A method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium

Abstract

The application discloses a method for quickly determining a diffusion coefficient of CO2 in a saturated oil-water porous medium. The diffusion coefficient of CO2 in a pure oil phase and a pure water phase is obtained through a diffusion experiment, and the diffusion coefficient of CO2 in a saturated pure oil phase and a saturated pure water phase porous medium is determined according to the diffusion coefficient. A diffusion model of the saturated oil phase and the porous medium saturated with the single-phase fluid is constructed, relevant parameters and conditions are set, and the diffusion amount change curve of the model is solved after grid division. The diffusion coefficient of CO2 in the saturated oil-water porous medium is obtained by changing the diffusion coefficient value to fit the curve to a required fitting degree. The method is reliable in principle, simple in operation, can quickly calculate the diffusion coefficient under different water saturation degrees, greatly saves time and economic cost compared with a traditional experimental method, conforms to an industry standard, is beneficial to industrial laboratory popularization, and provides important basic data for CO2 related projects.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas field development, and in particular to a method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium. Background Art

[0002] It is widely believed that the primary cause of rising global temperatures is greenhouse gas emissions from human activities. CO2 is the largest contributor to the greenhouse effect, and flue gas from fossil fuel plants is the largest source of CO2 emissions, accounting for 30% of total emissions. Therefore, controlling flue gas emissions from fossil fuel plants is an important approach to addressing the greenhouse effect. Furthermore, geological storage of CO2 is a potential way to reduce greenhouse gas emissions and address global climate change. Carbon capture, utilization, and storage (CCUS) is considered the most effective solution to mitigate global warming. CO2-enhanced oil recovery (EOR) technology can both reduce CO2 emissions and increase oil recovery, and has been widely used in oilfield development. Therefore, CO2-EOR is considered the most promising carbon capture, utilization, and storage technology. CO2-enhanced oil recovery primarily includes CO2 flooding and CO2 huff-and-puff. However, it is worth noting that molecular diffusion is a fundamental and important process in both approaches. CO2 molecules diffuse into the formation fluid, causing the crude oil to expand, reducing its viscosity and extracting lighter components. This reduces interfacial tension (IFT), significantly improving volumetric sweep efficiency and oil displacement efficiency, thereby increasing oil recovery. The gas mass transfer rate, defined as the diffusion coefficient, is the most fundamental parameter determining the mixing rate of injected CO2 with reservoir fluids during CO2 enhanced oil recovery. Furthermore, in numerical simulations of production performance predictions, the CO2 diffusion coefficient is crucial to the accuracy of the final prediction results. Therefore, it is essential to quickly and accurately determine the CO2 diffusion coefficient to accurately predict the dynamic behavior of CO2 development projects.

[0003] In actual underground reservoirs, there is not only crude oil but also formation water, such as bound water, injected water, and bottom water. Therefore, the CO2 injected in tertiary oil recovery should diffuse in porous media saturated with oil and water. Therefore, it is crucial to accurately obtain the diffusion coefficient of CO2 in porous media saturated with oil and water. Currently, there are two methods for measuring the diffusion coefficient of CO2 in the laboratory: direct method and indirect method. The direct method directly calculates the diffusion coefficient by extracting the oil sample in which CO2 is dissolved from the diffusion system during the test and performing component analysis. This method has large errors, high costs, and cumbersome operation procedures, so it is not suitable for measuring the diffusion coefficient of CO2 in porous media. The indirect method indirectly calculates the diffusion coefficient by measuring the changes in system parameters. Compared with the direct method, the indirect method is relatively low in cost, so it is often used to determine the diffusion coefficient in the laboratory.

[0004] The invention patent with patent number ZL202410011394.4 discloses a method for calculating the diffusion coefficient of CO2 in liquid phase under quasi-equilibrium boundary conditions. According to the CO2 diffusion experimental device, a diffusion experiment of CO2 in liquid phase is carried out, the experimental data is recorded, and the experimental pressure change curve is obtained; a physical three-dimensional model of CO2 diffusion is constructed, and the simulation parameters, initial values ​​of the diffusion coefficient, boundary conditions and initial conditions of the physical three-dimensional model of diffusion are determined to solve the concentration field distribution of CO2 at different times and spaces, and obtain the numerical simulation pressure change curve. By changing the CO2 diffusion coefficient in the numerical simulation process, the experimental pressure change curve and the numerical simulation pressure change curve are fitted. When the degree of fitting meets the requirements, the CO2 diffusion coefficient under the conditions is determined to be the actual diffusion coefficient. The simulation results can better meet the actual situation and fully meet the industry standard analysis and testing requirements, and are suitable for promotion and application in industrial laboratories.

