on groundwater seepage, but cracks and faults are important seepage channels in shale oil and gas reservoirs. Cracks and faults, as gaps and broken zones in the medium, have a significant impact on groundwater seepage and may change the direction, speed and path of the seepage channel. The existence of cracks and faults will lead to the unevenness and heterogeneity of the groundwater seepage channel and the incompleteness of the medium seepage model, which will lead to a decrease in the prediction accuracy of the groundwater flow direction, flow velocity and water level change, resulting in a large deviation between the simulation results and the actual situation.

[0004] The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not constitute the prior art that is already known to one of ordinary skill in the art. Summary of the invention

[0005] The purpose of the present invention is to provide a method for solving a matrix-natural fracture dual medium seepage model to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for solving a matrix-natural fracture dual medium seepage model, the specific steps of which include:

[0008] S1. Collect the porosity of the matrix system and the fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and the time data, and construct the continuity equations of the matrix system and the natural fracture system respectively according to the collected porosity of the matrix system and the fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and the time data;

[0009] S2. When the fracturing flowback fluid seeps in the dual medium, there is seepage inside the porous rock block, seepage along the cracks, and exchange of fracturing flowback fluid between the rock block and the cracks. The seepage in the matrix and the natural cracks conforms to Darcy's law. Based on Darcy's law, the motion equations of the seepage velocity of the fracturing flowback fluid in the matrix system and the natural crack system are constructed respectively. The influence of cracks and faults on the seepage channel is used to construct the coefficients that affect the seepage velocity of the fracturing flowback fluid in the matrix system and the natural crack system.

[0010] S3. combining the motion equations of the seepage velocity of the fracturing flowback fluid of the matrix system and the natural fracture system and the coefficient of the seepage velocity to generate new motion equations of the fracturing flowback fluid of the matrix system and the natural fracture system, respectively constructing the state equations of the density of the fracturing flowback fluid of the matrix system and the natural fracture system according to the new motion equations of the seepage velocity of the fracturing flowback fluid, and respectively obtaining the state equations of the porosity of the matrix system and the natural fracture system when the pressure is p0 through Maclaurin series expansion;

[0011] S4. constructing the state equation of the fluid of the fracturing flowback fluid according to the state equation of the porosity of the matrix system and the natural fracture system, and constructing the characteristic equation of the cross-flow rate between the fracture medium and the rock medium according to the state equation of the density of the fracturing flowback fluid of the matrix system and the natural fracture system and the state equation of the fluid of the fracturing flowback fluid;

[0012] S5. Input the motion equation of the seepage velocity of the fracturing flowback fluid in the matrix system and the natural fracture system, the state equation of the porosity of the matrix system and the natural fracture system, the state equation of the fluid of the fracturing flowback fluid, and the characteristic equation of the cross-flow rate between the fracture medium and the rock medium into the continuity equation of the matrix system and the natural fracture system, and obtain the seepage differential equation of the fracturing flowback fluid in the matrix medium and the fracture medium respectively;

[0013] S6. converting the seepage differential equations of the fracturing flowback fluid in the matrix medium and the fracture medium into the seepage differential equations based on polar coordinates, and constructing the initial conditions of the seepage phenomenon at the initial moment, the inner boundary conditions and the outer boundary conditions of the dual medium according to the seepage differential equations in polar coordinates;

[0014] S7. Construct a matrix-natural fracture dual medium seepage model based on the seepage differential equation of fracturing flowback fluid in matrix medium and fracture medium, the initial conditions of the seepage phenomenon at the initial time, the inner boundary conditions and outer boundary conditions of the dual medium, and the function of the fracture width changing with the spatial position;

[0015] S8. Use the finite difference method to solve the distribution of groundwater pressure field and velocity field under given initial conditions and boundary conditions, and predict the flow direction, velocity and water level changes of groundwater.

[0016] Furthermore, based on the collected porosity of the matrix system and fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow between the fracture medium and the rock medium, and time data, the continuity equation of the matrix system is constructed based on the following formula:

[0017]

[0018] Among them, φ m is the porosity of the matrix system;

[0019] ρ is the density of fracturing flowback fluid at any pressure (kg / m 3 );

[0020] v m is the flow velocity of fracturing flowback fluid in the matrix system (m / s),

[0021] q λ is the cross-flow rate between fracture medium and rock medium (m 3 / s);

[0022] t is time (s);

[0023] The continuity equation of the natural fracture system is constructed based on the following formula:

[0024]

[0025] Among them, φ f is the porosity of the fracture system;

[0026] ρ is the density of fracturing flowback fluid at any pressure (kg / m 3 );

[0027] v f is the seepage velocity of fracturing flowback fluid in the fracture system (m / s);

[0028] q λ is the cross-flow rate between fracture medium and rock medium (m 3 / s);

[0029] t is time (s).

[0030] Furthermore, the motion equation of the seepage velocity of the fracturing flowback fluid in the matrix system is constructed based on the following formula:

[0031]

[0032] Among them, v m is the seepage velocity of fracturing flowback fluid in the matrix system (m / s);

[0033] k m is the permeability of the matrix system (Dc);

[0034] μ is the viscosity of fracturing flowback fluid (Pa·s);

[0035] p m is the pressure of the matrix system (Pa);

[0036] The motion equation of the seepage velocity of the fracturing flowback fluid in the natural fracture system is constructed based on the following formula:

[0037]

[0038] Among them, v f is the seepage velocity of fracturing flowback fluid in natural fracture system (m / s);

[0039] k f is the permeability of the natural fracture system (Dc);

[0040] μ is the viscosity of fracturing flowback fluid (Pa·s);

[0041] p f is the pressure of the natural fracture system (Pa);

[0042] By using the influence of fractures and faults on seepage channels, the coefficients that affect the seepage velocity of fracturing flowback fluid in the matrix system and natural fracture system are constructed based on the following formula:

[0043] C=C0·f(w,ρ)

[0044] Where, C is the coefficient of seepage velocity;

[0045] C0 is the coefficient of seepage velocity when there are no cracks and faults;

[0046] w is the crack width;

[0047] ρ is the crack density;

[0048] f(w, ρ) is the function of the effect of fractures and faults on the seepage channel;

[0049] The motion equations of the flow velocity of the fracturing return fluid in the matrix system and the natural fracture system and the coefficients of the flow velocity are combined to generate new motion equations of the fracturing return fluid in the matrix system and the natural fracture system, based on the following formula:

[0050] ν m new =C·ν m

[0051] Among them, ν m new is the new seepage velocity of the fracturing flowback fluid in the matrix system (m / s);

[0052] ν f new =C·ν f

[0053] Among them, ν f new is the new seepage velocity of the fracturing flowback fluid in the natural fracture system (m / s).

