Method for rapidly determining diffusion coefficient of CO2 in saturated oil-water porous medium

Abstract

The invention discloses a method for rapidly determining a diffusion coefficient of CO2 in a saturated oil-water porous medium. Firstly, diffusion coefficients of CO2 in a pure oil phase and a pure water phase are obtained through a diffusion experiment, and accordingly the diffusion coefficients of CO2 in a saturated pure oil phase porous medium and a pure water phase porous medium are determined; a porous medium diffusion model of saturated oil phase and only saturated single-phase fluid is constructed, relevant parameters and conditions are set, and a diffusion amount change curve of the model is solved after grid division; by changing a diffusion coefficient value fitting curve, the diffusion coefficient of CO2 in the saturated oil-water porous medium is obtained when the required fitting degree is reached. The method is reliable in principle and easy and convenient to operate, diffusion coefficients under different water saturations can be rapidly calculated, compared with a traditional experimental method, time and economic cost are greatly saved, industrial standards are met, industrial laboratory popularization is facilitated, and important basic data are provided for CO2 related projects.

Description

Technical Field

[0001] The 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 generally believed that the main reason for the rise in global average temperature is greenhouse gas emissions caused by human activities. CO2 is the biggest factor causing the greenhouse effect, and the flue gas from fossil fuel plants is the largest source of CO2 emissions, accounting for 30% of the total emissions. Therefore, controlling the flue gas emissions from fossil fuel plants is one of the important ways to solve the greenhouse effect. In addition, geological storage of CO2 is also one of the potential ways to reduce greenhouse gas emissions to cope with global climate change. Carbon capture, utilization and storage (CCUS) is considered to be the most effective solution to mitigate climate warming. CO2 enhanced oil recovery technology (EOR) can not only reduce CO2 emissions, but also increase the recovery rate of oil reservoirs, and has been widely used in oil field development. Therefore, CO2-EOR is considered to be the most promising carbon capture, utilization and storage technology. CO2 enhanced oil recovery mainly includes two methods: CO2 flooding and CO2 huff and puff. But it is worth mentioning that molecular diffusion is a basic and important process in both methods. CO2 molecules diffuse into the formation fluid, causing crude oil to expand, reducing crude oil viscosity, extracting light components, reducing interfacial tension (IFT), greatly improving volumetric sweep efficiency and oil displacement efficiency, and thus increasing recovery. The gas mass transfer rate is defined as the diffusion coefficient, which is the most basic parameter that determines the mixing rate of injected CO2 and reservoir liquid during CO2 enhanced oil recovery. And in the numerical simulation of production dynamic prediction, the CO2 diffusion coefficient is crucial to the accuracy of the final prediction results. Therefore, it is necessary to quickly and accurately obtain the CO2 diffusion coefficient to achieve accurate prediction of 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 by 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. At present, there are two methods for measuring the diffusion coefficient of CO2 in laboratories: direct method and indirect method. The direct method directly obtains the diffusion coefficient by extracting the oil sample after CO2 dissolution from the diffusion system during the test and performing component analysis. This method has large errors, high costs, and cumbersome operation processes, 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 has a relatively low cost, so it is often used in the laboratory to determine the diffusion coefficient.

[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, and the concentration field distribution of CO2 at different times and spaces is solved to obtain the numerical simulation pressure change curve. The experimental pressure change curve and the numerical simulation pressure change curve are fitted by changing the CO2 diffusion coefficient in the numerical simulation process. When the degree of fit 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 the experimental measurement, it is necessary to establish water saturation in the core, and to simulate the actual situation, confining pressure needs to be added during the experiment, which is time-consuming and cumbersome. In addition, the water saturation in the formation changes at different development stages, so it is necessary to determine the diffusion coefficient of CO2 in porous media at different water saturations, which further increases the workload and difficulty of diffusion coefficient measurement. Summary of the invention

[0006] In view of the above problems, the present invention provides a method for quickly determining the diffusion coefficient of CO2 in a saturated oil-water porous medium, which can quickly calculate the diffusion coefficient of CO2 in a porous medium at different water saturations. The method has the advantages of reliable principle, convenient calculation, simple test, short time consumption, and high reliability of calculating the diffusion coefficient.

