Three-dimensional simulation method for artificially inflated cracks

Abstract

This invention relates to a method for simulating the three-dimensional propagation of artificially inflated fractures. It includes: based on drilling, logging, geological, and seismic data, completing layer division and correlation, establishing a three-dimensional stratigraphic model; using the ant-body tracking method to identify natural fractures and faults in tight reservoirs, establishing a three-dimensional structural model of the reservoir; based on triaxial stress experimental data and combined with logging data, establishing a reservoir rock mechanics model, inverting the reservoir stress field, and establishing a three-dimensional geostress model of the reservoir; based on fracturing operation data, and according to the three-dimensional structural model and the three-dimensional geostress model, establishing the equilibrium equations for the porous medium solid skeleton and the pore fluid continuity equations, obtaining displacement basis functions and pressure basis functions; based on linear elasticity theory and the law of conservation of mass, establishing the stress-viscous fluid pressure coupling equations for elastic media, and using the finite element solution gradient overapproximation method according to actual boundary conditions to obtain the solution of the stress-pressure coupling equations, characterizing the three-dimensional propagation of artificially inflated fractures.

Description

Technical Field

[0001] This invention relates to the field of oil and gas extraction technology, and in particular to a three-dimensional simulation method for artificial fracture propagation based on stress-fluid pressure coupling in tight reservoir elastic media. Background Technology

[0002] Tight reservoirs have poor porosity and permeability, necessitating large-scale hydraulic fracturing operations for oil and gas extraction. Horizontal well fracture monitoring reveals that the spatial propagation of hydraulically injected fractures is extremely complex. Due to the difficulty in characterizing the structure, geostress, and rock mechanics of tight reservoirs, as well as the interaction between injected fluid and the tight reservoir, the mechanisms of fracturing initiation and spatial propagation in tight reservoirs are highly complex. Currently, three-dimensional models characterizing fracture propagation include finite element, discrete element, and boundary element models. These models typically assume isotropic homogeneous media, two-dimensional or three-dimensional fluid flow within the fracture, the use of linear elastic fracture mechanics criteria for fracture propagation, and numerical simulation methods to achieve extension in the length, width, and height directions of the fracture. However, these models suffer from low computational efficiency and do not account for anisotropic, non-homogeneous media.

[0003] The fracturing stage of tight reservoirs generates changes in formation pore pressure and induces four-dimensional geostress evolution. Tight reservoirs have well-developed natural fractures, and the mechanisms of formation fluid movement and rock deformation are complex. Coupled simulation of gas reservoir seepage and rock deformation during extraction must consider the multi-media, multi-scale, and multi-dimensional issues of the matrix and fractures. Current hydraulic propagation models only analyze certain aspects in detail and have not yet formed a comprehensive simulation method.

[0004] In summary, a new model for the propagation of pressure fractures under complex stress-fluid pressure coupling control needs to be established. Summary of the Invention

[0005] To address the aforementioned problems, this invention overcomes the shortcomings of existing fluid-structure interaction simulation methods for hydraulic fracture propagation. The aim is to provide a novel method for simulating the coupling of elastic medium stress and fluid pressure, thereby realizing a three-dimensional characterization technique for artificial hydraulic fracture propagation in tight reservoirs.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for simulating the three-dimensional propagation of artificially inflated cracks includes the following steps:

[0008] Establish a three-dimensional structural model of the reservoir;

[0009] Establish a three-dimensional geostress model of the reservoir;

[0010] Based on fracturing operation data, and according to the three-dimensional structural model and the three-dimensional geostress model of the reservoir, the equilibrium equation of the porous medium solid skeleton was established, and the displacement basis function was obtained.

[0011] Based on fracturing operation data, and according to the three-dimensional structural model and the three-dimensional geostress model of the reservoir, a pore fluid continuity equation was established to obtain the pressure basis function.

[0012] Based on linear elasticity theory and the law of conservation of mass, a stress-pressure coupling equation for elastic media and viscous fluid is established. According to the actual boundary conditions, the finite element solution gradient overapproximation method is used to obtain the solution of the stress-pressure coupling equation based on the obtained displacement basis function and pressure basis function, which characterizes the three-dimensional propagation of artificial pressure cracks.

[0013] Establishing a three-dimensional structural model of the reservoir includes: based on drilling, logging, geological and seismic data, completing the sub-layer division and correlation, establishing a reservoir stratigraphic model, using the ant body tracking method to identify natural fractures and faults in the tight reservoir, and establishing a three-dimensional structural model of the reservoir.

