CO2 storage multi-field coupling safety evaluation method considering crack evolution

Abstract

The invention discloses a CO2 storage multi-field coupling safety evaluation method considering fracture evolution, and relates to the technical field of complex oil and gas reservoir development, and the CO2 storage multi-field coupling safety evaluation method comprises the following steps: s1, constructing and solving a multi-field coupling model; s2, virtual crack grid construction and expansion: according to the updated model stress field parameters, judging whether cracks are generated or not, and if so, constructing and expanding a corresponding virtual crack grid; s3, evaluating the integrity of the cover layer: after each time step is finished, evaluating the integrity of the cover layer; and continuing numerical simulation of the next time step until the simulation is finished. According to the method, comprehensive coupling of multiple physical fields such as crack generation and expansion, CO2 seepage, heat transfer between the injected CO2 fluid and the reservoir and between the injected CO2 fluid and the reservoir fluid, geological stress redistribution and the like in the CO2 storage process is fully considered, so that the spatial and temporal change of each physical field in the reservoir in the CO2 injection process can be truly represented, the simulation precision is greatly improved, and the simulation efficiency is improved. And the uncertainty caused by fixed parameters or simplified processing is effectively reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of complex oil and gas reservoir development, and in particular to a CO 2 Safety assessment method for multi-field coupling of storage. Background Art

[0002] Currently, CO 2 Geological storage technology has become an important means to mitigate greenhouse gas emissions and address climate change, and has therefore attracted widespread attention. Existing numerical simulation techniques mainly describe CO2 by solving the conservation equations of mass, energy, and momentum. 2 The seepage, heat transfer and geological stress redistribution process during the storage process. However, these methods only consider the multi-field coupling of heat, fluid and solid in the simulation and solution, and do not consider the fracture generation and evolution mechanism caused by high pressure and temperature difference during the injection process. 2 Injection not only changes the pressure and temperature field of the reservoir, but also induces local stress concentration, which leads to the generation and expansion of cracks. Traditional models have great limitations in describing the dynamic evolution of cracks. This leads to great uncertainty in the prediction of reservoir caprock integrity and long-term storage safety, thus affecting the CO 2 Optimal design and risk assessment of storage solutions.

[0003] To address the above problems, it is urgent to develop a numerical simulation method that can comprehensively couple multiple physical fields such as seepage, heat transfer and geological stress redistribution, and can accurately describe the process of crack generation and dynamic evolution. Summary of the invention

[0004] In order to solve the above technical problems, the present invention discloses a CO 2 A multi-field coupling safety evaluation method for storage is proposed. This method is based on the virtual fracture grid technology and constructs a CO2 storage safety evaluation model considering fracture evolution. 2 The storage heat-fluid-solid multi-field coupling model can accurately describe the CO 2 The evolution of cracks during the sealing process can be used to predict the integrity of the caprock and the safety of sealing.

[0005] A CO method considering crack evolution 2 The storage multi-field coupling safety assessment method includes the following steps:

[0006] s1. Construction and solution of multi-field coupling model: considering the generation and extension of fractures, the injected CO 2 Heat transfer between the reservoir and reservoir fluid, CO 2 The coupling relationship between seepage and geological stress is established to establish the CO 2The multi-field coupled numerical model of heat storage, fluid and solid is used to update the heat transfer field, stress field and seepage field parameters at each time step of the model by solving the conservation equations of mass, energy and momentum.

[0007] s2. Construction and expansion of virtual fracture grid: Based on the model stress field parameters updated in step s1, the pre-set rock fracture criterion is used to determine whether a fracture is generated. If a new fracture is generated, the corresponding virtual fracture grid is constructed and expanded;

[0008] s3. Cover integrity evaluation: After each time step, the cover integrity is evaluated to determine the CO 2 The storage safety is predicted; the numerical simulation of the next time step is continued until the simulation is completed.

