An acid liquid pointing quantitative evaluation method for carbonate rock geothermal reservoir preflush acid fracturing

Abstract

The present application relates to a kind of acid liquid fingering quantitative evaluation method for carbonate rock geothermal reservoir preflush acid fracturing, belong to the technical field of enhanced geothermal system reservoir stimulation.Establish three-dimensional model and multi-field coupling numerical model considering fingering effect, using numerical simulation method can obtain the three-dimensional morphological change of fracture surface, by calculating etching front fingering coefficient, the acid liquid fingering effect in the process of carbonate rock heat reservoir preflush acid fracturing is quantitatively evaluated, and the sensitivity of rock plate temperature, preflush viscosity, acid injection displacement and other parameters to fingering effect is also analyzed, which provides theoretical basis and technical support for geothermal reservoir depth acid fracturing technology.

Description

TECHNICAL FIELD

[0001] The application relates to an acid liquid fingering quantitative evaluation method for carbonate rock geothermal reservoir preflush acid fracturing. BACKGROUND

[0002] The enhanced geothermal system can greatly improve the geothermal extraction capacity and is an important means for large-scale utilization of geothermal resources. Reservoir stimulation is a key step. Due to the fast acid-rock reaction rate in a high-temperature environment, the conventional acid fracturing technology causes rapid acid consumption and limited action range. Therefore, deep acid fracturing technology is needed to achieve more far-reaching modification. The preflush acid fracturing is one of the deep acid fracturing technologies and has been widely used in carbonate rock reservoirs. The main principle is that a high-viscosity liquid is used as a preflush to press open the fractures in the formation, and then acid liquid is injected to etch the fracture wall. In this process, the low-temperature and low-viscosity acid liquid flows in the high-viscosity preflush to cause the fingering phenomenon. The more obvious the fingering effect is, the smaller the acid-rock contact area is, and the acid consumption rate is slowed down, which is beneficial to the migration of acid liquid to the deeper part of the fracture. Therefore, it is of great significance to accurately quantify the acid liquid fingering effect in the preflush acid fracturing process of the carbonate rock geothermal reservoir.

[0003] At present, there are few related researches. The Chinese patent document CN118774768A discloses an evaluation method for the viscous fingering effect in acid fracturing. The method uses a visual flat fracture device to quantitatively evaluate the viscous fingering flow effect. The Chinese patent document CN116378631A discloses an acid fracturing parameter design method based on the evaluation of the alternating injection fingering effect, which provides the optimal design of the acid fracturing fingering parameters. However, the existing technical achievements mainly focus on the acid liquid fingering evaluation of a single flow field and ignore the influence of the high-temperature environment of the geothermal reservoir. When the acid liquid contacts the high-temperature rock wall, a chemical reaction occurs, which further causes the deformation of the wall shape and the evolution of the rock porosity-permeability relationship and a series of complex multi-field coupling processes. Therefore, the coupling linkage effect between the acid liquid fingering and other physical fields such as the chemical reaction, heat transfer process and porosity change needs to be fully considered. SUMMARY

[0004] The application aims to solve the above problems and provides an acid liquid fingering quantitative evaluation method for the preflush acid fracturing of a carbonate rock geothermal reservoir. A three-dimensional model and a multi-field coupling numerical model considering the fingering effect are established. The three-dimensional shape change of the fracture wall can be obtained by using the numerical simulation method. The acid liquid fingering effect in the preflush acid fracturing process of the carbonate rock geothermal reservoir is quantitatively evaluated by calculating the etching front fingering coefficient. The sensitivity of parameters such as the rock plate temperature, preflush viscosity and acid injection displacement to the fingering effect is analyzed, which provides a theoretical basis and technical support for the deep acid fracturing technology of the geothermal reservoir.

