A method for constructing a pore-scale multi-field coupling model for salt precipitation caused by CO2 injection in saline strata

Abstract

The present application relates to the technical field of CO2 geological storage, and provides a method for constructing a pore-scale multi-field coupling model of salt precipitation caused by CO2 injection into saline layers. The method numerically simulates the real-time distribution of relevant variables such as flow rate, temperature, and concentration in the domain at the pore scale, and can accurately study the microscopic mechanism of salt precipitation during CO2 injection. The method comprises the following steps: S1, establishing a two-dimensional single-pore geometric model, wherein the two-dimensional single-pore geometric model comprises a parallel fluid inlet and a fluid outlet, and a pore wall connecting the fluid inlet and the fluid outlet; S2, establishing CO2 material and brine material respectively, adding a physical field, setting boundary conditions and initial conditions of the two-dimensional single-pore geometric model, and meshing the two-dimensional single-pore geometric model; S3, calculating parameters according to the two-dimensional single-pore geometric model, CO2 material, brine material, and physical field, and obtaining the microscopic mechanism according to the calculated results.

Description

Technical Field

[0001] The present invention relates to the technical field of CO2 geological storage, and in particular to a method for constructing a pore-scale multi-field coupling model of salt precipitation caused by CO2 injection into saline layers. Background Art

[0002] CO2 geological storage is one of the fastest and most effective measures to reduce carbon emissions. CO2 geological storage involves capturing CO2 from large emission sources and injecting it into geological structures in a supercritical state. It is generally believed that the best geological structures for CO2 storage are deep saline aquifers and depleted oil and gas reservoirs. After dry supercritical CO2 (scCO2) is injected into the target formation, the evaporation of liquid water will cause the concentration of dissolved salts in the brine to increase, and the dissolved salts will precipitate from the brine. Once the amount of precipitated salt precipitation exceeds the threshold, the connectivity of the reservoir will be severely damaged. The phenomenon of salt precipitation caused by injecting CO2 into saline aquifers is a complex multi-physical process problem involving multiphase fluid flow, gas-liquid phase change, heat transfer, material transport and other processes. Therefore, studying the microscopic mechanism of salt precipitation caused by injecting CO2 into saline aquifers can provide reasonable implementation methods and improvement measures for CO2 geological storage projects, effectively alleviate reservoir damage caused by salt precipitation, and improve the safety and efficiency of the project.

[0003] At present, the main methods for studying the salt precipitation caused by injecting CO2 into saline aquifers include core displacement experiments, microfluidic chip experiments, and macro-scale numerical simulations. Among them, core displacement experiments have been widely used due to their high authenticity and operability. For example, the "Multiphase Permeameter and Rock Permeability Characteristics Determination Method" (Patent No.: CN104897543A) uses scCO2 to displace the core saturated with brine, and then calculates the amount of salt precipitation according to the change in brine saturation. However, this method cannot visualize the distribution of CO2, brine and salt precipitation inside the core in real time, and the measurement methods of related variables such as flow rate, pressure and water vapor content are cumbersome and complicated. Compared with traditional core displacement experiments, microfluidic chip experiments can directly observe the physical and chemical phenomena and salt precipitation accumulation in porous media under high temperature and high pressure conditions, but this method is still challenging in reflecting complex pore geometry and the chemical properties of the real rock surface. Numerical simulations are also widely used to study salt precipitation during CO2 injection in porous media due to their high repeatability. However, most of them are macro-scale numerical simulations based on software such as CMG-GEM and TOUGH2-ECO2N. Macro-scale numerical simulators often assume that there is quasi-instantaneous equilibrium between the phases, do not model the water evaporation dynamics, and also ignore the properties of the CO2-brine interface, which is not conducive to in-depth study of the mechanism of salt precipitation during CO2 injection. Summary of the invention

[0004] In view of this, the present invention provides a method for constructing a pore-scale multi-field coupling model of salt precipitation caused by CO2 injection in saline layers. The model construction method provided in this application numerically simulates the real-time distribution of relevant variables such as flow rate, temperature, and concentration in the domain at the pore scale, which can realize accurate research on the microscopic mechanism of salt precipitation during CO2 injection.