[0054] Furthermore, the state equation of the density of the fracturing flowback fluid of the matrix system is constructed, and the state equation of the porosity of the matrix system when the pressure is p0 is obtained through the Maclaurin series expansion, and the formula is as follows:

[0055]

[0056] Where ρ0 is the density of the fracturing flowback fluid under pressure p0 (kg / m 3 );

[0057] φ m0 is the porosity of the matrix system at pressure p0;

[0058] C p is the compression coefficient of fracturing flowback fluid;

[0059] C m is the compressibility factor of the matrix system;

[0060] ρ m is the density of the fracturing flowback fluid in the matrix system (kg / m 3 );

[0061] The state equation of the porosity of the natural fracture system when the pressure is p0 is obtained based on the following formula:

[0062]

[0063] Where ρ0 is the density of the fracturing flowback fluid under pressure p0 (kg / m 3 );

[0064] φ f0 is the porosity of the matrix system at pressure p0;

[0065] C p is the compression coefficient of fracturing flowback fluid;

[0066] C f is the compression factor of the natural fracture system;

[0067] ρ f is the density of fracturing flowback fluid in natural fracture system (kg / m 3 );

[0068] According to the state equation of the porosity of the matrix system and the natural fracture system, the state equation of the fracturing flowback fluid is constructed based on the following formula:

[0069] ρ=ρ0[1+C p (p l -p0)]

[0070] Where ρ is the density of fracturing flowback fluid at any pressure (kg / m 3 );

[0071] ρ0 is the density of the fracturing flowback fluid at pressure p0 (kg / m 3 );

[0072] C p is the compression coefficient of fracturing flowback fluid;

[0073] p l is the pressure of the fracturing flowback fluid (Pa).

[0074] Furthermore, the state equation of the density of the fracturing flowback fluid of the matrix system is constructed, and the state equation of the porosity of the matrix system when the pressure is p0 is obtained through the Maclaurin series expansion, and the formula is as follows:

[0075]

[0076] Where ρ0 is the density of the fracturing flowback fluid under pressure p0 (kg / m 3 );

[0077] φ m0 is the porosity of the matrix system at pressure p0;

[0078] C p is the compression coefficient of fracturing flowback fluid;

[0079] C m is the compressibility factor of the matrix system;

[0080] ρ mis the density of the fracturing flowback fluid in the matrix system (kg / m 3 );

[0081] The state equation of the porosity of the natural fracture system when the pressure is p0 is obtained based on the following formula:

[0082]

[0083] Where ρ0 is the density of the fracturing flowback fluid under pressure p0 (kg / m 3 );

[0084] φ f0 is the porosity of the matrix system at pressure p0;

[0085] C p is the compression coefficient of fracturing flowback fluid;

[0086] C f is the compression factor of the natural fracture system;

[0087] ρ f is the density of fracturing flowback fluid in natural fracture system (kg / m 3 );

[0088] According to the state equation of the porosity of the matrix system and the natural fracture system, the state equation of the fracturing flowback fluid is constructed based on the following formula:

[0089] ρ=ρ0[1+C p (p l -p0)]

[0090] Where ρ is the density of fracturing flowback fluid at any pressure (kg / m 3 );

[0091] ρ0 is the density of the fracturing flowback fluid at pressure p0 (kg / m 3 );

[0092] C p is the compression coefficient of fracturing flowback fluid;

[0093] p l is the pressure of the fracturing flowback fluid (Pa).

[0094] Furthermore, according to the state equation of the density of the fracturing flowback fluid of the matrix system and the natural fracture system and the state equation of the fluid of the fracturing flowback fluid, the characteristic equation of the cross-flow between the fracture medium and the rock medium is constructed, and the formula is as follows:

[0095]

[0096] Among them, q λis the cross-flow rate between fracture medium and rock medium (m 3 / s);

[0097] ρ0 is the density of the fracturing flowback fluid at pressure p0 (kg / m 3 );

[0098] μ is the viscosity of fracturing flowback fluid (Pa·s);

[0099] p m is the pressure of the matrix system (Pa);

[0100] p f is the pressure of the natural fracture system (Pa).

[0101] Furthermore, the seepage differential equation of the fracturing flowback fluid in the matrix medium and the fracture medium is obtained based on the following formula:

[0102]

[0103] Where r is the distance from any point in the reservoir to the wellbore (m);

[0104] α is the shape factor;

[0105] is the compressibility factor of the matrix system;

[0106] is the compression factor of the natural fracture system;

[0107] is the second-order partial derivative;

[0108] The seepage differential equations of fracturing flowback fluid in matrix media and fracture media are converted into seepage differential equations based on polar coordinates, as follows:

[0109]

[0110] Furthermore, according to the seepage differential equation in polar coordinate form, the initial condition of the seepage phenomenon at the initial time is obtained. The initial condition is the value of the pressure in the entire area at the given initial time t = τ and τ = 0, based on the following formula:

[0111] p f (r,0)=p m (r,0)=p0

[0112] Where, p0 is the original reservoir pressure (Pa);

[0113] If the flow rate of fracturing return fluid per unit time is q, the internal boundary conditions of the dual medium at the initial time of the seepage phenomenon are obtained according to the seepage differential equation in polar coordinate form, based on the following formula:

[0114]

[0115] Among them, p ω is the pressure in the artificial fracture at a distance r from the fracture at time t;

[0116] p f is the pressure of the natural fracture at a distance r from the fracture at time t (Pa);

[0117] p m is the pressure of the matrix at a distance r from the crack at time t (Pa);

[0118] r w is the radius of the crack (m);

[0119] h is the reservoir thickness;

[0120] S is the effective permeability of the fracture;

[0121] B. Seepage between fracture medium and rock medium;

[0122] q Liquid flow rate of external injection / production;

[0123] C is the volumetric elastic coefficient of the system;

[0124] According to the seepage differential equation in polar coordinate form, the outer boundary conditions of the dual medium of the seepage phenomenon at the initial moment are constructed based on the following formula:

[0125]

[0126] Furthermore, a function of crack width changing with spatial position is constructed based on the following formula:

[0127]

[0128] Where w(x, y) is the crack width at position (x, y);

[0129] w0 is the reference crack width;

[0130] (x0, y0) is the position of the crack center;

[0131] σ x and σ y is the standard deviation of the crack in the horizontal and vertical directions;

[0132] According to the seepage differential equation of fracturing flowback fluid in matrix medium and fracture medium, the initial conditions of the seepage phenomenon at the initial time, the inner boundary conditions and outer boundary conditions of the dual medium, the matrix-natural fracture dual medium seepage model is constructed as follows:

[0133]

[0134] Furthermore, the finite difference method is used to solve the distribution of groundwater pressure field and velocity field under given initial conditions and boundary conditions, and the process of predicting groundwater flow direction, velocity and water level change is as follows:

[0135] Divide the solution area into grids or grid cells, where each grid cell represents a spatial point in the solution domain. This process is called meshing.

