Main control factor prediction method for crack initiation expansion of low-permeability reservoir

Abstract

The invention relates to a low-permeability reservoir crack initiation expansion main control factor prediction method comprising the following steps: S1, collecting geological, well drilling, well logging and seismic data of a target low-permeability reservoir, carrying out data analysis, correction and synthesis, analyzing the change rule of lithology and physical property parameters, and determining the change rule of the lithology and physical property parameters; establishing a three-dimensional geologic model of the target low-permeability reservoir based on the analysis result; s2, determining rock mechanical parameters of the target low-permeability reservoir based on a sample experiment method; s3, performing stress field analysis based on the three-dimensional geological model and the rock mechanical parameters, simulating regional stress field distribution by applying a finite element numerical simulation method, and analyzing a simulation result; s4, on the basis of stress analysis, crack generation and expansion simulation is carried out by adopting an expansion finite element method; and S5, executing global sensitivity analysis, and determining the influence of the input parameters of the three-dimensional geologic model on the crack generation and expansion simulation result, so as to identify dominant factors for controlling crack formation.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration and development, and particularly relates to a method for predicting the main controlling factors of fracture initiation and propagation in low-permeability reservoirs under the constraint of rock mechanics layers. Background Technique

[0002] Due to their special geological conditions, low-permeability reservoirs often contain abundant natural fractures, which have a significant impact on the water injection development effect of oilfields. To solve this problem, researchers have developed quantitative description techniques for fractures in low-permeability reservoirs, including fracture quantitative characterization, fracture intensity evaluation, fracture modeling, etc. These techniques not only solve the development problems caused by reservoir fracture development, but also achieve fine development adjustment of fractured reservoirs through different well pattern deployment methods, thus achieving better oil production increase effect. In addition, the fracture quantitative prediction method for low-permeability reservoirs can also help solve key problems in oil and gas exploration and development, such as the quantitative characterization of fracture characteristic parameters and the prediction of spatial distribution laws in reservoirs of oil and gas bearing basins. The application of these methods realizes the quantitative prediction of the spatial distribution of fractures in low-permeability oil and gas reservoirs by controlling the internal and external factors affecting fracture development. Proven by applications and practices in multiple oilfields, these methods have a high coincidence rate and are of great significance for guiding the exploration of hidden oilfields.

[0003] However, for the convenience of mathematical processing in the current existing technologies, existing models often simplify the geological complexity to varying degrees, such as ignoring the influence of microstructures and uniform fracture distribution, etc., which may deviate greatly from the actual reservoir situation. And different geological environments and reservoir types may require specific prediction methods. The existing general models may have reduced adaptability and accuracy when facing special geological phenomena or extreme conditions. Summary of the Invention

[0004] The present invention provides a method for predicting the main controlling factors of fracture initiation and propagation in low-permeability reservoirs, which can combine the prediction results of rock mechanics analysis, promote the cross-integration between rock mechanics and reservoir engineering disciplines, reduce the trial-and-error cost, and improve the development benefit.

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

[0006] In the first aspect, the present application provides a method for predicting the main controlling factors of fracture initiation and propagation in low-permeability reservoirs, including:

[0007] S1. Collect geological, drilling, logging and seismic data of the target low-permeability reservoir, analyze and correct and synthesize the data through data analysis, analyze the variation laws of lithology and physical property parameters, and establish a three-dimensional geological model of the target low-permeability reservoir based on the above analysis results;

[0008] S2. Determine the rock mechanical parameters of the target low-permeability reservoir based on the method of sample experiments;

[0009] S3. Conduct stress field analysis based on the three-dimensional geological model and the rock mechanical parameters, apply the finite element method numerical simulation method to simulate the regional stress field distribution, and analyze the simulation results;

[0010] S4. On the basis of stress analysis, use the extended finite element method to simulate the occurrence and propagation of fractures;

[0011] S5. Perform global sensitivity analysis to determine the influence of the input parameters of the three-dimensional geological model on the simulation results of fracture occurrence and propagation, so as to identify the dominant factors controlling fracture formation.

