Method for establishing numerical simulation model of large-scale fractured gas reservoirs

Abstract

The present invention discloses a method for establishing a large-scale fractured gas reservoir numerical simulation model, comprising the following steps: establishing a dual medium model, and converting fractures into DFN discrete fractures; discretizing the DFN discrete fractures and forming discrete grids, and numerically adjusting the discrete grids; coupling the discretized and adjusted DFN discrete fractures into the dual medium model to form a triple medium numerical simulation model; obtaining relative permeability curves of the matrix, fractures and fractures in the model; calibrating and optimizing the model; establishing the initial gas-water distribution of the matrix, fractures and fractures in the model; and performing trial calculations on the model until the initial water-gas distribution of the model is the same as the initial water-gas distribution of the gas reservoir or the calculation converges. The present invention can accurately characterize the physical process of rapid intrusion of edge and bottom water along large fractures, and the parameters of the entire simulation process are consistent with the actual geological parameters; the present invention can also simulate the physical phenomenon of rapid water intrusion of edge and bottom water along large DFN fractures.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field development, and in particular to a method for establishing a large-scale fractured gas reservoir numerical simulation model. Background Art

[0002] For fractured gas reservoirs, dual media are traditionally used for simulation. If large-scale fractures (connected fractures) are relatively developed, triple media (matrix-continuous fractures-fractures) simulation is used. The usual operation is dual media + embedded fractures (EDFM), that is, embedded fractures are used to represent fractures. Alternatively, local grid encryption is used: the grid where the fracture is located is encrypted, and small grids are used to represent the fracture.

[0003] Dual media + EDFM can accurately characterize large-scale fractured gas reservoirs. At the same time, in the existing technology, dedicated numerical simulation software, such as CMG software, is often used to establish dual media + EDFM. However, if there are many fractures, there will be a large number of EDFM embedded grids in the model, resulting in slow calculation speed and non-convergence of the model. If local encryption is used, the simulation effect of the development process of edge and bottom water intrusion along large fractures is poor, the calculation results have large deviations, and the calculation speed is slow and non-convergence. Summary of the invention

[0004] In order to solve at least one of the above problems, the present invention proposes a method for establishing a large-scale fractured gas reservoir numerical simulation model.

[0005] The technical solution of the present invention is: a method for establishing a large-scale fractured gas reservoir numerical simulation model, comprising the following steps:

[0006] S1. Establish a dual-medium model, set the model to dual-porosity and dual-permeability or dual-porosity and single-permeability based on fluid flow, and convert the fracture into DFN discrete fracture;

[0007] S2, discretize the DFN discrete fractures and form discrete grids, and numerically adjust the discrete grids based on fracture connectivity, conductivity and fracture filling conditions;

[0008] S3, coupling the discretized and adjusted DFN discrete fractures into the dual medium model to form a triple medium numerical simulation model;

[0009] S4, obtaining relative permeability curves of matrix, fractures and faults in the model;

[0010] S5. Calibrate and optimize the model based on the size of adjacent grids and the size of pore volume;

[0011] S6. According to the actual gas reservoir characteristics, the initial gas and water distribution of matrix, fractures and faults in the model is established;

[0012] S7. Perform trial calculations on the model. If the initial water-gas distribution of the model is different from the initial water-gas distribution of the gas reservoir or the calculation does not converge, repeat steps S4 to S6 until the initial water-gas distribution of the model is the same as the initial water-gas distribution of the gas reservoir or the calculation converges.

[0013] Beneficial effects: The present invention accurately characterizes the physical process of rapid intrusion of edge and bottom water along large fractures, and the entire simulation process and parameters are consistent with the actual geological parameters. At the same time, the present invention establishes a dual medium + discrete fracture numerical simulation method for large-scale fractured gas reservoirs, taking into account both the accuracy and speed of numerical simulation, and successfully simulates the physical phenomenon of rapid water intrusion of edge and bottom water along DFN large fractures, revealing the flow laws of different media in fractured gas reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a model history matching effect diagram of an embodiment of the present invention;

[0015] Figure 2 Schematic diagram of water channeling during the fitting process;

[0016] Figure 3 A schematic diagram of a model and embedding of an embodiment of the present invention;

[0017] Figure 4 It is the CMG model and embedding schematic diagram;

[0018] Figure 5 This is the simulated water intrusion effect diagram of the experimental group;

[0019] Figure 6 This is the simulated water intrusion effect diagram of the control group. DETAILED DESCRIPTION

[0020] The specific implementation modes of the present invention will be described clearly and completely below with reference to examples and drawings. Obviously, the described examples are only some embodiments of the present invention, rather than all embodiments.