[0005] The present application provides a method for constructing a pore-scale multi-field coupling model of salt precipitation caused by CO2 injection into saline layers, comprising the following steps:

[0006] S1. establishing a two-dimensional single pore geometric model, wherein the two-dimensional single pore geometric model includes a parallel fluid inlet and a fluid outlet, and a pore wall connecting the fluid inlet and the fluid outlet;

[0007] S2, respectively establishing CO2 material and brine material, adding physical fields, setting boundary conditions and initial conditions of the two-dimensional single-pore geometric model, and meshing the two-dimensional single-pore geometric model;

[0008] S3. Calculate parameters based on the two-dimensional single pore geometric model, CO2 material, brine material and physical field, and obtain the microscopic mechanism of salt precipitation caused by CO2 injection into the saline layer based on the calculated results.

[0009] In some specific implementations, in step S2, adding a physical field specifically includes:

[0010] S301, add the "laminar flow" physical field and the "fluid phase field" physical field, couple the "laminar flow" and "fluid phase field" physical fields through the multi-physics field "two-phase flow, phase field", set initial conditions and boundary conditions, and simulate the two-phase displacement process of CO2-brine in a single pore;

[0011] S302, add the "fluid heat transfer" physical field, couple the "laminar flow" physical field and the "fluid heat transfer" physical field through the multi-physics field "non-isothermal flow", set initial conditions and boundary conditions to simulate the heat transfer occurring during the CO2-brine two-phase displacement process in a single pore;

[0012] S303, adding the "water transport" physical field, and coupling the "laminar flow" physical field and the "water transport" physical field through the "water flow" multi-physics field, setting initial conditions and boundary conditions to simulate the generation and transport of water vapor in the CO2-brine two-phase displacement process in a single pore;

[0013] S304, adding the “Diluted Species Transport” physical field, and coupling the “Laminar Flow” physical field and the “Diluted Species Transport” physical field through the “Reacting Flow, Diluted Species” multi-physics field, setting initial conditions and boundary conditions, to simulate the transport and precipitation process of NaCl in the CO2-brine two-phase displacement process in a single pore;

[0014] S305. Add the "Deformed Geometry" physical field and set the initial and boundary conditions to simulate the accumulation of salt precipitation on the pore wall during the CO2-brine two-phase displacement process in a single pore.

[0015] There is no order restriction between steps S302 to S305.

[0016] In some specific implementations, in step S2, the CO2 material is established as a gas-added thermodynamic system, the substance CO2 is added, and the density of CO2 is calculated using the Peng-Robinson model, and the viscosity of CO2 is calculated using the Brokaw model;

[0017] The step S2 of establishing the salt water material specifically involves adding the material "water" to approximately calculate the viscosity of the salt water, and using formula (1) to calculate the density of the salt water at different temperatures and pressures:

[0018] Formula (1);

[0019] In formula (1), ρ l is the density of the salt water, kg / m 3 ρ ref is the reference density of salt water, kg / m 3 ; α is the coefficient of thermal expansion, 1 / K; β is the coefficient of compression, 1 / Pa; T is the temperature, K; P is the pressure, Pa; T ref is the reference temperature, K; P ref is the reference pressure, Pa.

[0020] In some specific implementations, in step S301, the above-mentioned “laminar flow” physical field controls the flow of the fluid through equations (2) and (3):

[0021] Formula (2);

[0022] In formula (2), u is the fluid velocity, m / s; S m is the mass source term, 1 / s, calculated according to formula (8):

[0023] Formula (8);

[0024] Among them, ρ g is the density of the gas phase, kg / m 3; ϕ is the dimensionless phase field variable; is the evaporation rate of liquid water;

[0025] Formula (3);

[0026] In formula (3), ρ is the fluid density, kg / m 3 ; t is the simulation time, s; p is the fluid pressure, Pa; I is the unit tensor; μ is the fluid dynamic viscosity, Pa∙s; F st is the surface tension acting on the CO2 / brine interface, N / m;