[0014] Establishing a three-dimensional reservoir geostress model includes: establishing a reservoir rock mechanics model based on triaxial stress experimental data and well logging data; using the rock mechanics model, inverting the reservoir stress field; and establishing a three-dimensional reservoir geostress model.

[0015] Obtaining the displacement basis functions includes:

[0016] Based on the equilibrium equation of the porous medium solid skeleton, the displacement basis function is solved, and the calculation formula is as follows:

[0017]

[0018] In the formula

[0019] f(x,t) is the stress per unit volume;

[0020] Pp represents the additional pore pressure.

[0021] G is the shear modulus;

[0022] t is the fluid injection time;

[0023] q(x,t) is the fluid injection rate;

[0024] μ is the translation vector;

[0025] ν is Poisson's ratio;

[0026] ∈ represents volumetric strain;

[0027] α is the Biot coefficient.

[0028] The formula for calculating the Biot coefficient is:

[0029]

[0030] In the formula

[0031] ν u It is a non-consolidated Poisson's ratio;

[0032] B is the Skempton coefficient.

[0033] The pressure basis functions are obtained as follows:

[0034] Based on the continuity equation for pore fluids, the pressure basis function is solved using the following formula:

[0035]

[0036] In the formula

[0037] q(x,t) is the fluid injection rate;

[0038] M is the Biot modulus;

[0039] k is the penetration rate;

[0040] η is the dynamic fluid viscosity.

[0041] The formulas for calculating Biot modulus and dynamic fluid viscosity are as follows:

[0042]

[0043]

[0044] In the formula

[0045] ν u It is a non-consolidated Poisson's ratio;

[0046] B is the Skempton coefficient;

[0047] D is the hydraulic conductivity value.

[0048] Obtaining the solution to the stress-pressure coupling equation involves: using the COMSOL Multiphysics finite element analysis tool, and establishing a pressure fracture propagation model based on the three-dimensional reservoir structural model and the three-dimensional geostress model.

[0049] Based on the three-dimensional structural model of the reservoir, actual strata of different thicknesses are set from top to bottom, and interpreted natural fractures and faults are set. Based on well logging experimental data, the porosity, permeability, and compressibility coefficient properties of the reservoir, fractures, and faults are set. Based on the three-dimensional geostress model, the Poisson's ratio and Young's modulus of the reservoir, fractures, and faults are set.

[0050] Based on the three-dimensional geostress model of the reservoir, the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress are set. The model boundary is set as a fixed boundary, the top is in a state of no traction, and the displacement of the lateral and bottom boundaries is kept to zero. It is assumed that the initial fluid system of the model is in a state of hydrostatic equilibrium, and the initial pore pressure and initial stress tensor are set to zero.

[0051] The present invention has the following advantages due to the adoption of the above technical solutions:

[0052] The elastic medium stress-fluid pressure coupling simulation method provided in this invention can comprehensively characterize the spatial expansion of hydraulic fracturing fractures during the hydraulic fracturing process, greatly improving the understanding of the fracturing effect in tight reservoirs. Attached Figure Description

[0053] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. In the drawings:

[0054] Figure 1 The research ideas and content diagrams provided for the embodiments of the present invention;

[0055] Figure 2 A three-dimensional structural model diagram provided for an embodiment of the present invention;

[0056] Figure 3 A three-dimensional geostress model diagram provided for an embodiment of the present invention;

[0057] Figure 4 A diagram showing the change in pore pressure during the fracturing process provided in an embodiment of the present invention;

[0058] Figure 5 This is a diagram showing the variation of additional stress during the fracturing process, provided in an embodiment of the present invention.

[0059] Figure 6 A three-dimensional expansion diagram of an artificial pressure fracture based on stress-fluid pressure coupling simulation of an elastic medium is provided for an embodiment of the present invention. Detailed Implementation

[0060] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0061] The purpose of this invention is to provide a three-dimensional simulation method for artificially propagated fractures based on the stress-fluid pressure coupling simulation of elastic media in tight reservoirs.

[0062] According to some embodiments of the present invention, a three-dimensional simulation method for artificial hydraulic crack propagation based on elastic medium stress-fluid pressure coupling includes:

[0063] Acquire drilling, logging and geological data, complete the sub-layer division and correlation, and establish reservoir stratigraphic models;

[0064] Based on high-resolution 3D seismic data, the ant tracking method is used to identify natural fractures and faults in the tight reservoir. Combined with the reservoir stratigraphic model, a 3D structural model of the reservoir is established.

