Method for evaluating fault dynamic sealing property based on fault slip index

Abstract

This invention provides a dynamic mechanical evaluation method for fault sealing based on the fault slip index. The method includes: acquiring rock mechanical parameters under a formation pressure state; establishing a geometric model in ANSYS software, assigning corresponding values ​​to the rock mechanical parameters, dividing the model into a mesh, and performing a three-dimensional numerical simulation of the geostress field; determining the unit normal vector of each node of the fault; assigning corresponding values ​​to the formation pore pressure data and node coordinates, calculating the effective stress, and calculating the fault slip index based on the unit normal vector of each node of the fault; sequentially modifying the rock mechanical parameters in the model to the parameters corresponding to other pressure states, repeating the steps of calculating the fault slip index to obtain the corresponding fault slip index; and evaluating the risk of fault sealing failure based on the fault slip index under different formation pressure states. This invention can accurately calculate quantitative mechanical evaluation indicators of fault sealing, effectively guide fault sealing evaluation, and has broad application prospects.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field exploration and development, and more specifically, to a method for assessing the dynamic sealing mechanical properties of faults based on the fault slip index. Background Technology

[0002] Fault sealing plays a crucial role in the formation and preservation of oil and gas reservoirs, and is of great value for predicting and assessing oil and gas resources. It is an indispensable study in oil and gas reservoir exploration and development. During oil and gas reservoir development, changes in pore pressure in the reservoir caused by oil and gas extraction and water injection can lead to disturbances in the regional geostress field, increasing the risk of fault activation and inducing lateral sealing failure of traps. Therefore, fully considering the impact of geostress disturbances caused by oil and gas reservoir development on fault sealing, and comprehensively evaluating the dynamic changes in fault sealing, is of great significance for reducing engineering risks.

[0003] Because actual fault conditions are highly complex, their morphology cannot be expressed using definite, specific mathematical relationships, and the mechanical properties of rocks change with pore pressure. Research indicates that slip index is commonly used in domestic and international studies to mechanically evaluate fault sealing. However, this method is currently mainly applied to the sealing evaluation of simple faults and does not provide a detailed evaluation of the sealing at various points along complex faults. The accuracy of research results under actual geological conditions needs improvement. Currently, a dynamic sealing evaluation method suitable for complex faults in oil and gas reservoir development has not yet been developed. Summary of the Invention

[0004] The purpose of this invention is to address at least one of the aforementioned deficiencies in the prior art. For example, one objective of this invention is to accurately calculate the quantitative mechanical evaluation index of fault sealing, and another objective is to achieve an accurate evaluation of fault sealing in oil and gas reservoir development.

[0005] To achieve the above objectives, the first aspect of the present invention provides a method for evaluating the dynamic closure of faults using mechanical methods.

[0006] The evaluation method includes: S10, determining the fault slippage index under a formation pressure state; S20, obtaining the fault slippage index of rock samples in the target area under several other pressure states; S30, classifying the degree of slippage instability risk based on the slippage index of each node of the fault under different formation pressure states, thereby realizing the dynamic closure evaluation of the fault.

[0007] Optionally, step S10 includes: S11, obtaining rock mechanical parameters of rock samples from the target area under a formation pressure state; S12, establishing a geometric model in ANSYS software according to the structural morphology of the target area, assigning corresponding values ​​to the rock mechanical parameters, and obtaining a three-dimensional finite element mechanical model after meshing; using the three-dimensional finite element mechanical model to numerically simulate the three-dimensional geostress field under the formation pressure state until the simulation results are reliable, and recording the loading conditions when the simulation results are reliable; S13, determining the unit normal vector of each node of the fault in the model; S14, assigning corresponding values ​​to the formation pore pressure data under the formation pressure state and the node coordinates in the model, calculating the effective stress of each node, and calculating the fault slip index of each node of the fault in combination with the unit normal vector of each node of the fault determined in step S13.

[0008] Alternatively, the rock mechanical parameters may include: rock density, Young's modulus, and Poisson's ratio.

[0009] Optionally, the reliability of the simulation results includes: the principal stress values ​​at the corresponding nodes have a consistency rate of over 85% with the well point in-situ stress test and core mechanics test results in the target area. Further, optionally, the core mechanics test includes tests capable of obtaining core principal stress data.

[0010] Optionally, step S13 includes: extracting: the coordinate data of key points of each fault plane and the coordinate data of each node of the fault; calculating the coordinates of the center point and the unit normal vector of each fault plane based on the coordinate data of the key points; determining the distance between each node and the center point of each fault plane based on the coordinate data of each node and the coordinates of the center point of each fault plane, and taking the unit normal vector of the nearest fault plane as the normal vector of the corresponding node.

[0011] Alternatively, the calculation process in step S13 is implemented based on ANSYS software and APDL command stream.