[0012] In one implementation, in S1, the steps of establishing the three-dimensional geological model of the target low-permeability reservoir include:

[0013] According to the corrected data, construct a stratigraphic framework in three-dimensional space, including top and bottom boundaries and fault planes;

[0014] Divide three-dimensional grids on the basis of the stratigraphic framework;

[0015] Assign lithology and physical property parameters to each grid to achieve continuous or discrete distribution of attributes in three-dimensional space, and compare with well logging data in actual data to verify the accuracy of the model;

[0016] Use geological modeling software to realize the three-dimensional visualization of the three-dimensional geological model and generate cross-sectional diagrams and three-dimensional diagrams.

[0017] In one implementation, in S2, the rock mechanical parameters include the elastic modulus, Poisson's ratio, and strength parameters of the rock.

[0018] In one implementation, in S2, estimate the elastic modulus and Poisson's ratio of the rock through uniaxial compression tests. The specific steps include:

[0019] Prepare standard rock samples to ensure that the rock samples are defect-free and the dimensions meet the test requirements;

[0020] Place the rock sample in a universal material testing machine, apply axial pressure, and simultaneously measure the axial strain and lateral strain of the rock sample;

[0021] Record the strain data at different pressure levels and draw a stress-strain curve;

[0022] The elastic modulus E is calculated by the slope of the initial straight line segment, and the Poisson's ratio ν is calculated by the ratio of the lateral strain to the axial strain under small strain conditions; the calculation formula for the elastic modulus E by the slope of the initial straight line segment is as follows:

[0023] E = σ / ε

[0024] Where E represents the elastic modulus, σ represents the stress, the force per unit area, and ε represents the strain, the relative change in the length of the material;

[0025] The Poisson's ratio ν is calculated by the ratio of the transverse strain to the axial strain:

[0026] ν = -εl / ε

[0027] The strain εl in the vertical direction and the strain ε in the loading direction.

[0028] In one implementation, in S2, the strength parameters of the rock are estimated through a triaxial compression test, and the specific steps include:

[0029] Place the rock sample in a triaxial compression apparatus that can control the radial pressure;

[0030] Increase the axial pressure in stages while maintaining or gradually increasing the radial pressure to simulate the in-situ stress state;

[0031] Record the axial force and confining pressure when the rock sample fails, and calculate the compressive strength, tensile strength, and Mohr-Coulomb strength of the rock based on this;

[0032] The mathematical formula for calculating the compressive strength of the rock is

[0033] a = P / A

[0034] Where a represents the compressive strength, P represents the maximum pressure, and A represents the compressed area;

[0035] The mathematical formula for calculating the tensile strength of the rock is

[0036] Where, in the formula: R t ── Tensile strength, MPa;

[0037] P max ── Maximum load at failure, N;

[0038] D ── Diameter of the cylindrical specimen, mm;

[0039] l ── Length of the cylindrical specimen, mm;

[0040] The mathematical formula for calculating the Mohr-Coulomb strength is

[0041]

[0042] Where T is the shear strength of the rock, c is the cohesion of the rock, and σ n is the normal stress, The internal friction angle of the rock.

[0043] In one implementation, in S3, the finite element method numerical simulation method is applied to simulate the regional stress field distribution, and the specific steps for analyzing the simulation results include:

[0044] According to the regional tectonic stress field, set the boundary conditions of the model, including the far-field stress boundary conditions, which reflect the plate movement or crustal load effect;

[0045] Considering the interlayer slip dynamics, apply the shear force along the slip surface, and the local stress concentration caused by fault locking, and select or define the initial position and shape of the crack based on the geological observation results;

[0046] Start the simulation calculation. The software will calculate the stress field distribution in the region by the finite element method based on the input geological structure and the set physical parameters;

[0047] Apply the extended finite element method XFEM to set the crack propagation criterion, and judge the initiation and propagation of the crack based on the maximum stress intensity factor, energy release rate or other fracture mechanics criteria.

[0048] In one implementation, in S4, the specific steps for crack occurrence and propagation simulation include:

[0049] Based on the original finite element model, select a suitable XFEM framework, and identify the boundaries and internal paths of the crack in the standard finite element mesh;

[0050] Initialize the crack, clarify the shape, position and size of the crack. XFEM allows the crack to pass through any number of elements without remeshing;

[0051] Add additional degrees of freedom to the nodes near the crack front to describe the singularity of the crack tip, and accurately describe the stress field and displacement field at the crack tip through the step function and the two-dimensional asymptotic crack tip displacement field function to ensure that the model can handle the singularity caused by the crack;

[0052] Determine the critical condition for crack propagation, based on the maximum stress intensity factor K I 、K II or K III , and set the critical value g. When the calculated stress intensity factor reaches this value, it is considered that the crack begins to propagate;

[0053] Perform calculations according to the additional degrees of freedom and the step function and the two-dimensional asymptotic crack tip displacement field function, analyze the distribution of the stress intensity factor in the calculation results, identify potential crack propagation paths and propagation rates, and record the changes in the model stress distribution before and after crack propagation.