[0005] However, before experimental measurements can be made, water saturation must be established in the core, and confining pressure must be applied during the experiment to simulate actual conditions. This process is time-consuming and cumbersome. Furthermore, the water saturation in the formation varies at different stages of development, requiring the determination of the CO2 diffusion coefficient in porous media at different water saturations. This further increases the workload and difficulty of diffusion coefficient measurement. Summary of the Invention

[0006] To address these issues, the present invention provides a method for rapidly determining the diffusion coefficient of CO2 in oil-water-saturated porous media. This method can rapidly calculate the diffusion coefficient of CO2 in porous media at varying water saturations. This method offers the advantages of a reliable principle, convenient calculation, simple testing, short test times, and high reliability in calculating the diffusion coefficient.

[0007] The embodiment of the present invention provides a method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium, comprising the following steps:

[0008] S1. Determine the diffusion coefficient of CO2 in pure oil phase and pure water phase through diffusion experiments;

[0009] S2. Determine the diffusion coefficient of CO2 in the porous medium saturated with pure oil phase and pure water phase based on the experimental data in step S1;

[0010] S3. Construct a porous medium diffusion model of saturated oil and water for CO2 diffusion;

[0011] S4. Construct a porous media diffusion model for CO2 diffusion that is only saturated with a single-phase fluid;

[0012] S5. Setting the initial value of the diffusion coefficient of CO2 in the porous medium saturated with only a single-phase fluid;

[0013] S6. Setting the initial conditions and boundary conditions of the porous media diffusion model constructed in steps S3 and S4;

[0014] S7, meshing the porous medium diffusion model obtained in steps S3 and S4;

[0015] S8. Solve the CO2 diffusion model for the porous medium saturated with oil and water obtained in step S3 based on the diffusion coefficient values, boundary conditions, and initial conditions obtained in steps S2 and S6, obtain the concentration field distribution of CO2 at different times and in different spaces, and obtain a diffusion amount change curve of CO2 in the porous medium saturated with oil and water;

[0016] S9. Solve the CO2 diffusion model obtained in step S4 for the porous medium saturated with a single-phase fluid based on the initial value of the diffusion coefficient, boundary conditions, and initial conditions obtained in steps S5 and S6, obtain the concentration field distribution of CO2 at different times and in different spaces, and obtain a diffusion amount change curve of CO2 in the porous medium saturated with a single-phase fluid;

[0017] S10. According to the diffusion change curve of CO2 in the porous medium saturated with oil and water obtained in step S8 and the diffusion change curve of CO2 in the porous medium saturated with only single-phase fluid obtained in step S9, fitting is performed by changing the diffusion coefficient value in step S5. When the fitting degree meets the requirements, the CO2 diffusion coefficient under this condition is determined to be the diffusion coefficient of CO2 in the porous medium saturated with oil and water.

[0018] The diffusion experiment in step S1 can refer to a method for calculating the diffusion coefficient of CO2 in a liquid phase under quasi-equilibrium boundary conditions in Chinese patent application number ZL202410011394.4.

[0019] In step S2, the diffusion coefficient of CO2 in the porous medium saturated with pure oil phase and pure water phase can be obtained by the diffusion coefficient of CO2 in the pure oil phase and pure water phase. The specific calculation formula is as follows:

[0020]

[0021] Where: D is the diffusion coefficient of CO2 in pure oil phase and pure water phase, m 2 / s;D eff is the diffusion coefficient of CO2 in porous media saturated with pure oil phase and pure water phase, m 2 / s; Γ is the tortuosity of the porous medium.

[0022] In step S3, the CO2 diffusion model of the porous medium saturated with oil and water is established by extracting the porous medium image after water displacement in the microscopic glass etching model;

[0023] In step S4, the porous medium diffusion model for CO2 diffusion that is saturated with only a single-phase fluid is established by deleting the water phase from the porous medium diffusion model saturated with oil and water obtained in step S3;

[0024] In step S5, the initial value of the diffusion coefficient of CO2 in the porous medium saturated with only a single-phase fluid is set to the diffusion coefficient value of CO2 in the porous medium saturated with a pure oil phase.

[0025] In step S6, the left boundary condition is a constant concentration boundary condition, and the distribution law is satisfied at the oil-water boundary, that is, the CO2 concentration is discontinuous at the oil-water interface, and the CO2 concentration in the water phase is significantly lower than that in the oil phase. The extent of the reduction depends on the value of the distribution coefficient. Essentially, this is caused by the different solubility of carbon dioxide in the oil phase and the water phase. The CO2 distribution coefficient can be obtained by using commercial phase equilibrium software (CMG WinProp, 2013). The remaining boundary conditions are no-flux boundary conditions. At the initial moment, the CO2 concentration in the porous medium is 0. The oil-water distribution coefficient calculation formula is as follows:

[0026]

[0027] Among them, k pc is the CO2 concentration distribution coefficient at the oil-water interface; c w is the concentration of CO2 in the water phase at the oil-water interface, mol / m 3 ;c o is the concentration of CO2 in the oil phase at the oil-water interface, mol / m 3 .