[0012] Alternatively, in step S14, the effective stress is calculated using a command stream written in APDL in ANSYS software.

[0013] Optionally, step S14 includes: calculating the normal stress and shear stress at each node of the fault, and then calculating the fault slip index at each node of the fault based on the normal stress and shear stress.

[0014] Alternatively, the normal stress and the shear stress can be calculated using the following formula:

[0015]

[0016] Where, σ nFor normal stress, σ1 is the maximum effective principal stress, σ2 is the intermediate effective principal stress, and σ3 is the minimum effective principal stress; τ n Let θ1 be the angle between the maximum effective principal stress and the normal vector at that point, θ2 be the angle between the intermediate effective principal stress and the normal vector at that point, and θ3 be the angle between the minimum effective principal stress and the normal vector at that point.

[0017] Alternatively, σ1, σ2, and σ3 can be calculated using principal stress and formation pore pressure data.

[0018] Alternatively, the numerical simulation performed in step S2 can obtain the principal stresses.

[0019] Alternatively, the fault slip index can be determined using the following formula:

[0020] Among them, T s This is the fault slip index.

[0021] Optionally, when the fault slip index is ≤0.3, the slip instability is of low risk; when the fault slip index is >0.3 and ≤0.6, the slip instability is of medium risk; and when the fault slip index is >0.6, the slip instability is of high risk.

[0022] Alternatively, the evaluation may include: evaluating the dynamic trend of fault sealing under different formation pressure states.

[0023] A second aspect of the present invention provides a method for determining the fault slip index under a single formation pressure state.

[0024] The method described above is essentially the same as step S10 in the aforementioned aspect, except that in step S12, it is not necessary to record the loading conditions when the simulation results are reliable. For example, the method includes the following steps: S11, obtaining the rock mechanical parameters of rock samples from the target area under a formation pressure state; S12, establishing a geometric model in ANSYS software according to the structural morphology of the target area, assigning corresponding values ​​to the rock mechanical parameters, and obtaining a three-dimensional finite element mechanical model after meshing; using the three-dimensional finite element mechanical model to numerically simulate the three-dimensional geostress field under the formation pressure state until the simulation results are reliable;

[0025] S13. Determine the unit normal vector of each node of the fault in the model; S14. Assign corresponding values ​​to the formation pore pressure data under the formation pressure state and the node coordinates in the model, calculate the effective stress of each node, and calculate the fault slip index of each node of the fault by combining the unit normal vector of each node of the fault determined in step S13.

[0026] The third aspect of this invention provides a method for determining the fault slip index under multiple formation pressure states.

[0027] The method may include steps S10 and S20 of the above-mentioned aspect. For example, the method includes: determining the fault slip index under a formation pressure state; obtaining rock mechanical parameters of rock samples in the target area under several other pressure states, and performing the following steps for each other pressure state: modifying the rock mechanical parameters of the three-dimensional finite element mechanical model in step S12 to the rock mechanical parameters corresponding to the other pressure state; applying loads according to the loading conditions recorded when the simulation results are reliable; calculating the geostress field distribution under the other pressure state; repeating step S14 to obtain the fault slip index of each node of the fault under the other pressure state.

[0028] Optionally, the plurality of formation pressure states include: under a certain confining pressure, setting a plurality of pore pressures, each pore pressure being a formation pressure state.

[0029] Compared with the prior art, the beneficial effects of the present invention include at least one of the following:

[0030] (1) The method of the present invention is simple and can clearly define the effective stress field distribution of complex three-dimensional geological bodies, accurately calculate the quantitative mechanical evaluation index of fault sealing - the fault slip index, and realize batch calculation and visualization analysis and evaluation in the ANSYS software platform. It has broad application prospects in oil and gas reservoir exploration and development research.

[0031] (2) This invention can improve the accuracy of dynamic fault sealing mechanical evaluation, effectively guide the fault sealing evaluation of multiple oil and gas reservoirs, and play an important role in the exploration and development of oil and gas reservoirs.

[0032] (3) This invention is applicable to the study of fault sealing evaluation in oil and gas reservoir exploration and development. The method of this invention can be used to carry out relevant geostress field studies and fault sealing evaluation under various stress states, which helps to reduce engineering risks. Attached Figure Description

[0033] The above and other objects and / or features of the present invention will become clearer from the following description taken in conjunction with the accompanying drawings, in which:

[0034] Figure 1 The diagram shows the distribution of the maximum principal stress obtained from the three-dimensional geostress field numerical simulation.

[0035] Figure 2 A schematic diagram of the slip index of two faults is shown.

[0036] Figure 3 The slip index of the fault node under different pressure conditions is shown. Detailed Implementation

[0037] The present invention will be described in detail below with reference to exemplary embodiments.

