Fluid-structure interaction reservoir numerical simulation method considering cracks and faults

Abstract

The invention discloses a fluid-solid coupling oil reservoir numerical simulation method considering cracks and faults, and relates to the technical field of complex oil reservoir development, and the method comprises the following steps: determining the relationship between fault permeability and fault sliding trend, and the critical sliding trend required by the first opening of the fault; a fluid-solid coupling oil reservoir model considering the cracks and the faults is constructed by considering the mutual coupling relation among the cracks, the faults and the rock matrix and the coupling relation between the seepage field and the stress field; simulating a fluid-solid coupling oil reservoir model, reading fault stress parameters at each time step, and calculating a fault sliding trend; in each time step, the permeability of the fault is determined according to the sliding trend of the fault and the relation between the sliding trend and the permeability of the fault; and updating the fault permeability, and continuing numerical simulation of the next time step until the simulation is finished. According to the method, modeling is adopted, the fracture network and the rock matrix are regarded as two overlapping media respectively, the complexity of numerical simulation calculation is reduced, and the operation efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of complex oil and gas reservoir development, and particularly to a fluid-solid coupling reservoir numerical simulation method considering fractures and faults. Background Art

[0002] In oil reservoir development, fractures and faults are key factors affecting seepage behavior and productivity. As high-permeability channels, fractures can significantly improve fluid flow efficiency; while faults divide the reservoir, control fluid migration paths, and undergo slip, opening or closing with stress changes, resulting in dynamic changes in permeability and connectivity. Traditional fluid-solid coupling simulation methods are usually limited to the assumption of linear elastic deformation and cannot describe non-linear or irreversible behaviors such as fault slip, rock fracture, and fracture opening and closing. On the other hand, current simulation methods mainly update fault permeability through pressure-permeability empirical formulas, ignoring the mechanical mechanism during fault slip. Existing experimental studies have shown that the permeability of faults can increase exponentially under the action of stress changes and cumulative shear slip, but it is difficult for fluid-solid coupling simulations based on linear elastic assumptions to capture this feature.

[0003] To improve the prediction accuracy of the production dynamics of fractured fault-block reservoirs, it is necessary to establish a fluid-solid coupling simulation method considering fracture distribution and fault dynamic evolution, so as to overcome the limitation of the insufficient prediction accuracy of traditional models. Summary of the Invention

[0004] To solve the above technical problems, the present invention discloses a fluid-solid coupling reservoir numerical simulation method considering fractures and faults. This method simultaneously considers the coupling relationship between fractures, faults and rock matrix, as well as the coupling relationship between the seepage field and the stress field, and combines the opening and closing mechanism of faults under the influence of stress to construct a fluid-solid coupling reservoir model that can accurately simulate the evolution behavior of faults in fractured reservoirs.

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

[0006] A fluid-solid coupling reservoir numerical simulation method considering fractures and faults, comprising the following steps:

[0007] s1. According to on-site or experimental data, determine the relationship between fault permeability and fault sliding trend, and the critical sliding trend required for the first opening of the fault (abbreviated as fault critical sliding trend);

[0008] s2. Consider the mutual coupling relationship between fractures, faults and rock matrix, as well as the coupling relationship between the seepage field and the stress field, and construct a fluid-solid coupling reservoir model considering fractures and faults;

[0009] s3. Simulate the fluid-solid coupling reservoir model, and read the stress parameters of the fault at each time step to calculate the sliding trend of the fault;

[0010] S4. At each time step, determine the permeability of the fault according to the sliding trend of the fault and its relationship with the fault permeability; update the permeability of the fault and continue with the numerical simulation of the next time step until the simulation ends.

[0011] Optionally, in step S2, consider the mutual coupling relationship between fractures, faults, and the rock matrix. The coupling process is as follows:

[0012] (1) The mass exchange between the fracture and the rock matrix and the fault is:

[0013] ;

[0014] In the formula, Fracture porosity, dimensionless; The saturation of phase j in the fracture, where j is o, g, or w, representing the oil phase, gas phase, and water phase respectively, dimensionless; The permeability of phase j in the fracture, with the unit of mD; The viscosity of phase j in the fracture, with the unit of mPa·s; The pressure of phase j in the fracture, with the unit of kPa; The source-sink term of phase j in the fracture, with the unit of m 3 / s; and The flow rates of phase j flowing from the rock matrix and the fault to the fracture respectively, with the unit of m 3 / s, and the calculation formula is:

[0015] ;

[0016] ;

[0017] In the formula, and The pressures of phase j in the rock matrix and the fault respectively, with the unit of kPa; and The inter-porosity mass exchange coefficients between the fracture and the rock matrix and between the fracture and the fault respectively, and the calculation formula is:

[0018] ;

[0019] ;

[0020] In the formula, and The densities of phase j in the rock matrix and the fault respectively, with the unit of kg / m 3 ; and are the permeability of phase j in the rock matrix and the fault, respectively, with the unit of mD; and are the viscosities of phase j in the rock matrix and the fault, respectively, with the unit of mPa·s; is the geometric characteristic parameter of the fracture, 1 / m 2 , and the calculation formula is:

[0021] ;

[0022] In the formula, , and are the spacings of the fracture in the three mutually perpendicular x, y, and z directions in three-dimensional space, respectively;

[0023] (2) The mass exchange between the fault and the rock matrix and the fracture is:

[0024] ;

[0025] In the formula, is the porosity of the fault, dimensionless; is the saturation of phase j in the fault, dimensionless; is the source-sink term of phase j in the fault, with the unit of m 3 / s; is the flow rate of phase j flowing from the rock matrix to the fault, with the unit of m 3 / s, and the calculation formula is:

[0026] ;

[0027] (3) The mass exchange between the rock matrix and the fracture and the fault is:

[0028] ;

[0029] In the formula, is the porosity of the rock matrix, dimensionless; is the saturation of phase j in the rock matrix, dimensionless; is the source-sink term of phase j in the rock matrix, with the unit of m 3 / s.

[0030] Optionally, in step s2, considering the coupling relationship between the seepage field and the stress field, the coupling process is:

[0031] ;

[0032] In the formula, is the unit time step, with the unit of s; is the Poisson's ratio, dimensionless; is the Biot coefficient, dimensionless; is the total pore pressure of the current grid, with the unit of kPa; is the body force, with the unit of N / m 3 ; is the normal mean stress, with the unit of kPa, The calculation formula of is:

[0033] ;

[0034] where , and are the maximum principal stress, intermediate principal stress and minimum principal stress respectively, with the unit of kPa.

[0035] Optionally, in step s3, the stress parameters of the fault (including shear stress and effective normal stress) are obtained, and the process of calculating the fault slip trend according to the mechanical properties of the fault (including parameters such as tangential stiffness and cohesion) is as follows:

[0036] ;

[0037] ;

[0038] ;

[0039] In the formula, is the shear stress of the fault plane, with the unit of kPa, where the fault plane is the cross-section passing through the center of the fault grid and parallel to the fault slip plane; is the tangential stiffness of the fault, with the unit of kPa / m; and are the sliding displacement and critical sliding displacement of the fault respectively, with the unit of m; , and are the relative sliding friction force, minimum relative sliding friction force and maximum relative sliding friction force of the fault respectively, dimensionless; is the cohesion of the formation rock, with the unit of kPa; is the effective normal stress of the fault plane, with the unit of kPa. The value range of ST is [0,1]. Before ST first reaches the critical slip trend of the fault , the fault is not opened, and the fault permeability is the initial fault permeability; when ST reaches the critical slip trend of the fault, the fault is opened, and the subsequent fault permeability is determined by the relationship between it and the fault slip trend; when ST reaches 1, the stress condition of the fault plane satisfies the Mohr-Coulomb shear failure criterion , and the fault is completely opened.

[0040] The beneficial effect of the present invention is that

[0041] (1) In the present invention, the fracture network and the rock matrix are regarded as two spatially overlapping media. The fracture network can be simulated only by relying on the original matrix grids in the numerical model without adding additional fracture grids, thereby significantly reducing the complexity of numerical simulation calculations and improving the operation efficiency while retaining the main physical characteristics of the fractures.

[0042] (2) The present invention establishes a coupled simulation method for fractures, faults, seepage fields, and stress fields, which can calculate the stress state and sliding trend of faults at each time step and dynamically adjust the permeability of faults according to the sliding trend, completely depicting the dynamic processes such as the transition of faults from static to sliding and the increase and decrease of permeability, thus more realistically reflecting the seepage-mechanical behavior of fractured fault-block oil reservoirs, significantly improving the accuracy and reliability of numerical simulation, and having important engineering application value for the production prediction of fractured fault-block oil reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is a schematic flow chart of a fluid-solid coupling oil reservoir numerical simulation method considering fractures and faults according to the present invention;

[0044] Figure 2 is a three-dimensional schematic diagram of fluid-solid coupling models I, II, and III of a three-dimensional fractured fault-block oil reservoir according to the present invention;

[0045] Figure 3 is a curve of the sliding trend and permeability change of fault I in model I shown in the embodiment of the present invention;