[0054] In one implementation, in S5, the specific steps of parameter sensitivity analysis include:

[0055] Use the Sobol sequence to generate a set of sample points that are evenly distributed within the parameter space, representing the variability of the entire parameter space;

[0056] For each input parameter or parameter combination, calculate the contribution of its variance to the total output variance to obtain the Sobol index;

[0057] Based on the calculated Sobol index, analyze the influence degree of each input parameter on the output result, compare the influence magnitudes of the input parameters on the output result, and determine the dominant factors of the input parameters on fracture formation.

[0058] Due to the adoption of the above technical solutions, the present invention has the following advantages:

[0059] (1) It not only considers geological factors but also deeply integrates the principles of rock mechanics, can more accurately reflect the fracture development law of the reservoir under in-situ stress, and improves the scientificity and practicality of prediction.

[0060] (2) By constructing a mechanical model considering the characteristics of layered media, it can more meticulously simulate the stress state of the reservoir and the process of fracture opening and closing, which helps to identify potential areas in low-permeability reservoirs and improve the oil and gas production efficiency.

[0061] (3) It can perform customized analysis according to the specific mechanical parameters of different geological horizons, adapt to various geological backgrounds and reservoir conditions, and this method has more obvious advantages especially in complex structural areas. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 It is a schematic flow chart of a method for predicting the main control factors of fracture initiation and propagation in a low-permeability reservoir under the constraint of a rock mechanics layer according to the present invention;

[0063] Figure 2 It is a schematic diagram of the stress-strain curve in the uniaxial compression test of a method for predicting the main control factors of fracture initiation and propagation in a low-permeability reservoir under the constraint of a rock mechanics layer according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0064] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present invention fall within the scope of protection of the present invention.

[0065] In view of the defects and problems of the prior art, the present application provides a method for predicting the main controlling factors of fracture initiation and propagation in low-permeability reservoirs, including:

[0066] S1. Collect geological, drilling, logging, and seismic data of the target low-permeability reservoir, analyze and correct and synthesize the data through data analysis, analyze the variation laws of lithological and physical property parameters, and establish a three-dimensional geological model of the target low-permeability reservoir based on the above analysis results;

[0067] S2. Determine the rock mechanical parameters of the target low-permeability reservoir based on the method of sample experiments;

[0068] S3. Conduct stress field analysis based on the three-dimensional geological model and the rock mechanical parameters, apply the finite element method numerical simulation method to simulate the regional stress field distribution, and analyze the simulation results;

[0069] S4. On the basis of stress analysis, use the extended finite element method to simulate fracture occurrence and propagation;

[0070] S5. Perform global sensitivity analysis to determine the influence of the input parameters of the three-dimensional geological model on the simulation results of fracture occurrence and propagation, so as to identify the main factors controlling fracture formation.

[0071] The above method is described in a more detailed embodiment in combination with more attached drawings. After a detailed embodiment of the present application, a calculation example is described to illustrate the practical effect of the present method.

[0072] Detailed Embodiment

[0073] As Figure 1 shown, the present invention provides a method for predicting the main controlling factors of fracture initiation and propagation in low-permeability reservoirs under the constraint of a rock mechanics layer, including the following steps:

[0074] S1. Data collection and geological modeling: Collect geological, drilling, logging, and seismic data of the target reservoir, analyze the data through geological data analysis and perform depth correction and synthesis, analyze the variation laws of lithological and physical property parameters, and establish a three-dimensional geological model based on the above analysis results;

[0075] S2. Determination of rock mechanical parameters: Estimate the elastic modulus, Poisson's ratio, and strength parameters of the rock by means of experimental measurement;

[0076] S3. Tectonic stress field analysis: Input the formation structure, fault distribution, and lithology distribution obtained in step S1 into the established three-dimensional geological model, apply the finite element method numerical simulation method to simulate the regional stress field distribution, and analyze the simulation results;

[0077] S4. Simulation of crack occurrence and propagation. The extended finite element method (XFEM) is used to describe the singularity at the crack tip by adding additional step functions and two-dimensional asymptotic crack tip displacement fields on the basis of the standard finite element method, and the crack is initialized. The critical value of crack propagation is set by choosing the maximum stress intensity factor and then the simulation is run.