[0021] In this embodiment, the fractures referred to are large cracks and faults, wherein a large crack refers to a crack with an opening greater than 100 μm.

[0022] A method for establishing a large-scale fractured gas reservoir numerical simulation model comprises the following steps:

[0023] S1. Establish a dual-medium model, set the model to dual-porosity and dual-permeability or dual-porosity and single-permeability based on fluid flow, and convert the fracture into DFN discrete fracture;

[0024] In this step, a dual medium model is established, and setting it to dual porosity and dual permeability or dual porosity and single permeability is a routine operation in the field. Technical personnel in this field can set it according to actual conditions: if the matrix is ​​relatively dense and has poor fluidity, a dual porosity and single permeability model is established; if the matrix has good permeability, a dual porosity and dual permeability model is established.

[0025] At the same time, in the dual medium model, it is usually matrix + continuous cracks, and the continuous cracks here usually refer to micro cracks. In some dual medium models, it does not include fractures. At the same time, based on the particularity of fractures, this embodiment also performs special treatment on fractures so that they can be better coupled with the dual medium model in the subsequent process. At the same time, the dual medium model needs to be meshed, which is a conventional operation in the dual medium model, so its specific steps are not repeated here.

[0026] For the dual medium model without fracture characterization, when processing fractures, the location of the fracture needs to be characterized based on the existing data, such as the location of the fracture can be determined by seismic data and ant body data. Then, the fracture is numerically processed, and finally, the numerically processed data is imported into the geological modeling software and converted into DFN discrete fractures. There are many geological modeling software that can achieve this function, such as the common Petrel or tNavigator, which can achieve the above purpose.

[0027] S2. Discretize the DFN discrete fractures and form discrete grids, and numerically adjust the discrete grids based on fracture connectivity, conductivity, and fracture filling conditions. This step mainly includes the following sub-steps:

[0028] S21. Discretize the DFN discrete cracks, and numerically characterize the grids through which the cracks pass to form discrete grids. The purpose of discretizing the cracks is to divide the DFN discrete cracks into grids with separate attributes to facilitate the subsequent numerical adjustment of the DFN discrete cracks. At the same time, the operation of discretizing the cracks belongs to the routine operation in the field. Those skilled in the art can discretize them according to actual conditions, and the specific operation will not be described in detail here.

[0029] S22. Adjust the conductivity of the discrete grid: Based on the connectivity of the discrete grid, set the conductivity parameter of the discrete grid to 0-1, where the better the connectivity of the discrete grid, the larger the conductivity parameter; for DFN discrete cracks, they are usually connected, and the discrete grid is obtained by discretizing the DFN discrete cracks, so the discrete grid is also connected, so its value is usually not 0. In this case, the better the connectivity of the discrete grid, the larger the value of the conductivity parameter of the discrete grid. When the discrete grid is completely connected, its conductivity parameter is set to 1.

[0030] S23. Adjust the permeability of discrete grids: Specifically, it can be adjusted according to experimental results, usually 10-1000 times that of continuous fracture media; during the setting process, it is also necessary to adjust according to the direction of the fracture. For example, if the large fracture is a high-angle fracture, the vertical permeability is larger; because the permeability of the fracture is difficult to measure, the permeability can also be set according to the grid size and the fracture conductivity: the conductivity of the laboratory measured fracture = the conductivity of the grid that characterizes the fracture = permeability × grid size.

[0031] In the actual production process, we can adjust the permeability of the discrete grid according to the experimental results or the conductivity. During the adjustment process, we can further adjust it in combination with the direction of the DFN discrete cracks.

[0032] S24, adjusting the porosity of the discrete grid: based on the opening and filling degree of the cracks, setting the porosity of the discrete grid to 0-1, wherein the higher the crack filling degree, the lower the porosity;

[0033] S3, coupling the discretized and adjusted DFN discrete fractures to the dual medium model to form a triple medium numerical simulation model; in S2, after adjusting the discrete grid, the combined DFN discrete fractures are more consistent with the actual fracture conditions, so after coupling it with the dual medium model, the obtained triple medium numerical simulation model is more consistent with the actual formation conditions. At the same time, the operation of coupling the fracture to the dual medium model is a conventional operation in the art, and its specific steps are not repeated here.

[0034] S4, obtaining relative permeability curves of matrix, fractures and faults in the model;

[0035] Specifically, in this step, according to the seepage characteristics of matrix-continuous cracks-fractures, the matrix phase permeability curve, the crack phase permeability curve and the fracture phase permeability curve are measured, and the relative permeability curves of the matrix, continuous cracks and fractures are set respectively. Since the physical properties of the matrix, continuous cracks and fractures are quite different, in this embodiment, the phase permeability curves of the three are set separately, so that the final result is more in line with the actual situation.