[0038] Fault sealing plays a crucial role in the formation and preservation of oil and gas reservoirs, and is of great value for predicting and assessing oil and gas resources. It is an indispensable study in oil and gas reservoir exploration and development. During oil and gas reservoir development, changes in reservoir pore pressure caused by oil and gas extraction and water injection can trigger disturbances in the regional geostress field, increasing the risk of fault activation. Therefore, the study of fault dynamic sealing is of great significance for oil and gas reservoir exploration and development.

[0039] This invention comprehensively considers the significant influence of factors such as fault morphology, formation pore pressure, and rock mechanical properties on the results of fault sealing studies. Utilizing existing data from the study area, and taking into full account the complex fault morphology and variations in formation pore pressure, this invention leverages the features of the ANSYS software platform to simulate and analyze the effective stress field distribution under various developmental states according to actual research needs. It also studies the stress state of the fault in conjunction with its attitude and evaluates the fault sealing performance under each state using geomechanical theory.

[0040] Exemplary Example 1

[0041] This exemplary embodiment provides a method for dynamic mechanical evaluation of fault closure. The method may include the following steps:

[0042] S10. Determine the fault slip index under a formation pressure state;

[0043] Specifically, step S10 may include:

[0044] S11. Obtain the rock mechanical parameters of rock samples from the target area under a formation pressure state.

[0045] S12. Based on the structural morphology of the target area, establish a geometric model in ANSYS software, assign corresponding values ​​to the rock mechanics parameters, and obtain a three-dimensional finite element mechanical model after meshing. Use the three-dimensional finite element mechanical model to perform numerical simulation of the three-dimensional geostress field under the pressure state of the formation until the simulation results are reliable. When the simulation results are reliable, record the loading conditions of the simulation.

[0046] S13. Determine the unit normal vector of each node of the interrupted layer in the three-dimensional finite element mechanical model.

[0047] S14. Based on the formation pore pressure data and the unit normal vector of each node of the fault under the formation pressure state, calculate the fault slip index under the formation pressure state.

[0048] In this embodiment, the rock mechanical parameters may include: rock density, Young's modulus, and Poisson's ratio.

[0049] In this embodiment, the reliability of the simulation results may include: the principal stress values ​​of the corresponding nodes and the results of well point in-situ stress tests and core mechanics tests in the target area both have a consistency rate of over 85%.

[0050] In this embodiment, step S13 may specifically include the following:

[0051] S131. Extract from the model: coordinate data of key points on different fault planes of each fault, and coordinate data of all nodes of each fault.

[0052] S132. Based on the key point coordinate data, calculate the center point coordinates and unit normal vector coordinates of each fault plane.

[0053] S133. For each fault, based on the coordinate data of each node and the coordinates of the center point of each fault plane, determine the distance between each node and the center point of each fault plane, and take the unit normal vector of the nearest fault plane as the normal vector of the corresponding node.

[0054] In this embodiment, the calculation process in step S13 can be implemented based on ANSYS software and APDL command stream.

[0055] In this embodiment, step S14 may specifically include: assigning corresponding values ​​to the formation pore pressure data and the node coordinates in the three-dimensional finite element mechanical model; then using APDL to write a command flow in ANSYS software to calculate the effective stress and extract the magnitude and direction cosine of each effective stress; combining the node normal vectors determined in step S13, calculating the normal stress and shear stress of each node of the fault, and using this to calculate the fault slip index of each node of the fault under the formation pressure state.

[0056] Furthermore, the normal stress and the shear stress can be calculated using the following formula:

[0057]

[0058] Where, σ n Let σ1 be the maximum effective principal stress at the node, σ2 be the intermediate effective principal stress at the node, and σ3 be the minimum effective principal stress at the node; τ n Let θ1 be the angle between the maximum effective principal stress and the normal vector at that point, θ2 be the angle between the intermediate effective principal stress and the normal vector at that point, and θ3 be the angle between the minimum effective principal stress and the normal vector at that point.

[0059] Furthermore, σ1, σ2, and σ3 can be calculated using principal stress and formation pore pressure data, wherein the principal stress is obtained based on the three-dimensional geostress field numerical simulation performed in step S2.

[0060] Specifically, the effective principal stress can be calculated using the principal stress magnitude and formation pore pressure data. The calculation method is as follows:

[0061] σ 1＝ S1-αP, σ 2＝ S2-αP, σ 3＝ S3-αP;

[0062] Where σ1, σ2, and σ3 are the maximum, intermediate, and minimum effective principal stresses, respectively; S1, S2, and S3 are the maximum, intermediate, and minimum principal stresses, respectively; α is the Biot coefficient, which is related to the lithological characteristics of the study area; and P is the formation pore pressure.