[0027] In the “laminar flow” physical field, the fluid inlet of the two-dimensional single pore geometric model is a velocity boundary condition, the fluid outlet of the two-dimensional single pore geometric model is a pressure boundary condition, and the pore wall of the two-dimensional single pore geometric model is a wall condition;

[0028] The “fluid phase field” physics field tracks the movement of the CO2-brine two-phase interface through equation (4):

[0029] Formula (4);

[0030] In formula (4), ϕ is the dimensionless phase field variable; γ is the mobility, m 3 ∙s / kg; λ is the mixing energy density, N; ε is the interface thickness parameter, m; S f is the phase field source term, 1 / s, calculated according to formula (9):

[0031] Formula (9);

[0032] In the "fluid phase field" physical field, the fluid inlet of the two-dimensional single pore geometric model is the inlet boundary condition; the fluid outlet of the two-dimensional single pore geometric model is the outlet boundary condition; and the pore wall of the two-dimensional single pore geometric model is the wetting wall boundary condition.

[0033] In some specific implementations, in step S302, the “fluid heat transfer” physical field calculates the change in fluid temperature by equation (6):

[0034] Formula (6);

[0035] In formula (6), C p is the heat capacity, J / (kg∙K); k is the thermal conductivity, W / (m∙K); Q is the heat source, W / (m 3 ∙s), Q is the heat source term, calculated according to formula (10):

[0036] Formula (10);

[0037] In formula (10), L is the latent heat of evaporation, J / kg;

[0038] In the "fluid heat transfer" physical field, the fluid inlet of the two-dimensional single pore geometric model is an inflow boundary condition, the fluid outlet of the two-dimensional single pore geometric model is an outflow boundary condition, and the pore wall of the two-dimensional single pore geometric model is an adiabatic boundary condition.

[0039] In some specific implementations, in step S303, the liquid water evaporation dynamics is modeled in the “water transport” physical field using equation (7):

[0040] Formula (7)

[0041] in, is the evaporation rate of liquid water, kg / m 2 ∙s;k v is the evaporation rate constant, 1 / s; V f,l is the volume fraction of the liquid phase; ρ w is the density of water, kg / m 3 ;p sat is the water vapor saturation pressure, Pa; p par is the water vapor partial pressure, Pa;

[0042] The “water transport” physics field controls the generation and transport of water vapor through equation (12):

[0043] Formula (12);

[0044] In formula (12), M v is the molar mass of water, kg / mol, c v is the water vapor concentration, mol / m 3 ;D v is the water vapor diffusion coefficient, m 2 / s; G is the water vapor source term, calculated according to formula (11):

[0045] Formula (11)

[0046] In formula (11), ρ v is the density of water vapor, kg / m 3 ;

[0047] In the "water transport" physical field, the fluid inlet of the two-dimensional single pore geometric model is an inflow boundary condition, the fluid outlet of the two-dimensional single pore geometric model is an outflow boundary condition, and the pore wall of the two-dimensional single pore geometric model is an insulating boundary condition.

[0048] In some specific implementations, in step S304, the “diluted species transfer” physical field controls the transport of NaCl in the brine through equation (13):

[0049] Formula (13)

[0050] In formula (13), c s is the concentration of NaCl, mol / m 3 ;D s is the diffusion coefficient of NaCl in salt water, m 2 / s;

[0051] In the “Transfer of Diluted Species” physical field, the fluid inlet of the two-dimensional single pore geometric model is the inflow boundary condition, the fluid outlet of the two-dimensional single pore geometric model is the outflow boundary condition, and the pore wall of the two-dimensional single pore geometric model is the flux boundary condition, which represents the process of dissolved salt precipitation from brine and is calculated by formula (14):

[0052] Formula (14)

[0053] In formula (14), n is the normal vector; b is the pore diameter, m; r m is the sedimentation rate coefficient, 1 / s; X NaCl(aq) is the mass fraction of NaCl, kg / kg; X L is the solubility limit of NaCl, kg / kg; M m is the molar mass of NaCl, kg / mol.