[0036] S5. Calibrate and optimize the model based on the size of adjacent grids and the size of pore volume;

[0037] After completing the above model, it is necessary to proofread and optimize it to increase the calculation accuracy and speed. The specific proofreading and optimization methods are as follows: If the size change of adjacent grids of the two media is greater than 80 to 150 times, at least one of the adjacent grids is processed so that its change range is less than 80 to 150 times; if the pore volume change of adjacent grids of the two media is greater than 800 to 1500 times, at least one of the adjacent grids is processed so that its change range is less than 800 to 1500 times.

[0038] In this step, the 80 to 150 times and 800 to 1500 times are all empirical data, and the specific values ​​are different for different models. At the same time, the specific values ​​can be adjusted according to the trial calculation results of step S7.

[0039] In this process, the pore volume can be calculated by the following formula: pore volume = mesh size × porosity × net-to-gross ratio.

[0040] S6. According to the actual gas reservoir characteristics, the initial gas and water distribution of matrix, fractures and faults in the model is established;

[0041] In this step, the initial water-gas distribution can be established by the following methods: Set equilibrium partitions: According to the development characteristics of the gas reservoir, different index numbers are set for the faults, cracks and matrix respectively. According to experimental measurements or logging data, the initial gas-water distribution of the matrix, cracks and faults is established in the equilibrium partition according to the actual gas reservoir characteristics, and the initial equilibrium conditions of the pressure and fluid saturation of the fault, crack and matrix system are completed.

[0042] S7. Calculate the model. If the initial water vapor distribution of the model is different from the initial water vapor distribution of the gas reservoir or the calculation does not converge, repeat steps S4 to S6 until the initial water vapor distribution of the model is the same as the initial water vapor distribution of the gas reservoir or the calculation converges.

[0043] In order to further illustrate the effects of the embodiments of the present invention, a specific example is used for illustration below.

[0044] 1. Taking the fractured edge water gas reservoir in Kuche Sag, Tarim Basin as an example, a three-dimensional two-phase (gas-water) black oil numerical model was established using RFD's tNavigator reservoir simulation software. The numerical simulation model uses a uniform grid (dx=dy=50m, dz=3m), is divided into 102 layers vertically, and the number of grids in the dual medium model is 10.65 million. The model is 31km long from east to west and 4km wide from north to south, with an average porosity of 5.12%, an average pressure of 116MPa, and an average gas saturation of 60%. Since the gas reservoir faults are relatively developed, a large number of high-angle fractures are developed around the faults and in the region. Therefore, the DFN method is used, mainly along the fault development periphery, with the fault as a constraint, and high-angle discrete fractures are randomly established. The permeability of large-scale high-angle fractures ranges from 78-4320mD, and the average permeability of the high permeability zone in the reservoir plane is about 198mD.

[0045] The gas reservoir model of this embodiment is used to fit the production of fixed gas volume, and the history fitting is completed by fitting the water production and bottom hole pressure. The final result is as follows Figure 1 As shown. Figure 1 It can be seen that the overall fitting rate of this embodiment is greater than 85%, the fitting effect is good, and the subsequent process requirements are met.

[0046] Figure 2 is the process diagram of edge water intruding along the large cracks during the fitting process, where: Figure 2 a represents the initial gas-water distribution diagram, Figure 2 b represents the gas-water distribution diagram of the uniform lifting of edge water through the matrix at a certain moment in the simulation process. Figure 2 c represents the gas-water distribution diagram of the rapid intrusion of edge water along the large cracks at a certain moment during the experiment. It can be seen from the figure that the model of the embodiment of the present invention can simulate the water breakthrough process in the production process.

[0047] 2. According to the method of the embodiment of the present invention, a numerical model is established as an experimental group, such as Figure 3 As shown, the numerical model was then established using CMG simulation software as a control group, as shown in Figure 4 The model parameters of the experimental group and the control group are shown in Table 1.

[0048] Table 1 Numerical simulation model parameters

[0049] Numerical model parameters Experimental Group Control group Unit / Remarks Double medium plane grid quantity 300×300 300×300 indivual Number of vertical grids for dual media 10 10 indivual Plane mesh step 10 10 m Vertical grid step 1 1 m Matrix porosity 5 5 ％ Fracture Porosity 2 2 ％ Fault Porosity 0.8 0.8 ％ Matrix permeability 0.1 0.1 mD Fracture permeability 20 20 mD Fault permeability 1000 1000 mD Water body type Edge water Edge water 5 times water

[0050] Finally, the established model was used to simulate the water intrusion phenomenon during gas production. The final simulation results are as follows: Figure 5 , Figure 6 shown.