[0063] In this embodiment, the fault slip index is determined using the following formula:

[0064] Among them, T s This is the fault slip index.

[0065] S20. Obtain the fault slip index of rock samples in the target area under several other pressure states.

[0066] In this embodiment, for any other pressure state, the following steps are performed:

[0067] S21. Obtain the rock mechanical parameters of rock samples from the target area under a different formation pressure state.

[0068] S22. Modify the rock mechanics parameters of the three-dimensional finite element mechanical model in step S12 to the rock mechanics parameters corresponding to the other formation pressure state, apply load according to the loading conditions recorded when the simulation results are reliable, and calculate the geostress field distribution under the other formation pressure state.

[0069] S23. Repeat step S14 to calculate the fault slip index under another pressure state.

[0070] In this embodiment, the multiple formation pressure states include: under a certain confining pressure, setting multiple pore pressures, with each pore pressure representing a formation pressure state.

[0071] S30. Based on the slip index of each node of the fault under different formation pressure states, the degree of slippage instability risk is classified, thereby realizing the dynamic closure evaluation of the fault.

[0072] In this embodiment, when the fault slip index is ≤0.3, the slip instability is of low risk; when the fault slip index is >0.3 and ≤0.6, the slip instability is of medium risk; and when the fault slip index is >0.6, the slip instability is of high risk.

[0073] In this embodiment, the evaluation includes: evaluating the dynamic trend of fault sealing under different formation pressure states.

[0074] Exemplary Example 2

[0075] This exemplary embodiment provides a method for evaluating the dynamic closure mechanical properties of faults, which can be performed based on ANSYS.

[0076] This exemplary embodiment fully considers the changes in rock mechanical properties and effective stress with formation pore pressure, and combines the characteristics of the ANSYS software platform to analyze and evaluate the dynamic sealing of faults under complex fault morphology conditions. The steps are as follows:

[0077] Step 1: Three-dimensional numerical simulation of geostress field

[0078] Determine the pressure state of the formation (state A) of the oil and gas reservoir under its current condition, and use rock mechanics testing methods to obtain the elastic modulus, Poisson's ratio, and density under this state.

[0079] Based on the structural morphology of the study area, the morphological characteristics of the main target layer and faults were identified. A geometric model was established in ANSYS software, and the rock mechanics parameters under condition A were assigned corresponding values. A three-dimensional geostress field under condition A was numerically simulated. The results were considered reliable when the principal stress values ​​at the corresponding nodes showed a consistency rate of over 85% with the wellpoint geostress test and core mechanics test results in the study area. The loading conditions (load F) during the simulation were recorded. The core mechanics test results include, but are not limited to, acoustic emission and differential strain measurements, which yielded the principal stress data of the core.

[0080] It should be noted that this invention first establishes a geometric model based on the structural morphology, then assigns rock mechanical parameters to it, and meshes it to obtain a mechanical model (i.e., a three-dimensional finite element mechanical model), which can then be referred to as a three-dimensional geological model. The difference between the mechanical model and the geometric model is that the mechanical model contains rock mechanical properties, and its three-dimensional shape is completely consistent with the geometric model, while the geometric model does not contain mechanical parameters. The three-dimensional geological model is a three-dimensional solid model, including continuous block solid models and fault zone solid models. Meshing is performed in the above solid models, and the resulting nodes are also nodes in the solid models.

[0081] Step 2: Calculate the unit normal vector at each point along the fault.

[0082] The three-dimensional coordinate data of key points of fault planes are extracted from the model. The coordinates of the center point and the unit normal vector coordinates of each fault plane are calculated according to the connection method of each fault plane in the model. The model includes continuous block models and fault zone models. The coordinate data of each node in the fault zone model are extracted. The distance between each node and the center point of each fault plane is calculated. The unit normal vector of the closest point is selected as the normal vector of the node.

[0083] Step 3: Evaluate the fault closure under the current condition.

[0084] The formation pore pressure data under state A in the study area were mapped to the node coordinates in the finite element model (i.e., the three-dimensional finite element mechanical model when the simulation results are reliable). In ANSYS software, an APDL command stream was used to calculate the effective stress at each node, extracting the magnitude and direction cosines of each effective stress. Combined with the fault node normal vectors obtained in the second step, the normal stress and shear stress at each fault node were calculated, and the fault slip index under state A was calculated. A batch data calculation and map output were achieved using an APDL command stream in ANSYS software.