[0046] Figure 4 is a curve of the sliding trend and permeability change of fault III in model I shown in the embodiment of the present invention;

[0047] Figure 5 is a cumulative gas production curve of models I, II, and III shown in the embodiment of the present invention;

[0048] Figure 6 is a cumulative oil production curve of models I, II, and III shown in the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0049] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0050] The present invention discloses a fluid-solid coupling reservoir numerical simulation method considering fractures and faults, which simulates a three-dimensional fractured fault-block reservoir model, as Figure 1 shown. The specific steps are as follows:

[0051] S1. According to field or experimental data, determine the relationship between fault permeability and fault slip trend, as well as the critical fault slip trend. The relationship between fault permeability and slip trend is shown in Table 1. The permeability multipliers corresponding to the values between adjacent slip trends in Table 1 are determined by linear interpolation. In this embodiment, the critical fault slip trend ST critical = 0.8.

[0052] Table 1

[0053]

[0054] S2. Considering the mutual coupling relationship between fractures, faults, and rock matrix, as well as the coupling relationship between the seepage field and the stress field, construct a fluid-solid coupling reservoir model considering fractures and faults, and denote it as Model I, as Figure 2As shown in the figure. Let \(x\) and \(y\) represent two mutually perpendicular horizontal directions, and \(z\) represent the longitudinal direction perpendicular to the \(x\) - direction and \(y\) - direction. The total number of grids in Model I is \(15\times10\times10\) (\(x\times y\times z\)), the size of the grid in the \(z\) - direction is \(10\ m\), and in the \(x\) and \(y\) - directions is \(0.1 - 10\ m\). The size of the grid is smaller near the fault. The total size of Model I is \(66.3\times72.1\times200\ m\). Model I contains two production wells perforated at the bottom of the reservoir (the arrow direction at the top of the wellbore is upward), two \(CO_2\) injection wells perforated at the top of the reservoir (the arrow direction at the top of the wellbore is downward), and two fault blocks formed by three faults. Among them, Fault I and Fault II jointly form Fault Block I, and Fault III and the reservoir boundary jointly form Fault Block II; Fault Block I is controlled by Fault I, and Fault Block II is controlled by Fault III. For comparison, two other models with the same physical size and parameters as Model I are established, denoted as Model II and Model III respectively; among them, Model II only considers the dynamic evolution of faults and does not contain a fracture network; Model III considers the fracture network based on the dual - porosity model but does not consider the dynamic change of faults, and its fault permeability is always the initial permeability. The key parameters of Model I, Model II, and Model III are shown in Table 2.

[0055] Table 2

[0056]

[0057] Considering the mutual coupling relationship among fractures, faults, and rock matrix, the coupling process includes:

[0058] (1) The mass exchange between the fracture and the rock matrix and the fault is:

[0059] ;

[0060] In the formula, Fracture porosity, dimensionless; is the saturation of the \(j\) - phase in the fracture, where \(j\) is \(o\), \(g\), or \(w\), representing the oil phase, gas phase, and water phase respectively, dimensionless; is the permeability of the \(j\) - phase in the fracture, with the unit of \(mD\); is the viscosity of the \(j\) - phase in the fracture, with the unit of \(mPa\cdot s\); is the pressure of the \(j\) - phase in the fracture, with the unit of \(kPa\); is the source - sink term of the \(j\) - phase in the fracture, with the unit of \(m\) 3 / s; and are the flow rates of the \(j\) - phase flowing from the rock matrix and the fault to the fracture respectively, with the unit of \(m\) 3 / s, and the calculation formula is:

[0061] ;

[0062] ;

[0063] In the formula, and are the pressures of the j-phase in the rock matrix and the fault, respectively, with the unit of kPa; and are the mass exchange coefficients between the fracture and the rock matrix and between the fracture and the fault pores, respectively, and the calculation formula is:

[0064] ;

[0065] ;

[0066] In the formula, and are the densities of the j-phase in the rock matrix and the fault, respectively, with the unit of kg / m 3 ; and are the permeabilities of the j-phase in the fault, respectively, with the unit of mD; and is the viscosity of the j-phase in the fault, with the unit of mPa·s; is the geometric characteristic parameter of the fracture, 1 / m 2 , and the calculation formula is:

[0067] ;

[0068] In the formula, , and are the spacings of the fracture in the three mutually perpendicular x, y, and z directions in three-dimensional space, respectively.