[0054] In some specific implementations, in step S305, in the “deformed geometry” physical field, the normal mesh displacement of the fluid inlet and fluid outlet of the two-dimensional single pore geometric model is 0 m, and the accumulation process of salt precipitation on the pore wall is simulated by specifying the normal mesh displacement speed of the pore wall of the two-dimensional single pore geometric model. The normal mesh displacement speed of the pore wall of the two-dimensional single pore geometric model is calculated by formula (15):

[0055] Formula (15);

[0056] In formula (15), ρ m is the density of the salt precipitate, kg / m 3 .

[0057] In some specific implementations, step S3 includes:

[0058] Add a study step, set the output time step to solve, and obtain the CO2 / brine and salt precipitation distribution map, temperature distribution map, water vapor saturation distribution map and / or NaCl concentration distribution map;

[0059] The microscopic mechanism of salt precipitation caused by CO2 injection into saline layers is analyzed based on the CO2 / brine and salt precipitation distribution map, temperature distribution map, water vapor saturation distribution map and / or NaCl concentration distribution map.

[0060] In some specific implementations, the height of the two-dimensional single-pore geometric model is 40 μm to 120 μm, and the width is 100 μm to 200 μm.

[0061] This application establishes a pore-scale two-dimensional single-pore geometric model. The numerical simulation at the pore scale can easily obtain the real-time distribution of relevant variables such as flow rate, temperature, and concentration in the domain. At the same time, it can also consider the properties of the CO2-brine interface, and model the water evaporation dynamics and salt precipitation dynamics, which can deeply study the microscopic mechanism of salt precipitation caused by injecting CO2 into the saline layer; further, this application performs fully coupled numerical simulations on multi-phase fluid flow, gas-liquid phase change, heat transfer, material transport and other multi-physical processes, and examines the influence of relevant variables on salt precipitation, as well as the interaction between various physical fields, to provide technical means for studying the mechanism of salt precipitation in the process of underground CO2 injection. Therefore, the method provided in this application constructs a multi-physical field coupling model at the pore scale, which can numerically simulate the CO2-brine two-phase displacement, liquid water evaporation, heat transfer, dissolved salt transport and accumulation of salt precipitation on the pore wall in the process of CO2 saline layer storage, so as to realize in-depth research on the microscopic mechanism of salt precipitation caused by injecting CO2 into the saline layer. The experimental results show that the method provided in this application can couple multiple physical processes such as fluid flow, heat transfer, gas-liquid phase change and material transport involved in the salt precipitation phenomenon caused by CO2 injection, and can easily obtain the real-time spatial distribution of relevant research variables such as fluid volume fraction, fluid temperature, water vapor and NaCl concentration, so as to provide technical means for studying the microscopic mechanism of salt precipitation caused by CO2 injection, and then provide measures to alleviate the hazards of salt precipitation for CO2 geological storage projects, so as to solve the problem that existing research technology cannot accurately study the microscopic mechanism of salt precipitation caused by CO2 injection. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0063] Figure 1 It is a schematic flow chart of a method for constructing a pore-scale multi-field coupling model for salt precipitation caused by CO2 injection into saline layers provided by an embodiment of the present invention;

[0064] Figure 2 is a schematic diagram of a single micron-scale pore model in a saline layer provided by an embodiment of the present invention;

[0065] Figure 3 It is a flow chart of a method for setting physical fields, initial conditions and boundary conditions of a pore-scale multi-field coupling model of salt precipitation caused by CO2 injection in saline layers provided by an embodiment of the present invention;

[0066] Figure 4 This is a distribution diagram of CO2, brine and salt precipitation after 6.1 ms of gas injection provided by an embodiment of the present invention;

[0067] Figure 5 is a temperature distribution diagram after 6.1 ms of gas injection provided by an embodiment of the present invention;

[0068] Figure 6 is a water vapor saturation distribution diagram after 6.1 ms of gas injection provided by an embodiment of the present invention;

[0069] Figure 7 This is a NaCl concentration distribution diagram after 6.1 ms of gas injection provided by an embodiment of the present invention.

[0070] in:

[0071] 1- fluid inlet; 2- fluid outlet; 3- pore wall.