[0065] Obtain triaxial stress experimental data and combine it with well logging data to establish a reservoir rock mechanics model;

[0066] Using the aforementioned rock mechanics model, the reservoir stress field is inverted, and a three-dimensional geostress model of the reservoir is established.

[0067] Based on the obtained fracturing construction data, and the aforementioned three-dimensional structural model and three-dimensional geostress model, a coupled equation of elastic medium stress-viscosity fluid pressure was established based on linear elasticity theory and the law of conservation of mass. Furthermore, based on the actual boundary conditions, a complex stress-fluid pressure coupled control fracturing propagation model was established using the finite element solution gradient super-approximation method.

[0068] A pressure fracture propagation model was established using the COMSOL Multiphysics finite element analysis tool, based on the aforementioned three-dimensional structure and geostress model.

[0069] Regarding model dimensions, the dimensions in the x, y, and z directions are set to represent the influence range of the horizontal well group.

[0070] Regarding the model medium, based on the aforementioned structural model, actual strata of varying thicknesses are set from top to bottom, along with interpreted natural fractures and faults. Based on well logging and experimental data, the porosity, permeability, and compressibility coefficient properties of the reservoir, fractures, and faults are set. Based on the aforementioned geostress model, elastic mechanical parameters such as Poisson's ratio and Young's modulus of the reservoir, fractures, and faults are set.

[0071] Regarding model stress, based on the aforementioned geostress model, the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress are set.

[0072] Regarding the model boundary conditions, the model boundary is set as a fixed boundary, that is, the top is in a state of no traction, and the displacement of the lateral and bottom boundaries remains zero.

[0073] Regarding the initial conditions of the model, it is assumed that the initial fluid system of the model is in a state of hydrostatic equilibrium, that is, the initial pore pressure and the initial stress tensor are set to zero.

[0074] Therefore, the results of the coupled simulation are the corresponding changes in pore pressure and stress tensor. The injection volume of fracturing fluid in each fracturing stage is converted into the net injection pressure of each stage, which serves as the driving force in the coupled simulation process. The duration of the net injection pressure is consistent with the actual construction time of each fracturing stage. After the previous stage of fracturing is completed, the net pressure at the end of the fracturing fracture approaches 0 (close to the formation pore pressure), and the next stage of fracturing begins, thus cycling until the final stage of fracturing is completed.

[0075] The coupled model is set to be governed by linear elasticity and Darcy's law, and the mesh is refined near the hydraulic fractures and faults. Through coupled simulation of solid mechanics and porous media fluid flow, stress disturbances and pore pressure changes during the hydraulic fracturing process are calculated, thereby characterizing the spatial propagation of the hydraulic fractures.

[0076] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0077] Figure 1 A flowchart illustrating an embodiment of the three-dimensional propagation simulation method for artificial hydraulic cracks based on elastic medium stress-fluid pressure coupling provided by the present invention is shown below. Figure 1 As shown, as one possible implementation method, a three-dimensional simulation method for artificial hydraulic crack propagation based on elastic medium stress-fluid pressure coupling includes the following steps:

[0078] Based on drilling, logging, geological and seismic data, S1 completes the sub-layer division and correlation, establishes reservoir stratigraphic model, and uses the ant body tracking method to identify natural fractures and faults in tight reservoir development, and establishes a three-dimensional structural model of the reservoir.

[0079] Based on triaxial stress experimental data and well logging data, S2 establishes a reservoir rock mechanics model. Using the rock mechanics model, the reservoir stress field is inverted to establish a three-dimensional reservoir geostress model.

[0080] Based on fracturing construction data, S3 establishes the equilibrium equation of the porous medium solid skeleton according to the three-dimensional structural model and the three-dimensional geostress model, and obtains the displacement basis function.

[0081] Based on fracturing construction data, S4 establishes a pore fluid continuity equation and obtains the pressure basis function according to the aforementioned three-dimensional structural model and three-dimensional geostress model;

[0082] Based on linear elasticity theory and the law of conservation of mass, S5 established a stress-pressure coupling equation for elastic media and a viscous fluid. According to the actual boundary conditions, the solution of the stress-pressure coupling equation was obtained by using the finite element solution gradient over-approximation method.

