Fractured reservoir acidification method based on three-dimensional embedded discrete fracture model

Abstract

The invention discloses a fractured reservoir acidification method based on a three-dimensional embedded discrete fracture model. The method comprises the following steps: S1, constructing a three-dimensional fractured reservoir physical model containing natural fractures; s2, carrying out structured grid division on the matrix, carrying out unstructured grid division on the crack by utilizing a matrix grid boundary, endowing physical parameters to the physical model, and setting acidification construction time; s3, fluid channeling coefficients between the matrix and the fractures and between the fractures are calculated; s4, based on the characteristics of the three-dimensional embedded discrete fracture model, constructing an acid liquor seepage model and an acid liquor chemical reaction model in the matrix and the fracture, and constructing a pore permeability evolution auxiliary equation of the matrix and the fracture; s5, coupling and solving the model and the equation in the S4; and S6, if the current time step reaches the construction time, obtaining a three-dimensional fractured reservoir acidification numerical simulation result. According to the method, high-precision dynamic coupling simulation of the three-dimensional complex fracture network and the acidification process is realized, and the method has a wide market application prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of matrix acidizing, and in particular relates to a fractured reservoir acidizing method based on a three-dimensional embedded discrete fracture model. Background Art

[0002] In the field of numerical simulation of acidizing fractured reservoirs, existing technologies mostly rely on simplified two-dimensional models or equivalent medium assumptions, simulating the flow of acid in the matrix and fractures through continuous medium theory or discrete fracture characterization. However, natural fractures in actual reservoirs have complex three-dimensional spatial distribution characteristics, and the dynamic coupling of their topological structure and the acid diversion effect significantly affects the wormhole branching pattern and acidizing uniformity. Traditional methods often approximate fracture characterization through structured grids or equivalent permeability fields. This makes it difficult to accurately describe the morphology of three-dimensional fractures and their dynamic mass exchange with the matrix, resulting in an underestimated rate of acid advancement along high-conductivity fractures and deviations in the prediction of the extension length of the wormhole main channel.

[0003] This paper proposes a numerical simulation method for acidizing fractured reservoirs based on a three-dimensional embedded discrete fracture model (3D-EDFM). This method overcomes the technical limitations of traditional models in terms of dimensionality simplification, extensive fracture characterization, and grid adaptability, providing theoretical innovation and practical engineering support for the design of acidizing schemes for highly heterogeneous fractured reservoirs. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for acidizing fractured reservoirs based on a three-dimensional embedded discrete fracture model. The method has a reliable principle and is easy to operate. It realizes for the first time a high-precision dynamic coupling simulation of a three-dimensional complex fracture network and the acidizing process, overcomes the defects and shortcomings of the existing technology, and has broad market application prospects.

[0005] In order to achieve the above technical objectives, the present invention adopts the following technical solutions.

[0006] A fractured reservoir acidizing method based on a three-dimensional embedded discrete fracture model includes the following steps: S1: Construct a three-dimensional fracture reservoir physical model containing natural fractures; S2: performing structured grid division on the matrix, performing unstructured grid division on the fracture using the matrix grid boundary, assigning physical parameters to the physical model, and setting the acidizing construction time; S3: Calculate the cross-flow coefficient NNC between matrix and cracks and between cracks; S4: Based on the characteristics of the three-dimensional embedded discrete fracture model (3D-EDFM), the acid seepage model and acid chemical reaction model in the matrix and fractures are constructed, and the auxiliary equations for the porosity and permeability evolution of the matrix and fractures are constructed; S5: Coupled solution of the model and equations described in S4; S6: Determine whether the current time step has reached the construction time. If not, repeat steps S3 - S5. If so, obtain the numerical simulation results of acidizing the three - dimensional fractured reservoir.

[0007] Preferably, in step S1, the natural fractures of the three - dimensional fractured reservoir physical model are generated by random or explicit definition.

[0008] Preferably, in step S2, the physical parameters include the initial porosity, permeability, pore - specific surface area, pore - throat radius, reservoir pressure, acid concentration, and acid reaction rate of the matrix and fractures.