[0005] The technical solutions of the present application are as follows:

[0006] An acid liquid fingering quantitative evaluation method for carbonate rock geothermal reservoir preflush acid fracturing, comprising the following steps:

[0007] (1) A three-dimensional model considering the fingering effect of multiphase flow and a multi-field coupled numerical model are established, the multi-field coupled numerical model includes a porous medium seepage module, a chemical reaction module, a porous medium heat transfer module and a mathematical calculation module;

[0008] (2) Numerical simulation experiment is carried out; high-temperature rock plate etching results under different conditions are obtained according to experimental requirements, which is convenient for subsequent analysis;

[0009] (3) The fracture etching boundary of the rock plate after acid etching is determined, and the data of the points on the boundary are extracted; the collected data are imported into data processing software, which is convenient for subsequent calculation;

[0010] (4) Data arrangement and calculation; the upper rock plate etching front edge length L1, the lower rock plate etching front edge length L2, the total etching front edge length L0 and the fracture width D are calculated by using MATLAB;

[0011] (5) Calculate the etching front edge fingering coefficient F; the coefficient evaluates the strength of the fingering effect and the randomness of the etching area distribution on the fracture wall surface, indirectly evaluates the non-uniform etching degree of the fracture, the larger the coefficient, the longer the etching front edge length, the stronger the fingering effect, the smaller the coefficient, the closer the etching front edge to the fracture width, the closer to the piston displacement process, at the same time, the fingering coefficient is also used to represent the non-uniform etching degree of the fracture wall surface, the larger the coefficient, the more complex the etching front edge, the stronger the randomness of the etching area distribution, and the stronger the non-uniform etching degree of the wall surface;

[0012] (6) Sensitivity factor analysis; based on the experimental results, the influence of rock plate temperature, preflush viscosity, acid injection displacement and other parameters on the etching front edge fingering coefficient is analyzed.

[0013] Preferably, in step (1), the three-dimensional model is distributed on the upper and lower parallel rock plates with the same size, and the fracture area is added in the middle, the free tetrahedron network is used, the subdivision method is Delaunay method, and the fracture network system is encrypted and constructed with smaller grid size than the rock plate matrix.

[0014] Preferably, in step (1), the porous medium seepage module is used to describe the fluid flow process of the whole model, wherein, in the fracture area, the porous medium multiphase flow is followed, and the flow equation and the continuity equation are:

[0015]

[0016]

[0017]

[0018]

[0019]

[0020] where u p , u a are the fluid velocity vectors of the preflush and the acid, respectively; , are the pressures of the two phases in the fracture; ▽ represents the gradient operator, i.e. grad, k is the absolute permeability of the reservoir; k p (S), k a (S) are the permeabilities of the preflush and the acid, respectively; k rp (S), k ra (S) are the relative permeabilities of the two phases, which are affected by the acid saturation S; , are the net exchange terms of the preflush and the acid between the media, respectively; is the equivalent porosity of the fracture region; S p , S a are the saturations of the two phases, respectively;

[0021] In the rock plate matrix, the Darcy's law of porous media is followed, and the flow equation and the continuity equation are respectively:

[0022]

[0023]

[0024] where u m is the fluid velocity vector in the porous medium determined by the Darcy's law, , k are the matrix porosity and permeability, respectively, is the net exchange term of the acid between the fracture and the matrix, and the calculation method is as follows:

[0025]

[0026] where, are the fluid pressures in the fracture and the matrix, respectively, infinitely close to the fracture wall surface.

[0027] Preferably, in step (1), the chemical reaction module is used to describe the chemical reaction process of the whole model, wherein the solute transport mass transfer and chemical reaction consumption equation in the fracture region is as follows:

[0028]

[0029] where d f is the fracture width, c f is the concentration of the solute in the fracture, R f is the reaction rate of the solute in the fracture, is the diffusion coefficient of the solute in the fracture, is the convection and diffusion exchange term between the fracture and the porous medium, and the calculation method is:

[0030]

[0031] In the formula: represents the fracture wall surface;

[0032] The material balance equation of the solute in the liquid phase in the matrix is:

[0033]

[0034] In the formula, c a is the concentration of the acid in the liquid phase, c s is the concentration of the acid at the acid-rock interface, is the diffusion coefficient of the solute in the matrix, k c is the mass transfer coefficient, a v is the specific surface area; is the matrix porosity;

[0035] The chemical reaction will dissolve the carbonate rock mineral components, resulting in an increase in the porosity of the rock. The relationship between the porosity and the chemical reaction is described by the continuity equation of the rock skeleton:

[0036]

[0037] In the formula, ρ s is the rock density, α represents the dissolution ability of the acid, a v is defined as the mass of the solid that can be dissolved per mole of acid reaction, R(c s ) is the reaction dynamic, is the matrix porosity.