[0083] S6 characterizes the three-dimensional propagation of artificial pressure fractures based on the solution of the stress-pressure coupling equation.

[0084] The specific steps of the three-dimensional structural modeling described in step S1 include:

[0085] Based on drilling, logging and geological data, and according to the characteristics of logging curves, the top and bottom surfaces of the target layer are identified, multi-well sub-layer division and comparison are completed, and a three-dimensional stratigraphic model is established.

[0086] Based on high-resolution 3D seismic data, the ant-body tracking method was used to identify natural fractures and faults in the reservoir. Combined with the aforementioned stratigraphic model, a 3D structural model was established, such as... Figure 2 As shown;

[0087] The specific steps of the three-dimensional geostress modeling described in step S2 include:

[0088] Based on triaxial stress experimental data and combined with sonic logging data, Poisson's ratio and Young's modulus parameters were obtained, and a reservoir rock mechanics model was established.

[0089] Based on the aforementioned rock mechanics model and well logging data, the vertical, maximum, and minimum principal stresses of the reservoir are inverted, and a three-dimensional geostress model of the reservoir is established, as follows: Figure 3 As shown;

[0090] The specific steps for obtaining the displacement basis function as described in step S3 include:

[0091] Based on the equilibrium equation of the porous medium solid skeleton, the displacement basis function is solved, and its calculation formula is as follows:

[0092]

[0093] Where f(x,t) is the stress per unit volume, N / m 3 q(x,t) is the fluid injection rate, m 3 / s; μ is the translation vector, m; ν is Poisson's ratio, dimensionless; ∈ is the volumetric strain, dimensionless; α is the Biot coefficient, dimensionless;

[0094] The formula for calculating the Biot coefficient is as follows:

[0095]

[0096] The specific steps for obtaining the pressure basis function as described in step S4 include:

[0097] Based on the continuity equation for pore fluids, the pressure basis function is solved, and its calculation formula is as follows:

[0098]

[0099] Where M is the Biot modulus, which is dimensionless; k is the permeability, in μm. -2 η is the dynamic fluid viscosity, cp;

[0100] The formulas for calculating Biot modulus and mobility ratio are as follows:

[0101]

[0102]

[0103] Where, ν u Where is the unconsolidated Poisson's ratio, B is the Skempton coefficient, and D is the hydraulic conductivity.

[0104] The steps for solving the elastic medium stress-fluid pressure coupling in step S5 are as follows:

[0105] Using the COMSOL Multiphysics finite element analysis tool, the above equations were solved based on the three-dimensional structure and geostress model to establish a pressure fracture propagation model.

[0106] Regarding model dimensions, the dimensions in the x, y, and z directions are set to represent the influence range of the horizontal well group.

[0107] Regarding the model medium, based on the aforementioned structural model, actual strata of different thicknesses are set from top to bottom, and interpretable natural fractures and faults are also included.

[0108] Based on well logging, experimental, and other data, the porosity, permeability, and compressibility coefficient attributes of reservoirs, fractures, and faults are set.

[0109] Based on the aforementioned geostress model, elastic mechanical parameters such as Poisson's ratio and Young's modulus of reservoirs, fractures, and faults are set.

[0110] Regarding model stress, based on the aforementioned geostress model, the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress are set.

[0111] Regarding the model boundary conditions, the model boundary is set as a fixed boundary, that is, the top is in a state of no traction, and the displacement of the lateral and bottom boundaries remains zero.

[0112] Regarding the initial conditions of the model, it is assumed that the initial fluid system of the model is in a state of hydrostatic equilibrium, that is, the initial pore pressure and the initial stress tensor are set to zero.

[0113] Therefore, the results of the coupled simulation are the corresponding changes in pore pressure and stress tensor.

[0114] The volume of fracturing fluid injected into each fracturing stage is converted into the net injection pressure of each stage, which is then used as the driving force in the coupled simulation process. The duration of the net injection pressure is consistent with the actual construction time of each fracturing stage. After the previous fracturing stage is completed, the net pressure at the end of the fracturing fracture approaches 0 (close to the formation pore pressure), and the next fracturing stage is initiated, thus cycling until the final stage of fracturing is completed.

[0115] The coupled model is set to be governed by linear elasticity and Darcy's law, and the mesh is refined near the pressure fractures and faults.

[0116] Simulations were performed by coupling solid mechanics with fluid flow in porous media. For example... Figure 4 As shown, the porosity changes during the fracturing process were calculated. Figure 5 As shown, the variation of the additional stress is calculated.