[0009] Preferably, in step S3, calculating the cross - flow coefficients between the matrix and fractures and between fractures and fractures includes: Cross - flow coefficient between the matrix and fractures: In the formula, is the cross - flow coefficient between the matrix and fractures, m 3 ; is the fracture area in the grid block, m 2 ; is the matrix permeability, m 2 ; is the average distance between the matrix and fracture grids, m; Cross - flow coefficient between two intersecting fractures: In the formula, is the cross - flow coefficient between two intersecting fractures, m 3 ; and are the permeabilities of two fractures, m 2 ; and are the widths of two fractures, m; is the length of the intersection line of two fractures, m; and are the average normal distances from two fractures to the intersection line, m.

[0010] Preferably, in step S4, constructing the acid seepage model in the matrix and fractures based on the characteristics of the three - dimensional embedded discrete fracture model (3D - EDFM) includes: Matrix pressure control equation: Fracture pressure control equation: In the formula, and are matrix porosity and fracture porosity, with superscripts and subscripts and represent matrix and fracture respectively, and the same below; t is time, s; is the divergence operator, dimensionless; is the gradient operator, dimensionless; and are matrix permeability and fracture permeability respectively, m 2 ; is the viscosity of acid solution, Pa·s; and are matrix pressure and fracture pressure respectively, Pa; and are flux transfer functions from matrix to fracture and from fracture to matrix respectively, m 3 / s; is the flux transfer function between intersecting fractures, m 3 / s; and are matrix grid volume and fracture grid volume respectively, m 3 ; In the formula, and are pressures of two intersecting fracture grids respectively, Pa.

[0011] Preferably, in step S4, the acid solution chemical reaction models in matrix and fracture are constructed based on the characteristics of the three-dimensional embedded discrete fracture model (3D-EDFM), including: Matrix acid solution concentration control equation: Fracture acid solution concentration control equation: In the formula, and are acid concentrations in matrix and fracture respectively, kmol / m 3 ; and are fluid velocities in matrix and fracture respectively, m / s; is the effective diffusion coefficient in different directions, m / s, represents , and directions; is the effective diffusion coefficient, m / s; is The function, kmol / m 3 ; and are the specific surface areas of pores in the matrix and fractures respectively, m 2 / m 3 ; and are the surface reaction rates in the matrix and fractures respectively, kmol / (s·m 2 ); Surface reaction rates of the matrix and fractures: In the formula, the subscript represents the matrix or fracture , the same hereinafter; is the mass transfer coefficient, m / s; is the reaction rate constant, m / s; is the specific surface area of pores, m 2 / m 3 ; Mass transfer rate of acid solution: In the formula, is the asymptotic Sherwood number; is the pore Reynolds number, expressed as ; is the Schmidt number, expressed as ; is the ratio of pore length to pore diameter; is the average pore throat radius, m; is the density of acid solution, kg / m 3 ; is in magnitude, m / s.

[0012] Preferably, in step S4, the steps of constructing the pore permeability evolution auxiliary equations for the matrix and fractures based on the characteristics of the three-dimensional embedded discrete fracture model (3D-EDFM) include: Diffusion coefficient control equation: Permeability control equation: Pore throat radius control equation: Specific surface area of pores control equation: Porosity control equation: In the formula, and are constants related to the pore structure, = 0.5, = 0.1, = 0.1; is the Péclet number, expressed as ; and are the initial permeability and the current permeability, m 2 ; and are the initial porosity and the current porosity; and are the initial average pore throat radius and the current average pore throat radius, m; is the pore expansion parameter; and are the initial pore specific surface area and the current pore specific surface area, m 2 / m 3 ; is the dissolution power of the acid, defined as the grams of dissolved solids in each mole of acid reaction, kg / kmol; is the rock density, kg / m 3 .