[0038] Preferably, in step (1), the porous medium heat transfer module is used to describe the heat transfer process of the entire model, including heat convection and heat conduction.

[0039] The heat transfer equation considering heat convection and heat conduction in the fracture is:

[0040]

[0041]

[0042]

[0043] T is temperature, is the equivalent density of the fracture, , are the acid density and the preflush density, respectively, is the equivalent heat volume of the fracture, is the specific heat capacity of the rock, is the specific heat capacity of the acid, is the specific heat capacity of the preflush, is the equivalent heat transfer coefficient of the fracture, is the heat transfer coefficient of the rock, is the heat transfer coefficient of the acid, is the heat transfer coefficient of the preflush, is the heat exchange between the two media.

[0044] The energy conservation equation of the matrix considering the heat convection, heat conduction and chemical reaction heat effect is:

[0045]

[0046]

[0047]

[0048]

[0049] In the formula: is the equivalent density of the matrix, is the equivalent heat volume of the matrix, is the equivalent heat transfer coefficient of the matrix, is the chemical reaction heat release, H is the enthalpy change, is the heat exchange between the two media, and the calculation method is:

[0050]

[0051] In the formula: z→d f represents the fracture wall surface.

[0052] Preferably, in step (1), the mathematical calculation module is used to describe the evolution of the pore permeability relationship of the rock matrix part, and the pore permeability change will further affect other physical processes. The calculation formula of the relationship between permeability, pore radius, specific surface and porosity is:

[0053]

[0054]

[0055]

[0056] In the formula, and are the porosity and initial porosity of the rock, respectively; and are the permeability and initial permeability of the rock, respectively; and are the pore radius and initial pore radius of the rock, respectively; and are the specific surface and initial specific surface of the rock, respectively; is a constant related to the pore structure;

[0057] The core-scale thermal-water-chemical multi-field coupling numerical model is finally constructed by coupling and calculating the variables between multiple physical field control equations.

[0058] Preferably, in step (2), different experimental conditions are set to carry out parameter sensitivity analysis, and to explore the influence law of rock plate temperature on the etching front advance coefficient, 5 different initial temperature values are set from 353K / 80℃ to 453K / 180℃, and other factors are fixed, and the acid etching process under different temperatures within the same time is simulated;

[0059] To explore the influence law of the viscosity of the preflush on the etching front advance coefficient, 5 graduation values are selected from 20 mPa·s to 60 mPa·s, other factors are fixed, and the acid etching process under different viscosity differences within the same time is simulated;

[0060] To explore the influence law of the acid injection displacement on the etching front advance coefficient, 5 graduation values are selected from 2 mL·min -1 to 6 mL·min -1 , other factors are fixed, and the acid etching process under different displacements within the same time is simulated.

[0061] Preferably, in step (3), the determination method of the etching boundary is to indirectly calculate the fracture width by using the area with relatively high porosity in the porosity distribution, instead of directly calculating the fracture width, and the calculation method is: wherein, is the porosity of the etching boundary, is a coefficient determined according to experiments, is the maximum value of porosity set in the numerical model, which is 0.99, and the isosurface with a porosity of can be used to determine the etching boundary.

[0062] Preferably, in step (3), the data extraction is to extract the three-dimensional coordinates of all discrete points on the fracture boundary by means of the post-processing module of the numerical simulation software, and the data processing software is software capable of realizing data screening and sorting functions, such as Excel, Origin, etc.

[0063] Preferably, in step (4), the etching front is defined as the portion of the etching boundary curve at the same height as the original crack wall surface on the etching boundary;

[0064] The calculation method of the etching front length L1 of the upper rock plate and the etching front length L2 of the lower rock plate is to select all the points on the fracture boundary that meet the definition of the finger front, draw the etching front curve, derive the etching front curve, and calculate the curve length using MATLAB;

[0065] The total length L0 of the etching front is: L0 = L1 + L2.

[0066] Preferably, in step (5), the etching front index is defined as the ratio of the length of the etching front to the crack width;

[0067] The calculation formula of etching front edge index F is: .