[0009] Optionally, in step s1, the generation and expansion of cracks, the injected CO 2 Heat transfer between the reservoir and reservoir fluid, CO 2 The coupling relationship between seepage and geological stress, the coupling method is:

[0010] For the mass conservation equation:

[0011] ;

[0012] In the formula, is the simulation duration, in seconds; is the fluid pore volume, dimensionless; is the density of phase j, in kg / m 3 , where j is w, o or g, representing water phase, oil phase or gas phase, respectively; is the j-phase saturation, dimensionless; is the total number of matrix and fracture grids adjacent to the current matrix or fracture grid; is the source and sink term of phase j, in m 3 / s; It represents the velocity difference of phase j between the current grid and its adjacent grid, in m / s; It represents the mass internal energy of phase j, in J / kg; is the volume internal energy of the inert rock matrix, in J / m 3 ; is the mass enthalpy of phase j, in J / kg; is the comprehensive thermal conductivity of the saturated rock matrix, in W / (m·K); is the temperature difference between the current grid and its adjacent grid, in K;

[0013] For the energy conservation equation:

[0014] ;

[0015] In the formula, is the mass fraction of component c in phase j, dimensionless; is the difference in mass fraction of component c in phase j between the current grid and the adjacent grid, dimensionless;

[0016] For the momentum conservation equation:

[0017] ;

[0018] In the formula, is Poisson's ratio, dimensionless; is the Biot coefficient, dimensionless; is the total pore pressure of the current grid, in kPa; is the linear thermal expansion coefficient, in 1 / ℃ or 1 / K; is the bulk modulus, in kPa; is the body force, in N / m 3 ; is the temperature in °C or K; is the normal average stress, in kPa, The calculation method is:

[0019] ;

[0020] In the formula, , and They are the maximum principal stress, intermediate principal stress and minimum principal stress respectively, and the unit is kPa.

[0021] Optionally, in step s2, a pre-set rock fracture criterion is used to determine whether a crack is generated, and the rock fracture criterion is:

[0022] If the stress condition of the mesh satisfies the following equation, the mesh will be tensilely damaged:

[0023] ;

[0024] In the formula, is the tensile strength of the rock matrix, is the minimum effective principal stress of the rock matrix;

[0025] If the stress condition of the mesh satisfies the following equation, the mesh will be sheared:

[0026] ;

[0027] In the formula, is the shear stress, For cohesion, is the normal stress, is the internal friction angle, shear stress and normal stress The calculation formulas are:

[0028] ;

[0029] ;

[0030] In the formula, and are the maximum total principal stress and the minimum total principal stress, respectively. is the rock failure surface and the minimum total principal stress The angle between.

[0031] Optionally, in step s2, the steps of constructing and expanding the corresponding virtual fracture grid specifically include:

[0032] s21. Add a new mesh as a virtual fracture mesh in the same layer of the matrix mesh where the fracture is generated. The permeability and porosity of the newly added virtual fracture mesh are equal to the permeability and porosity of the newly generated fracture.

[0033] s22. Establishing a mass, energy and momentum coupling relationship between the newly added virtual crack grid and the matrix grid that generates the crack, the establishment method is the same as step s1;

[0034] s23. If the matrix grid where the newly generated fractures are generated contains source and sink terms, in order to ensure the energy and mass exchange between the source and sink terms and the newly generated fractures, the same source and sink terms are also added to the newly added virtual fracture grid;

[0035] s24. If the matrix grid of the newly generated fracture is adjacent to the matrix grid of the existing fracture, energy and mass exchange is established between the newly added virtual fracture grid and the existing virtual fracture grid, and the establishment method is the same as step s22.

[0036] Optionally, in step s3, the cap layer integrity evaluation criteria are: if cracks are detected in the cap layer, resulting in CO 2 If the cap layer is broken through and migrates upward, the integrity of the cap layer is judged to have failed.

[0037] The beneficial effect of the present invention is that the present invention fully considers CO 2 The generation and expansion of cracks during storage, CO 2 Seepage, injected CO 2 The comprehensive coupling of multi-physics fields such as heat transfer between fluid and reservoir and reservoir fluid, as well as geological stress redistribution, can realistically reproduce the CO 2The temporal and spatial changes of various physical fields in the reservoir during the injection process greatly improve the accuracy of the simulation and effectively reduce the uncertainty caused by fixed or simplified parameter processing, providing a reliable quantitative tool for the long-term safety and risk assessment of the storage system, and thus providing scientific and effective decision-making support for engineering design, injection scheme optimization and risk management. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The present invention is a CO considering crack evolution 2 Schematic diagram of the process of evaluating the safety of storage multi-field coupling;

[0039] Figure 2 CO 2 Schematic diagram of the two-dimensional longitudinal numerical model I of the thermal-fluid-solid multi-field coupling of storage;