[0013] Compared with the prior art, the present invention has the following beneficial effects: By constructing a three-dimensional embedded discrete fracture model (3D-EDFM) and adopting a hybrid discretization strategy of matrix structured grids and fracture unstructured grids, the present invention accurately characterizes three-dimensional fractures, solves the problems of acid vertical sweep and fracture competitive dissolution distortion caused by geometric simplification in traditional two-dimensional models, and significantly improves the simulation accuracy of acid diversion paths in complex fracture networks; by dynamically correcting the cross-flow coefficients (NNC) between the matrix and fractures and between fractures in real time, the present invention effectively enhances the dynamic control ability of acid diversion, optimizes the reliability of acid breakthrough path prediction, provides a precise basis for process parameter design, provides a high-precision technical means for rapid evaluation of oilfield block-level acidification programs, significantly enhances the prediction effect of acid sweep efficiency and wormhole diversion ability in complex fracture systems, and has important engineering application value for reducing acidification operation costs and improving oil and gas recovery rates. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions of the present invention, the drawings required for implementation examples or prior art descriptions will be briefly introduced below. The drawings in the following descriptions are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0015] Figure 1This is the flow chart of the acidification method for fractured reservoirs based on the three-dimensional embedded discrete fracture model of the present invention; Figure 2 This is a schematic diagram of the grid division result of a three-dimensional formation core with a complex fracture network in a specific embodiment; Figure 3 This is a pressure distribution nephogram of a three-dimensional fractured formation core at the acidification time of 600 s in a specific embodiment; Figure 4 This is a pressure distribution nephogram of a three-dimensional fractured formation core at the acidification time of 1200 s in a specific embodiment; Figure 5 This is a pressure distribution nephogram of a three-dimensional fractured formation core at the acidification time of 1700 s in a specific embodiment; Figure 6 This is an acid fluid concentration distribution nephogram of a three-dimensional fractured formation core at the acidification time of 600 s in a specific embodiment; Figure 7 This is an acid fluid concentration distribution nephogram of a three-dimensional fractured formation core at the acidification time of 1200 s in a specific embodiment; Figure 8 This is an acid fluid concentration distribution nephogram of a three-dimensional fractured formation core at the acidification time of 1700 s in a specific embodiment; Figure 9 This is the wormhole development structure of a three-dimensional fractured formation core at the acidification time of 600 s in a specific embodiment; Figure 10 This is the wormhole development structure of a three-dimensional fractured formation core at the acidification time of 1200 s in a specific embodiment; Figure 11 This is the wormhole development structure of a three-dimensional fractured formation core at the acidification time of 1700 s in a specific embodiment. Detailed implementation manners

[0016] The present invention will be further described below in conjunction with the drawings and embodiments. It should be noted that, without conflict, the embodiments in the present application and the technical features in the embodiments may be combined with each other. It should be pointed out that unless otherwise specified, all the technologies and scientific terms used in the present application have the same meanings as those commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0017] As Figure 1 shown, the acidification method for fractured reservoirs based on the three-dimensional embedded discrete fracture model successively includes the following steps: S1: Construct a physical model of a three-dimensional fractured reservoir with natural fractures, and the generation of natural fractures in this physical model is obtained through random or explicit definition; S2: Conduct structured grid division on the matrix, perform unstructured grid division on the fractures using the matrix grid boundaries, assign physical parameters to the physical model, and set the acidizing time to 1700 s; S3: Calculate the NNC between the matrix and fractures and between fractures; S4: Based on the characteristics of the three-dimensional embedded discrete fracture model (3D-EDFM), construct the acid fluid seepage model and acid fluid chemical reaction model in the matrix and fractures; construct the auxiliary equations for pore permeability evolution in the matrix and fractures; S5: Coupled solution of the models and equations described in S4; S6: Determine whether the current time step has reached the construction time: if not, repeat steps S3 - S5; if so, obtain the numerical simulation results of acidizing the three-dimensional fractured reservoir.