[0068] The beneficial effects of the present invention are:

[0069] 1. The numerical simulation is scientific and accurate, and more in line with the actual situation: The thermo-hydraulic-chemical (THC) multi-field coupled numerical model used in this invention introduces multiphase flow theory. It considers the fingering flow effect of low-temperature and low-viscosity acid in the fracture when a high-viscosity pre-fluid is present. At the same time, it simulates a series of physical processes such as the chemical etching reaction after acid-rock contact, the heat transfer process, and the evolution of the porosity and permeability relationship. The simulation results are more scientific and accurate.

[0070] 2. Digital processing of the surface morphology before and after acid etching cracks: The present invention uses the post-processing module of numerical simulation software to extract the three-dimensional coordinates of all discrete points on the crack boundary, uses data processing software to implement data screening, and accurately obtains the etching front boundary data.

[0071] 3. Quantitative evaluation of the fingering effect of high-temperature carbonate rock thermal storage acid fluid: The present invention defines the etching front fingering coefficient and reflects the fingering effect by quantitatively evaluating the etching morphology of the fracture wall.

[0072] 4. Multi-factor sensitivity analysis: Based on simulation and data processing results, this paper clarifies the influence of parameters such as rock plate temperature, pre-fluid viscosity, and acid injection displacement on the etching front fingering coefficient, providing a theoretical basis and technical support for the optimization of process parameters for acid fracturing transformation of carbonate rock thermal storage pre-fluid. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 is a process flow chart of the present invention;

[0074] Figure 2 3D model diagram of the present invention, wherein A is the injection end and B is the outlet end;

[0075] Figure 3 Determination of the fracture etching boundary map from the porosity field in the present application, wherein, Figure 3 (a) is the global porosity distribution map in the present application; Figure 3 (b) is the high porosity area distribution map in the present application; Figure 3 (c) is the etching boundary schematic diagram in the present application;

[0076] Figure 4 Extraction of the etching front curve map from the fracture etching boundary in the present application;

[0077] Figure 5 The etching front advance coefficient changes with different factors in the present application, wherein, Figure 5 (a) is the influence of the rock plate temperature on the etching front advance coefficient in the present application; Figure 5 (b) is the influence of the preflush viscosity on the etching front advance coefficient in the present application; Figure 5 (c) is the influence of the acid injection displacement on the etching front advance coefficient in the present application. DETAILED DESCRIPTION

[0078] The present application will be further described below by examples and in conjunction with the accompanying drawings, but is not limited thereto.

[0079] In the embodiments of the present application:

[0080] The numerical simulation uses COMSOL Multiphysics multi-physical field simulation software;

[0081] Data processing and analysis are analyzed by using MATLAB and Origin software.

[0082] Example 1:

[0083] As Figure 1 shown, the present embodiment provides an acid liquid advance quantitative evaluation method for carbonate rock geothermal reservoir preflush acid fracturing, which includes the following steps:

[0084] (1) Establish a three-dimensional model considering the multiphase flow advance effect and a multi-field coupled numerical model, and the multi-field coupled numerical model includes a porous medium seepage module, a chemical reaction module, a porous medium heat transfer module and a mathematical calculation module;

[0085] A three-dimensional model was established using COMSOL Multiphysics multi-physical field simulation software, and the three-dimensional model was composed of parallel rock plates of the same size distributed on the upper and lower holding middle fracture regions. The calculation domain size of a single rock plate was a three-dimensional grid of 10 cm × 2 cm × 1 cm, and the fracture domain size was 10 cm × 2 cm × 0.1 cm. A free tetrahedral network was used, and the subdivision method was the Delaunay method. The fracture network system was encrypted and constructed using a smaller grid size than the rock plate matrix, as shown in FIG. 1. Figure 2

[0086] A porous medium seepage module was used to describe the fluid flow process of the entire model, wherein the porous medium multiphase flow was followed in the fracture region, and the flow equation and the continuity equation were as follows:

[0087]

[0088]

[0089]

[0090]

[0091]

[0092] wherein u p and u a are fluid flow velocity vectors of the preflush and the acid liquid, respectively; , are two-phase fluid pressures in the fracture, respectively; ▽ represents a gradient operator, that is, grad, k is an absolute permeability of the reservoir; k p (S), k a (S) are permeabilities of the preflush and the acid liquid, respectively; k rp (S), k ra (S) are relative permeabilities of the two-phase fluids, and the values thereof are affected by the acid liquid saturation S; , are net exchange terms of the preflush and the acid liquid between the media, respectively; is an equivalent porosity of the fracture region; S p and S a are saturations of the two-phase fluids, respectively;