[0040] Figure 3 This is a schematic diagram of reliability verification of Model I of the present invention;

[0041] Figure 4A This is a crack distribution diagram of Model I after 38 years of gas injection shown in an embodiment of the present invention;

[0042] Figure 4B The crack distribution diagram of Model I after 42 years of gasification is shown in the embodiment of the present invention;

[0043] Figure 4C This is a crack distribution diagram of Model I after 46 years of gasification according to an embodiment of the present invention. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0045] A CO method considering crack evolution 2 The storage multi-field coupling safety assessment method includes the following steps:

[0046] s1. Construction and solution of multi-field coupling model: considering the generation and extension of fractures, the injected CO2 Heat transfer between the reservoir and reservoir fluid, CO 2 The coupling relationship between seepage and geological stress is established to establish the CO 2 The multi-field coupled numerical model of heat storage, fluid and solid is used to update the heat transfer field, stress field and seepage field parameters at each time step of the model by solving the conservation equations of mass, energy and momentum.

[0047] Considering the formation and expansion of fractures, the injected CO 2 Heat transfer between the reservoir and reservoir fluid, CO 2 The coupling relationship between seepage and geological stress, the coupling method is:

[0048] For the mass conservation equation:

[0049] ;

[0050] In the formula, is the simulation duration, in seconds; is the fluid pore volume, dimensionless; is the density of phase j, in kg / m 3 , where j is w, o or g, representing water phase, oil phase or gas phase, respectively; is the j-phase saturation, dimensionless; is the total number of matrix and fracture grids adjacent to the current matrix or fracture grid; is the source and sink term of phase j, in m 3 / s; It represents the velocity difference of phase j between the current grid and its adjacent grid, in m / s; It represents the mass internal energy of phase j, in J / kg; is the volume internal energy of the inert rock matrix, in J / m 3 ; is the mass enthalpy of phase j, in J / kg; is the comprehensive thermal conductivity of the saturated rock matrix, in W / (m·K); is the temperature difference between the current grid and its adjacent grid, in K;

[0051] For the energy conservation equation:

[0052] ;

[0053] In the formula, is the mass fraction of component c in phase j, dimensionless; is the difference in mass fraction of component c in phase j between the current grid and the adjacent grid, dimensionless;

[0054] For the momentum conservation equation:

[0055] ;

[0056] In the formula, is Poisson's ratio, dimensionless; is the Biot coefficient, dimensionless; is the total pore pressure of the current grid, in kPa; is the linear thermal expansion coefficient, in 1 / ℃ or 1 / K; is the bulk modulus, in kPa; is the body force, in N / m 3 ; is the temperature in °C or K; is the normal average stress, in kPa, The calculation method is:

[0057] ;

[0058] In the formula, , and They are the maximum principal stress, intermediate principal stress and minimum principal stress respectively, and the unit is kPa.

[0059] s2. Construction and expansion of virtual fracture grid: Based on the model stress field parameters updated in step s1, the pre-set rock fracture criterion is used to determine whether a fracture is generated. If a new fracture is generated, the corresponding virtual fracture grid is constructed and expanded;

[0060] The rock fracture criterion is:

[0061] If the stress condition of the mesh satisfies the following equation, the mesh will be tensilely damaged:

[0062] ;

[0063] In the formula, is the tensile strength of the rock matrix, is the minimum effective principal stress of the rock matrix;

[0064] If the stress condition of the mesh satisfies the following equation, the mesh will be sheared:

[0065] ;

[0066] In the formula, is the shear stress, For cohesion, is the normal stress, is the internal friction angle, shear stress and normal stress The calculation formulas are:

[0067] ;

[0068] ;

[0069] In the formula, and are the maximum total principal stress and the minimum total principal stress, respectively. is the rock failure surface and the minimum total principal stress The angle between.

[0070] The steps of constructing and expanding the corresponding virtual fracture grid include:

[0071] s21. Add a new mesh as a virtual fracture mesh in the same layer of the matrix mesh where the fracture is generated. The permeability and porosity of the newly added virtual fracture mesh are equal to the permeability and porosity of the newly generated fracture.