[0078] S5. Parameter sensitivity analysis. Global sensitivity analysis is performed through the Sobol index to determine the influence of input parameters on the crack prediction results and identify the dominant factors controlling crack formation.

[0079] In step S1, the steps for establishing the three-dimensional geological model are as follows:

[0080] Based on the depth-corrected data, a stratigraphic framework is constructed in three-dimensional space, including the top and bottom boundaries and fault planes.

[0081] On the basis of the framework model, three-dimensional grids are divided, and the grid size needs to be determined according to the geological complexity and subsequent simulation requirements.

[0082] Lithological and physical property parameters are assigned to the grids to achieve continuous or discrete distribution of properties in three-dimensional space, and compared with the logging data in the actual data to verify the accuracy of the model.

[0083] Three-dimensional visualization is carried out using geological modeling software such as Petrel, GOCAD or Move to generate cross-sectional diagrams and three-dimensional diagrams.

[0084] In step S2, the elastic modulus and Poisson's ratio of the rock are estimated through uniaxial compression tests, and the specific steps are as follows:

[0085] Prepare standard rock samples to ensure that the rock samples are defect-free and the dimensions meet the test requirements.

[0086] Place the rock samples in a universal material testing machine, apply axial pressure, and simultaneously measure the axial strain and lateral strain of the rock samples.

[0087] Record the strain data at different pressure levels and draw the stress-strain curve.

[0088] The elastic modulus E is calculated through the slope of the initial straight line segment, and Poisson's ratio ν is calculated under small strain conditions through the ratio of the lateral strain to the axial strain. The calculation formula for the elastic modulus E through the slope of the initial straight line segment is as follows:

[0089] E = σ / ε

[0090] Where E represents the elastic modulus, σ represents the stress, the force per unit area, and ε represents the strain, the relative change in the length of the material.

[0091] The calculation formula for Poisson's ratio ν through the ratio of the lateral strain to the axial strain:

[0092] ν = -εl / ε

[0093] The strain εl in the vertical direction and the strain ε in the load direction.

[0094] In step S2, the strength parameters of the rock are estimated through a triaxial compression test, and the specific steps are as follows:

[0095] Place the rock sample in a triaxial compression apparatus that can control the radial pressure;

[0096] Increase the axial pressure in stages while maintaining or gradually increasing the radial pressure to simulate the in-situ stress state;

[0097] Record the axial force and confining pressure when the rock sample fails, and calculate the compressive strength, tensile strength, and Mohr-Coulomb strength of the rock based on this;

[0098] The mathematical formula for calculating the compressive strength of the rock is

[0099] a = P / A

[0100] Where, a represents the compressive strength, P represents the maximum pressure, and A represents the compressed area;

[0101] The mathematical formula for calculating the tensile strength of the rock is

[0102]

[0103] Where, in the formula: R t ── Tensile strength, MPa;

[0104] P max ── The maximum load at failure, N;

[0105] D ── The diameter of the cylindrical specimen, mm;

[0106] l ── The length of the cylindrical specimen, mm;

[0107] The mathematical formula for calculating the Mohr-Coulomb strength is

[0108]

[0109] Where, T is the shear strength of the rock, c is the cohesion of the rock, σn is the normal stress, The internal friction angle of the rock.

[0110] In step S3, the finite element method numerical simulation method is applied to simulate the regional stress field distribution, and the specific steps for analyzing the simulation results are as follows:

[0111] Set the boundary conditions of the model according to the regional tectonic stress field, including far-field stress boundary conditions, which reflect plate movement or crustal loading effects;

[0112] Consider interlayer slip dynamics, apply shear forces along the slip surface, and local stress concentration due to fault locking, and select or define the initial position and shape of the crack based on geological observation results;

[0113] Start the simulation calculation. The software will calculate the stress field distribution in the region by the finite element method based on the input geological structure and set physical parameters;

[0114] Apply the extended finite element method XFEM to set the crack propagation criterion. Based on the maximum stress intensity factor, energy release rate, or other fracture mechanics criteria, judge the initiation and propagation of the crack.