[0051] in, Figure 5 a is the crack simulation water intrusion effect diagram of the experimental group. Figure 5b is a matrix simulation water invasion effect diagram of the experimental group, in which the black dots are production wells, the green part is water, and the red part is natural gas. It can be seen from the figure that when the model of this embodiment is used for simulation, the water invasion phenomenon in the gas production process can be well reproduced.

[0052] Figure 6 a is the crack simulation water intrusion effect diagram of the control group. Figure 6 b is the matrix simulation water invasion effect diagram of the control group. In the figure, the green dots are production wells, the blue part is water, and the red part is natural gas. It can be seen from the figure that the simulation effect of the control group is relatively poor, and the similarity with the actual production situation is low.

[0053] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for establishing a numerical simulation model of a large-scale fractured gas reservoir, characterized in that: The following steps are involved: S1. Establish a dual-medium model, set the model to dual-porosity and dual-permeability or dual-porosity and single-permeability based on fluid flow characteristics, and convert the fracture into DFN discrete fractures; S2, discretizing the DFN discrete fractures and forming a discrete grid, and numerically adjusting the discrete grid in combination with fracture connectivity, conductivity and fracture filling conditions; this step includes the following sub-steps: S21, discretizing the DFN discrete cracks, and numerically characterizing the grids through which the cracks pass to form discrete grids; S22, adjusting the conductivity of the discrete grid: based on the connectivity of the DFN discrete fractures, setting the conductivity parameter of the discrete grid to 0-1, wherein the better the connectivity of the DFN discrete fractures, the greater the conductivity parameter; S23, adjusting the permeability of the discrete grid: adjusting the permeability based on the measured permeability value, the fracture orientation and the conductivity; S24, adjusting the porosity of the discrete grid: based on the opening and filling degree of the DFN discrete fractures, the porosity of the discrete grid is set to 0-1, wherein the higher the filling degree of the DFN discrete fractures, the lower the porosity of the discrete grid; S3, coupling the discretized and adjusted DFN discrete fractures into the dual medium model to form a triple medium numerical simulation model; S4, obtaining relative permeability curves of matrix, fractures and faults in the model; S5. Based on the size of adjacent grids and the size of pore volume, calibrate and optimize the grids with large size changes in the model: if the size change of adjacent grids of two media is greater than 80~150 times, at least one of the adjacent grids is processed to make its change less than 80~150 times; if the pore volume change of adjacent grids of two media is greater than 800~1500 times, at least one of the adjacent grids is processed to make its change less than 800~1500 times; S6. According to the actual gas reservoir characteristics, the initial water and gas distribution of the matrix, fractures and faults in the model is established; S7. Calculate the model. If the initial water vapor distribution of the model is different from the initial water vapor distribution of the gas reservoir or the calculation does not converge, repeat steps S4 to S6 until the initial water vapor distribution of the model is the same as the initial water vapor distribution of the gas reservoir or the calculation converges.

2. The method according to claim 1, characterized in that In S1, the steps of converting the fracture into DFN discrete fractures are as follows: characterize the fracture in the model, then digitize the fracture data, and then import the digitized data into the geological modeling software to convert the fracture data into DFN discrete fractures.

3. The method according to claim 1, characterized in that S23 includes the following steps: for the permeability of the discrete grid, assign a value according to 10 to 1000 times the fracture permeability; or, set it according to the grid size combined with the fracture conductivity, wherein the grid conductivity characterizing the fracture = permeability × grid size; in the process of adjusting the permeability, adjust the size of the fracture according to its direction.

4. The method according to claim 1, characterized in that S4 includes the following sub-steps: according to the seepage characteristics of matrix-continuous fractures-fractures, the matrix phase permeability curve, the fracture phase permeability curve and the fracture phase permeability curve are measured, and the relative permeability curves of the matrix, continuous fractures and fractures are set respectively.

5. The method according to claim 1, characterized in that The pore volume is calculated using the following formula: pore volume = mesh size × porosity × net-to-gross ratio.

6. The method according to claim 1, characterized in that S6 includes the following sub-steps: according to the seepage characteristics of matrix-continuous fractures-fractures, based on physical experiments, the matrix phase permeability curve, the fracture phase permeability curve and the fracture phase permeability curve are obtained, and the relative permeability curves of the matrix, continuous fractures and fractures are set respectively.

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