[0072] s22. Establishing mass, energy and momentum exchange between the newly added virtual crack grid and the matrix grid generating the crack, the establishment method is the same as step s1;

[0073] s23. If the matrix grid where the newly generated fractures are generated contains source and sink terms, in order to ensure the energy and mass exchange between the source and sink terms and the newly generated fractures, the same source and sink terms are also added to the newly added virtual fracture grid; for example, a matrix grid A contains source and sink terms (such as CO 2 Injection well), then the matrix grid A generates a new fracture grid B, then the same source and sink items as those of the matrix grid A are added to the fracture grid B.

[0074] s24. If the matrix grid of the newly generated fracture is adjacent to the matrix grid of the existing fracture, energy and mass exchange is established between the newly added virtual fracture grid and the existing virtual fracture grid, and the establishment method is the same as step s22.

[0075] s3. Cover integrity evaluation: After each time step, the cover integrity is evaluated to determine the CO 2 The safety of the storage is predicted; the numerical simulation of the next time step is continued until the simulation is completed. The evaluation criteria for the integrity of the caprock are: if cracks are detected in the caprock, resulting in CO 2 If the cap layer is broken through and migrates upward, the integrity of the cap layer is judged to have failed.

[0076] Application Examples

[0077] A prediction of CO 2 A multi-field coupling numerical simulation method for storage safety is used to simulate CO 2 The storage heat-fluid-solid multi-field coupling model is used for simulation and prediction, such as Figure 1 As shown, the specific steps are as follows:

[0078] s1. Construction and solution of multi-field coupling model: Based on the In Salah CO 2 For onshore storage projects, the generation and propagation of fractures, the injected CO 2 Heat transfer between the reservoir and reservoir fluid, CO 2 The coupling relationship between seepage and geological stress is established to establish the CO 2 The storage heat-fluid-solid multi-field coupled two-dimensional longitudinal numerical model is recorded as Model I. The key parameters of Model I are shown in Table 1. X and Y represent two mutually perpendicular horizontal directions, and Z represents the longitudinal direction perpendicular to the X and Y directions. The number of grids in the fine-scale Model I is 200×1×30 (X×Y×Z), and the size of each grid is 20×5900×1 m. The upper 10 layers of Model I are cap layers, and the lower 20 layers are saline layers, as shown in Figure 2 The model contains 1 CO 2 Injection wells, such as Figure 2 The gray circles in the figure are shown. By solving the mass, energy and momentum conservation equations, the heat transfer field, stress field and seepage field parameters of model I at each time step are updated.

[0079] Table 1 Key parameters of Model I

[0080]

[0081] s2. Construction and expansion of virtual fracture grid: Based on the stress field parameters of the model updated in step s1, the pre-set rock fracture criterion is used to determine whether a fracture is generated. If a new fracture is generated, the corresponding virtual fracture grid is constructed and expanded;

[0082] s3. Cover integrity evaluation: After each time step, the cover integrity is evaluated to determine the CO 2 The storage safety is predicted; the numerical simulation of the next time step is continued until the simulation is completed.

[0083] like Figure 3 As shown, the black dot is In Salah CO 2 The surface displacement data of the storage project site. The black curve is the surface displacement data of Model I. By comparison, it is found that Model I is 2 The field data of the storage project is in good agreement, which verifies the reliability of Model I. FIG. 4A to FIG. 4C In the figure, the black grid represents the virtual crack grid. Figure 4A As shown in Figure 1, after 38 years of gas injection in Model I, cracks appeared at the junction of the cap rock and the saline layer (layer 10), and the integrity of the cap rock was destroyed. Since the fracture permeability is much greater than the matrix permeability, the fracture becomes a CO 2Therefore, according to the current geological parameters and gas injection parameters of the In Salah project, the safe gas injection time of the project is up to 38 years. Figure 4B As shown in the figure, between 42 and 46 years of steam injection, cracks appeared on the top of the caprock (layer 1), and the cracks in the saline layer (layers 11 to 30) gradually expanded to the right. 2 The method of the present invention can accurately simulate the dynamic evolution process of cracks and has an important impact on the safety of CO storage. 2 Safety and design of CO storage 2 The storage solution has important application value.

[0084] The method proposed in the present invention solves the mass, energy and momentum conservation equations in each time step to update the temperature, pressure, stress and seepage parameters of each grid in the model in real time; at the same time, it determines whether a crack is generated according to the pre-set mechanical fracture criterion, and constructs a newly generated fracture grid based on the virtual fracture grid method proposed in the present invention, thereby establishing a fracture network that truly reflects the reservoir conditions. The advantage of this method is that it realizes the dynamic coupling between fractures and thermal fluid-solid multi-physical fields, so that the interaction between the physical fields inside the reservoir can be more accurately described, thereby providing a basis for caprock integrity evaluation and CO 2 Long-term storage safety prediction provides a more reliable scientific basis.