[0115] In step S4, the specific steps of crack occurrence and propagation simulation are as follows:

[0116] Based on the original finite element model, select a suitable XFEM framework and identify the boundaries and internal paths of the crack in the standard finite element mesh;

[0117] Initialize the crack, clarify the shape, position, and size of the crack. XFEM allows the crack to pass through any number of elements without remeshing;

[0118] Add additional degrees of freedom for the nodes near the crack front to describe the singularity at the crack tip. Accurately describe the stress field and displacement field at the crack tip through the step function and two-dimensional asymptotic crack tip displacement field function to ensure that the model can handle the singularity caused by the crack;

[0119] Determine the critical condition for crack propagation, based on the maximum stress intensity factor KI, KII, or KIII, and set the critical value g. When the calculated stress intensity factor reaches this value, it is considered that the crack begins to propagate;

[0120] Perform calculations based on the additional degrees of freedom and the step function and two-dimensional asymptotic crack tip displacement field function, analyze the distribution of the calculated stress intensity factor, identify potential crack propagation paths and propagation rates, and record the changes in the stress distribution of the model before and after crack propagation.

[0121] The mathematical expression of the step function is as follows:

[0122]

[0123] Among them, is the test function;

[0124] The mathematical expression of the two-dimensional asymptotic crack tip displacement field function is as follows:

[0125]

[0126] Among them, X Z and Y z are displacement components respectively, r and θ are the radius and angle in the polar coordinate system respectively, r represents the distance from the crack tip, and θ represents the azimuth angle from the crack tip.

[0127] The specific steps of the parameter sensitivity analysis are as follows:

[0128] Use the Sobol sequence to generate a set of sample points that are evenly distributed within the parameter space, representing the variability of the entire parameter space;

[0129] For each input parameter or parameter combination, calculate the contribution of its variance to the total output variance to obtain the Sobol index;

[0130] Based on the calculated Sobol indices, analyze the influence degree of each input parameter on the output result, compare the influence magnitudes of the input parameters on the output result, and determine the dominant factors of the input parameters on crack formation.

[0131] Run the XFEM solver to calculate the overall response of the structure, including stress and strain distributions, monitor the state of the crack tip according to the crack propagation criterion. If the propagation condition is met, automatically update the crack path and the model, and continue the iterative solution without changing the original mesh.

[0132] Next, in a practical example, illustrate the effect of this method.

[0133] I. Data collection and geological modeling

[0134] Taking a certain target reservoir as an example, some of its data are as follows:

[0135] The data after depth correction shows that the depths of the top and bottom boundaries of the reservoir are 2000 meters and 2500 meters respectively, and there is a fault plane with a strike of 45° northeast, a dip of 60° southeast.

[0136] When dividing the 3D mesh, considering the relatively high geological complexity, the mesh size is determined to be 50 m × 50 m × 10 m.

[0137] The lithology mainly includes sandstone and mudstone. Among the physical property parameters, the porosity is 10% in sandstone and 5% in mudstone. After allocating these parameters to the mesh and comparing with the actual logging data, it is found that the average error between the porosity predicted by the model and the actual logging porosity is within 3%, indicating that the model is relatively accurate.

[0138] II. Determination of rock mechanics parameters

[0139] Uniaxial compression test

[0140] Prepare standard rock samples with a diameter of 25 mm and a height of 50 mm, without natural fractures.

[0141] Apply axial pressure and record the strain data at different pressure levels as shown in Table 1 below:

[0142] Table 1

[0143]

[0144] When the pressure is 60 MPa, the axial strain is 0.0024 and the lateral strain is -0.00072.

[0145] As Figure 2 shown, plot the stress-strain curve and calculate the elastic modulus E = σ / ε = 50 / 0.002 = 25000 MPa from the slope of the initial straight line segment. The Poisson's ratio ν = -εl / ε = -(-0.0006) / 0.002 = 0.3.

[0146] Triaxial compression test

[0147] Place the rock sample in a triaxial compression apparatus that can control the radial pressure. Gradually increase the axial pressure and radial pressure to simulate the in-situ stress state. When the rock sample fails, the axial force is 53 kN and the confining pressure is 30 MPa.