[0018] In a specific embodiment, the acidizing method for fractured reservoirs based on the three-dimensional embedded discrete fracture model of the present invention is adopted. As Figure 2 shown, a physical model of a fractured reservoir core is set. The length, width, and height of this model are 0.3 m, 0.1 m, and 0.1 m respectively, and 121 fractures are randomly generated. Structured division (hexahedron) is performed on the matrix grid, the number of matrix grids is 75×25×25, the number of grids obtained by dividing the fractures using the edges of the matrix grids is 9270, and the total number of grids is 56145. The set physical parameters include the initial average porosity of the core matrix of 0.1, the average permeability of 10 mD, the pore specific surface area of 5000 m 2 / m 3 , the pore throat radius of 0.0001 m; the initial average porosity of the fractures is 0.8, the average permeability is 200 mD, the pore specific surface area is 5000 m 2 / m 3 , the pore throat radius of 0.0001 m. The initial conditions of the model are a reservoir pressure of 0 MPa, an acid fluid concentration of 20 wt% HCl, and an acid fluid reaction rate of 0.02 cm / s. The left boundary of the physical model is the injection boundary of the acid fluid, the acid injection rate is 50 ml / min, and the right boundary is a constant pressure boundary. Simulate the acidizing process of a reservoir with a complex fracture network, and set the acidizing time to 1700 s.

[0019] Figures 3 to 5These are three-dimensional pressure distribution cloud maps of fractured formation cores at different times during the acidizing process. This 3D pressure distribution map reveals the dynamic evolution of the reservoir pressure field during the acidizing process. In fractured reservoirs, acid preferentially advances rapidly along high-conductivity fractures, leading to significant pressure gradients near the fracture channels. As acidizing progresses, the pressure field gradually expands toward the ends of the fractures, widening the high-pressure zone. However, in some areas, new conductive channels form due to the development of wormholes, resulting in a non-uniform reconstruction of the pressure distribution. This dynamic pressure change intuitively demonstrates the control of the three-dimensional fracture network on the acid sweep path and the mechanism by which wormhole expansion enhances reservoir conductivity.

[0020] Figures 6 to 8 This is a 3D cloud map of the acid concentration distribution in a fractured formation core at different times during the acidizing process. This 3D visualization of the acid concentration distribution clearly demonstrates the differential migration patterns of acid within fractures and the matrix. High-concentration areas extend in strips along the primary fractures, indicating preferential long-range acid transport through high-conductivity fractures. Low-concentration areas are primarily distributed in the matrix or in fractures away from the primary wormholes, reflecting limited acid diffusion in low-permeability areas. Notably, the concentration field exhibits patchy characteristics at fracture intersections and secondary fracture branching points, revealing the dynamic diversion of acid within the complex fracture network. As acidizing progresses, the acid concentration in the primary fracture gradually decreases, indicating continuous consumption of active acid due to acid-rock reactions. Furthermore, the persistence of locally high concentrations at the ends of wormhole branches confirms the enhanced acid transport efficiency of wormhole expansion. This concentration distribution provides important insights for optimizing acid injection rates and targeted fracture stimulation.

[0021] Figures 9 to 11 The following are three-dimensional wormhole structures of fractured formation cores at different times during the acidizing process. The dynamic evolution of the three-dimensional wormhole development structure intuitively illustrates the transformation of reservoir diversion channels during acidizing. Initially, wormholes are primarily distributed linearly along natural fractures, reflecting the preferential dissolution of fracture walls by acid. As acidizing progresses, dense clusters of wormholes form at fracture intersections. The wormhole network exhibits multi-level branching and three-dimensional connectivity. The diameter of wormholes in primary fractures increases significantly, forming an interlaced and interconnected diversion system with secondary fractures. This evolution of the three-dimensional topological structure reveals the non-uniform etching patterns of acid in the fracture-matrix system and the enhancement of reservoir permeability anisotropy by the self-organized expansion of wormholes.