[0093] In the rock plate matrix, the porous medium Darcy law is followed, and the flow equation and the continuity equation are as follows:

[0094]

[0095] ​

[0096] where, u m is the fluid velocity vector determined by Darcy's law in the porous medium, , k are the matrix porosity and permeability, respectively, is the net exchange term between the fracture and the matrix, which is calculated as follows:

[0097]

[0098] where, are the fluid pressures in the fracture and the matrix, respectively, when approaching the fracture wall.

[0099] The chemical reaction module is used to describe the chemical reaction process of the whole model, in which the transport-mass transfer and chemical reaction consumption equations of the solute in the fracture area are as follows:

[0100]

[0101] where, d f is the fracture width, c f is the solute concentration in the fracture, R f is the reaction rate of the solute in the fracture, is the diffusion coefficient of the solute in the fracture, is the convective and diffusive exchange term between the fracture and the porous medium, which is calculated as follows:

[0102]

[0103] where: z→d f represents the fracture wall;

[0104] The material balance equation of the solute in the liquid phase in the matrix is:

[0105]

[0106] where, c a is the acid concentration in the liquid phase, c s is the acid concentration at the acid-rock interface, is the diffusion coefficient of the solute in the matrix, is the mass transfer coefficient, is the specific surface area; is the matrix porosity;

[0107] The chemical reaction will dissolve the carbonate rock mineral components, resulting in an increase in the porosity of the rock. The relationship between the porosity and the chemical reaction is described by the continuity equation of the rock skeleton:

[0108]

[0109] wherein, ρ s is the rock density, a represents the acid dissolution ability, a v defined as the mass of solid that can be dissolved per mole of acid reacted, R(c s ) is the reaction kinetics, is the matrix porosity.

[0110] The porous medium heat transfer module is used to describe the heat transfer process of the entire model, including heat convection and heat conduction;

[0111] The heat transfer equation considering heat convection and heat conduction in the fracture is:

[0112]

[0113]

[0114]

[0115] wherein: T is the temperature, is the equivalent density of the fracture, , are the acid density and the preflush density, respectively, is the equivalent heat volume of the fracture, is the rock specific heat capacity, is the acid specific heat capacity, is the preflush specific heat capacity, is the equivalent heat transfer coefficient of the fracture, is the rock heat transfer coefficient, is the acid heat transfer coefficient, is the preflush heat transfer coefficient, is the heat exchange between the two media.

[0116] The energy conservation equation of the matrix considering heat convection, heat conduction and chemical reaction heat effect is:

[0117]

[0118]

[0119]

[0120]

[0121] wherein: is the equivalent density of the matrix, is the equivalent heat volume of the matrix, is the equivalent heat transfer coefficient of the matrix, is the chemical reaction heat release, H is the enthalpy change, For heat exchange between two media, the calculation method is:

[0122]

[0123] In the formula: z→d f is represented as the crack wall surface.

[0124] In step (1), the mathematical calculation module is used to describe the evolution of the pore and permeability relationship of the rock matrix part, and the change of pore and permeability will further affect other physical processes. The calculation formula of the relationship between permeability, pore radius, specific surface and porosity is:

[0125]

[0126]

[0127]

[0128] In the formula, and porosity and initial porosity of the rock, respectively; and permeability and initial permeability of the rock, respectively; and pore radius and initial pore radius of the rock, respectively; and specific surface and initial specific surface of the rock, respectively; is a constant related to the pore structure;

[0129] By coupling and linking the variables between multiple physical field control equations, a core-scale thermal-water-chemical multi-field coupled numerical model is finally established.