[0069] (2) The mass exchange between the fault and the rock matrix and the fracture is:

[0070] ;

[0071] In the formula, is the porosity of the fault, dimensionless; is the saturation of the j-phase in the fault, dimensionless; is the source-sink term of the j-phase in the fault, with the unit of m 3 / s; is the flow rate of the j-phase flowing from the rock matrix to the fault, with the unit of m 3 / s, and the calculation formula is:

[0072] ;

[0073] (3) The mass exchange between the rock matrix and the fracture and the fault is:

[0074] ;

[0075] In the formula, is the porosity of the rock matrix, dimensionless; is the saturation of phase j in the rock matrix, dimensionless; is the source-sink term of phase j in the rock matrix, with the unit of m 3 / s.

[0076] Considering the coupling relationship between the seepage field and the stress field, the coupling method is:

[0077] ;

[0078] In the formula, is the unit time step, with the unit of s; is the Poisson's ratio, dimensionless; is the Biot coefficient, dimensionless; is the total pore pressure of the current grid, with the unit of kPa; is the body force, with the unit of N / m 3 ; is the normal mean stress, with the unit of kPa, The calculation formula of

[0079] ;

[0080] In the formula, , and are the maximum principal stress, the intermediate principal stress and the minimum principal stress respectively, with the unit of kPa.

[0081] s3. Simulate the fluid-solid coupling reservoir model, and read the stress parameters of the fault (including shear stress and effective normal stress) at each time step. According to the mechanical properties of the fault (including parameters such as tangential stiffness and cohesion), calculate the sliding trend of the fault, including:

[0082] ;

[0083] ;

[0084] ;

[0085] In the formula, is the shear stress of the fault plane, with the unit of kPa, where the fault plane is the cross-section passing through the center of the fault grid and parallel to the fault slip plane; is the tangential stiffness of the fault, with the unit of kPa / m; and are the sliding displacement and the critical sliding displacement of the fault respectively, with the unit of m; , and They are the relative sliding friction force, the minimum relative sliding friction force, and the maximum relative sliding friction force of the fault, dimensionless; is the cohesion of the formation rock, with the unit of kPa; is the effective normal stress of the fault plane, with the unit of kPa. The value range of ST is [0,1]. Before ST first reaches the critical sliding trend of the fault, the fault is not opened, and the fault permeability is the initial fault permeability; when ST reaches the critical sliding trend of the fault, the fault is opened, and the subsequent fault permeability is determined by its relationship with the fault sliding trend; when ST reaches 1, the stress condition of the fault plane satisfies the Mohr-Coulomb shear failure criterion , and the fault is completely opened.

[0086] S4. In each time step, determine the fault permeability according to the sliding trend of the fault and its relationship with the fault permeability; update the fault permeability, and continue the numerical simulation of the next time step until the simulation ends.

[0087] As Figure 3 shown, with the continuous injection of CO2, the sliding trend of Fault I gradually increases, and it first exceeds the critical value of 0.8 on the 422nd day, prompting its permeability to suddenly increase from 1×10 -6 mD to 0.5 mD. After that, the sliding trend quickly drops, and the permeability also decreases accordingly. On the 580th day, the sliding trend rises again and reaches a peak of 0.8 on the 942nd day, corresponding to a permeability increase to 12 mD. In contrast, Figure 4 shows that the sliding trend of Fault III fluctuates violently. Although it rises to a maximum of 0.6 on the 2819th day, it does not reach the critical value of 0.8, so its permeability is always low (1×10 -6 mD).

[0088] As Figure 5 and Figure 6 shown, Model I containing a fracture network is significantly superior to Model II without a fracture network in terms of cumulative oil and gas production. Due to the lack of connected fractures in Model II, its overall permeability is much lower than that of Model I, resulting in a slow seepage velocity of the injected CO2 and reservoir crude oil in Model II, so the cumulative production is low. This result shows that the fracture network has a key impact on the development performance of fractured fault-block oil reservoirs, and the method of the present invention is of great significance for the production prediction of fractured fault-block oil reservoirs. On the other hand, compared with Model III that does not consider the dynamic evolution of faults, the permeability of Fault I in Model I considering the fault evolution mechanism is significantly improved between the 414th day and the 1386th day ( Figure 3), so that the CO2 injected into Model I from the injection well quickly flows through Fault I to the production well. Therefore, the cumulative gas production of Model I is relatively high and the cumulative oil production is relatively low. In summary, the dynamic evolution of faults has a significant impact on the development effect of fractured fault-block reservoirs. The method of the present invention can accurately simulate the temporal evolution of fault stress states, sliding trends, and permeabilities, providing a powerful tool for reservoir dynamic analysis and development plan design.