[0085] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A multi-field coupling safety assessment method for CO2 storage considering fracture evolution, characterized in that: The steps include: s1. Construction and solution of multi-field coupling model: Considering the coupling relationship between the generation and expansion of fractures, the heat transfer between the injected CO2 and the reservoir and reservoir fluid, the CO2 seepage and geological stress, a multi-field coupling numerical model of CO2 storage heat-fluid-solid is established. By solving the conservation equations of mass, energy and momentum, the heat transfer field, stress field and seepage field parameters of each time step of the model are updated; s2. Construction and expansion of virtual fracture grid: Based on the model stress field parameters updated in step s1, the pre-set rock fracture criterion is used to determine whether a fracture is generated. If a new fracture is generated, the corresponding virtual fracture grid is constructed and expanded; s3. Cap rock integrity evaluation: After each time step, the cap rock integrity is evaluated to predict the safety of CO2 storage; the numerical simulation of the next time step is continued until the simulation is completed.

2. A CO2 storage multi-field coupling safety assessment method considering crack evolution as claimed in claim 1, characterized in that: In step s1, the coupling relationship between the generation and expansion of fractures, the heat transfer between the injected CO2 and the reservoir and reservoir fluid, the CO2 seepage and the geological stress is considered, and the coupling method is: For the mass conservation equation: ； In the formula, is the simulation duration, in seconds; is the fluid pore volume, dimensionless; is the density of phase j, in kg / m 3 , where j is w, o or g, representing water phase, oil phase or gas phase, respectively; is the j-phase saturation, dimensionless; is the total number of matrix and fracture grids adjacent to the current matrix or fracture grid; is the source and sink term of phase j, in m 3 / s; It represents the velocity difference of phase j between the current grid and its adjacent grid, in m / s; It represents the mass internal energy of phase j, in J / kg; is the volume internal energy of the inert rock matrix, in J / m 3 ; is the mass enthalpy of phase j, in J / kg; is the comprehensive thermal conductivity of the saturated rock matrix, in W / (m·K); is the temperature difference between the current grid and its adjacent grid, in K; For the energy conservation equation: ； In the formula, is the mass fraction of component c in phase j, dimensionless; is the difference in mass fraction of component c in phase j between the current grid and the adjacent grid, dimensionless; For the momentum conservation equation: ； In the formula, is Poisson's ratio, dimensionless; is the Biot coefficient, dimensionless; is the total pore pressure of the current grid, in kPa; is the linear thermal expansion coefficient, in 1 / ℃ or 1 / K; is the bulk modulus, in kPa; is the body force, in N / m 3 ; is the temperature in °C or K; is the normal average stress, in kPa, The calculation method is: ； In the formula, , and They are the maximum principal stress, intermediate principal stress and minimum principal stress respectively, and the unit is kPa.

3. A CO2 storage multi-field coupling safety assessment method considering crack evolution as claimed in claim 1, characterized in that: In step s2, the steps of constructing and expanding the corresponding virtual fracture grid specifically include: s21. Add a new mesh as a virtual fracture mesh in the same layer of the matrix mesh where the fracture is generated. The permeability and porosity of the newly added virtual fracture mesh are equal to the permeability and porosity of the newly generated fracture. s22. Establishing a mass, energy and momentum coupling relationship between the newly added virtual crack grid and the matrix grid generating the crack; s23. If the matrix grid where the newly generated fractures are generated contains source and sink terms, in order to ensure the energy and mass exchange between the source and sink terms and the newly generated fractures, the same source and sink terms are also added to the newly added virtual fracture grid; s24. If the matrix grid of the newly generated fracture is adjacent to the matrix grid of the existing fracture, energy and mass exchange is established between the newly added virtual fracture grid and the existing virtual fracture grid, and the establishment method is the same as step s22.

4. A CO2 storage multi-field coupling safety assessment method considering crack evolution as claimed in claim 1, characterized in that: In step s3, the cap layer integrity evaluation standard is: if cracks are detected in the cap layer, causing CO2 to break through the cap layer and migrate upward, the cap layer integrity is determined to be failed.

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