[0072] 01-Salt precipitation location. DETAILED DESCRIPTION

[0073] It should be understood that the expression "one or more of..." includes each of the items recited after the expression individually and in various different combinations of two or more of the recited items, unless otherwise understood from the context and usage. The expression "and / or" in combination with three or more recited items should be understood to have the same meaning, unless otherwise understood from the context.

[0074] The use of the terms "comprising", "having" or "containing", including their grammatical synonyms, should generally be understood as open and non-restrictive, for example not excluding other unrecited elements or steps, unless otherwise specifically stated or otherwise understood from the context.

[0075] It should be understood that the order of steps or the order in which certain actions are performed is not important as long as the present invention remains operable. In addition, two or more steps or actions may be performed simultaneously.

[0076] The use of any and all examples or exemplary language such as "for example" or "including" herein is intended only to better illustrate the present invention and is not intended to limit the scope of the present invention. Any language in this specification should not be construed as indicating that any unclaimed element is essential to the practice of the present invention.

[0077] In addition, the numerical ranges and parameters used to define the present invention are approximate values, and the relevant values ​​in the specific embodiments have been presented as accurately as possible. However, any numerical value inherently inevitably contains standard deviations due to individual test methods. Therefore, unless otherwise expressly stated, it should be understood that all ranges, quantities, values ​​and percentages used in this disclosure are modified by "about". Here, "about" generally means that the actual value is within plus or minus 10%, 5%, 1% or 0.5% of a specific value or range.

[0078] The present invention belongs to the technical field of CO2 geological storage, and in particular to a method for constructing a pore-scale multi-field coupling model of salt precipitation caused by CO2 injection into saline layers. Excessive CO2 emissions cause global warming, and CO2 geological storage is an effective measure to mitigate climate change. Deep saline layers are one of the best places for CO2 geological storage, but injecting CO2 into saline layers may cause salt precipitation, block pore channels, and reduce the connectivity of the reservoir. Therefore, studying the microscopic mechanism of salt precipitation caused by injecting CO2 into saline layers can provide reasonable implementation methods and improvement measures for CO2 geological storage projects, effectively alleviate reservoir damage caused by salt precipitation, and improve the safety and efficiency of the project.

[0079] In view of the problem that existing research methods cannot accurately study the microscopic mechanism of salt precipitation caused by CO2 injection in saline water layers, the present invention provides a pore-scale multi-field coupling model construction method for salt precipitation caused by CO2 injection in saline water layers, so as to realize accurate research on the microscopic mechanism of salt precipitation during CO2 injection.

[0080] In one embodiment of the present invention, Figure 1 As shown, a flow chart of a method for constructing a pore-scale multi-field coupling model of salt precipitation caused by CO2 injection into saline layers is provided, which is used to accurately study the microscopic mechanism of salt precipitation caused by CO2 injection into saline layers.

[0081] In this embodiment, the method includes the following steps:

[0082] S1. establishing a two-dimensional single pore geometric model, wherein the two-dimensional single pore geometric model includes a parallel fluid inlet and a fluid outlet, and a pore wall connecting the fluid inlet and the fluid outlet;

[0083] S2, respectively establishing CO2 material and brine material, adding physical fields, setting boundary conditions and initial conditions of the two-dimensional single-pore geometric model, and meshing the two-dimensional single-pore geometric model;

[0084] S3. Calculate parameters based on the two-dimensional single pore geometric model, CO2 material, brine material and physical field, and obtain the microscopic mechanism of salt precipitation caused by CO2 injection into the saline layer based on the calculated results.

[0085] The method provided in this application can couple multiple physical processes such as fluid flow, heat transfer, gas-liquid phase change and material transport involved in the salt precipitation phenomenon caused by CO2 injection, and can easily obtain the real-time spatial distribution of relevant research variables such as fluid volume fraction, fluid temperature, water vapor and NaCl concentration, so as to provide technical means for studying the microscopic mechanism of salt precipitation caused by CO2 injection, and then provide measures and methods to alleviate the hazards of salt precipitation for CO2 geological storage projects, so as to solve the problem that existing research technology cannot accurately study the microscopic mechanism of salt precipitation caused by CO2 injection.