[0130] (2) Numerical simulation experiment; according to the experimental requirements, the high-temperature rock plate etching results under different conditions are obtained, which is convenient for subsequent analysis;

[0131] Different experimental conditions are set to carry out parameter sensitivity analysis, and to explore the influence of rock plate temperature on the etching front advance coefficient. Five different initial temperature values (80℃, 105℃, 130℃, 155℃, 180℃) are set from 353K / 80℃ to 453K / 180℃, and other factors are fixed. The acid etching process under different temperatures in the same time is simulated;

[0132] In order to explore the influence of the viscosity of the preflush on the etching front advance coefficient, five graduation values (20 mPa·s, 30 mPa·s, 40 mPa·s, 50 mPa·s, 60 mPa·s) are selected from 20 mPa·s to 60 mPa·s, and other factors are fixed. The acid etching process under different viscosity differences in the same time is simulated;

[0133] To explore the influence of acid injection rate on the etching front finger coefficient, five discrete values (2 mL·min -1 , 3 mL·min -1 , 4 mL·min -1 , 5 mL·min -1 , 6 mL·min -1 ) were selected from 2 mL·min -1 to 6 mL·min -1 , and the acid etching process under different injection rates was simulated for the same time, and the results are shown in Table 1.

[0134] Table 1: Reaction conditions corresponding to rock plate number

[0135]

[0136] (3) Determine the etching boundary of the fractured rock plate after acid etching, and extract the data of the points on the boundary; import the collected data into the data processing software for subsequent calculation;

[0137] The method for determining the etching boundary is to indirectly calculate the fracture width in the region with relatively high porosity in the porosity distribution, rather than directly calculating the fracture width. The calculation method is: wherein, is the porosity of the etching boundary, is a coefficient determined according to the experiment, is the maximum value of porosity set in the numerical model, which is 0.99, and the isosurface with a porosity of can be used to determine the etching boundary.

[0138] Data extraction is to extract the three-dimensional coordinates of all discrete points on the fracture boundary by means of the post-processing module of the numerical simulation software. The data processing software is Origin, which can realize data filtering and sorting functions, as shown in Figure 3 .

[0139] (4) Data arrangement and calculation; use MATLAB to calculate the upper rock plate etching front length L1, the lower rock plate etching front length L2, the total etching front length L0 and the fracture width D;

[0140] The etching front is defined as the part of the etching boundary curve at the same height position as the original fracture wall surface on the etching boundary;

[0141] The calculation method of the upper rock plate etching front length L1 and the lower rock plate etching front length L2 is to draw the etching front curve by screening the points that meet the definition of the finger front from all the point coordinates of the fracture boundary, and then export the etching front curve and calculate the curve length using MATLAB, as shown inFigure 4 as shown in the figure;

[0142] The total length of the etching front edge L0 is: L0=L1+L2.

[0143] (5) Calculate the etching front edge fingering coefficient F; the coefficient evaluates the strength of the fingering effect and the randomness of the etching area distribution on the fracture wall surface, indirectly evaluates the degree of non-uniform etching of the fracture, the larger the coefficient, the longer the etching front edge, the stronger the fingering effect, the smaller the coefficient, the closer the etching front edge to the fracture width, the closer to the piston displacement process, at the same time, the fingering coefficient is also used to represent the degree of non-uniform etching of the fracture wall surface, the larger the coefficient, the more complex the etching front edge, the stronger the randomness of the etching area distribution, and the stronger the degree of non-uniform etching of the wall surface;

[0144] The etching front edge fingering coefficient is defined as the ratio of the length of the etching front edge to the fracture width;

[0145] The calculation formula of the etching front edge fingering coefficient F is: .

[0146] (6) Sensitivity factor analysis; based on the experimental results, analyze the influence of rock plate temperature, preflush viscosity, acid injection displacement and other parameters on the etching front edge fingering coefficient. The results are shown in the figure, it can be seen that the non-uniform etching coefficient has a certain linear correlation with the rock plate temperature, preflush viscosity and acid injection displacement. Figure 5

[0147] For those skilled in the art, it is obvious that the present application is not limited to the details of the above-mentioned specific embodiments, and the present application can be realized in other specific forms without departing from the spirit or essential characteristics of the present application. Therefore, from any point of view, the embodiments should be regarded as exemplary rather than limiting, and the scope of the present application is defined by the appended claims rather than the above description, and therefore all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present application. Any reference signs in the claims should not be considered as limiting the claims involved.