[0085] Step 4: Evaluate the dynamic sealing of the fault

[0086] Based on the other pressure states (state B, state C, etc.) required for the analysis of this oil and gas reservoir, mechanical tests are conducted on core samples using pore pressure and confining pressure under each state as constraints to obtain the elastic modulus, Poisson's ratio, and density under states B, C, etc. On the basis of the three-dimensional finite element mechanical model established in the first step, the mechanical parameters are modified to the rock mechanical parameters of state B. A load F is applied, and the geostress field distribution of state B is calculated. An APDL command stream is then used to assign the formation pore pressure under state B to the model. Repeating the third step yields the fault slip index for state B. Repeating the above operations obtains the fault slip index for a series of states, including state C. It should be noted that the second step to calculate the unit normal vector at each fault point is not necessary, as the geometric model for each state is identical, and the unit normal vector at each fault point is also the same in the models for each state.

[0087] By applying a load F, the stress field distribution in state B can be calculated through numerical simulation, including the magnitude and direction of the principal stresses. The effective principal stress is calculated using the principal stress magnitudes and formation pore pressure data, and the calculation method is as follows:

[0088] σ 1＝ S1-αP, σ 2＝ S2-αP, σ 3＝ S3-αP;

[0089] Where σ1, σ2, and σ3 are the maximum, intermediate, and minimum effective principal stresses, respectively; S1, S2, and S3 are the maximum, intermediate, and minimum principal stresses, respectively; α is the Biot coefficient, which is related to the lithological characteristics of the study area; and P is the formation pore pressure.

[0090] The first to third steps described above are necessary steps for calculating the fault slip index under one of the formation pressure states. The fourth step is to assign rock mechanical parameters under other formation pressure states to the model. After modifying the rock mechanical parameters, the principal stress obtained under the same loading conditions will also change, and the corresponding fault slip index will also change.

[0091] This analysis examines fault sealing under different formation pore pressure conditions to assess the risk of sealing failure due to fault slip instability during oil and gas reservoir development. Based on actual studies in multiple oil and gas basins in China, a fault slip index of 0–0.3 (inclusive) indicates a low risk of slip instability; an index of 0.3 (exclusive)–0.6 (inclusive) indicates a moderate risk requiring enhanced monitoring and analysis; and an index greater than 0.6 indicates a high risk necessitates timely development of corresponding countermeasures.

[0092] Exemplary Example 3

[0093] This exemplary embodiment provides a method for determining the fault slip index under a single formation pressure state.

[0094] The method includes steps that are substantially the same as step S10 in Exemplary Embodiment 1, except that in step S12, it is not necessary to record the loading conditions when the simulation results are reliable. For example, the method includes the following steps:

[0095] S11. Obtain the rock mechanical parameters of rock samples from the target area under a formation pressure state.

[0096] S12. Based on the structural morphology of the target area, establish a geometric model in ANSYS software, assign corresponding values ​​to the rock mechanics parameters, and obtain a three-dimensional finite element mechanical model after meshing. Use the three-dimensional finite element mechanical model to perform numerical simulation of the three-dimensional geostress field under the pressure state of the formation until the simulation results are reliable.

[0097] S13. Determine the unit normal vector of each node of the interrupted layer in the three-dimensional finite element mechanical model.

[0098] S14. Based on the formation pore pressure data and the unit normal vector of each node of the fault under the formation pressure state, calculate the fault slip index under the formation pressure state.

[0099] Exemplary Example 4

[0100] This exemplary embodiment provides a method for determining the fault slip index under a single formation pressure state.

[0101] The method may include steps one through three as described in Exemplary Example 2.

[0102] Exemplary Example 5

[0103] This exemplary embodiment provides a method for determining the fault slip index under multiple formation pressure states.

[0104] The method may include steps S10 and S20 as described in Exemplary Example 1. For example, the method includes:

[0105] Determine the fault slip index under formation pressure;

[0106] For any other pressure condition, the following steps are performed to obtain the corresponding fault slip index.

[0107] S21. Obtain the rock mechanical parameters of rock samples from the target area under a different formation pressure state.

[0108] S22. Modify the rock mechanics parameters of the three-dimensional finite element mechanical model in step S12 to the rock mechanics parameters corresponding to the other formation pressure state, apply load according to the loading conditions recorded when the simulation results are reliable, and calculate the geostress field distribution under the other formation pressure state.

[0109] S23. Repeat step S14 to calculate the fault slip index under another pressure state.

[0110] Exemplary Example 6

[0111] This exemplary embodiment provides a method for determining the fault slip index under multiple formation pressure states.

[0112] The method may include the first to third steps in Exemplary Example 2, and the calculation of the fault slip index under other pressure states in the fourth step.

[0113] To better understand the exemplary embodiments described above, further explanation will be provided below with specific examples.

[0114] This example demonstrates a fault dynamic closure mechanical evaluation method based on ANSYS and includes the following steps:

[0115] Step 1: Rock mechanics tests under different conditions

[0116] Three states were identified for calculation and analysis: a confining pressure of 30 MPa and pore pressures of 10 (state A), 5 (state B), and 0 (state C), where state A represents the current formation state of the gas reservoir. Triaxial mechanical tests were performed on rock samples under these three conditions to obtain rock density, Young's modulus, and Poisson's ratio. The results are shown in Table 1.