[0089] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A numerical simulation method for fluid-solid coupling reservoir considering fractures and faults, characterized in that, It includes the following steps: S1. Determine the relationship between the fault permeability and the fault slip trend based on on-site or experimental data, as well as the critical slip trend required for the initial opening of the fault; S2. Consider the mutual coupling relationship between fractures, faults, and rock matrix, as well as the coupling relationship between the seepage field and the stress field, and construct a fluid-solid coupling reservoir model considering fractures and faults; S3. Simulate the fluid-solid coupling reservoir model, and read the stress parameters of the fault at each time step to calculate the fault slip trend; S4. In each time step, determine the permeability of the fault according to the fault slip trend and its relationship with the fault permeability; update the permeability of the fault, and continue the numerical simulation of the next time step until the simulation ends.

2. The fluid-solid coupling reservoir numerical simulation method considering fractures and faults according to claim 1, wherein, In step S2, considering the mutual coupling relationship between fractures, faults, and rock matrix, the coupling process is as follows: (1) The mass exchange between the fracture and the rock matrix and the fault is: ； In the formula, Fracture porosity, dimensionless; is the saturation of phase j in the fracture, where j is o, g, or w, representing the oil phase, gas phase, and water phase respectively, dimensionless; is the permeability of phase j in the fracture, with the unit of mD; is the viscosity of phase j in the fracture, with the unit of mPa·s; is the pressure of phase j in the fracture, with the unit of kPa; is the source-sink term of phase j in the fracture, with the unit of m 3 / s; and are the flow rates of phase j flowing from the rock matrix and fault to the fracture respectively, with the unit of m 3 / s, and the calculation formula is: ； ； Wherein, and are the pressures of phase j in the rock matrix and the fault respectively, with the unit of kPa; and are the mass exchange coefficients between the fracture and the rock matrix and between the fracture and the fault pores respectively, and the calculation formula is: ； ； Wherein, and are the density of phase j in the rock matrix and the fault respectively, with the unit of kg / m 3 ; and are the permeability of phase j in the rock matrix and the fault respectively, with the unit of mD; and are the viscosity of phase j in the rock matrix and the fault respectively, with the unit of mPa·s; is the geometric characteristic parameter of the fracture, 1 / m 2 , and the calculation formula is: ； In the formula, , and are the spacings of the cracks in the three mutually perpendicular x, y, and z directions in three-dimensional space, respectively; (2) The mass exchange between the fault and the rock matrix and the fracture is: ； wherein, is the porosity of the fault, dimensionless; is the saturation of phase j in the fault, dimensionless; is the source-sink term of phase j in the fault, with the unit of m 3 / s; is the flow rate of phase j flowing from the rock matrix to the fault, with the unit of m 3 / s, and the calculation formula is: ； (3) The mass exchange between the rock matrix and the fracture and the fault is: ； In the formula, is the porosity of the rock matrix, dimensionless; is the saturation of phase j in the rock matrix, dimensionless; is the source-sink term of phase j in the rock matrix, with the unit of m 3 / s.

3. A fluid-solid coupling reservoir numerical simulation method considering fractures and faults according to claim 1, characterized in that, In step S2, considering the coupling relationship between the seepage field and the stress field, the coupling process is as follows: ； In the formula, is the unit time step, with the unit of s; is the Poisson's ratio, dimensionless; is the Biot coefficient, dimensionless; is the total pore pressure of the current grid, with the unit of kPa; is the body force, with the unit of N / m 3 ; is the normal mean stress, with the unit of kPa, The calculation formula of is: ； Among them, , and are the maximum principal stress, the intermediate principal stress, and the minimum principal stress, respectively, with the unit of kPa.

4. The fluid-solid coupling reservoir numerical simulation method considering fractures and faults according to claim 1, characterized in that In step S3, the process of calculating the fault slip trend ST is: ； ； ； In the formula, is the shear stress of the fault plane, with the unit of kPa, where the fault plane is the cross-section passing through the center of the fault grid and parallel to the fault slip plane; is the tangential stiffness of the fault, with the unit of kPa / m; and are the sliding displacement and critical sliding displacement of the fault respectively, with the unit of m; 、 and are the relative sliding friction force, minimum relative sliding friction force and maximum relative sliding friction force of the fault respectively, dimensionless; is the cohesion of the formation rock, with the unit of kPa; is the effective normal stress of the fault plane, with the unit of kPa; The value range of ST is [0,1]. Before ST first reaches the critical sliding trend of the fault , the fault is not opened, and the fault permeability is the initial fault permeability; When ST reaches the critical sliding trend of the fault , the fault is opened, and the subsequent fault permeability is determined by its relationship with the fault sliding trend; When ST reaches 1, the stress condition of the fault plane satisfies the Mohr-Coulomb shear failure criterion , and the fault is completely opened.

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