[0148] In addition, it should be understood that although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description manner of the specification is only for the sake of clarity, those skilled in the art should consider the specification as a whole, and the technical solutions in the embodiments can also be properly combined to form other embodiments that those skilled in the art can understand.​

Claims

1. An acid liquid fingering quantitative evaluation method for carbonate rock geothermal reservoir preflush acid fracturing, characterized in that, The steps include the following: (1) Establishing a three-dimensional model and a multi-field coupling numerical model considering the fingering effect of multiphase flow, the multi-field coupling numerical model including a porous medium seepage module, a chemical reaction module, a porous medium heat transfer module and a mathematical calculation module; (2) Carrying out numerical simulation experiments; obtaining high-temperature rock plate etching results under different conditions according to experimental requirements; (3) Determining the fracture etching boundary of the rock plate after acid etching, extracting data of points on the boundary; importing the collected data into a data processing software; (4) Data arrangement and calculation, using MATLAB to calculate the upper rock plate etching front length L1, the lower rock plate etching front length L2, the total etching front length L0 and the fracture width D; The etching front is defined as the part of the etching boundary curve at the same height position as the original fracture wall surface on the etching boundary; The calculation method of the upper rock plate etching front length L1 and the lower rock plate etching front length L2 is to draw the etching front curve by screening the points of the fracture boundary coordinates that meet the definition of the fingering front, exporting the etching front curve and using MATLAB to calculate the curve length; The total etching front length L0 is L0 = L1 + L2; (5) Calculating the etching front fingering coefficient F; The larger the coefficient, the longer the etching front length, the stronger the fingering effect, the smaller the coefficient, the closer the etching front to the fracture width, the closer to the piston displacement process, at the same time, the fingering coefficient is also used to represent the degree of non-uniform etching of the fracture wall surface, the larger the coefficient, the more complex the etching front, the stronger the randomness of the etching area distribution, the stronger the degree of non-uniform etching of the wall surface; The etching front fingering coefficient is defined as the ratio of the length of the etching front to the fracture width; The calculation formula of the etching front edge pointing coefficient F is as follows: (6) Sensitivity factor analysis; based on the experimental results, analyzing the influence of rock plate temperature, preflush viscosity and acid injection displacement on the etching front fingering coefficient.

2. The acid liquid fingering quantification evaluation method for the prepad acid fracturing of the carbonate rock geothermal reservoir of claim 1, characterized in that, In step (1), the three-dimensional model is composed of parallel rock plates of the same size distributed above and below, with a fracture region in the middle.

3. The acid fluid diversion quantification evaluation method for carbonate rock geothermal reservoir prepad acid fracturing of claim 2, wherein, In step (1), the porous medium seepage module is used to describe the fluid flow process of the whole model, wherein, in the fracture region, the porous medium multiphase flow is followed, and the flow equation and the continuity equation are as follows: where u p , u a are the fluid flow rate vectors of the preflush and the acid, respectively; are the two-phase fluid pressures in the fracture; denotes the gradient operator, and k is the absolute permeability of the reservoir; k p (S), k a (S) are the permeabilities of the preflush and the acid, respectively; k rp (S), k ra (S) are the relative permeabilities of the two-phase fluids; are the net exchange terms between the media for the preflush and the acid, respectively; is the equivalent porosity of the fracture region; S p , S a are the saturations of the two-phase fluids; In the rock plate matrix, the porous medium Darcy's law is followed, and the flow equation and the continuity equation are as follows: where u m is the fluid flow velocity vector in the matrix determined by Darcy's law, k are the matrix porosity and permeability, respectively, is the net exchange term between the fracture and the matrix for the acid fluid, which is calculated as follows: wherein respectively, the fluid pressure in the fracture and matrix, respectively, as the fracture wall is approached infinitely closely.