[0086] The present application first establishes a two-dimensional single pore geometric model, which includes a parallel fluid inlet and a fluid outlet, and a pore wall connecting the fluid inlet and the fluid outlet. In some specific implementations, the two-dimensional single pore geometric model can be a rectangle, see Figure 2 , Figure 2 A schematic diagram of the structure of a two-dimensional single pore geometric model provided in an embodiment of the present application, wherein 1 is a fluid inlet, 2 is a fluid outlet, and 3 is a pore wall connecting the fluid inlet 1 and the fluid outlet 2. Specifically, the two-dimensional dimension can be obtained by selecting a two-dimensional space dimension in the COMSOL model wizard.

[0087] In some specific implementations, the two-dimensional single-pore geometric model is a rectangular geometry with a height of 40 μm to 120 μm, preferably 80 μm, and a width of 100 μm to 200 μm, preferably 120 μm.

[0088] After establishing the two-dimensional single-pore geometric model, CO2 material and brine material are established respectively, physical fields are added, boundary conditions and initial conditions of the two-dimensional single-pore geometric model are set, and the two-dimensional single-pore geometric model is meshed.

[0089] Specifically, the CO2 material is created by adding a gas thermodynamic system, adding the substance CO2, and using the Peng-Robinson model to calculate the density of CO2 and the Brokaw model to calculate the viscosity of CO2. The salt water material is created by adding the material "water" from the COMSOL material library to approximate the viscosity of the salt water, and using formula (1) to calculate the density of the salt water at different temperatures and pressures:

[0090] (1)

[0091] In formula (1), ρ l is the density of the salt water, kg / m 3 ρ ref is the reference density of salt water at reference temperature and reference pressure, kg / m 3 ; α is the thermal expansion coefficient of the salt water, 1 / K; β is the compressibility coefficient of the salt water, 1 / Pa; T is the temperature, K; P is the pressure, Pa; T ref is the reference temperature, K; P ref is the reference pressure, Pa.

[0092] This application also adds physical fields and sets initial conditions and boundary conditions to simulate the salt precipitation process caused by CO2 injection into the saline layer. In some specific implementations, adding physical fields specifically includes:

[0093] Add the "laminar flow" physics field and the "fluid phase field" physics field, and couple the "laminar flow" and "fluid phase field" physics fields through the multi-physics field "two-phase flow, phase field", set the initial conditions and boundary conditions, and simulate the two-phase displacement process of CO2-brine.

[0094] Add the "Fluid Heat Transfer" physics field, and couple the "Laminar Flow" physics field and the "Fluid Heat Transfer" physics field through the multiphysics field "Non-Isothermal Flow". Set initial conditions and boundary conditions to simulate the heat transfer that occurs during the CO2-brine two-phase displacement process.

[0095] Add the "Water Transport" physics field, and couple the "Laminar Flow" physics field and the "Water Transport" physics field through the "Water Flow" multiphysics field, set initial conditions and boundary conditions to simulate the generation and transport of water vapor in the CO2-brine two-phase displacement process;

[0096] Add the "Diluted Species Transport" physics field, and couple the "Laminar Flow" physics field and the "Diluted Species Transport" physics field through the "Reacting Flow, Diluted Species" multiphysics field, set initial conditions and boundary conditions to simulate the transport and precipitation process of NaCl in the CO2-brine two-phase displacement process;

[0097] Add the Deformed Geometry physics field and set the initial and boundary conditions to simulate the accumulation of salt precipitation on the pore walls during the CO2-brine two-phase flooding process.

[0098] See also Figure 3 , Figure 3This is a schematic diagram of a method for adding a physical field provided in an embodiment of the present application. There is no order restriction between adding steps S302 to S305. In this embodiment, a specific implementation method for adding a physical field includes the following steps:

[0099] S301. Add the "laminar flow" physics field and the "fluid phase field" physics field, and couple the "laminar flow" physics field and the "fluid phase field" physics field through the multi-physics field "two-phase flow, phase field". Reasonably set the initial conditions and boundary conditions to simulate the two-phase displacement process of CO2-brine in a single micron-sized pore.