[0148] The compressed area A = π×(12.5 mm)2 = 490.625 mm 2 ;

[0149] The compressive strength a = P / A = 53000 N / 490.625 mm 2 = 108.025 MPa.

[0150] For a cylindrical specimen with a diameter of 25 mm and a length of 5 mm, and a maximum load of 1200 N at failure, the tensile strength = 2×1200 N / (π×25 mm×5 mm) ≈ 6.11 MPa.

[0151] Through the test, the cohesion c of the rock is 10 MPa, the normal stress σn is 40 MPa, and the internal friction angle Then the Mohr-Coulomb strength

[0152] Based on the above uniaxial compression test and triaxial compression test, the following parameters are obtained as shown in Table 2:

[0153] Table 2

[0154] Test type Parameter Value Uniaxial compression test Elastic modulus E (MPa) 25000 Uniaxial compression test Poisson's ratio ν 0.3 Triaxial compression test Compressive strength a (MPa) 108.025 Triaxial compression test Tensile strength (MPa) 6.11 Triaxial compression test Mohr-Coulomb strength T (MPa) 33.87

[0155] III. Tectonic stress field analysis

[0156] Set the model boundary conditions: The far-field stress boundary conditions are a horizontal maximum principal stress of 80 MPa, a horizontal minimum principal stress of 60 MPa, and a vertical stress of 70 MPa. Considering the interlayer slip dynamics, the shear force along the slip surface is 20 MPa. Based on the geological observation results, a location near a fault is selected as the initial position of the crack, with an elliptical shape having a major axis of 100 meters and a minor axis of 50 meters. After starting the simulation calculation, the regional stress field distribution is obtained, with a maximum stress value of 120 MPa and a minimum stress value of 40 MPa.

[0157] IV. Simulation of Crack Initiation and Propagation

[0158] Based on the standard finite element model, the XFEM framework is selected to identify the boundaries and internal paths where the cracks are located. The initial crack is a straight line with a length of 20 meters and is located in the middle of the reservoir.

[0159] Additional degrees of freedom are added to the nodes near the crack front, and the singularity at the crack tip is described by the step function and the two-dimensional asymptotic crack tip displacement field function. At a certain moment, the distance r from the crack tip is 10 mm and the azimuth angle θ is 45°, then the displacement components u1 and u2 in the two-dimensional asymptotic crack tip displacement field function can be obtained according to specific calculations.

[0160] The critical condition for crack propagation is determined as the maximum stress intensity factor KI = 5 MPa√m. When the calculated stress intensity factor reaches this value, it is considered that the crack begins to propagate.

[0161] After performing the calculation, analyze the distribution of the calculated stress intensity factor. In a certain area, the stress intensity factor reaches the critical value, and the potential crack propagation path is identified as extending in the northeast direction with a propagation rate of 1 meter per hour. Record the changes in the model stress distribution before and after crack propagation and find that the stress concentration area moves in the direction of crack extension after propagation.

[0162] The numerical values of each data are obtained through tectonic stress field analysis and crack initiation and propagation simulation, as shown in Table 3:

[0163] Table 3

[0164] Step Data description Value Analysis of tectonic stress field Maximum horizontal principal stress (MPa) 80 Analysis of tectonic stress field Minimum horizontal principal stress (MPa) 60 Analysis of tectonic stress field Vertical stress (MPa) 70 Analysis of tectonic stress field Shearing force (MPa) 20 Simulation of crack initiation and propagation Initial crack length (m) 20 Simulation of crack initiation and propagation Critical value of crack propagation KI (MPa√m) 5 Simulation of crack initiation and propagation Propagation rate (m / hour) 1

[0165] V. Parameter Sensitivity Analysis

[0166] Use the Sobol sequence to generate a set of sample points, with three input parameters: elastic modulus E, Poisson's ratio ν, and internal friction angle Specifically, as shown in Table 4:

[0167] Table 4

[0168]

[0169]

[0170] For each parameter or parameter combination, the contribution of its variance to the total output variance is calculated to obtain the Sobol index. The calculation results are as follows: the Sobol index of the elastic modulus E is 0.4, the Sobol index of the Poisson's ratio ν is 0.3, and the Sobol index of the internal friction angle is 0.2. Analysis shows that the elastic modulus E has the greatest influence on the crack prediction result and is the dominant factor controlling crack formation.