4. The acid liquid fingering quantification evaluation method for the prepad acid fracturing of the carbonate rock geothermal reservoir of claim 3, characterized in that, In step (1), the chemical reaction module is used to describe the chemical reaction process of the whole model, wherein, in the fracture region, the solute transport mass transfer and chemical reaction consumption equation is as follows: where d f is the fracture width, c f is the concentration of the solute in the fracture, R f is the reaction rate of the solute in the fracture, is the diffusion coefficient of the solute in the fracture, is the convection and diffusion exchange term between the fracture and the porous medium, calculated as: wherein: z→d f is represented as a fracture wall surface; The mass balance equation for solute transport in the liquid phase in the matrix is: where c a is the concentration of acid in the liquid phase, c s is the concentration of acid at the acid-rock interface, is the diffusion coefficient of the solute in the matrix, k c is the mass transfer coefficient, a v is the specific surface area; is the matrix porosity; Chemical reaction will dissolve carbonate rock mineral components, resulting in an increase in the porosity of the rock, and the relationship between the porosity and the chemical reaction is described by the continuity equation of the rock skeleton: where p s is the rock density, a represents the acid's solubility, a v defined as the mass of solid that can be dissolved per mole of acid reacted, R(c s ) is the reaction rate, is the matrix porosity.

5. The acid fluid diversion quantification evaluation method for carbonate rock geothermal reservoir prepad acid fracturing of claim 4, wherein, In step (1), the porous medium heat transfer module is used to describe the heat transfer process of the whole model, including heat convection and heat conduction; The heat transfer equation considering heat convection and heat conduction in the fracture is: where T is temperature, p f is the equivalent density of the fracture, p a , p p are the acid and preflush densities, respectively, C f is the equivalent heat capacity of the fracture, C s is the rock specific heat capacity, C a is the acid specific heat capacity, C p is the preflush specific heat capacity, l f is the equivalent heat transfer coefficient of the fracture, l s is the rock heat transfer coefficient, l a is the acid heat transfer coefficient, l p is the preflush heat transfer coefficient, Q t is the heat exchange between the two media; The energy conservation equation considering heat convection, heat conduction and chemical reaction heat effect in the matrix is: Q c = -R(c s ) · H where: p m C is the equivalent density of the matrix, C m Cv is the equivalent heat capacity of the matrix, λ m K is the equivalent heat transfer coefficient of the matrix, Q c H is the enthalpy change, Q t Q is the heat exchange between the two media, calculated as: where: z→d f is represented as a fracture wall surface.

6. The acid fluid diversion quantification evaluation method for carbonate rock geothermal reservoir prepad acid fracturing of claim 5, wherein, In step (1), the mathematical calculation module is used to describe the evolution of the pore and permeability relationship of the rock matrix part, and the change of the pore and permeability will further affect other physical processes, and the relationship calculation formula among the permeability, pore radius, specific surface and porosity is as follows: wherein, and are the porosity and the initial porosity of the rock, respectively; K and K0are the permeability and the initial permeability of the rock, respectively; r p and r0are the pore radius and the initial pore radius of the rock, respectively; a v and a0are the specific surface and the initial specific surface of the rock, respectively; β is a constant related to the pore structure; Finally, the core-scale heat-water-chemical multi-field coupling numerical model is established by coupling the variables among the multiple physical field control equations.

7. The acid fluid diversion quantification evaluation method for carbonate rock geothermal reservoir prepad acid fracturing of claim 6, wherein, In step (2), different experimental conditions are set to carry out parameter sensitivity analysis and explore the influence of rock plate temperature on the etching front advance coefficient. Five different initial temperature values are set from 353K / 80℃ to 453K / 180℃, and other factors are fixed to simulate the acid etching process under different temperatures within the same time. In order to explore the influence of the viscosity of the preflush on the etching front advance coefficient, five graduation values are selected from 20mPa·s to 60mPa·s, and other factors are fixed to simulate the acid etching process under different viscosity differences within the same time. To explore the influence of acid injection displacement on the etching front edge pointing coefficient, five scale values were selected from 2 mL·min -1 to 6 mL·min -1 , and the acid etching process under different displacements in the same time was simulated by fixing other factors.

8. The acid fluid diversion quantification evaluation method for carbonate rock geothermal reservoir prepad acid fracturing of claim 7, wherein, In step (3), the method for determining the etching boundary is to indirectly calculate the fracture width by using the region with relatively high porosity in the porosity distribution, and the calculation method is: wherein, is the porosity of the etching boundary, and ε is a coefficient determined according to experiments, is the maximum value of the porosity set in the numerical model, which is 0.99, and the isosurface with the porosity of can determine the etching boundary. Data extraction is to extract the three-dimensional coordinates of all discrete points on the fracture boundary, and the data processing software is the software that realizes the data filtering and sorting functions.

Images