[0028] In step S7, the grid is divided into triangular grid units;

[0029] In step S8, the constitutive equation for CO2 diffusion simulation is obtained based on Fick's law, and the calculation formula is as follows:

[0030]

[0031] Where C represents the concentration of CO2, mol / m 3 ; t is the diffusion time, s; D is the diffusion coefficient of CO2, m 2 / s.

[0032] In step S8, when CO2 diffuses in the porous medium, the formula for calculating the diffusion amount of CO2 in the porous medium at any time is as follows:

[0033]

[0034] Where M is the diffusion amount of CO2 in the porous medium at time t, mol; x min、x max 、y min and y max are the positions of the four end points of the porous medium, m.

[0035] In step S9, the solution method is the same as that in step S8;

[0036] In step S10, the diffusion coefficient value in step S5 is changed to fit the diffusion amount change curve obtained in step S8 and step S9. When the goodness of fit R 2 When it is >0.95, the required degree of fit is considered to be achieved, and the CO2 diffusion coefficient under this condition is determined to be the diffusion coefficient of CO2 in saturated oil-water porous media.

[0037] The embodiments of the present invention have at least the following advantages or beneficial effects:

[0038] 1. This paper provides a complete process for determining the diffusion coefficient of CO2 in saturated oil-water porous media. From basic data acquisition to model construction, calculation and verification, each step is closely linked, ensuring the scientific nature and reliability of the entire process, laying the foundation for accurately obtaining the diffusion coefficient, and helping to promote the research and application of the diffusion behavior of CO2 in complex media in related fields.

[0039] 2. The present invention involves constructing a porous media diffusion model for saturated oil and water and saturated single-phase fluid. It can compare the diffusion of CO2 in different media environments, comprehensively consider the influence of multiple factors, deeply analyze the diffusion mechanism of CO2 in saturated oil and water porous media, improve the accuracy and comprehensiveness of the diffusion coefficient determination, and provide more valuable reference data for engineering practice.

[0040] 3. The present invention calculates the diffusion coefficient of CO2 in porous media saturated with pure oil phase and pure water phase through a specific formula, taking into account the key factor of porous medium tortuosity. It can more accurately reflect the influence of the actual porous medium structure on the diffusion process, making the calculation results closer to the actual situation, and enhancing the applicability and scientificity of the entire method under different porous media conditions.

[0041] 4. The present invention uses the image of the porous medium after water displacement in the microscopic glass etching model to establish a diffusion model. It can highly restore the microstructure of the porous medium and the oil-water distribution state in the actual formation, making the model more representative and realistic, so that the determined diffusion coefficient can more accurately reflect the diffusion characteristics in the actual formation, providing a reliable theoretical basis for practical applications such as oil field development.

[0042] 5. This invention establishes a diffusion model of saturated single-phase fluid by deleting the water phase from the saturated oil-water model, which facilitates comparative study with the saturated oil-water model and clearly demonstrates the effect of the presence or absence of the water phase on CO2 diffusion. This contributes to a deeper understanding of the diffusion law of CO2 in oil-water multiphase systems, further improves the accuracy of diffusion coefficient determination, and provides an important comparative basis for CO2-related research in complex oil-water systems.

[0043] 6. The present invention uses the diffusion coefficient of CO2 in a porous medium saturated with pure oil phase as the initial value of the saturated single-phase fluid model, providing a reasonable starting point for subsequent fitting calculations, reducing calculation blindness, improving calculation efficiency, and ensuring that the final diffusion coefficient is within a reasonable numerical range, which helps to quickly and accurately converge to the correct diffusion coefficient value.

[0044] 7. The boundary conditions in the present invention are clearly defined, including a constant concentration boundary condition, an oil-water distribution law that conforms to physical reality, and a no-flux boundary condition. These conditions can accurately simulate the actual boundary conditions of CO2 diffusion in the formation, making the model calculation more consistent with the actual physical process. This greatly improves the accuracy of the diffusion coefficient calculation, provides key conditions for accurately evaluating the diffusion behavior of CO2 in saturated oil-water porous media, and has important guiding significance for prediction and decision-making in engineering practice.

