Method for determining diffusion coefficient of diffusion mass transfer of CO2 in multiphase fluid

Abstract

The invention discloses a method for determining a diffusion coefficient of diffusion mass transfer of CO2 in a multiphase fluid. The method comprises the following steps: establishing a multiphase system physical model for CO2 diffusion mass transfer; dividing diffusion stages according to the position of the diffusion front edge of the CO2 in the physical model, and establishing diffusion mathematical models of the CO2 in different diffusion stages; solving the diffusion mathematical model to obtain the spatial concentration distribution of CO2 in different diffusion stages; calculating the diffusion amount of CO2 according to the concentration distribution; representing a pressure drop model of CO2 in the multiphase system by using a gas state equation, and substituting the diffusion amount into the pressure drop model to obtain a theoretical pressure drop curve; testing an actual pressure drop curve of CO2 through a pressure drop method; fitting the actual pressure drop curve by using the theoretical pressure drop curve to obtain diffusion coefficients of CO2 in the heterogeneous system; the problem that in the prior art, a method for determining the diffusion coefficient of diffusion mass transfer of CO2 in multiphase fluid is lacked is solved.

Description

Technical Field

[0001] The present disclosure relates to the technical field of oil and gas production engineering, and more particularly to a calculation method for determining a diffusion coefficient of CO2 diffusion and mass transfer in a multiphase system. Background Art

[0002] As the development of conventional oil and gas resources enters a bottleneck stage, low-permeability and ultra-low-permeability oil and gas reservoirs have also become a hot spot for future development. my country is rich in low-permeability and ultra-low-permeability carbonate oil and gas resources, and acid fracturing technology has become an indispensable means to achieve the effective development of this type of oil and gas reservoirs. For carbonate oil and gas reservoirs with low pressure and strong reservoir sensitivity, how to quickly return liquid after the acid fracturing construction is an important factor affecting production capacity. CO2 gas is often used to assist acid fracturing construction during on-site construction. The purpose is to use the expansion of CO2 gas to quickly return liquid after the construction is completed, and at the same time, it can also replenish energy to the formation. During the acid fracturing construction process, the injection timing, injection volume and liquid return timing of CO2 gas after acid fracturing are key factors for carbonate reservoirs to achieve better transformation effects. In the acid fracturing design process, the determination of CO2 gas injection timing and post-pressure liquid discharge timing is particularly important. Therefore, it is necessary to calculate the diffusion and mass transfer law of CO2 in saturated multiphase fluid media, and the accurate calculation of its diffusion coefficient is even more important.

[0003] At present, the diffusion and mass transfer laws of gas in fluids are mostly studied using the pressure drop method in the indirect method, and the basic research is on the diffusion and mass transfer laws of gas in single-phase fluids, such as the determination of the diffusion coefficient of CO2 in oil phase or water phase. However, there are no reports on the research on the diffusion and mass transfer laws of CO2 in multiphase fluid media. Summary of the invention

[0004] In view of this, the present disclosure provides a method for determining the diffusion coefficient of CO2 diffusion mass transfer in a multiphase fluid, which solves the problem of lack of a method for determining the diffusion coefficient of CO2 diffusion mass transfer in a multiphase fluid in the prior art.

[0005] To achieve the above-mentioned object of the invention, the method for determining the diffusion coefficient of CO2 diffusion and mass transfer in a multiphase fluid comprises:

[0006] Establish a physical model of a multiphase system consisting of n-hexanol, crude oil and water for CO2 diffusion and mass transfer;

[0007] Dividing the diffusion stages according to the position of the diffusion front of CO2 in the multiphase system physical model, and establishing a mathematical model of CO2 diffusion in different diffusion stages;

[0008] Solving the diffusion mathematical model, obtaining the spatial concentration distribution of CO2 at different diffusion stages; and obtaining the diffusion amounts of CO2 in n-hexanol, crude oil and water phases respectively according to the concentration distribution;

[0009] The PR gas state equation is used to characterize the pressure change of CO2 diffusion in n-hexanol, crude oil and water phases, respectively, to obtain a pressure drop model of the microscopic diffusion process, and the diffusion amount is substituted into the pressure drop model to obtain a theoretical pressure drop curve;

[0010] The actual pressure drop curve of CO2 in n-hexanol, crude oil and water phases was tested by the pressure drop method;

[0011] The actual pressure drop curve is fitted using the theoretical pressure drop curve, and when the set fitting accuracy is reached, the diffusion coefficients of the CO2 in n-hexanol, crude oil and water phases are obtained.