[0117] Table 1

[0118]

[0119] Step 2: Three-dimensional numerical simulation of geostress field

[0120] Based on the tectonic morphology of the study area, a geometric model was established in ANSYS software. The rock mechanics parameters obtained in step 1 under condition A were assigned corresponding values, and a finite element mechanical model was obtained after meshing. Numerical simulation of the tectonic stress field under condition A was performed. The results were considered reliable when the principal stress values ​​at the corresponding nodes showed a consistency rate of over 85% with the well point in-situ stress test and core mechanics test results in the study area. The loading conditions during the simulation were recorded (load F: 88 MPa in the north-south direction, 50 MPa in the east-west direction, and vertical gravitational acceleration of 9.8 m / s²). 2 ).

[0121] Figure 1 This is a maximum principal stress distribution map obtained from the three-dimensional geostress field numerical simulation in step 2. The map shows the distribution of the maximum principal stresses, with color codes at the bottom corresponding to the range of maximum principal stresses: dark blue for 55.0 MPa to 65.5 MPa, blue for 45.0 MPa to 55.0 MPa, and so on, with red for 16.3 MPa to 23.0 MPa. Figure 1 As shown in the figure, the high structural area is shown in yellow, representing the maximum principal stress of 23.0 MPa to 26.3 MPa, while the area near the fault on the left is shown in dark blue, representing the maximum principal stress of 55.0 MPa to 65.5 MPa.

[0122] Step 3: Calculate the unit normal vector at each point along the fault.

[0123] There are two faults within the study area, displayed as two fault entities in the model. These two fault entities can be composed of multiple sequentially connected planes, such as multiple sequentially connected triangular planes. The coordinate data of key points on each fault plane are extracted. Based on the connection method of each plane in the model, the coordinates of its center point and the unit normal vector coordinates of that plane are calculated. The coordinate data of each node in the fault zone model are extracted, and the distance between each node and the center point of each fault plane is calculated. The unit normal vector of the closest point is selected as the normal vector of that node. In establishing the geometric model, each plane is a plane composed of three key points connected together. The key points are the data points input during the geometric model establishment.

[0124] Three key points were selected from each fault plane in the above three-dimensional geological model, and their coordinates are as follows:

[0125] M(mx,my,mz),N(nx,ny,nz),P(px,py,pz)

[0126] Then the direction vector of that surface

[0127]

[0128] The coordinates of the center point of this surface are:

[0129]

[0130] normal vector

[0131]

[0132] Based on the coordinates of each node of the fault, compare the distance between the node and the center point of each surface, select the center point of the surface that is closest to the node, and use its normal vector as the normal vector of the node.

[0133] The above calculations can be performed using the ANSYS software platform and APDL command stream.

[0134] Taking a fault as an example, which is composed of multiple sequentially connected triangular planes, step 3 includes: calculating the unit normal vector of each triangular plane (fault plane); calculating the distance between each node in the fault entity (including the fault plane) and the center point of each triangular plane, and selecting the unit normal vector of the nearest center point as the unit normal vector of that node.

[0135] Step 4: Evaluate fault sealing under single conditions

[0136] The formation pore pressure data under state A in the study area were assigned corresponding values ​​to the node coordinates in the finite element model. The effective stress of each node was calculated using the APDL command flow in ANSYS software. The magnitude and direction cosine of each effective stress were extracted. The normal stress and shear stress of each node of the fault were calculated by combining the normal vector of each node of the fault obtained in step 3. The fault slip index under state A was also calculated.

[0137] The normal stress and shear stress at each node of the fault are calculated according to the following formula:

[0138] σ n =σ1cos 2 θ1+σ2cos 2 θ2+σ3cos 2 θ3

[0139]

[0140] Where, σ n Let σ1 be the maximum effective principal stress at the node, σ2 be the intermediate effective principal stress at the node, and σ3 be the minimum effective principal stress at the node; τ n Let θ1 be the angle between the maximum effective principal stress and the normal vector at that point, θ2 be the angle between the intermediate effective principal stress and the normal vector at that point, and θ3 be the angle between the minimum effective principal stress and the normal vector at that point.

[0141] The angles θ1, θ2, and θ3 mentioned above can be calculated using the direction cosines of the three effective principal stresses in the global coordinate system and the aforementioned unit normal vector.

[0142] Fault slip index:

[0143] Among them, T s The slippage index.

[0144] In ANSYS software, APDL is used to write command streams to achieve batch data calculation and graph output.

[0145] Finally, the slip index of all nodes in each fault entity was calculated.