[0045] 8. This paper simulates CO2 diffusion based on the constitutive equation of Fick's law, adhering to the physical nature of the simulation and ensuring the scientific and rationality of the simulation. The diffusion calculation formula provided can accurately calculate the diffusion volume at any time, providing accurate data for subsequent analysis. This will facilitate in-depth research on the dynamic process of CO2 diffusion in porous media, reveal the diffusion laws, and provide theoretical support for applications such as optimizing oilfield development.

[0046] 9. Steps S9 and S8 in the present invention use a consistent solution method, ensuring the consistency and comparability of the calculation processes of the two models. This facilitates comparative analysis of CO2 diffusion under different models, reduces errors caused by differences in solution methods, improves the accuracy and reliability of the entire method, and helps to more accurately determine the diffusion coefficient of CO2 in saturated oil-water porous media.

[0047] 10. The present invention sets a higher goodness of fit (R 2 >0.95) as the judgment condition, ensuring that the final CO2 diffusion coefficient is highly reliable and accurate, meeting the strict requirements of engineering practice and scientific research on accuracy, and providing reliable data support for related engineering applications such as the study of CO2 diffusion behavior in saturated oil-water porous media, numerical simulation, and oilfield development, thus ensuring the scientific and rationality of decision-making.

[0048] In summary, the present invention simulates the diffusion of CO2 in the established porous medium model and calculates the diffusion coefficient of CO2 in the saturated oil-water porous medium. It can truly simulate the actual diffusion of CO2 in the saturated oil-water porous medium. By visualizing the CO2 diffusion process, the CO2 concentration field distribution image at different times and positions in the porous medium can be obtained. In addition, compared with traditional experimental methods, the present invention can greatly save time and economic costs. In addition, the present invention can also quickly calculate the diffusion coefficient of CO2 in porous media at different water saturations, and can quickly provide basic data for the diffusion of CO2 in water-bearing formations at different development stages of oil fields. At the same time, the numerical simulation of the present invention can be carried out under high temperature and high pressure, and can simulate the actual situation of high temperature and high pressure formations. Therefore, the present invention fully meets the industry standardization analysis and testing requirements and is suitable for promotion and application in industrial laboratories. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0050] Figure 1 is a flow chart of the present invention;

[0051] Figure 2 The porous medium diffusion model of saturated oil and water constructed in the specific embodiment 1 of the present invention;

[0052] Figure 3 The porous medium diffusion model for saturated single-phase fluid constructed in the specific embodiment 1 of the present invention;

[0053] Figure 4 A grid division diagram of the oil-water saturated porous medium diffusion model constructed in specific embodiment 1 of the present invention;

[0054] Figure 5 A grid division diagram of the porous medium diffusion model for saturated single-phase fluid constructed in specific embodiment 1 of the present invention;

[0055] Figure 6 This is a diagram showing the fitting of a diffusion curve of CO2 in a porous medium saturated with oil and water and a diffusion curve of CO2 in a porous medium saturated with only a single-phase fluid in Example 1 of the present invention;

[0056] Figure 7This is a comparison chart of the diffusion coefficient of CO2 in porous media with different water saturations calculated by the present invention in specific embodiment 2 of the present invention and the diffusion coefficient in the published literature (Wang and Hou (2021)). DETAILED DESCRIPTION

[0057] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0058] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are intended to fall within the scope of protection of the present invention.

[0059] like Figures 1 to 7 As shown, a specific embodiment of the present invention provides a method for quickly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium, comprising the following steps:

[0060] S1. Determine the diffusion coefficient of CO2 in pure oil phase and pure water phase through diffusion experiments;

[0061] S2. Determine the diffusion coefficient of CO2 in the porous medium saturated with pure oil phase and pure water phase based on the experimental data in step S1;

[0062] S3. Construct a porous medium diffusion model of saturated oil and water for CO2 diffusion;

[0063] S4. Construct a porous media diffusion model for CO2 diffusion that is only saturated with a single-phase fluid;

[0064] S5. Setting the initial value of the diffusion coefficient of CO2 in the porous medium saturated with only a single-phase fluid;

[0065] S6. Setting the initial conditions and boundary conditions of the porous media diffusion model constructed in steps S3 and S4;

[0066] S7, meshing the porous medium diffusion model obtained in steps S3 and S4;

[0067] S8. Solve the CO2 diffusion model for the porous medium saturated with oil and water obtained in step S3 based on the diffusion coefficient values, boundary conditions, and initial conditions obtained in steps S2 and S6, obtain the concentration field distribution of CO2 at different times and in different spaces, and obtain a diffusion amount change curve of CO2 in the porous medium saturated with oil and water;