[0012] In the present disclosure and possible embodiments, the fitting accuracy is greater than 0.99.

[0013] In the present disclosure and possible embodiments, the multiphase system physical model sets the initial n-hexanol and CO2 interface z to 0, the initial n-hexanol and crude oil interface z0 to 0.02m, the oil-water interface z1 to 0.04m, and the model bottom zt to 0.08m.

[0014] In the present disclosure and possible embodiments, the method of dividing the diffusion stage is:

[0015] The first diffusion stage is when the CO2 diffusion front does not reach the interface between n-hexanol and crude oil, and CO2 diffuses only in n-hexanol. The second diffusion stage is when the CO2 diffusion front does not reach the oil-water interface, and CO2 diffuses in n-hexanol and crude oil. The third diffusion stage is when the CO2 diffusion front reaches the oil-water interface, and CO2 diffuses in n-hexanol, crude oil, and water phases.

[0016] In the present disclosure and possible embodiments, the method for establishing a mathematical model of CO2 diffusion at different diffusion stages includes:

[0017] A mathematical model of CO2 diffusion in the first diffusion stage, the second diffusion stage and the third diffusion stage is established based on Fick's law.

[0018] In the present disclosure and possible embodiments, the diffusion mathematical model is solved based on a moving grid numerical simulation method.

[0019] In the present disclosure and possible embodiments, when solving using the moving grid numerical simulation method, the number of spatial grids remains unchanged, and the time grids are differentiated.

[0020] In the present disclosure and possible embodiments, a mathematical model of the diffusion of CO2 in n-hexanol in the first diffusion stage is discretized based on a finite difference method, and a central difference format is used for grid difference. After the mathematical model of the diffusion of CO2 in n-hexanol in the first diffusion stage, boundary conditions and initial conditions are differentially gridded, the solution is performed using a Matlab programming method to obtain the concentration distribution of CO2 in n-hexanol in the first diffusion stage, and the time t0 when the CO2 diffusion front reaches the interface between n-hexanol and crude oil and the concentration distribution of CO2 in n-hexanol at t0 are obtained. The concentration distribution of CO2 in n-hexanol at t0 is used as the initial condition for calculation of the second diffusion stage, and the concentration distribution of CO2 in the second and third diffusion stages is obtained by analogy.

[0021] The present disclosure has the following beneficial effects:

[0022] The present invention discloses a calculation method for the diffusion and mass transfer of CO2 in a multiphase system, which takes into account the volume expansion caused by the dissolution of carbon dioxide in n-hexanol and crude oil. This is because in actual underground conditions, the diffusion of CO2 into crude oil will cause volume expansion. In the actual diffusion process, the dissolution of CO2 in n-hexanol and crude oil will cause volume expansion. Therefore, the diffusion coefficient of CO2 in n-hexanol, crude oil and water phases obtained by the method disclosed in the present invention can more accurately reflect the actual diffusion process, and considering the expansion of the crude oil volume can more realistically simulate the underground diffusion situation, providing guidance for on-site program design. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0024] Figure 1 Schematic diagram of a microscopic physical model of CO2 diffusion in a multiphase system in an embodiment of the present disclosure;

[0025] Figure 2 It is a schematic diagram of using a moving grid numerical simulation method to track the change of a phase interface over time in an embodiment of the present disclosure;

[0026] Figure 3 A schematic diagram of an experimental device for a CO2 diffusion experiment in a multiphase system designed in an embodiment of the present disclosure;

[0027] Figure 4 It is a schematic diagram of a pressure drop curve when the multiphase system includes n-hexanol, crude oil and water in a ratio of 1:1:2 in an embodiment of the present disclosure. DETAILED DESCRIPTION

[0028] The present disclosure is described below based on embodiments, but it is worth noting that the present disclosure is not limited to these embodiments. In the detailed description of the present disclosure below, some specific details are described in detail. However, for the parts that are not described in detail, those skilled in the art can also fully understand the present disclosure.