[0171] In the present invention, the crack propagation criterion is a rule or standard used to describe and predict how cracks in a material expand over time and under external loads. These criteria are based on the principles of fracture mechanics and aim to understand and control the crack propagation behavior in materials to ensure the safety and reliability of materials and structures. The establishment of the crack propagation criterion is based on a deep understanding of the processes of crack initiation, propagation, and instability, which involve the mechanical properties of the material, the geometry of the crack, and the conditions of the external load.

[0172] In the present invention, the stress intensity factor criterion can be adopted. The stress intensity factor criterion is a method in fracture mechanics for judging whether a crack will expand. When there is a crack in a material, the stress field near the crack tip will exhibit a high degree of concentration, which has an important influence on the crack propagation and the fracture behavior of the material. To quantitatively describe the intensity of this stress field, a physical quantity, the stress intensity factor (K), is introduced. When the stress intensity factor reaches its critical value (referred to as the fracture toughness), the crack begins to expand. Therefore, by measuring and analyzing the stress intensity factor, it is possible to judge whether the crack will undergo unstable expansion and thus lead to the fracture of the material.

[0173] Therefore, the present invention adopts the above-mentioned method for predicting the main controlling factors of crack initiation and propagation in low-permeability reservoirs under the constraint of rock mechanics layers. It not only considers geological factors but also deeply integrates the principles of rock mechanics, can more accurately reflect the crack development law of the reservoir under in-situ stress, and improves the scientificity and practicality of the prediction. By constructing a mechanical model considering the characteristics of layered media, it is possible to more carefully simulate the stress state of the reservoir and the crack opening and closing process, which helps to identify potential areas in low-permeability reservoirs and improve the oil and gas production efficiency.

[0174] In several embodiments provided by the present invention, it should be understood that the disclosed method can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the above division of units is only a logical function division. In actual implementation, there may be other division methods. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed coupling or direct coupling or communication connection between each other can be through some interfaces. The indirect coupling or communication connection of devices or units can be in electrical, mechanical or other forms.

[0175] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A prediction method for the main controlling factors of fracture initiation and propagation in low-permeability reservoirs, characterized in that Including: S1. Collect geological, drilling, logging, and seismic data of the target low-permeability reservoir. Through data analysis, calibration, and synthesis, analyze the variation laws of lithology and physical property parameters, and establish a three-dimensional geological model of the target low-permeability reservoir based on the above analysis results; S2. Determine the rock mechanical parameters of the target low-permeability reservoir based on the method of sample experiments; S3. Conduct stress field analysis based on the three-dimensional geological model and the rock mechanical parameters. Apply the finite element method numerical simulation method to simulate the regional stress field distribution and analyze the simulation results; S4. On the basis of stress analysis, use the extended finite element method to simulate the occurrence and propagation of fractures; S5. Perform global sensitivity analysis to determine the influence of the input parameters of the three-dimensional geological model on the simulation results of fracture occurrence and propagation, so as to identify the dominant factors controlling fracture formation.

2. The prediction method for the main controlling factors of fracture initiation and propagation in low-permeability reservoirs according to claim 1, characterized in that In S1, the steps of establishing a three-dimensional geological model of the target low-permeability reservoir include: According to the calibrated data, construct a stratigraphic framework in three-dimensional space, including top and bottom boundaries and fault planes; Divide three-dimensional grids on the basis of the stratigraphic framework; Assign lithology and physical property parameters to each grid to achieve continuous or discrete distribution of attributes in three-dimensional space, and compare with the logging data in the actual data to verify the accuracy of the model; Use geological modeling software to realize the three-dimensional visualization of the three-dimensional geological model and generate cross-sectional views and stereograms.

3. The prediction method for the main controlling factors of fracture initiation and propagation in low-permeability reservoirs according to claim 1, characterized in that In S2, the rock mechanical parameters include the elastic modulus, Poisson's ratio, and strength parameters of the rock.