[0136] The derivative term in the partial differential equation is replaced by a difference approximation. In the two-dimensional case, the partial derivative can be approximated by the central difference as follows:

[0137]

[0138] Among them, (u i+1，j ) is the quantity to be solved at the grid point (i+1, j), (u i-1，j ) is the quantity to be solved at the grid point (i-1, j), and Δx is the grid spacing;

[0139] Substituting the gridded partial differential equation into the grid points, we get the corresponding difference equation:

[0140]

[0141] Where u is the groundwater pressure, D is the water diffusion coefficient, is the gradient operator, where is the groundwater pressure at the grid point (i, j) at the nth time step, Δt is the time step, Δx and Δy are the space step;

[0142] Using the initial conditions and boundary conditions, the differential equation is used for iterative calculation to solve the pressure and velocity values ​​in the groundwater medium. By analyzing the pressure and velocity values ​​in the groundwater medium, the process of predicting the flow direction, velocity and water level change of groundwater is as follows:

[0143] Gradient analysis of the pressure field:

[0144] The gradient of the pressure field indicates the rate and direction of pressure change. By calculating the gradient of the pressure field in space, the flow direction of groundwater is determined. Groundwater in the high-gradient area of ​​the pressure field flows to the area with lower pressure. Since the direction of groundwater flow is opposite to the direction of the pressure gradient, the flow direction of groundwater is determined.

[0145] Analysis of flow velocity field:

[0146] The velocity field directly provides information about the velocity of groundwater in space. By analyzing the size and direction of the velocity field, the distribution of groundwater velocity can be determined. Areas with larger velocity indicate faster groundwater flow, while areas with smaller velocity indicate slower groundwater flow.

[0147] Inference of water level changes:

[0148] The water level is closely related to the pressure field. As time goes by, the pressure field in the groundwater system will change, leading to changes in the water level. By observing the changes in the pressure field at different time points, the changes in the groundwater level can be inferred. An increase in the pressure field corresponds to an increase in the water level, and a decrease in the pressure field corresponds to a decrease in the water level.

[0149] Compared with the prior art, the present invention has the following beneficial effects:

[0150] The present invention constructs continuity equations of a matrix system and a natural fracture system, motion equations of the seepage velocity of the fracturing return fluid of the matrix system and the natural fracture system, utilizes the influence of fractures and faults on the seepage channel, constructs coefficients that affect the seepage velocity of the fracturing return fluid of the matrix system and the natural fracture system, constructs new motion equations of the fracturing return fluid of the matrix system and the natural fracture system, constructs state equations of the density of the fracturing return fluid of the matrix system and the natural fracture system according to the new motion equations of the seepage velocity of the fracturing return fluid, and obtains the state equations of the porosity of the matrix system and the natural fracture system when the pressure is p0 through Maclaurin series expansion. Equation is used to construct the state equation of the fracturing return fluid, the characteristic equation of the crossflow rate between the fracture medium and the rock medium, and the seepage differential equation of the fracturing return fluid in the matrix medium and the fracture medium. According to the seepage differential equation of the fracturing return fluid in the matrix medium and the fracture medium, the initial conditions of the seepage phenomenon at the initial moment, the inner and outer boundary conditions of the dual media, and the function of the fracture width changing with the spatial position, a matrix-natural fracture dual media seepage model is constructed. The finite difference method is used to solve the distribution of the pressure field and velocity field of groundwater under given initial conditions and boundary conditions, and the flow direction, velocity and water level change of groundwater are predicted. Therefore, by incorporating the influence of cracks and faults on groundwater seepage, according to the seepage differential equation of fracturing return fluid in matrix medium and fracture medium, the initial conditions of seepage phenomenon at the initial moment, the inner and outer boundary conditions of dual media, and the function of crack width changing with spatial position, the groundwater seepage channel is made more uniform and homogeneous, thereby improving the integrity of the seepage model and the prediction accuracy of groundwater flow direction, flow velocity and water level change, avoiding large deviations between simulation results and actual conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0151] Figure 1 It is a schematic diagram of the overall method flow of the present invention;

[0152] Figure 2 It is a schematic diagram of the dual medium seepage model of the present invention. DETAILED DESCRIPTION

[0153] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments.

[0154] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0155] Example:

[0156] See also Figure 1 to Figure 2 , the present invention provides a technical solution:

[0157] A method for solving a matrix-natural fracture dual medium seepage model, the specific steps of which include:

[0158] S1. Collect the porosity of the matrix system and the fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and the time data, and construct the continuity equations of the matrix system and the natural fracture system respectively according to the collected porosity of the matrix system and the fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and the time data;

[0159] S2. When the fracturing flowback fluid seeps in the dual medium, there is seepage inside the porous rock block, seepage along the cracks, and exchange of fracturing flowback fluid between the rock block and the cracks. The seepage in the matrix and the natural cracks conforms to Darcy's law. Based on Darcy's law, the motion equations of the seepage velocity of the fracturing flowback fluid in the matrix system and the natural crack system are constructed respectively. The influence of cracks and faults on the seepage channel is used to construct the coefficients that affect the seepage velocity of the fracturing flowback fluid in the matrix system and the natural crack system.