[0029] In addition, those skilled in the art should understand that the drawings are provided only to illustrate the purpose, features and advantages of the present disclosure, and the drawings are not actually drawn to scale. At the same time, unless the context clearly requires, the words "include", "comprise" and the like in the entire specification and claims should be interpreted as inclusive rather than exclusive or exhaustive; that is, "including but not limited to" meaning.

[0030] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the present disclosure describes the method of the present invention in detail through the following preferred embodiments:

[0031] 1: Establish a mathematical model for CO2 diffusion and mass transfer in a multiphase system

[0032] 1. Establish a multiphase system physical model for CO2 diffusion and mass transfer

[0033] The present disclosure embodiment Figure 1 The physical model of the n-hexanol, crude oil and water phases shown is used as a multiphase system for CO2 diffusion mass transfer. The initial n-hexanol and CO2 interface is set to z = 0, the initial n-hexanol and crude oil interface is set to z = z0 = 0.02m, the oil-water interface is set to z = z1 = 0.04m, and the bottom of the model is set to z = zt = 0.08m.

[0034] 2. Dividing the diffusion stages of CO2 in the physical model of multiphase system

[0035] In the physical model of multiphase system, due to the differences in the diffusion coefficients of CO2 in n-hexanol, crude oil and water phases, it is necessary to discuss in stages when establishing the mathematical model of CO2 diffusion and mass transfer. Based on the position of the CO2 diffusion front, the diffusion process is divided into three diffusion stages. The first diffusion stage is when the CO2 diffusion front does not reach the interface between n-hexanol and crude oil, and CO2 diffuses only in n-hexanol; the second diffusion stage is when the CO2 diffusion front does not reach the oil-water interface, and CO2 diffuses in n-hexanol and crude oil; the third diffusion stage is when the CO2 diffusion front reaches the oil-water interface, and CO2 diffuses in n-hexanol, crude oil and water phases.

[0036] 3. Simplified conditions for diffusion modeling

[0037] When considering the expansion of the liquid phase, since the diffusion coefficient and solubility of CO2 in n-hexanol and crude oil are much greater than those in the water phase, only the expansion of n-hexanol and crude oil is considered in the simulation process.

[0038] In order to simplify the modeling process of CO2 diffusion in n-hexanol, crude oil and water phases, the following simplified conditions are proposed:

[0039] (1) The temperature is constant during the diffusion process;

[0040] (2) The concentration of CO2 at the interface between n-hexanol and CO2 is always equal to the equilibrium concentration ceq;

[0041] (3) The diffusion coefficient of CO2 in n-hexanol is D1, the diffusion coefficient in crude oil is D2, and the diffusion coefficient in water phase is Dw, and D1, D2 and Dw are constants;

[0042] (4) The effects of natural convection caused by the expansion and density change of n-hexanol and crude oil are ignored;

[0043] (5) Ignore evaporation and gravity of the liquid phase;

[0044] (6) The influence of capillary force is ignored during the expansion of crude oil.

[0045] 4. Modeling in stages

[0046] (1) Mathematical model of CO2 diffusion in n-hexanol in the first diffusion stage

[0047] Diffusion is ubiquitous in nature. The formula that characterizes the diffusion process was first derived experimentally by Fick in 1855. Fick's law defines the diffusion flux generated by the concentration gradient. In order to characterize the expansion of crude oil caused by the diffusion of CO2 into crude oil, the interface position of n-hexanol and CO2 and the interface position of n-hexanol and crude oil are expressed as time-varying functions Z0(t) and Z1(t).

[0048] Based on Fick's law, the diffusion differential equation of CO2 in n-hexanol in the first diffusion stage is:

[0049]

[0050] Where: c1(z,t)—the concentration of CO2 in n-hexanol in the first diffusion stage, mol / m 3 ;

[0051] D1—Diffusion coefficient of CO2 in n-hexanol, m 2 / s;

[0052] Z0(t) - position function of the interface between n-hexanol and CO2, m;

[0053] t0—The time when the CO2 diffusion front just reaches the interface separating n-hexanol and crude oil, s.