[0068] S9. Solve the CO2 diffusion model obtained in step S4 for the porous medium saturated with a single-phase fluid based on the initial value of the diffusion coefficient, boundary conditions, and initial conditions obtained in steps S5 and S6, obtain the concentration field distribution of CO2 at different times and in different spaces, and obtain a diffusion amount change curve of CO2 in the porous medium saturated with a single-phase fluid;

[0069] S10. According to the diffusion change curve of CO2 in the porous medium saturated with oil and water obtained in step S8 and the diffusion change curve of CO2 in the porous medium saturated with only single-phase fluid obtained in step S9, fitting is performed by changing the diffusion coefficient value in step S5. When the fitting degree meets the requirements, the CO2 diffusion coefficient under this condition is determined to be the diffusion coefficient of CO2 in the porous medium saturated with oil and water.

[0070] Example 1:

[0071] The diffusion experiment in step S1 can refer to a method for calculating the diffusion coefficient of CO2 in a liquid phase under quasi-equilibrium boundary conditions in Chinese patent application number ZL202410011394.4.

[0072] In step S2, the diffusion coefficient of CO2 in the porous medium saturated with pure oil phase and pure water phase can be obtained by the diffusion coefficient of CO2 in the pure oil phase and pure water phase. The specific calculation formula is as follows:

[0073]

[0074] Where: D is the diffusion coefficient of CO2 in pure oil phase and pure water phase, m 2 / s;D eff is the diffusion coefficient of CO2 in porous media saturated with pure oil phase and pure water phase, m 2 / s; Γ is the tortuosity of the porous medium.

[0075] In step S3, the CO2 diffusion model of the porous medium saturated with oil and water is established by extracting the porous medium image after water displacement in the microscopic glass etching model;

[0076] In step S4, the porous medium diffusion model for CO2 diffusion that is saturated with only a single-phase fluid is established by deleting the water phase from the porous medium diffusion model saturated with oil and water obtained in step S3;

[0077] In step S5, the initial value of the diffusion coefficient of CO2 in the porous medium saturated with only a single-phase fluid is set to the diffusion coefficient value of CO2 in the porous medium saturated with a pure oil phase.

[0078] In step S6, the left boundary condition is a constant concentration boundary condition, and the distribution law is satisfied at the oil-water boundary, that is, the CO2 concentration is discontinuous at the oil-water interface, and the CO2 concentration in the water phase is significantly lower than that in the oil phase. The extent of the reduction depends on the value of the distribution coefficient. Essentially, this is caused by the different solubility of carbon dioxide in the oil phase and the water phase. The CO2 distribution coefficient can be obtained by using commercial phase equilibrium software (CMG WinProp, 2013). The remaining boundary conditions are no-flux boundary conditions. At the initial moment, the CO2 concentration in the porous medium is 0. The oil-water distribution coefficient calculation formula is as follows:

[0079]

[0080] Among them, k pc is the CO2 concentration distribution coefficient at the oil-water interface; c w is the concentration of CO2 in the water phase at the oil-water interface, mol / m 3 ;c o is the concentration of CO2 in the oil phase at the oil-water interface, mol / m 3 .

[0081] In step S7, the grid is divided into triangular grid units;

[0082] In step S8, the constitutive equation for CO2 diffusion simulation is obtained based on Fick's law, and the calculation formula is as follows:

[0083]

[0084] Where C represents the concentration of CO2, mol / m 3 ; t is the diffusion time, s; D is the diffusion coefficient of CO2, m 2 / s.

[0085] In step S8, when CO2 diffuses in the porous medium, the formula for calculating the diffusion amount of CO2 in the porous medium at any time is as follows:

[0086]

[0087] Where M is the diffusion amount of CO2 in the porous medium at time t, mol; x min 、x max 、y min and y max are the positions of the four end points of the porous medium, m.

[0088] In step S9, the solution method is the same as that in step S8;

[0089] In step S10, the diffusion coefficient value in step S5 is changed to fit the diffusion amount change curve obtained in step S8 and step S9. When the goodness of fit R 2 When it is >0.95, the required degree of fit is considered to be achieved, and the CO2 diffusion coefficient under this condition is determined to be the diffusion coefficient of CO2 in saturated oil-water porous media.