[0160] S3. combining the motion equations of the seepage velocity of the fracturing flowback fluid of the matrix system and the natural fracture system and the coefficient of the seepage velocity to generate new motion equations of the fracturing flowback fluid of the matrix system and the natural fracture system, respectively constructing the state equations of the density of the fracturing flowback fluid of the matrix system and the natural fracture system according to the new motion equations of the seepage velocity of the fracturing flowback fluid, and respectively obtaining the state equations of the porosity of the matrix system and the natural fracture system when the pressure is p0 through Maclaurin series expansion;

[0161] S4. constructing the state equation of the fluid of the fracturing flowback fluid according to the state equation of the porosity of the matrix system and the natural fracture system, and constructing the characteristic equation of the cross-flow rate between the fracture medium and the rock medium according to the state equation of the density of the fracturing flowback fluid of the matrix system and the natural fracture system and the state equation of the fluid of the fracturing flowback fluid;

[0162] S5. Input the motion equation of the seepage velocity of the fracturing flowback fluid in the matrix system and the natural fracture system, the state equation of the porosity of the matrix system and the natural fracture system, the state equation of the fluid of the fracturing flowback fluid, and the characteristic equation of the cross-flow rate between the fracture medium and the rock medium into the continuity equation of the matrix system and the natural fracture system, and obtain the seepage differential equation of the fracturing flowback fluid in the matrix medium and the fracture medium respectively;

[0163] S6. converting the seepage differential equations of the fracturing flowback fluid in the matrix medium and the fracture medium into the seepage differential equations based on polar coordinates, and constructing the initial conditions of the seepage phenomenon at the initial moment, the inner boundary conditions and the outer boundary conditions of the dual medium according to the seepage differential equations in polar coordinates;

[0164] S7. Construct a matrix-natural fracture dual medium seepage model based on the seepage differential equation of fracturing flowback fluid in matrix medium and fracture medium, the initial conditions of the seepage phenomenon at the initial time, the inner boundary conditions and outer boundary conditions of the dual medium, and the function of the fracture width changing with the spatial position;

[0165] S8. Use the finite difference method to solve the distribution of groundwater pressure field and velocity field under given initial conditions and boundary conditions, and predict the flow direction, velocity and water level changes of groundwater.

[0166] In this embodiment, Figure 2 As shown in the figure, there is cross-flow between the fracture system of the shale matrix, so the dual medium seepage process of the fracturing return fluid between the shale matrix and the fracture system is considered. During the hydraulic fracturing process, the horizontal wellbore is blocked from the formation except for the perforation, so the seepage process of the fracturing return fluid through the shale matrix to the horizontal wellbore is ignored. Based on the above considerations, the physical model of the return of fracturing return fluid in shale gas reservoirs selects the matrix-natural fracture coupling model.

[0167] On the basis of the above embodiment, according to the collected porosity of the matrix system and the fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and the time data, the continuity equation of the matrix system is constructed, and the formula is as follows:

[0168]

[0169] Among them, φ m is the porosity of the matrix system;

[0170] ρ is the density of fracturing flowback fluid at any pressure (kg / m 3 );

[0171] v m is the seepage velocity of fracturing flowback fluid in the matrix system (m / s);

[0172] q λ is the cross-flow rate between fracture medium and rock medium (m 3 / s);

[0173] t is time (s);

[0174] The continuity equation of the natural fracture system is constructed based on the following formula:

[0175]

[0176] Among them, φ f is the porosity of the fracture system;

[0177] ρ is the density of fracturing flowback fluid at any pressure (kg / m 3 );

[0178] v f is the seepage velocity of fracturing flowback fluid in the fracture system (m / s);

[0179] q λ is the cross-flow rate between fracture medium and rock medium (m 3 / s);

[0180] t is time (s).

[0181] The continuity equation is an equation that describes the conservation of mass in a medium. Continuity equations are established for the matrix system and the natural fracture system respectively to describe the transfer and change process of water flow in these two media. The continuity equations of the matrix system and the natural fracture system are usually based on the principle of conservation of mass, considering the inflow, outflow and accumulation of water in different media. The continuity equation can analyze the water flow exchange between the matrix system and the fracture system.

[0182] On the basis of the above-mentioned embodiment, the motion equation of the seepage velocity of the fracturing flowback fluid of the matrix system is constructed, and the formula according to which is as follows:

[0183]

[0184] Among them, v m is the seepage velocity of fracturing flowback fluid in the matrix system (m / s);

[0185] k m is the permeability of the matrix system (Dc);

[0186] μ is the viscosity of fracturing flowback fluid (Pa·s);

[0187] p m is the pressure of the matrix system (Pa);

[0188] The motion equation of the seepage velocity of the fracturing flowback fluid in the natural fracture system is constructed based on the following formula:

[0189]

[0190] Among them, v f is the seepage velocity of fracturing flowback fluid in natural fracture system (m / s);

[0191] k f is the permeability of the natural fracture system (Dc);

[0192] μ is the viscosity of fracturing flowback fluid (Pa·s);

[0193] p f is the pressure of the natural fracture system (Pa).

[0194] The motion equation of the seepage velocity of fracturing return fluid in the matrix system and the natural fracture system is constructed to describe the variation of the seepage velocity of fracturing return fluid in the matrix system and the natural fracture system over time. The motion equation is intended to take into account the different seepage paths of fracturing return fluid in the dual medium, including seepage inside porous rock blocks, seepage along fractures, and exchange of fracturing return fluid between rock blocks and fractures. By solving the motion equation, the seepage process of fracturing return fluid in the matrix system and the natural fracture system can be simulated, thereby predicting the distribution of seepage velocity at different time points and spatial positions, which is helpful to evaluate the migration path and potential diffusion range of fracturing return fluid.

[0195] On the basis of the above embodiments, the influence of cracks and faults on the seepage channel is used to construct the coefficient affecting the seepage velocity of the fracturing return fluid in the matrix system and the natural fracture system, and the formula is as follows:

[0196] C=C0·f(w,ρ)

[0197] Where, C is the coefficient of seepage velocity;

[0198] C0 is the coefficient of seepage velocity when there are no cracks and faults;

[0199] w is the crack width;

[0200] ρ is the crack density;

[0201] f(w, ρ) is a function of the effect of cracks and faults on the seepage channel.

[0202] The purpose of using the influence of cracks and faults on seepage channels to construct coefficients that affect the seepage velocity of fracturing return fluid in matrix systems and natural fracture systems is: cracks and faults, as gaps and broken zones in the medium, have a significant impact on groundwater seepage. They may change the direction, velocity and path of the seepage channel. The existence of cracks and faults will lead to unevenness and inhomogeneity of groundwater seepage channels. Therefore, when simulating the seepage process in the matrix-crack dual medium, the influence of cracks and faults needs to be considered.