[0054] At the initial moment, the concentration of CO2 in n-hexanol is 0. At the interface separating the n-hexanol and CO2 gas phases, the concentration of CO2 is the equilibrium concentration c eq The diffusion flux is 0 at the bottom of the model. The initial and boundary conditions of the model can be expressed as:

[0055]

[0056] In each diffusion stage, the interface position between n-hexanol and CO2 and the interface position between n-hexanol and crude oil can be calculated by the following formula:

[0057]

[0058] Where: CO2 -density of CO2, kg / m 3 ;

[0059] M CO2 - molar mass of CO2, kg / mol;

[0060] The rate of change at which molecules diffuse across a phase interface into the other phase.

[0061] (2) Mathematical model of CO2 diffusion in n-hexanol and crude oil phases in the second diffusion stage

[0062] As the diffusion process progresses, the CO2 diffusion front reaches the interface between n-hexanol and crude oil and begins to diffuse into the crude oil. In this process, the diffusion phenomenon occurs in the two liquid phases of n-hexanol and crude oil, resulting in changes in the diffusion boundary conditions and initial conditions, and new initial conditions and boundary conditions need to be defined.

[0063] ① Diffusion of CO2 in n-hexanol

[0064] The diffusion control equation of CO2 in n-hexanol in the second diffusion stage is expressed by Fick's law:

[0065]

[0066] Where: z0(T1)—interface position function of n-hexanol and CO2 in the second diffusion stage, m;

[0067] z1(T1)—position function of the interface between n-hexanol and crude oil in the second diffusion stage, m;

[0068] c2(z,t)—CO2 concentration distribution function in n-hexanol in the second diffusion stage, mol / m 3 ;

[0069] T1—Diffusion time of the second diffusion stage, T1=t-t0, s.

[0070] In this process, the initial condition of CO2 in n-hexanol is no longer 0 concentration, but becomes the concentration at the end of the first diffusion stage. The concentration distribution can be solved by the first diffusion stage. The initial condition is expressed as follows:

[0071] c2(z,T1=0)=f(z),z0(T1) <z<z0 (5)

[0072] Where: f(z)—CO2 concentration distribution function in n-hexanol at the end of the first diffusion stage, mol / m 3 .

[0073] At the interface separating the n-hexanol and CO2 gas phases, the concentration of CO2 is always the equilibrium concentration c eq :

[0074] c2(z0(T1),T1)=c eq ,T1>0 (6)

[0075] At the interface between n-hexanol and crude oil, the concentration is a function of time and increases with time:

[0076] c2(z1(T1),T1)=c z (T1),T1>0 (7)

[0077] Where: c z (T1)—CO2 concentration at the interface between n-hexanol and crude oil during the second diffusion stage, mol / m 3 .

[0078] ② Diffusion of CO2 in crude oil

[0079] The diffusion control equation of CO2 in crude oil in the second diffusion stage is also expressed by Fick's law:

[0080]

[0081] Where: c3(z,T1)—CO2 concentration distribution function in crude oil in the second diffusion stage, mol / m 3 .

[0082] In the second diffusion stage, CO2 diffuses in crude oil. Initially, the CO2 concentration in crude oil is 0, so the initial condition is expressed as:

[0083] c3(z,T1=0)=0,z1(T1) <z<z1 (9)

[0084] At the bottom of the model, CO2 cannot pass through the closed boundary, so the diffusion flux is 0, expressed as:

[0085]

[0086] At the interface between n-hexanol and crude oil, it is assumed that the concentrations at the interface between n-hexanol and crude oil are equal, that is, the concentration at the interface between n-hexanol and crude oil is continuous and there is no concentration mutation:

[0087] c3(z1(T1),T1)=c z (T1),T1>0 (11)

[0088] At the interface between n-hexanol and crude oil, according to the law of conservation of mass, the diffusion flux of CO2 leaving n-hexanol through the interface between n-hexanol and crude oil should be equal to the diffusion flux of CO2 entering crude oil through the interface between n-hexanol and crude oil, expressed as:

[0089]

[0090] (3) Mathematical model of CO2 diffusion in the third diffusion stage among n-hexanol, crude oil and water

[0091] As the CO2 diffusion front reaches the interface between n-hexanol and crude oil, CO2 diffuses in the multiphase system and enters the third diffusion stage. In the third diffusion stage, CO2 diffuses in n-hexanol, crude oil and water phases at the same time, and the initial conditions and boundary conditions of CO2 diffusion in n-hexanol and crude oil change further.