[0022] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the technical content disclosed above within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for acidizing fractured reservoirs based on a three-dimensional embedded discrete fracture model, successively including the following steps: S1: Construct a physical model of a three-dimensional fractured reservoir with natural fractures; S2: Conduct structured grid division on the matrix, conduct unstructured grid division on the fractures using the matrix grid boundaries, assign physical parameters to the physical model, and set the construction time of acidizing; S3: Calculate the interporosity coefficients between the matrix and fractures and between fractures and fractures, including: The interporosity coefficient between the matrix and fractures: ； In the formula, is the cross-flow coefficient between the matrix and the fracture, m 3 ; is the fracture area in the grid block, m 2 ; is the matrix permeability, m 2 ; is the average distance between the matrix and the fracture grid, m; The interporosity coefficient between two intersecting fractures: ； In the formula, is the crossflow coefficient between two intersecting fractures, m 3 ; and are the permeabilities of two fractures, m 2 ; and are the widths of two fractures, m; is the length of the intersection line of two fractures, m; and are the average normal distances from two fractures to the intersection line, m; S4: Based on the characteristics of the three-dimensional embedded discrete fracture model, construct an acid fluid seepage model and an acid fluid chemical reaction model in the matrix and fractures, and construct an auxiliary equation for pore permeability evolution in the matrix and fractures. The process is as follows: Construct an acid fluid seepage model in the matrix and fractures, including: The matrix pressure control equation: ； The fracture pressure control equation: ； In the formula, and are the matrix porosity and the fracture porosity respectively, and the superscripts and subscripts and represent the matrix and the fracture respectively, and the same applies hereinafter; t is the time, s; is the divergence operator, dimensionless; is the gradient operator, dimensionless; and are the matrix permeability and the fracture permeability respectively, m 2 ; is the acid viscosity, Pa·s; and are the matrix pressure and the fracture pressure respectively, Pa; and are the flux transfer functions from the matrix to the fracture and from the fracture to the matrix respectively, m 3 / s; is the flux transfer function between intersecting fractures, m 3 / s; and are the matrix grid volume and the fracture grid volume respectively, m 3 ; ； In the formula, and are the pressures of two intersecting crack meshes respectively, in Pa; Construct an acid fluid chemical reaction model in the matrix and fractures, including: The matrix acid fluid concentration control equation: ； The fracture acid fluid concentration control equation: ； In the formula, and are the acid concentrations in the matrix and the fracture, respectively, kmol / m 3 ; and are the fluid velocities in the matrix and the fracture, respectively, m / s; is the effective diffusion coefficient in different directions, m / s, represents , and directions; is the effective diffusion coefficient, m / s; and are the pore specific surface areas of the matrix and the fracture, respectively, m 2 / m 3 ; and are the surface reaction rates in the matrix and the fracture, respectively, kmol / (s·m 2 ); The surface reaction rate of the matrix and fractures: ； ； In the formula, the subscript represents the matrix or the fracture , and the same applies hereinafter; is the mass transfer coefficient, m / s; is the reaction rate constant, m / s; is the specific surface area of pores, m 2 / m 3 ; is the asymptotic Sherwood number; is the pore Reynolds number, ; is the Schmidt number, ; is the ratio of the pore length to the pore diameter; is the average pore throat radius, m; is the density of the acid solution, kg / m 3 ; is in magnitude, m / s; Construct an auxiliary equation for pore permeability evolution in the matrix and fractures, including: The diffusion coefficient control equation: ； The permeability control equation: ； The pore throat radius control equation: ； The pore specific surface area control equation: ； The porosity control equation: ； In the formula, and are constants related to the pore structure, = 0.5, = 0.1, = 0.1; is the Peclet number, expressed as ; and are the initial permeability and the current permeability, m 2 ; and are the initial porosity and the current porosity; and are the initial average pore throat radius and the current average pore throat radius, m; is the pore expansion parameter; and are the initial pore specific surface area and the current pore specific surface area, m 2 / m 3 ; is the dissolution power of the acid, defined as the grams of dissolved solids per mole of acid reaction, kg / kmol; is the rock density, kg / m 3 ; S5: Coupled solution of the models and equations described in S4; S6: Determine whether the current time step has reached the construction time: If not, repeat steps S3 - S5; if so, obtain the numerical simulation results of acidizing the three-dimensional fractured reservoir.

2. The acidizing method for fractured reservoirs based on a three-dimensional embedded discrete fracture model according to claim 1, wherein In step S1, the generation of natural fractures in the physical model of the three-dimensional fractured reservoir is obtained through random or explicit definition.

3. The acidification method for fractured reservoirs based on a three-dimensional embedded discrete fracture model according to claim 1, wherein In step S2, the physical parameters include the initial porosity, permeability, pore specific surface area, pore throat radius, reservoir pressure, acid fluid concentration, and acid fluid reaction rate of the matrix and fractures.

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