[0203] Specifically, by incorporating the effects of cracks and faults on seepage channels, coefficients that affect the seepage velocity of fracturing flowback fluids in matrix systems and natural fracture systems can be constructed. The coefficients can be used to adjust the motion equation of the seepage velocity of fracturing flowback fluids constructed based on Darcy's law to more accurately reflect the effects of cracks and faults on the seepage velocity.

[0204] The motion equations of the flow velocity of the fracturing return fluid in the matrix system and the natural fracture system and the coefficients of the flow velocity are combined to generate new motion equations of the fracturing return fluid in the matrix system and the natural fracture system, based on the following formula:

[0205] ν m new =C·ν m

[0206] Among them, ν m new is the new seepage velocity of the fracturing flowback fluid in the matrix system (m / s);

[0207] ν f new =C·ν f

[0208] Among them, ν f new is the new seepage velocity of the fracturing flowback fluid in the natural fracture system (m / s).

[0209] On the basis of the above embodiment, the state equation of the density of the fracturing flowback fluid of the matrix system is constructed, and the state equation of the porosity of the matrix system when the pressure is p0 is obtained through the Maclaurin series expansion, and the formula is as follows:

[0210]

[0211] Where ρ0 is the density of the fracturing flowback fluid under pressure p0 (kg / m 3 );

[0212] φ m0 is the porosity of the matrix system at pressure p0;

[0213] C p is the compression coefficient of fracturing flowback fluid;

[0214] C m is the compressibility factor of the matrix system;

[0215] ρ m is the density of the fracturing flowback fluid in the matrix system (kg / m 3 );

[0216] The state equation of the porosity of the natural fracture system when the pressure is p0 is obtained based on the following formula:

[0217]

[0218] Where ρ0 is the density of the fracturing flowback fluid under pressure p0 (kg / m 3 );

[0219] φ f0 is the porosity of the matrix system at pressure p0;

[0220] C p is the compression coefficient of fracturing flowback fluid;

[0221] C f is the compression factor of the natural fracture system;

[0222] ρ f is the density of fracturing flowback fluid in natural fracture system (kg / m 3 ).

[0223] The purpose of constructing the state equation for the porosity of the matrix system and the natural fracture system is to describe the relationship between the porosity of the matrix system and the natural fracture system and the pressure. Porosity describes the proportion of pore space in the rock or medium, which changes with increasing pressure because the porosity decreases when the rock is compressed. Therefore, the state equation is used to quantify the relationship between porosity and pressure, so as to understand how the pore structure in the medium changes with pressure.

[0224] On the basis of the above embodiment, according to the state equation of the porosity of the matrix system and the natural fracture system, the state equation of the fluid of the fracturing flowback fluid is constructed, and the formula is as follows:

[0225] ρ=ρ0[1+C p (p l -p0)]

[0226] Where ρ is the density of fracturing flowback fluid at any pressure (kg / m 3 );

[0227] ρ0 is the density of the fracturing flowback fluid at pressure p0 (kg / m 3 );

[0228] C p is the compression coefficient of fracturing flowback fluid;

[0229] p l is the pressure of the fracturing flowback fluid (Pa).

[0230] On the basis of the above embodiments, according to the state equation of the density of the fracturing flowback fluid of the matrix system and the natural fracture system and the state equation of the fluid of the fracturing flowback fluid, the characteristic equation of the crosstalk between the fracture medium and the rock medium is constructed, and the formula is as follows:

[0231]

[0232] Among them, q λ is the cross-flow rate between fracture medium and rock medium (m 3 / s);

[0233] ρ0 is the density of the fracturing flowback fluid at pressure p0 (kg / m 3 );

[0234] μ is the viscosity of fracturing flowback fluid (Pa·s);

[0235] p m is the pressure of the matrix system (Pa);

[0236] p f is the pressure of the natural fracture system (Pa).

[0237] The purpose of constructing the characteristic equation of the crossflow rate between fracture medium and rock medium is to quantify the crossflow rate between fracture medium and rock medium and understand the variation law of the crossflow rate under different conditions (such as pressure and temperature), so as to more accurately describe and predict the seepage model.

[0238] Based on the above embodiment, the seepage differential equation of the fracturing flowback fluid in the matrix medium and the fracture medium is obtained according to the following formula:

[0239]

[0240] Where r is the distance from any point in the reservoir to the wellbore (m);

[0241] α is the shape factor;

[0242] is the compressibility factor of the matrix system;

[0243] is the compression factor of the natural fracture system;

[0244] is the second-order partial derivative.

[0245] The purpose of constructing the seepage differential equation of fracturing flowback fluid in matrix media and fracture media is to simulate and predict the changes in parameters such as seepage velocity and pressure distribution of fracturing flowback fluid in the media under different conditions.

[0246] On the basis of the above embodiment, the seepage differential equations of the fracturing flowback fluid in the matrix medium and the fracture medium are converted into the seepage differential equations based on polar coordinates, and the formula is as follows:

[0247]

[0248] The purpose of constructing the seepage differential equation of fracturing return fluid in matrix medium and fracture medium based on polar coordinates is that circular or annular structures can be expressed more naturally under polar coordinates, and fractures often present this geometric shape, and the equation in polar coordinate form can usually simplify the form of the seepage differential equation of fracturing return fluid in matrix medium and fracture medium. Through polar coordinate transformation, the original two-dimensional space problem can be converted into a one-dimensional problem, thereby simplifying the solution process of the mathematical model.

[0249] On the basis of the above embodiment, according to the seepage differential equation in polar coordinate form, the initial condition of the seepage phenomenon at the initial time is obtained. The initial condition is the value of the pressure in the entire area at the given initial time t=τ (usually τ=0), and the formula is as follows:

[0250] p f (r,0)=p m (r,0)=p0

[0251] Where, p0 is the original reservoir pressure (Pa);

[0252] If the flow rate of fracturing return fluid per unit time is q, the internal boundary conditions of the dual medium at the initial time of the seepage phenomenon are obtained according to the seepage differential equation in polar coordinate form, based on the following formula:

[0253]

[0254] Among them, p ω is the pressure in the artificial fracture at a distance r from the fracture at time t;

[0255] p f is the pressure of the natural fracture at a distance r from the fracture at time t (Pa);

[0256] p m is the pressure of the matrix at a distance r from the crack at time t (Pa);

[0257] r w is the radius of the crack (m);

[0258] h is the reservoir thickness;

[0259] S is the effective permeability of the fracture;

[0260] B. Seepage between fracture medium and rock medium;

[0261] q Liquid flow rate of external injection / production;

[0262] C is the volumetric elastic coefficient of the system;

[0263] According to the seepage differential equation in polar coordinate form, the outer boundary conditions of the dual medium of the seepage phenomenon at the initial moment are constructed based on the following formula:

[0264]

[0265] The purpose of constructing the initial conditions of the seepage phenomenon at the initial moment, the internal boundary conditions of the dual medium, and the external boundary conditions is to provide a complete input for the groundwater model to simulate the seepage phenomenon in the groundwater system and ensure that the model can accurately reflect the actual geological and hydrological conditions, as well as the interactions at different boundaries.