[0092] ① Diffusion of CO2 in n-hexanol

[0093] The diffusion control equation of CO2 in n-hexanol in the third diffusion stage can be expressed as:

[0094]

[0095] Where: T2—diffusion time of the third diffusion stage, s;

[0096] T 11 —The time when the CO2 diffusion front reaches the oil-water interface, s;

[0097] z0(T2)—position function of the interface between n-hexanol and CO2 in the third diffusion stage, m;

[0098] z1(T2)—position function of the interface between n-hexanol and crude oil in the third diffusion stage, m;

[0099] c4(z,t)—Concentration distribution function of CO2 in n-hexanol in the third diffusion stage, mol / m 3 .

[0100] The initial condition of CO2 in n-hexanol in the third diffusion stage becomes the concentration of CO2 in it at the end of the second diffusion stage, expressed as:

[0101] c4(z,T2=0)=g(z),z0(T2) <z<z1(T2) (14)

[0102] Where: g(z)—CO2 concentration distribution function in n-hexanol at the end of the second diffusion stage, mol / m 3 .

[0103] At the interface separating the n-hexanol and CO2 gas phases, the concentration of CO2 is always the equilibrium concentration c eq :

[0104] c4(z0(T2),T2)=c eq ,T2>0 (15)

[0105] At the interface between n-hexanol and crude oil, the concentration is a function of time and increases with time:

[0106] c4(z1(T2),T2)=c z (T2),T2>0 (16)

[0107] Where: c z (T2)—CO2 concentration at the interface between n-hexanol and crude oil during the third diffusion stage as a function of time, mol / m 3 .

[0108] ② Diffusion of CO2 in crude oil

[0109] The diffusion control equation of CO2 in crude oil in the third diffusion stage is also expressed by Fick's law:

[0110]

[0111] Where: c5(z,T2)—CO2 concentration distribution function in crude oil during the third diffusion stage, mol / m 3 .

[0112] The initial condition of CO2 diffusion in crude oil in the third diffusion stage becomes the concentration of CO2 in it at the end of the second diffusion stage, expressed as:

[0113] c5(z,T2=0)=h(z),z1(T2) <z<z1 (18)

[0114] Where: h(z)—CO2 concentration distribution function in crude oil at the end of the second diffusion stage, mol / m 3 .

[0115] At the interface between n-hexanol and crude oil, the CO2 concentration is continuous:

[0116] c5(z1(T2),T2)=c z (T2),T2>0 (19)

[0117] In the third diffusion stage, the CO2 concentration at the oil-water interface is greater than 0 and increases with time:

[0118] c5(z1,T2)=c o (T2),T2>0 (20)

[0119] Where: c o (T2)—CO2 concentration in the oil phase at the oil-water interface in the third diffusion stage as a function of time, mol / m 3 .

[0120] At the interface between n-hexanol and crude oil, according to the law of conservation of mass, the diffusion flux of CO2 leaving n-hexanol through the interface between n-hexanol and crude oil should be equal to the diffusion flux of CO2 entering crude oil through the interface between n-hexanol and crude oil, expressed as:

[0121]

[0122] ③ Diffusion of CO2 in water phase

[0123] The diffusion control equation of CO2 in the water phase in the third diffusion stage is expressed by Fick's law:

[0124]

[0125] Where: c6(z,T2)—CO2 concentration distribution function in the water phase during the third diffusion stage, mol / m 3 .