[0007] The 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:

[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 according to 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 medium diffusion model constructed in steps S3 and S4;

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

[0015] S8, solving the porous medium diffusion model of CO2 saturated with oil and water obtained in step S3 according to the diffusion coefficient values, boundary conditions and initial conditions obtained in steps S2 and S6, obtaining the concentration field distribution of CO2 at different times and in different spaces, and obtaining the diffusion amount change curve of CO2 in the porous medium saturated with oil and water;

[0016] S9, solving the porous medium diffusion model of CO2 diffusion in the saturated single-phase fluid obtained in step S4 according to the initial value of the diffusion coefficient, boundary conditions and initial conditions obtained in steps S5 and S6, obtaining the concentration field distribution of CO2 at different times and in different spaces, and obtaining the diffusion amount change curve of CO2 in the porous medium saturated with the single-phase fluid;

[0017] S10. According to the diffusion amount change curve of CO2 in the porous medium saturated with oil and water obtained in step S8 and the diffusion amount 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, it is determined that the CO2 diffusion coefficient under this condition is the diffusion coefficient of CO2 in the porous medium saturated with oil and water.

[0018] In the step S1, the diffusion experiment can refer to a method for calculating the diffusion coefficient of CO2 in a liquid phase under quasi-equilibrium boundary conditions in a Chinese patent with 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 porous medium diffusion model of saturated oil and water for CO2 diffusion 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 of CO2 diffusion saturated with only single-phase fluid is established by deleting the water phase in the porous medium diffusion model saturated with oil and water obtained in step S3;

[0024] In the 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 value of the diffusion coefficient 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 magnitude of the reduction depends on the value of the distribution coefficient. In essence, 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 diffusion amount of CO2 in the porous medium at any time is calculated 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 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, by changing the diffusion coefficient value in step S5 to fit step S8 and step S9 to obtain the diffusion amount change curve, when the goodness of fit R 2 When >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. The present invention 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 scientificity 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 only, which can compare the diffusion of CO2 in different medium environments, comprehensively consider the influence of various factors, deeply analyze the diffusion mechanism of CO2 in saturated oil and water porous media, improve the accuracy and comprehensiveness of the determination of the diffusion coefficient, and provide more valuable reference data for engineering practice.

[0040] 3. The present invention calculates the diffusion coefficient of CO2 in porous media of saturated pure oil phase and pure water phase through a specific formula, taking into account the key factor of porous medium tortuosity, which can more accurately reflect the influence of 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 whole method under different porous media conditions.

[0041] 4. The present invention uses the porous medium image after water-driven oil displacement in the microscopic glass etching model to establish a diffusion model, which can highly restore the porous medium microstructure and 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. The present invention establishes a diffusion model of saturated single-phase fluid only by deleting the water phase in the saturated oil-water model, which is convenient for comparative study with the saturated oil-water model, and clearly shows the influence of the presence or absence of the water phase on the diffusion of CO2. It helps to deeply understand the diffusion law of CO2 in the oil-water multiphase system, further improve the accuracy of the determination of the diffusion coefficient, and provide 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, and improving calculation efficiency. At the same time, it ensures 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 set, including constant concentration boundary conditions, oil-water distribution laws that conform to physical reality, and no-flux boundary conditions. They can accurately simulate the actual boundary conditions of CO2 diffusion in the formation, making the model calculation more in line with the actual physical process, greatly improving the accuracy of diffusion coefficient calculation, and providing key conditions for accurately evaluating the diffusion behavior of CO2 in saturated oil-water porous media, which has important guiding significance for prediction and decision-making in engineering practice.