[0146] Figure 2 The figure shows the slip indices of two faults obtained under state A. The color codes at the bottom represent the slip index ranges: 0–0.05 is dark blue, 0.05–0.1 is blue, and so on, with 0.35–0.4 being red. Different locations of the faults in the figure have different colors, representing different slip indices at those locations. Figure 2Taking the fault on the left side as an example, based on the calculation results, a comprehensive analysis of the distribution characteristics of the slip index of the fault in the horizontal and vertical directions shows that the slip index at both ends of the fault is smaller and that in the middle is larger. Therefore, the risk of slip instability in the middle of the fault is relatively high, but its value is small (the slip index is less than 0.3), indicating that the overall risk of slip instability under this study condition is relatively small.

[0147] Step 5: Evaluate the dynamic sealing of the fault

[0148] Based on the three-dimensional finite element mechanical model established in step 2, the mechanical parameters are changed to the rock mechanical parameters of state B in step 1. A load F is applied, and the distribution of the geostress field in state B is calculated. The formation pore pressure under state B is assigned to the model. Step 4 is repeated to obtain the fault slip index of state B. Repeating the above operation can obtain the fault slip index under a series of state conditions such as state C. The results are shown in Table 2.

[0149] Table 2. Slip index of fault nodes under different conditions.

[0150]

[0151] Based on the slip index calculation results of each fault node, the fault sealing performance under different formation pore pressure conditions is analyzed, and the risk of sealing failure caused by fault slip instability during oil and gas reservoir development is evaluated.

[0152] Figure 3 Table 1 shows the slippage index of interrupted layer node 1 in various states. For example... Figure 3 As shown, in each study state, from state A to state C, the formation pore pressure decreases and the fault slip index decreases, indicating that as the gas reservoir development process progresses, the fault sealing increases and the risk of slippage and instability decreases.

[0153] This invention focuses on the evaluation of fault sealing under different effective stress states. It fully considers factors such as the complexity of actual fault morphology, formation pore pressure, effective rock stress, and rock mechanical properties. Through comprehensive analysis, it develops a dynamic mechanical evaluation method for fault sealing, which can effectively guide fault sealing evaluation during oil and gas reservoir development, improve evaluation accuracy, and has the following significant characteristics:

[0154] (1) Analysis of effective stress field under different stress states: Through actual rock sample testing under different effective stress states and numerical simulation of stress field under corresponding states, the distribution of effective stress field under different states is calculated and analyzed.

[0155] (2) Evaluation of fault dynamic sealing: Based on the attitude of the fault and the effective stress field under different conditions, calculate the normal stress, shear stress and slip index of the fault, and comprehensively evaluate the dynamic change trend of fault sealing.

[0156] Although the present invention has been described above in conjunction with exemplary embodiments and accompanying drawings, those skilled in the art should understand that various modifications can be made to the above embodiments without departing from the spirit and scope of the claims.

Claims

1. A method for evaluating the dynamic closure mechanical properties of faults, characterized in that, The method includes: S10. Determine the fault slip index under a formation pressure state; S20. Obtain the fault slip index of rock samples in the target area under several other pressure states; S30. Based on the slip index of each node of the fault under different formation pressure states, the degree of slippage instability risk is classified, thereby realizing the dynamic closure evaluation of the fault.

2. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 1, characterized in that, Step S10 includes: S11. Obtain the rock mechanical parameters of rock samples from the target area under formation pressure. S12. Based on the structural morphology of the target area, establish a geometric model in ANSYS software, assign corresponding values ​​to the rock mechanics parameters, and obtain a three-dimensional finite element mechanical model after meshing. Use the three-dimensional finite element mechanical model to perform numerical simulation of the three-dimensional geostress field under the pressure state of the formation until the simulation results are reliable, and record the loading conditions when the simulation results are reliable. S13. Determine the unit normal vector of each node in the interrupted layer of the model; S14. Assign values ​​to the formation pore pressure data under the formation pressure state and the corresponding node coordinates in the model, calculate the effective stress of each node, and calculate the fault slip index of each node of the fault by combining the unit normal vector of each node of the fault determined in step S13.

3. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 2, characterized in that, The rock mechanical parameters include: rock density, Young's modulus, and Poisson's ratio.

4. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 2, characterized in that, The reliability of the simulation results is indicated by the fact that the principal stress values ​​of the corresponding nodes have a consistency rate of over 85% with the well point in-situ stress test and core mechanics test results in the target area.

5. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 4, characterized in that, The core mechanics test includes tests that can obtain core principal stress data.

6. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 2, characterized in that, Step S13 includes: Extract: Coordinate data of key points on each fault plane of the fault, and coordinate data of each node of the fault; Based on the key point coordinate data, calculate the center point coordinates and unit normal vector coordinates of each fault plane; Based on the coordinate data of each node and the coordinates of the center point of each fault plane, the distance between each node and the center point of each fault plane is determined, and the unit normal vector of the nearest fault plane is taken as the normal vector of the corresponding node.

7. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 2, characterized in that, The calculation process in step S13 is implemented based on ANSYS software and APDL command flow.

8. The fault dynamic sealing mechanical evaluation method according to claim 2, wherein step S14 includes: Calculate the normal stress and shear stress at each node of the fault, and then calculate the fault slip index at each node based on the normal stress and shear stress.

9. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 8, characterized in that, The normal stress and the shear stress are calculated using the following formula: Where, σ n For normal stress, σ1 is the maximum effective principal stress, σ2 is the intermediate effective principal stress, and σ3 is the minimum effective principal stress. τ n Let θ1 be the angle between the maximum effective principal stress and the normal vector at that point, θ2 be the angle between the intermediate effective principal stress and the normal vector at that point, and θ3 be the angle between the minimum effective principal stress and the normal vector at that point.

10. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 9, characterized in that, The values ​​σ1, σ2, and σ3 are calculated using the principal stress and the formation pore pressure data.

11. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 2, wherein, The numerical simulation performed in step S12 can obtain the principal stress.

12. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 9, characterized in that, The fault slip index is calculated using the following formula: Among them, T s This is the fault slip index.

13. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 1, characterized in that, Step S10 includes: S11. Obtain the rock mechanical parameters of rock samples from the target area under formation pressure. S12. Based on the structural morphology of the target area, establish a geometric model in ANSYS software, assign corresponding values ​​to the rock mechanics parameters, and obtain a three-dimensional finite element mechanical model after meshing. Use the three-dimensional finite element mechanical model to perform numerical simulation of the three-dimensional geostress field under the pressure state of the formation until the simulation results are reliable, and record the loading conditions when the simulation results are reliable. S13. Determine the unit normal vector of each node in the interrupted layer of the model; S14. Assign values ​​to the formation pore pressure data under the formation pressure state and the corresponding node coordinates in the model, calculate the effective stress of each node, and calculate the fault slip index of each node of the fault by combining the unit normal vector of each node of the fault determined in step S13. Step S20 includes: obtaining rock mechanical parameters of rock samples from the target area under several other pressure states, and performing the following steps for each other pressure state: The rock mechanics parameters of the three-dimensional finite element mechanical model in step S12 are modified to the rock mechanics parameters corresponding to another pressure state. The load is applied according to the loading conditions recorded when the simulation results are reliable, and the distribution of the geostress field under the other pressure state is calculated. Repeat step S14 to obtain the fault slip index of each node of the fault under another pressure state.

14. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 1, characterized in that, When the fault slip index is ≤0.3, the slip instability is of low risk; when the fault slip index is >0.3 and ≤0.6, the slip instability is of medium risk; when the fault slip index is >0.6, the slip instability is of high risk.

15. The method for evaluating the dynamic sealing mechanical properties of faults according to claim 1, characterized in that, The evaluation includes: evaluating the dynamic trend of fault sealing under different formation pressure states.

16. A method for determining the fault slip index under a single formation pressure state, characterized in that, The method includes the following steps: S11. Obtain the rock mechanical parameters of rock samples from the target area under formation pressure. S12. Based on the structural morphology of the target area, establish a geometric model in ANSYS software, assign corresponding values ​​to the rock mechanics parameters, and obtain a three-dimensional finite element mechanical model after meshing; use the three-dimensional finite element mechanical model to perform numerical simulation of the three-dimensional geostress field under the pressure state of the formation until the simulation results are reliable. S13. Determine the unit normal vector of each node in the interrupted layer of the model; S14. Assign values ​​to the formation pore pressure data under the formation pressure state and the corresponding node coordinates in the model, calculate the effective stress of each node, and calculate the fault slip index of each node of the fault by combining the unit normal vector of each node of the fault determined in step S13.

17. A method for determining the fault slip index under multiple formation pressure states, characterized in that, The method is based on the method of claim 16 and includes: Determine a fault slip index under formation pressure; and if the simulation results in step S12 are reliable, record the simulated loading conditions. Obtain the rock mechanical parameters of rock samples from the target area under several other pressure states, and perform the following steps for each other pressure state: The rock mechanics parameters of the three-dimensional finite element mechanical model in step S12 are modified to the rock mechanics parameters corresponding to another pressure state. The load is applied according to the loading conditions recorded when the simulation results are reliable, and the distribution of the geostress field under the other pressure state is calculated. Repeat step S14 to obtain the fault slip index of each node of the fault under another pressure state.

18. The method for determining the fault slip index under multiple formation pressure states according to claim 17, characterized in that, The multiple formation pressure states include: under a certain confining pressure, setting multiple pore pressures, with each pore pressure representing a formation pressure state.

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