4. The prediction method for the main controlling factors of fracture initiation and propagation in low-permeability reservoirs according to claim 3, characterized in that In S2, estimate the elastic modulus and Poisson's ratio of the rock through uniaxial compression tests. The specific steps include: Prepare standard rock samples to ensure that the rock samples are defect-free and the dimensions meet the test requirements; Place the rock samples in a universal material testing machine, apply axial pressure, and simultaneously measure the axial strain and lateral strain of the rock samples; Record the strain data at different pressure levels and draw a stress-strain curve; The elastic modulus E is calculated by the slope of the initial straight line segment, and the Poisson's ratio ν is calculated by the ratio of the lateral strain to the axial strain under small strain conditions. The calculation formula for the elastic modulus E by the slope of the initial straight line segment is as follows: E = σ / ε Where E represents the elastic modulus, σ represents the stress, the force per unit area, and ε represents the strain, the relative change in the length of the material; The calculation formula for the Poisson's ratio ν by the ratio of the lateral strain to the axial strain: ν = -εl / ε The strain εl in the vertical direction and the strain ε in the loading direction.

5. The prediction method for the main controlling factors of fracture initiation and propagation in low-permeability reservoirs according to claim 3, characterized in that In S2, estimate the strength parameters of the rock through triaxial compression tests. The specific steps include: Place the rock samples in a triaxial compression instrument that can control the radial pressure; Increase the axial pressure in stages while maintaining or gradually increasing the radial pressure to simulate the in-situ stress state; Record the axial force and confining pressure when the rock samples rupture, and calculate the compressive strength, tensile strength, and Mohr-Coulomb strength of the rock; The mathematical formula for calculating the compressive strength of the rock is a = P / A Where a represents the compressive strength, P represents the maximum pressure, and A represents the compressed area; The mathematical formula for calculating the tensile strength of the rock is Among them, in the formula: R t ── Tensile strength, MPa; P max ── Maximum load at rupture, N; D ── the diameter of the cylindrical specimen, mm; l ── the length of the cylindrical specimen, mm; The mathematical formula for calculating the Mohr-Coulomb strength is Among them, T is the shear strength of the rock, c is the cohesion of the rock, and σ n is the normal stress, and φ is the internal friction angle of the rock.

6. The prediction method for the main controlling factors of fracture initiation and propagation in low-permeability reservoirs according to claim 1, wherein In S3, the specific steps for applying the finite element method numerical simulation method to simulate the regional stress field distribution and analyze the simulation results include: According to the regional tectonic stress field, set the boundary conditions of the model, including the far-field stress boundary conditions, to reflect the plate movement or crustal load effect; Consider the interlayer slip dynamics, apply the shear force along the slip surface, and the local stress concentration caused by fault locking, and select or define the initial position and shape of the crack based on the geological observation results; Start the simulation calculation. The software will calculate the stress field distribution in the region through the finite element method based on the input geological structure and set physical parameters; Apply the extended finite element method XFEM to set the crack propagation criterion, and judge the initiation and propagation of the crack based on the maximum stress intensity factor, energy release rate or other fracture mechanics criteria.

7. The prediction method for the main controlling factors of fracture initiation and propagation in low-permeability reservoirs according to claim 1, characterized in that In S4, the specific steps for crack occurrence and propagation simulation include: On the basis of the original finite element model, select a suitable XFEM framework, and identify the boundaries and internal paths of the crack in the standard finite element mesh; Initialize the crack, clarify the shape, position and size of the crack. XFEM allows the crack to pass through any number of elements without remeshing; Add additional degrees of freedom to the nodes near the crack front to describe the singularity of the crack tip, and accurately describe the stress field and displacement field at the crack tip through the step function and the two-dimensional asymptotic crack tip displacement field function to ensure that the model can handle the singularity caused by the crack; Determine the critical conditions for crack propagation, based on the maximum stress intensity factor K I , K II or K III , and set a critical value g. When the calculated stress intensity factor reaches this value, it is considered that the crack begins to propagate; Perform calculations according to the additional degrees of freedom and the step function and the two-dimensional asymptotic crack tip displacement field function, analyze the distribution of the stress intensity factor in the calculation results, identify potential crack propagation paths and propagation rates, and record the changes in the model stress distribution before and after crack propagation.

8. The prediction method for the main controlling factors of fracture initiation and propagation in low-permeability reservoirs according to claim 1, wherein In S5, the specific steps for parameter sensitivity analysis include: Use the Sobol sequence to generate a set of sample points that are evenly distributed in the parameter space and represent the variability of the entire parameter space; For each input parameter or parameter combination, calculate the contribution of its variance to the total output variance to obtain the Sobol index; According to the calculated Sobol index, analyze the influence degree of each input parameter on the output result, compare the influence sizes of the input parameters on the output result, and determine the dominant factors of the input parameters on crack formation.

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