[0126] At the beginning of the third diffusion stage, no CO2 has entered the water phase, so the initial condition is expressed as:

[0127] c6(z,T2=0)=0,z1 <z<z t (twenty three)

[0128] At the bottom of the model, CO2 cannot pass through the closed boundary, so the diffusion flux is 0, expressed as:

[0129]

[0130] At the interface between crude oil and water phases, the solubility and diffusion coefficient of CO2 in crude oil and water phases are quite different, so the concentration of CO2 in the oil phase and water phase will suddenly change at this interface. In order to describe this concentration mutation phenomenon at this interface, the oil-water concentration distribution coefficient k is defined as pc , the oil-water concentration distribution coefficient is the ratio of the concentration of CO2 in the oil phase to the concentration in the water phase at the oil-water interface, expressed as:

[0131]

[0132] Where: k pc —Oil-water concentration distribution coefficient at the oil-water interface;

[0133] c w (T2)—The concentration of CO2 in the water phase at the oil-water interface in the third diffusion stage as a function of time, mol / m 3 .

[0134] In the third diffusion stage, at the oil-water interface, the CO2 concentration in the water phase is greater than 0 and increases with time:

[0135] c6(z1,T2)=k pc -1 ·c o (T2),T2>0 (26)

[0136] At the oil-water interface, according to the law of conservation of mass, the diffusion flux of CO2 leaving the heavy oil through the oil-water interface should be equal to the diffusion flux of CO2 entering the water phase through the oil-water interface, expressed as:

[0137]

[0138] At this point, a mathematical model of CO2 diffusion in a multiphase system was established, which divided the different diffusion stages according to the position of the CO2 diffusion front. The movement of the interface between n-hexanol and CO2 and the interface between n-hexanol and crude oil were used to simulate the expansion of n-hexanol and oil phases.

[0139] 2. Solve the above diffusion mathematical model based on the moving grid numerical simulation method to obtain the spatial concentration distribution of CO2 at different diffusion stages

[0140] The diffusion mathematical model established above includes a moving interface, and an analytical solution cannot be obtained, and can only be solved by a numerical solution. The embodiment of the present disclosure uses a moving grid numerical simulation method to solve the diffusion mathematical model.

[0141] When solving the problem using the moving mesh numerical simulation method, the number of spatial grids remains unchanged, and the time grid is differentiated. In this process, the step size of the spatial grid changes with time to describe the movement of the interface position. Figure 2 shown.

[0142] The diffusion equation of CO2 in n-hexanol in the first diffusion stage is discretized based on the finite difference method. The central difference format is used for grid difference, and the following difference format is obtained:

[0143]

[0144] After differentiating the boundary conditions at the bottom of the diffusion chamber in the first diffusion stage, we can obtain:

[0145]

[0146] The discretization of the boundary conditions at the interface between n-hexanol and CO2 yields:

[0147]

[0148] The initial conditions can be expressed as:

[0149]

[0150] The discrete form of the interface migration equation between n-hexanol and CO2 (Equation 3) was obtained using the central difference format:

[0151]

[0152] The grid step size of the n-hexanol region can be expressed as:

[0153]

[0154] The grid step size of the crude oil area can be expressed as:

[0155]

[0156] As shown in equations (33) and (34), as n-hexanol and crude oil expand during the diffusion process, the interface position Z0(t) between n-hexanol and CO2 and the interface position Z1(t) between n-hexanol and crude oil move, resulting in an increase in the area of ​​the n-hexanol and crude oil regions, but the number of grids N remains unchanged, so the grid step size becomes larger. After differentially meshing the diffusion differential equation, boundary conditions and initial conditions of CO2 in n-hexanol in the first diffusion stage, the above equations are solved using the Matlab programming method to obtain the concentration distribution of CO2 in n-hexanol in the first diffusion stage, and the time t0 when the CO2 diffusion front reaches the interface between n-hexanol and crude oil and the concentration distribution f(z) of CO2 in n-hexanol at time t0 are obtained, and used as the initial conditions for the calculation of the second diffusion stage. The differential format of the second and third diffusion stages is similar to that of the first diffusion stage, and will not be repeated here.

[0157] 3. Establishment of theoretical pressure drop model

[0158] After CO2 is injected into the formation, the well is soaked. During the soaking process, the CO2 at the bottom of the well will gradually diffuse into the multiphase system of the formation, causing the CO2 pressure near the wellbore to gradually decrease. The PR gas state equation is used to characterize the pressure change of CO2 diffusion in n-hexanol, crude oil and water phases, and the pressure drop model of the microscopic diffusion process is obtained as follows:

[0159]

[0160]

[0161]

[0162] a=a c α(T r ,ω) (38)

[0163]

[0164]

[0165]

[0166] P and P c are pressure and critical pressure, R is Avogadro's constant, V is volume, T and T c are temperature and critical temperature respectively, and α(Tr,ω) is the α function determined by Tr and eccentricity factor ω.