[0266] On the basis of the above embodiment, a function of crack width changing with spatial position is constructed, and the formula is as follows:

[0267]

[0268] Where w(x, y) is the crack width at position (x, y);

[0269] w0 is the reference crack width;

[0270] (x0, y0) is the position of the crack center;

[0271] σ x and σ y is the standard deviation of the crack in the horizontal and vertical directions.

[0272] Since the function of fracture width variation with spatial position provides a quantitative description of the variation of fracture width with spatial position, the function of fracture width variation with spatial position can be used as part of the model parameters to calibrate and optimize the matrix-natural fracture dual medium seepage model, so as to better capture the actual situation; the function of fracture width variation with spatial position can be used to adjust the scale and morphology of the seepage channel, so as to improve the accuracy of seepage simulation. By accurately describing the spatial variation of fracture width, the movement and transmission process of fluid in the fracture can be more accurately simulated.

[0273] On the basis of the above embodiments, according to the seepage differential equation of fracturing flowback fluid in matrix medium and fracture medium, the initial conditions of the seepage phenomenon at the initial time, the inner boundary conditions and outer boundary conditions of the dual medium, and the function of the fracture width changing with the spatial position, the matrix-natural fracture dual medium seepage model is constructed as follows:

[0274]

[0275] On the basis of the above-mentioned embodiment, the process of using the finite difference method to solve the distribution of the pressure field and velocity field of groundwater under given initial conditions and boundary conditions, and predicting the flow direction, velocity and water level change of groundwater is as follows:

[0276] Divide the solution area into grids or grid cells, where each grid cell represents a spatial point in the solution domain. This process is called meshing.

[0277] The derivative term in the partial differential equation is replaced by a difference approximation. In the two-dimensional case, the partial derivative can be approximated by the central difference as follows:

[0278]

[0279] Among them, (u i+1，j ) is the quantity to be solved at the grid point (i+1, j), (u i-1，j ) is the quantity to be solved at the grid point (i-1, j), and Δx is the grid spacing;

[0280] Substituting the gridded partial differential equation into the grid points, we get the corresponding difference equation:

[0281]

[0282] Where u is the groundwater pressure, D is the water diffusion coefficient, is the gradient operator, where is the groundwater pressure at the grid point (i, j) at the nth time step, Δt is the time step, Δx and Δy are the space steps;

[0283] Using the initial conditions and boundary conditions, the differential equation is used for iterative calculation to solve the pressure and velocity values ​​in the groundwater medium. By analyzing the pressure and velocity values ​​in the groundwater medium, the process of predicting the flow direction, velocity and water level change of groundwater is as follows:

[0284] Gradient analysis of the pressure field:

[0285] The gradient of the pressure field indicates the rate and direction of pressure change. By calculating the gradient of the pressure field in space, the flow direction of groundwater can be determined. Groundwater in the high-gradient area of ​​the pressure field flows to the area with lower pressure. Since the direction of groundwater flow is opposite to the direction of the pressure gradient, the flow direction of groundwater can be determined.

[0286] Analysis of flow velocity field:

[0287] The velocity field directly provides information about the velocity of groundwater in space. By analyzing the size and direction of the velocity field, the distribution of groundwater velocity can be determined. Generally, areas with higher velocity indicate faster groundwater flow, while areas with lower velocity indicate slower groundwater flow.

[0288] Inference of water level changes:

[0289] Water level is usually closely related to the pressure field. As time goes by, the pressure field in the groundwater system will change, resulting in changes in the water level. By observing the changes in the pressure field at different time points, the changes in groundwater level can be inferred. An increase in the pressure field corresponds to an increase in the water level, and a decrease in the pressure field corresponds to a decrease in the water level.

[0290] The above formulas are all dimensionless and numerical calculations. The formula is a formula for the most recent real situation obtained by collecting a large amount of data and performing software simulation. The preset parameters in the formula are set by technicians in this field according to actual conditions.

[0291] The above embodiments may be implemented in whole or in part by software, hardware, firmware or any other combination thereof. When implemented by software, the above embodiments may be implemented in whole or in part in the form of a computer program product. Those skilled in the art may appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein may be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed by hardware or software methods depends on the specific application and design constraints of the technical solution.

[0292] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, and may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0293] The above description is only a specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application.