[0090] The specific implementation is as follows:

[0091] Step 1: A diffusion experiment was conducted with reference to a method for calculating the diffusion coefficient of CO2 in a liquid phase under quasi-equilibrium boundary conditions, as described in Chinese patent application number ZL202410011394.4. The diffusion coefficients of CO2 in pure oil phase and pure water phase at 6 MPa and 50°C were 7.68×10 -9 m 2 / s and 2.85×10 -10 m 2 / s;

[0092] Step 2: The diffusion coefficient of CO2 in the porous medium saturated with pure oil phase and pure water phase is obtained by the diffusion coefficient of CO2 in pure oil phase and pure water phase. The specific calculation formula is as follows:

[0093]

[0094] Where: D is the diffusion coefficient of CO2 in pure oil phase and pure water phase, m 2 / s;D eff is the diffusion coefficient of CO2 in porous media saturated with pure oil phase and pure water phase, m 2 / s; Γ is the tortuosity of the porous medium.

[0095] The tortuosity of the porous medium is 1.32, and the diffusion coefficients of CO2 in the porous medium saturated with pure oil phase and pure water phase are calculated to be 5.82×10 -9 m 2 / s and 2.16×10 -10 m 2 / s;

[0096] Step 3: The CO2 diffusion model of the porous medium saturated with oil and water is established by extracting the porous medium image after water displacement in the microscopic glass etching model;

[0097] Step 4: A porous medium diffusion model saturated with only a single-phase fluid is obtained by deleting the water phase from the porous medium diffusion model saturated with oil and water obtained in step 3;

[0098] Step 5: The initial value of the diffusion coefficient of CO2 in the porous medium saturated with only single-phase fluid is set to the diffusion coefficient of CO2 in the porous medium saturated with pure oil phase, that is, the initial value of the diffusion coefficient is 5.82×10 -9 m 2 / s;

[0099] Step 6: Determine the boundary conditions and initial conditions for solving the porous media diffusion model. The left boundary condition is a constant concentration boundary condition, and the boundary concentration is set to 1000 mol / m 3 The distribution law is satisfied at the oil-water boundary, meaning that the CO2 concentration is discontinuous at the oil-water interface. The CO2 concentration in the water phase is significantly lower than that in the oil phase, and the magnitude of this reduction depends on the distribution coefficient. The CO2 distribution coefficient was calculated using commercial phase equilibrium software (CMG WinProp, 2013) and is 3.32. The remaining boundary conditions are no-flux boundary conditions. Initially, the CO2 concentration in the porous medium is zero.

[0100] Step 7: Mesh the porous media diffusion model into triangular mesh units;

[0101] Step 8: Use finite element analysis software to solve the diffusion model of the porous medium saturated with oil and water, and obtain the concentration field distribution of CO2 at different times and spaces. By integrating the CO2 concentration, the diffusion curve of CO2 in the porous medium saturated with oil and water at any time is obtained. The specific calculation formula is as follows:

[0102]

[0103] Where M is the diffusion amount of CO2 in the porous medium at time t, mol; x min 、x max 、y min and y max are the positions of the four end points of the porous medium, m.

[0104] Step 9: Use finite element analysis software to solve the porous medium diffusion model saturated with only single-phase fluid to obtain the concentration field distribution of CO2 at different times and in different spaces. By integrating the CO2 concentration, the diffusion curve of CO2 in the porous medium saturated with only single-phase fluid at any time is obtained.

[0105] Step 10: By changing the diffusion coefficient value in step 5, the diffusion change curve obtained in step 9 is continuously changed, so that the diffusion change curve obtained in step 9 is continuously close to the diffusion change curve obtained in step 8. The fitting result of the diffusion change curve is as follows: Figure 6 As shown, the goodness of fit R 2=0.9995>0.95, it is considered that the required fitting accuracy has been achieved. The CO2 diffusion coefficient at this time is 2.4×10 -9 m 2 / s is the diffusion coefficient of CO2 in the oil-water saturated core. It should be noted that the water saturation in the porous medium is 22.4% at this time.

[0106] Example 2:

[0107] In order to further illustrate the feasibility and accuracy of the diffusion coefficient determination method proposed in this invention, the diffusion coefficient of CO2 in porous media at different water saturations was further calculated using data from published literature (Wang and Hou (2021)), and the calculated results were compared and verified with the data tested in the published literature.

[0108] The specific implementation is as follows:

[0109] Step 1: Since the diffusion coefficient of CO2 in saturated pure oil phase and pure water phase was experimentally tested in the published literature (Wang and Hou (2021)), step 1 can be omitted in specific embodiment 2.

[0110] Step 2: Based on the experimental data tested in the published literature (Wang and Hou (2021)), the diffusion coefficients of CO2 in porous media saturated with pure oil phase and pure water phase at 50 MPa and 393 K were 6.16×10 -9 m 2 / s and 1.48×10 -10 m 2 / s;

[0111] Step 3: The CO2 diffusion model for oil-water saturated porous media was constructed by extracting images of the porous media after water displacement from the microscopic glass etching model. It is important to note that the water saturation in the porous media was 22.4% at this point. When simulating different water saturations, water phases were randomly generated within the porous media to simulate these conditions. This random generation ensures that the distribution of the water phase within the porous media is random, representing the actual oil-water distribution in the subsurface.