[0167] 4: Test the actual pressure drop curve by pressure drop method

[0168] (1) Take n-hexanol, formation water and crude oil samples and carry out the diffusion experiment of CO2 in a multiphase system under laboratory simulated formation conditions (temperature and pressure). The design structure diagram of the experimental device is shown in the figure below: Figure 3 As shown, the actual pressure drop curve of CO2 in n-hexanol, crude oil and water phases was tested by the pressure drop method;

[0169] (2) The established pressure drop model was used to fit the actual pressure drop curve obtained from the experiment. Combining the experiment and other parameters, the diffusion coefficients D1, D2 and D3 of CO2 in n-hexanol, crude oil and water phases were obtained respectively. w ;

[0170] Among them, the diffusion experiment of CO2 in a multiphase system has the following experimental process:

[0171] (1) Diffusion chamber airtightness check: Before the experiment, close all valves, open valves 1, 2, 3, and 8, inject CO2 into the diffusion chamber, then close valves 1, 2, 3, and 8, turn on the computer pressure data acquisition system to record the pressure of the diffusion chamber, and monitor the pressure change of the diffusion chamber within 3 hours. If the pressure does not change, the system is airtight and subsequent experiments can be carried out;

[0172] (2) Water phase injection: open valves 4, 7, and 11, and use a horizontal flow pump to inject the water in the upper part of the water injection intermediate container into the lower part of the diffusion chamber. The injection volume is monitored by the horizontal flow pump. After the specified volume of water phase is injected, close valves 4, 7, and 11;

[0173] (3) Crude oil injection: Open valves 4, 6, and 10, and use a horizontal flow pump to inject the oil in the right part of the oil injection intermediate container into the lower middle part of the diffusion chamber. The injection volume is monitored by the horizontal flow pump. After the specified volume of oil phase is injected, close valves 4, 6, and 10;

[0174] (4) n-hexanol injection: open valves 4, 5, and 9, and use a horizontal flow pump to inject the n-hexanol in the upper part of the n-hexanol injection intermediate container into the upper middle part of the diffusion chamber. The injection volume is monitored by the horizontal flow pump. After the specified volume of n-hexanol is injected, close valves 4, 5, and 9;

[0175] (5) System temperature adjustment: Open valves 1 and 2 to introduce CO2 into the CO2 high-pressure gas tank, close valves 1 and 2, and turn on the constant temperature system to make the temperature of the CO2 gas and the n-hexanol, crude oil, and water phase in the diffusion chamber reach the experimental temperature;

[0176] (6) CO2 pressurization: Open valves 3 and 8, introduce CO2 in the high-pressure gas tank into the diffusion chamber, and use the high-pressure gas tank to pressurize the CO2 to the experimental pressure;

[0177] (7) Experiment start: Close valves 3 and 8, CO2 diffuses into the multiphase system, turn on the computer pressure data acquisition system, and start recording the experimental pressure with a sampling interval of 10 s;

[0178] (8) End of the experiment: After collecting the pressure data, turn off the computer pressure data acquisition system, vent the CO2 gas in the diffusion chamber, remove the diffusion chamber, and clean out the n-hexanol, crude oil and water phases.

[0179] In the embodiment of the present invention, the actual pressure drop curve of the diffusion device is obtained as follows: Figure 4 shown.

[0180] Five: Fit the data to get the diffusion coefficient

[0181] The established pressure drop model is used to fit the actual pressure drop curve. When the fitting accuracy R2>0.99, the required fitting accuracy is achieved. At this time, D1 is the diffusion coefficient of CO2 in n-hexanol. The diffusion coefficients of CO2 in n-hexanol, crude oil and water phase are 3.24×10 -6 m 2 / s, 2.14×10 -8 m 2 / s and 1.65×10 -9 m 2 / s.

[0182] The above-described embodiments are only embodiments of the present disclosure, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the present disclosure. It should be noted that, for a person of ordinary skill in the art, without departing from the concept of the present disclosure, several variations, equivalent substitutions, improvements, etc. may be made, all of which fall within the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure shall be subject to the attached claims.