Claims

1. A method for solving a matrix-natural fracture dual medium seepage model, characterized in that: The specific steps include: S1. Collect the porosity of the matrix system and the fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and the time data, and construct the continuity equations of the matrix system and the natural fracture system respectively according to the collected porosity of the matrix system and the fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow rate between the fracture medium and the rock medium, and the time data; S2. When the fracturing flowback fluid seeps in the dual medium, there is seepage inside the porous rock block, seepage along the cracks, and exchange of fracturing flowback fluid between the rock block and the cracks. The seepage in the matrix and the natural cracks conforms to Darcy's law. Based on Darcy's law, the motion equations of the seepage velocity of the fracturing flowback fluid in the matrix system and the natural crack system are constructed respectively. The influence of cracks and faults on the seepage channel is used to construct the coefficients that affect the seepage velocity of the fracturing flowback fluid in the matrix system and the natural crack system. S3. combining the motion equations of the seepage velocity of the fracturing flowback fluid of the matrix system and the natural fracture system and the coefficient of the seepage velocity to generate new motion equations of the fracturing flowback fluid of the matrix system and the natural fracture system, respectively constructing the state equations of the density of the fracturing flowback fluid of the matrix system and the natural fracture system according to the new motion equations of the seepage velocity of the fracturing flowback fluid, and respectively obtaining the state equations of the porosity of the matrix system and the natural fracture system when the pressure is p0 through Maclaurin series expansion; S4. constructing the state equation of the fluid of the fracturing flowback fluid according to the state equation of the porosity of the matrix system and the natural fracture system, and constructing the characteristic equation of the cross-flow rate between the fracture medium and the rock medium according to the state equation of the density of the fracturing flowback fluid of the matrix system and the natural fracture system and the state equation of the fluid of the fracturing flowback fluid; S5. Input the motion equation of the seepage velocity of the fracturing flowback fluid in the matrix system and the natural fracture system, the state equation of the porosity of the matrix system and the natural fracture system, the state equation of the fluid of the fracturing flowback fluid, and the characteristic equation of the cross-flow rate between the fracture medium and the rock medium into the continuity equation of the matrix system and the natural fracture system, and obtain the seepage differential equation of the fracturing flowback fluid in the matrix medium and the fracture medium respectively; S6. converting the seepage differential equations of the fracturing flowback fluid in the matrix medium and the fracture medium into the seepage differential equations based on polar coordinates, and constructing the initial conditions of the seepage phenomenon at the initial moment, the inner boundary conditions and the outer boundary conditions of the dual medium according to the seepage differential equations in polar coordinates; S7. Construct a matrix-natural fracture dual medium seepage model based on the seepage differential equation of fracturing flowback fluid in matrix medium and fracture medium, the initial conditions of the seepage phenomenon at the initial time, the inner boundary conditions and outer boundary conditions of the dual medium, and the function of the fracture width changing with the spatial position; S8. Use the finite difference method to solve the distribution of groundwater pressure field and velocity field under given initial conditions and boundary conditions, and predict the flow direction, velocity and water level change of groundwater; By using the influence of fractures and faults on seepage channels, the coefficients that affect the seepage velocity of fracturing flowback fluid in the matrix system and natural fracture system are constructed based on the following formula: C=C0·f(w,ρ) Where, C is the coefficient of seepage velocity; C0 is the coefficient of seepage velocity when there are no cracks and faults; w is the crack width; ρ is the crack density; f(w, ρ) is a function of the effect of cracks and faults on the seepage channel.

2. The method for solving the matrix-natural fracture dual medium seepage model according to claim 1, characterized in that: According to the collected porosity of the matrix system and fracture system, the density of the fracturing flowback fluid at any pressure, the crossflow between the fracture medium and the rock medium, and the time data, the continuity equation of the matrix system is constructed based on the following formula: Among them, φ m is the porosity of the matrix system; ρ is the density of fracturing flowback fluid at any pressure, kg / m 3 ; v m is the seepage velocity of fracturing flowback fluid in the matrix system, m / s; q λ is the cross flow rate between fracture medium and rock medium, m 3 / s; t is time, s; The continuity equation of the natural fracture system is constructed based on the following formula: Among them, φ f is the porosity of the fracture system; ρ is the density of fracturing flowback fluid at any pressure, kg / m 3 ; v f is the flow velocity of fracturing flowback fluid in the fracture system, m / s; q λ is the cross flow rate between fracture medium and rock medium, m 3 / s; t is time, s.

3. The method for solving the matrix-natural fracture dual medium seepage model according to claim 2, characterized in that: The motion equation of the seepage velocity of the fracturing flowback fluid in the matrix system is constructed based on the following formula: Among them, v m is the seepage velocity of fracturing flowback fluid in the matrix system, m / s; k m is the permeability of the matrix system, Dc; μ is the viscosity of fracturing flowback fluid, Pa·s; p m is the pressure of the matrix system, Pa; The motion equation of the seepage velocity of the fracturing flowback fluid in the natural fracture system is constructed based on the following formula: Among them, v f is the flow velocity of fracturing flowback fluid in natural fracture system, m / s; k f is the permeability of the natural fracture system, Dc; μ is the viscosity of fracturing flowback fluid, Pa·s; p f is the pressure of the natural fracture system, Pa; The motion equations of the flow velocity of the fracturing return fluid in the matrix system and the natural fracture system and the coefficients of the flow velocity are combined to generate new motion equations of the fracturing return fluid in the matrix system and the natural fracture system, based on the following formula: n m new =C·ν m Among them, ν m new is the new seepage velocity of the fracturing flowback fluid in the matrix system, m / s; n f new =C·ν f Among them, ν f new is the new seepage velocity of fracturing flowback fluid in the natural fracture system, m / s.

4. The method for solving the matrix-natural fracture dual medium seepage model according to claim 1, characterized in that: The state equation of the density of the fracturing flowback fluid of the matrix system is constructed, and the state equation of the porosity of the matrix system when the pressure is p0 is obtained through the Maclaurin series expansion, based on the following formula: Where ρ0 is the density of the fracturing flowback fluid under pressure p0, kg / m 3 ; φ m0 is the porosity of the matrix system at pressure p0; C p is the compression coefficient of fracturing flowback fluid; C m is the compressibility factor of the matrix system; ρ m is the density of fracturing flowback fluid in the matrix system, kg / m 3 ; The state equation of the porosity of the natural fracture system when the pressure is p0 is obtained based on the following formula: Where ρ0 is the density of the fracturing flowback fluid under pressure p0, kg / m 3 ; φ f0 is the porosity of the matrix system at pressure p0; C p is the compression coefficient of fracturing flowback fluid; C f is the compression factor of the natural fracture system; ρ f is the density of fracturing flowback fluid in natural fracture system, kg / m 3 ; According to the state equation of the porosity of the matrix system and the natural fracture system, the state equation of the fracturing flowback fluid is constructed based on the following formula: ρ=ρ0[1+C p (p l -p0)] Where ρ is the density of fracturing flowback fluid at any pressure, kg / m 3 ; ρ0 is the density of fracturing flowback fluid under pressure p0, kg / m 3 ; C p is the compression coefficient of fracturing flowback fluid; p l is the pressure of the fracturing flowback fluid, Pa.