[0112] Step 4: A porous medium diffusion model saturated with only a single-phase fluid is obtained by deleting the water phase from the porous medium diffusion model saturated with oil and water obtained in step 3;

[0113] Step 5: The initial value of the diffusion coefficient of CO2 in a porous medium saturated with a single-phase fluid is set to the diffusion coefficient of CO2 in a porous medium saturated with a pure oil phase, that is, the initial value of the diffusion coefficient is 6.16×10 -9 m 2 / s;

[0114] Step 6: Determine the boundary conditions and initial conditions for solving the porous media diffusion model. The left boundary condition is a constant concentration boundary condition, and the boundary concentration is set to 1000 mol / m 3 The distribution law is satisfied at the oil-water boundary, meaning that the CO2 concentration is discontinuous at the oil-water interface. The CO2 concentration in the water phase is significantly lower than that in the oil phase, and the magnitude of this reduction depends on the distribution coefficient. The CO2 distribution coefficient was calculated using commercial phase equilibrium software (CMG WinProp, 2013) and is 3.08. The remaining boundary conditions are no-flux boundary conditions. Initially, the CO2 concentration in the porous medium is zero.

[0115] Step 7: Mesh the porous media diffusion model into triangular mesh units;

[0116] Step 8: Use finite element analysis software to solve the diffusion model of the porous medium saturated with oil and water, and obtain the concentration field distribution of CO2 at different times and spaces. By integrating the CO2 concentration, the diffusion curve of CO2 in the porous medium saturated with oil and water at any time is obtained. The specific calculation formula is as follows:

[0117]

[0118] Where M is the diffusion amount of CO2 in the porous medium at time t, mol; x min 、x max 、y min and y max are the positions of the four end points of the porous medium, m.

[0119] Step 9: Use finite element analysis software to solve the porous medium diffusion model saturated with only single-phase fluid to obtain the concentration field distribution of CO2 at different times and in different spaces. By integrating the CO2 concentration, the diffusion curve of CO2 in the porous medium saturated with only single-phase fluid at any time is obtained.

[0120] Step 10: By changing the diffusion coefficient value in step 5, the diffusion change curve obtained in step 9 is continuously changed, so that the diffusion change curve obtained in step 9 is continuously close to the diffusion change curve obtained in step 8. Finally, when the goodness of fit R 2 When the CO2 diffusion coefficient is greater than 0.95, the required fitting accuracy is considered to have been achieved. The CO2 diffusion coefficient at this time is the diffusion coefficient of CO2 in the saturated oil-water core under a certain water saturation. The final water saturation values ​​of 10.74%, 22.36%, 33.52%, 38.06%, 47%, 53%, 61.66%, 66.19%, 77.35% and 88.94% respectively, and the CO2 diffusion coefficient in the saturated oil-water porous medium is 4.5×10-9 m 2 / s, 3×10 -9 m 2 / s, 2.5×10 -9 m 2 / s, 1.9×10 -9 m 2 / s, 1.7×10 -9 m 2 / s, 1.02×10 -9 m 2 / s, 0.81×10 -9 m 2 / s, 0.63×10 -9 m 2 / s, 0.51×10 -9 m 2 / s and 0.25×10 -9 m 2 / s. The diffusion coefficient of CO2 in porous media with different water saturations calculated by the present invention is compared with the diffusion coefficient in the published literature (Wang and Hou (2021)). Figure 7 As shown, the CO2 coefficient change curve calculated by the present invention is consistent with the diffusion coefficient change curve in the published literature (Wang and Hou (2021)), which further illustrates the feasibility and accuracy of the diffusion coefficient determination method proposed in the present invention.

[0121] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

[0122] It will be apparent to those skilled in the art that the present application is not limited to the details of the exemplary embodiments described above, and that the present application can be implemented in other specific forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the present application is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