Claims

1. A method for determining the diffusion coefficient of CO2 diffusion mass transfer in a multiphase fluid, characterized in that: include: Establish a physical model of a multiphase system consisting of n-hexanol, crude oil and water for CO2 diffusion and mass transfer; Dividing the diffusion stages according to the position of the diffusion front of CO2 in the multiphase system physical model, and establishing a mathematical model of CO2 diffusion in different diffusion stages; Solving the diffusion mathematical model, obtaining the spatial concentration distribution of CO2 at different diffusion stages; and obtaining the diffusion amounts of CO2 in n-hexanol, crude oil and water phases respectively according to the concentration distribution; The PR gas state equation is used to characterize the pressure change of CO2 diffusion in n-hexanol, crude oil and water phases, respectively, to obtain a pressure drop model of the microscopic diffusion process, and the diffusion amount is substituted into the pressure drop model to obtain a theoretical pressure drop curve; The actual pressure drop curve of CO2 in n-hexanol, crude oil and water phases was tested by the pressure drop method; The actual pressure drop curve is fitted using the theoretical pressure drop curve, and when the set fitting accuracy is reached, the diffusion coefficients of the CO2 in n-hexanol, crude oil and water phases are obtained.

2. The method for determining the diffusion coefficient of CO2 diffusion and mass transfer in a multiphase fluid according to claim 1, characterized in that: The fitting accuracy is greater than 0.

99.

3. The method for determining the diffusion coefficient of CO2 diffusion and mass transfer in a multiphase fluid according to claim 2, characterized in that: The multiphase system physical model sets the initial interface z between n-hexanol and CO2 to 0, the initial interface z0 between n-hexanol and crude oil to 0.02 m, the oil-water interface z1 to 0.04 m, and the model bottom zt to 0.08 m.

4. The method for determining the diffusion coefficient of CO2 diffusion mass transfer in a multiphase fluid according to claim 3, characterized in that: The division method of the diffusion stage is: The first diffusion stage is when the CO2 diffusion front does not reach the interface between n-hexanol and crude oil, and CO2 diffuses only in n-hexanol. The second diffusion stage is when the CO2 diffusion front does not reach the oil-water interface, and CO2 diffuses in n-hexanol and crude oil. The third diffusion stage is when the CO2 diffusion front reaches the oil-water interface, and CO2 diffuses in n-hexanol, crude oil, and water phases.

5. The method for determining the diffusion coefficient of CO2 diffusion mass transfer in a multiphase fluid according to claim 4, characterized in that: The methods for establishing the mathematical model of CO2 diffusion at different diffusion stages include: A mathematical model of CO2 diffusion in the first diffusion stage, the second diffusion stage and the third diffusion stage is established based on Fick's law.

6. The method for determining the diffusion coefficient of CO2 diffusion and mass transfer in a multiphase fluid according to claim 5, characterized in that: The diffusion mathematical model is solved based on a moving grid numerical simulation method.

7. The method for determining the diffusion coefficient of CO2 diffusion and mass transfer in a multiphase fluid according to claim 6, characterized in that: When solving using the moving grid numerical simulation method, the number of spatial grids remains unchanged, and the time grid is differentiated.

8. The method for determining the diffusion coefficient of CO2 diffusion mass transfer in a multiphase fluid according to claim 7, characterized in that: The mathematical model of CO2 diffusion in n-hexanol in the first diffusion stage is discretized based on the finite difference method, and the central difference format is used for grid difference. The mathematical model of CO2 diffusion in n-hexanol in the first diffusion stage, boundary conditions and initial conditions are differentially gridded, and then solved using the Matlab programming method to obtain the concentration distribution of CO2 in n-hexanol in the first diffusion stage, and the time t0 when the CO2 diffusion front reaches the interface between n-hexanol and crude oil and the concentration distribution of CO2 in n-hexanol at t0 are obtained. The concentration distribution of CO2 in n-hexanol at t0 is used as the initial condition for the calculation of the second diffusion stage, and the concentration distribution of CO2 in the second and third diffusion stages is obtained by analogy.

Images