5. The method for solving the matrix-natural fracture dual medium seepage model according to claim 4 is characterized in that: The state equation of the density of the fracturing flowback fluid of the matrix system is constructed, and the state equation of the porosity of the matrix system when the pressure is p0 is obtained through the Maclaurin series expansion, based on the following formula: Where ρ0 is the density of the fracturing flowback fluid under pressure p0, kg / m 3 ; φ m0 is the porosity of the matrix system at pressure p0; C p is the compression coefficient of fracturing flowback fluid; C m is the compressibility factor of the matrix system; ρ m is the density of fracturing flowback fluid in the matrix system, kg / m 3 ; The state equation of the porosity of the natural fracture system when the pressure is p0 is obtained based on the following formula: Where ρ0 is the density of the fracturing flowback fluid under pressure p0, kg / m 3 ; φ f0 is the porosity of the matrix system at pressure p0; C p is the compression coefficient of fracturing flowback fluid; C f is the compression factor of the natural fracture system; ρ f is the density of fracturing flowback fluid in natural fracture system, kg / m 3 ; According to the state equation of the porosity of the matrix system and the natural fracture system, the state equation of the fracturing flowback fluid is constructed based on the following formula: ρ=ρ0[1+C p (p l -p0)] Where ρ is the density of fracturing flowback fluid at any pressure, kg / m 3 ; ρ0 is the density of fracturing flowback fluid under pressure p0, kg / m 3 ; C p is the compression coefficient of fracturing flowback fluid; p l is the pressure of the fracturing flowback fluid, Pa.

6. The method for solving the matrix-natural fracture dual medium seepage model according to claim 1, characterized in that: According to the state equation of the density of the fracturing flowback fluid in the matrix system and the natural fracture system, and the state equation of the fluid of the fracturing flowback fluid, the characteristic equation of the cross-flow between the fracture medium and the rock medium is constructed, and the formula is as follows: Among them, q λ is the cross flow rate between fracture medium and rock medium, m 3 / s; ρ0 is the density of fracturing flowback fluid under pressure p0, kg / m 3 ; μ is the viscosity of fracturing flowback fluid, Pa·s; p m is the pressure of the matrix system, Pa; p f is the pressure of the natural fracture system, Pa; α is the shape factor.

7. The method for solving the matrix-natural fracture dual medium seepage model according to claim 1, characterized in that: The differential equation of the seepage of fracturing flowback fluid in the matrix medium and fracture medium is obtained based on the following formula: Where r is the distance from any point in the reservoir to the wellbore, m; α is the shape factor; is the compressibility factor of the matrix system; is the compression factor of the natural fracture system; is the second-order partial derivative; The seepage differential equations of fracturing flowback fluid in matrix media and fracture media are converted into seepage differential equations based on polar coordinates, as follows:

8. The method for solving the matrix-natural fracture dual medium seepage model according to claim 7, characterized in that: According to the seepage differential equation in polar coordinate form, the initial condition of the seepage phenomenon at the initial time is obtained. The initial condition is the value of the pressure in the entire area at the given initial time t = τ and τ = 0, based on the following formula: p f (r,0)=p m (r,0)=p0 Where p0 is the original reservoir pressure, Pa; If the flow rate of fracturing return fluid per unit time is q, the internal boundary conditions of the dual medium at the initial time of the seepage phenomenon are obtained according to the seepage differential equation in polar coordinate form, based on the following formula: Among them, p ω is the pressure in the artificial fracture at a distance r from the fracture at time t; p f (r,t) is the pressure of the natural fracture at a distance r from the fracture at time t, Pa; p m (r,t) is the pressure of the matrix at time t, r from the crack, Pa; r w is the radius of the crack, m; h is the reservoir thickness; S is the effective permeability of the fracture; B. Seepage between fracture medium and rock medium; q Liquid flow rate of external injection / production; C is the volumetric elastic coefficient of the system; According to the seepage differential equation in polar coordinate form, the outer boundary conditions of the dual medium of the seepage phenomenon at the initial moment are constructed based on the following formula:

9. The method for solving the matrix-natural fracture dual medium seepage model according to claim 1, characterized in that: The function of crack width changing with spatial position is constructed based on the following formula: Where w(x, y) is the crack width at position (x, y); w0 is the reference crack width; (x0, y0) is the position of the crack center; σ x and σ y is the standard deviation of the crack in the horizontal and vertical directions; According to the seepage differential equation of fracturing flowback fluid in matrix medium and fracture medium, the initial conditions of the seepage phenomenon at the initial time, the inner boundary conditions and outer boundary conditions of the dual medium, the matrix-natural fracture dual medium seepage model is constructed as follows:

10. The method for solving the matrix-natural fracture dual medium seepage model according to claim 9, characterized in that: The process of using the finite difference method to solve the distribution of groundwater pressure field and velocity field under given initial conditions and boundary conditions, and predicting the flow direction, velocity and water level change of groundwater is as follows: Divide the solution area into grids or grid cells, where each grid cell represents a spatial point in the solution domain. This process is called meshing. The derivative term in the partial differential equation is replaced by a difference approximation. In the two-dimensional case, the partial derivative can be approximated by the central difference as follows: Among them, (u i+1，j ) is the quantity to be solved at the grid point (i+1, j), (u i-1，j ) is the quantity to be solved at the grid point (i-1, j), and Δx is the grid spacing; Substituting the gridded partial differential equation into the grid points, we get the corresponding difference equation: Where u is the groundwater pressure, D is the water diffusion coefficient, is the gradient operator, where is the groundwater pressure at the grid point (i, j) at the nth time step, Δt is the time step, Δx and Δy are the space step; Using the initial conditions and boundary conditions, the differential equation is used for iterative calculation to solve the pressure and velocity values ​​in the groundwater medium. By analyzing the pressure and velocity values ​​in the groundwater medium, the process of predicting the flow direction, velocity and water level change of groundwater is as follows: Gradient analysis of the pressure field: The gradient of the pressure field indicates the rate and direction of pressure change. By calculating the gradient of the pressure field in space, the flow direction of groundwater is determined. Groundwater in the high-gradient area of ​​the pressure field flows to the area with lower pressure. Since the direction of groundwater flow is opposite to the direction of the pressure gradient, the flow direction of groundwater is determined. Analysis of flow velocity field: The velocity field directly provides information about the velocity of groundwater in space. By analyzing the size and direction of the velocity field, the distribution of groundwater velocity can be determined. Areas with larger velocity indicate faster groundwater flow, while areas with smaller velocity indicate slower groundwater flow. Inference of water level changes: The water level is closely related to the pressure field. As time goes by, the pressure field in the groundwater system will change, leading to changes in the water level. By observing the changes in the pressure field at different time points, the changes in the groundwater level can be inferred. An increase in the pressure field corresponds to an increase in the water level, and a decrease in the pressure field corresponds to a decrease in the water level.

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