[0045] 8. The present invention simulates CO2 diffusion based on the constitutive equation of Fick's law, follows the physical essence, and ensures the scientificity and rationality of the simulation calculation. At the same time, the diffusion amount calculation formula given can accurately calculate the diffusion amount at any time, providing an accurate data basis for subsequent analysis, which is helpful for in-depth research on the dynamic process of CO2 diffusion in porous media, revealing the diffusion law, and providing theoretical support for optimizing oil field development and other applications.

[0046] 9. Steps S9 and S8 in the present invention use a consistent solution method, which ensures the consistency and comparability of the calculation processes of the two models, facilitates comparative analysis of the diffusion of CO2 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, providing reliable data support for the diffusion behavior of CO2 in saturated oil-water porous media, numerical simulation, oilfield development and other related engineering applications, and ensuring the scientificity 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, which 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 requirements of industry standardization analysis and testing, 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 drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without 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 only constructed in the specific embodiment 1 of the present invention;

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

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

[0055] Figure 6 It is a fitting diagram of the diffusion amount change curve of CO2 in a porous medium saturated with oil and water and the diffusion amount change curve of CO2 in a porous medium saturated with only a single-phase fluid in the specific embodiment 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] 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, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here 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 claimed for protection, but merely represents 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 creative work are 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 according to 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 medium diffusion model constructed in steps S3 and S4;

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

[0067] S8, solving the porous medium diffusion model of CO2 saturated with oil and water obtained in step S3 according to the diffusion coefficient values, boundary conditions and initial conditions obtained in steps S2 and S6, obtaining the concentration field distribution of CO2 at different times and in different spaces, and obtaining the diffusion amount change curve of CO2 in the porous medium saturated with oil and water;

[0068] S9, solving the porous medium diffusion model of CO2 diffusion in the saturated single-phase fluid obtained in step S4 according to the initial value of the diffusion coefficient, boundary conditions and initial conditions obtained in steps S5 and S6, obtaining the concentration field distribution of CO2 at different times and in different spaces, and obtaining the diffusion amount change curve of CO2 in the porous medium saturated with the single-phase fluid;

[0069] S10. According to the diffusion amount change curve of CO2 in the porous medium saturated with oil and water obtained in step S8 and the diffusion amount 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, it is determined that the CO2 diffusion coefficient under this condition is the diffusion coefficient of CO2 in the porous medium saturated with oil and water.

[0070] Embodiment 1:

[0071] In the step S1, the diffusion experiment can refer to a method for calculating the diffusion coefficient of CO2 in a liquid phase under quasi-equilibrium boundary conditions in a Chinese patent with 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 porous medium diffusion model of saturated oil and water for CO2 diffusion 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 of CO2 diffusion saturated with only single-phase fluid is established by deleting the water phase in the porous medium diffusion model saturated with oil and water obtained in step S3;

[0077] In the 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 value of the diffusion coefficient 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 magnitude of the reduction depends on the value of the distribution coefficient. In essence, 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 diffusion amount of CO2 in the porous medium at any time is calculated 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 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, by changing the diffusion coefficient value in step S5 to fit step S8 and step S9 to obtain the diffusion amount change curve, when the goodness of fit R 2 When >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 in a Chinese patent with patent application number ZL202410011394.4. The diffusion coefficients of CO2 in pure oil phase and pure water phase under 6MPa and 50℃ 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 porous medium diffusion model of CO2 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: The porous medium diffusion model saturated with only 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 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 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, 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 magnitude of the reduction depends on the value of 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. At the initial moment, the CO2 concentration in the porous medium is 0;

[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 in different 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 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 of only saturated single-phase fluid, obtain the concentration field distribution of CO2 at different times and in different spaces, and obtain the diffusion amount change curve of CO2 in the porous medium saturated with only single-phase fluid at any time by integrating the CO2 concentration;

[0105] Step 10: By changing the diffusion coefficient value in step 5, the diffusion amount change curve obtained in step 9 is continuously changed, so that the diffusion amount change curve obtained in step 9 is continuously close to the diffusion amount change curve obtained in step 8. The fitting result of the diffusion amount 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. At this time, the CO2 diffusion coefficient value is 2.4×10 -9 m 2 / s is the diffusion coefficient of CO2 in the saturated oil and water core. It should be noted that the water saturation in the porous medium is 22.4% at this time.