[0117] Based on the results of the elastic medium stress-fluid pressure coupling solution, the three-dimensional propagation simulation results of artificial pressure cracks are characterized, such as... Figure 6 As shown.

[0118] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for simulating the three-dimensional propagation of artificially inflated cracks, characterized in that, Includes the following steps: Establish a three-dimensional structural model of the reservoir; Establish a three-dimensional geostress model of the reservoir; Based on fracturing operation data, and according to the reservoir three-dimensional structural model and the reservoir three-dimensional geostress model, the equilibrium equation of the porous medium solid skeleton is established, and the displacement basis function is obtained. Based on fracturing operation data, and according to the reservoir three-dimensional structural model and the reservoir three-dimensional geostress model, a pore fluid continuity equation is established to obtain the pressure basis function; Based on linear elasticity theory and the law of conservation of mass, a stress-pressure coupling equation for elastic medium and viscous fluid is established. According to the actual boundary conditions, the finite element solution gradient overapproximation method is adopted. Based on the obtained displacement basis function and pressure basis function, the solution of stress-pressure coupling equation is obtained, which characterizes the three-dimensional propagation of artificial pressure cracks. The obtained displacement basis functions include: Based on the equilibrium equation of the porous medium solid skeleton in equation (1), solve for the displacement basis function: (1) In the formula f(x,t) is the stress per unit volume; To add pore pressure; Shear modulus; For fluid injection time; u is the translation vector; v Poisson's ratio; For volumetric strain; For Biot coefficients; The obtained pressure basis function includes: Solve for the pressure basis function based on the pore fluid continuity equation (3) below: (3) In the formula q(x,t) is the fluid injection rate; M is the Biot modulus; k is the penetration rate; η is the dynamic fluid viscosity.

2. The method for simulating the three-dimensional propagation of artificially inflated cracks according to claim 1, characterized in that, The establishment of the reservoir three-dimensional structural model includes: based on drilling, logging, geological and seismic data, completing the sub-layer division and comparison, establishing the reservoir stratigraphic model, using the ant body tracking method to identify natural fractures and faults in the tight reservoir, and establishing the reservoir three-dimensional structural model.

3. The method for simulating the three-dimensional propagation of artificially inflated cracks according to claim 1, characterized in that, The establishment of the reservoir three-dimensional geostress model includes: establishing a reservoir rock mechanics model based on triaxial stress experimental data and well logging data; using the rock mechanics model, inverting the reservoir stress field; and establishing the reservoir three-dimensional geostress model.

4. The method for simulating the three-dimensional propagation of artificially inflated cracks according to claim 1, characterized in that, The formula for calculating the Biot coefficient is as follows: (2) In the formula ν u It is a non-consolidated Poisson's ratio; B is the Skempton coefficient.

5. The method for simulating the three-dimensional propagation of artificially inflated cracks according to claim 1, characterized in that, The formulas for calculating the Biot modulus and dynamic fluid viscosity are as follows: (4) (5) In the formula ν u It is a non-consolidated Poisson's ratio; B is the Skempton coefficient; D is the hydraulic conductivity value.

6. The method for simulating the three-dimensional propagation of artificially inflated cracks according to claim 1, characterized in that, The process of obtaining the solution to the stress-pressure coupling equation includes: using the COMSOL Multiphysics finite element analysis tool, a pressure fracture propagation model is established based on the three-dimensional reservoir structural model and the three-dimensional geostress model.

7. The method for simulating the three-dimensional propagation of artificially inflated cracks according to claim 1, characterized in that, Based on the three-dimensional reservoir structure model, actual formations of different thicknesses are set from top to bottom, and interpreted natural fractures and faults are set. Based on well logging experimental data, the porosity, permeability, and compressibility coefficient properties of the reservoir, fractures, and faults are set. Based on the three-dimensional geostress model, the Poisson's ratio and Young's modulus of the reservoir, fractures, and faults are set.

8. The method for simulating the three-dimensional propagation of artificially inflated cracks according to claim 1, characterized in that, Based on the three-dimensional geostress model of the reservoir, the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress are set. The model boundary is set as a fixed boundary, the top is in a state of no traction, and the displacement of the lateral and bottom boundaries is kept to zero. It is assumed that the initial fluid system of the model is in a state of hydrostatic equilibrium, and the initial pore pressure and initial stress tensor are set to zero.

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