Claims

1. A method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium, characterized in that The steps include: S1. Determine the diffusion coefficient of CO2 in pure oil phase and pure water phase through diffusion experiments; S2. Determine the diffusion coefficient of CO2 in the porous medium saturated with pure oil phase and pure water phase based on the experimental data in step S1; S3. Construct a porous medium diffusion model of saturated oil and water for CO2 diffusion; S4. Construct a porous media diffusion model for CO2 diffusion that is only saturated with a single-phase fluid; S5. Setting the initial value of the diffusion coefficient of CO2 in the porous medium saturated with only a single-phase fluid; S6. Setting the initial conditions and boundary conditions of the porous media diffusion model constructed in steps S3 and S4; S7, meshing the porous medium diffusion model obtained in steps S3 and S4; S8. Solve the CO2 diffusion model for the porous medium saturated with oil and water obtained in step S3 based on the diffusion coefficient values, boundary conditions, and initial conditions obtained in steps S2 and S6, obtain the concentration field distribution of CO2 at different times and spaces, and obtain a diffusion amount change curve of CO2 in the porous medium saturated with oil and water; S9. Solve the CO2 diffusion model obtained in step S4 for the porous medium saturated with a single-phase fluid based on the initial value of the diffusion coefficient, boundary conditions, and initial conditions obtained in steps S5 and S6, obtain the concentration field distribution of CO2 at different times and spaces, and obtain a diffusion amount change curve of CO2 in the porous medium saturated with a single-phase fluid; S10. According to the diffusion change curve of CO2 in the porous medium saturated with oil and water obtained in step S8 and the diffusion change curve of CO2 in the porous medium saturated with only single-phase fluid obtained in step S9, fitting is performed by changing the diffusion coefficient value in step S5. When the fitting degree meets the requirements, the CO2 diffusion coefficient under this condition is determined to be the diffusion coefficient of CO2 in the porous medium saturated with oil and water.

2. A method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium according to claim 1, characterized in that: In step S2, the diffusion coefficient of CO2 in the porous medium saturated with pure oil phase and pure water phase can be obtained by the diffusion coefficient of CO2 in the pure oil phase and pure water phase. The specific calculation formula is as follows: Where: D is the diffusion coefficient of CO2 in pure oil phase and pure water phase, m 2 / s;D eff is the diffusion coefficient of CO2 in porous media saturated with pure oil phase and pure water phase, m 2 / s; Γ is the tortuosity of the porous medium.

3. The method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium according to claim 1, characterized in that: In step S3, the CO2 diffusion model of the porous medium saturated with oil and water is established by extracting the porous medium image after water displacement from the microscopic glass etching model.

4. The method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium according to claim 1, characterized in that: In step S4, a porous medium diffusion model for CO2 diffusion that is saturated with only a single-phase fluid is established by deleting the water phase from the porous medium diffusion model for saturated oil and water obtained in step S3.

5. The method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium according to claim 1, characterized in that: In step S5, the initial value of the diffusion coefficient of CO2 in the porous medium saturated with only a single-phase fluid is set to the diffusion coefficient value of CO2 in the porous medium saturated with a pure oil phase.

6. The method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium according to claim 1, characterized in that: In step S6, the left boundary condition is a constant concentration boundary condition, and the distribution law is satisfied at the oil-water boundary, that is, the CO2 concentration is discontinuous at the oil-water interface. The CO2 concentration in the water phase is significantly lower than that in the oil phase. The extent of the reduction depends on the value of the distribution coefficient. Essentially, this is caused by the different solubility of carbon dioxide in the oil phase and the water phase. The CO2 distribution coefficient is obtained by using commercial phase equilibrium software, CMG WinProp, 2013. The remaining boundary conditions are no-flux boundary conditions. At the initial moment, the CO2 concentration in the porous medium is 0. The oil-water distribution coefficient is calculated as follows: Among them, k pc is the CO2 concentration distribution coefficient at the oil-water interface; c w is the concentration of CO2 in the water phase at the oil-water interface, mol / m 3 ;c o is the concentration of CO2 in the oil phase at the oil-water interface, mol / m 3 .

7. The method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium according to claim 1, characterized in that: In step S8, the constitutive equation for CO2 diffusion simulation is obtained based on Fick's law, and the calculation formula is as follows: Where C represents the concentration of CO2, mol / m 3 ; t is the diffusion time, s; D is the diffusion coefficient of CO2, m 2 / s, In step S8, when CO2 diffuses in the porous medium, the formula for calculating the diffusion amount of CO2 in the porous medium at any time is as follows: Where M is the diffusion amount of CO2 in the porous medium at time t, mol; x min 、x max 、y min and y max are the positions m of the four end points of the porous medium.

8. The method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium according to claim 1, characterized in that: The solution method of step S9 is consistent with that of step S8.

9. The method for rapidly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium according to claim 1, characterized in that: In step S10, the diffusion coefficient value in step S5 is changed to fit the diffusion amount change curve obtained in step S8 and step S9. When the goodness of fit R 2 When it is >0.95, the required degree of fit is considered to be achieved, and the CO2 diffusion coefficient under this condition is determined to be the diffusion coefficient of CO2 in saturated oil-water porous media.

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