[0106] Embodiment 2:

[0107] In order to further illustrate the feasibility and accuracy of the diffusion coefficient determination method proposed in the present invention, the diffusion coefficient of CO2 in porous media at different water saturations was further calculated using the data in the published literature (Wang and Hou (2021)), and the calculated results were compared and verified with the test data 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 under 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 porous media diffusion model of CO2 diffusion saturated with oil and water is established by extracting the porous media image after water displacement in the microscopic glass etching model. It should be noted that the water saturation in the porous media is 22.4% at this time. When simulating different water saturations, the water phase is randomly generated in the porous media to simulate different water saturations. Random generation can ensure that the distribution of the water phase in the porous media is random and can represent the actual oil and water distribution in a wide range of underground areas.

[0112] Step 4: The porous medium diffusion model saturated with only 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 only 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, 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 magnitude of the reduction depends on the value of 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. At the initial moment, the CO2 concentration in the porous medium is 0;

[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 in different 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 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 of only saturated single-phase fluid, obtain the concentration field distribution of CO2 at different times and in different spaces, and obtain the diffusion amount change curve of CO2 in the porous medium saturated with only single-phase fluid at any time by integrating the CO2 concentration;

[0120] Step 10: By changing the diffusion coefficient value in step 5, the diffusion amount change curve obtained in step 9 is continuously changed, so that the diffusion amount change curve obtained in step 9 is continuously close to the diffusion amount change curve obtained in step 8. Finally, when the goodness of fit R 2 When the fitting accuracy is >0.95, the required fitting accuracy is considered to be 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 is 10.74%, 22.36%, 33.52%, 38.06%, 47%, 53%, 61.66%, 66.19%, 77.35% and 88.94%, and the diffusion coefficient of CO2 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 only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

[0122] It is obvious 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 features of the present application. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects, and the scope of the present application is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present application. Any reference numeral in a claim should not be regarded as limiting the claim to which it relates.

Claims

1. A method for rapidly determining the diffusion coefficient of CO2 in a porous medium saturated with oil and water, 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 according to 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 medium diffusion model constructed in steps S3 and S4; S7, meshing the porous medium diffusion model obtained in steps S3 and S4; S8, solving the porous medium diffusion model of CO2 saturated with oil and water obtained in step S3 according to the diffusion coefficient values, boundary conditions and initial conditions obtained in steps S2 and S6, obtaining the concentration field distribution of CO2 at different times and in different spaces, and obtaining the diffusion amount change curve of CO2 in the porous medium saturated with oil and water; S9, solving the porous medium diffusion model of CO2 diffusion in the saturated single-phase fluid obtained in step S4 according to the initial value of the diffusion coefficient, boundary conditions and initial conditions obtained in steps S5 and S6, obtaining the concentration field distribution of CO2 at different times and in different spaces, and obtaining the diffusion amount change curve of CO2 in the porous medium saturated with the single-phase fluid; S10. According to the diffusion amount change curve of CO2 in the porous medium saturated with oil and water obtained in step S8 and the diffusion amount 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, it is determined that the CO2 diffusion coefficient under this condition is 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 porous medium diffusion model of saturated oil and water for CO2 diffusion is established by extracting the porous medium image after water displacement in 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 of CO2 diffusion that is saturated with only a single-phase fluid is established by deleting the water phase from the porous medium diffusion model of 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 the 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 value of the diffusion coefficient 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, and the CO2 concentration in the water phase is significantly lower than that in the oil phase. The magnitude of the reduction depends on the value of the distribution coefficient. In essence, 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. 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. 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 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 diffusion amount of CO2 in the porous medium at any time is calculated 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 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. A method for rapidly determining the diffusion coefficient of CO2 in a porous medium saturated with oil and water 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 >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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