[0100] Specifically, the above-mentioned "laminar flow" physical field simulates and controls the flow of fluid through equations (2) and (3):

[0101] (2)

[0102] In formula (2), u is the fluid velocity, m / s; S m is the mass source term, 1 / s, S m This will be described in detail in step S303.

[0103] (3)

[0104] In formula (3), ρ is the fluid density, kg / m 3 ; t is the simulation time, s; p is the fluid pressure, Pa; I is the unit tensor; μ is the fluid dynamic viscosity, Pa∙s; F st is the surface tension acting on the CO2 / brine interface, N / m.

[0105] In some specific implementations, in the above-mentioned "laminar flow" physical field, the model reference pressure is set to 15 MPa. The fluid inlet 1 of the single pore model is set as a velocity boundary condition, and the normal inflow velocity is 0.01 m / s. The fluid outlet 2 of the single pore model is set as a pressure boundary condition, and the static pressure is 0 Pa. The pore wall 3 of the single pore model is set as a wall condition, and there is no mass exchange on the pore wall 3.

[0106] The above “fluid phase field” physics field tracks the movement of the CO2-brine two-phase interface through equation (4):

[0107] (4)

[0108] In formula (4), ϕ is the dimensionless phase field variable; γ is the mobility, m 3 ∙s / kg; λ is the mixing energy density, N; ε is the interface thickness parameter, m; S f is the phase field source term, 1 / s, S f This will be described in detail in step S303.

[0109] In some specific implementations, in the above-mentioned "fluid phase field" physical field, the single pore model is set to be saturated with brine in the initial state. The fluid inlet 1 of the single pore model is set as the inlet boundary condition, and the injected fluid is CO2. The fluid outlet 2 of the single pore model is set as the outlet boundary condition. The pore wall 3 of the single pore model is set as the wetting wall boundary condition, and the contact angle is 80°.

[0110] In some specific implementations, in the "fluid phase field" physical field, the first weak contribution and the second weak contribution are added to describe the mass transfer caused by water evaporation. The first weak contribution expression is: test(p)*S m , the second weak contribution expression is: test(psi)*S f .

[0111] In the above “Two-Phase Flow, Phase Field” multiphysics field, fluid 1 is set to CO2 and fluid 2 is set to brine. The “Two-Phase Flow, Phase Field” multiphysics field calculates the volume fraction V of the gas phase through equation (5): f,g and the volume fraction of the liquid phase V f,l :

[0112] (5)

[0113] In some specific implementations, the CO2-brine surface tension coefficient is set to 0.03 N / m according to the target saltwater layer temperature and pressure conditions (50°C, 15 MPa).

[0114] S302. Add the "fluid heat transfer" physical field, and couple the "laminar flow" and "fluid heat transfer" physical fields through the multi-physics field "non-isothermal flow", and reasonably set the initial conditions and boundary conditions to simulate the heat transfer occurring during the CO2-brine two-phase displacement process.

[0115] The above “fluid heat transfer” physics field calculates the change in fluid temperature through equation (6):

[0116] (6)

[0117] In formula (6), C p is the heat capacity, J / (kg∙K); k is the thermal conductivity, W / (m∙K); Q is the heat source, W / (m 3 ∙s), Q describes the heat transfer caused by the evaporation of liquid water, which will be described in detail in step S303.

[0118] In some specific implementations, in the above-mentioned "fluid heat transfer" physical field, the initial temperature of the brine is set to 50°C, the fluid inlet 1 of the single pore model is set as the inflow boundary condition, and the upstream temperature is 50°C. The fluid outlet 2 of the single pore model is set as the outflow boundary condition. The pore wall 3 of the single pore model is set as the adiabatic boundary condition. A heat source is added, and the generalized heat source expression is set to Q.

[0119] S303. Add the "water transport" physical field, and couple the "laminar flow" and "water transport" physical fields through the "water flow" multi-physics field. Reasonably set the initial conditions and boundary conditions to simulate the generation and transport of water vapor during the CO2-brine two-phase displacement process.

[0120] In the pore-scale multi-field coupling model of salt precipitation caused by CO2 injection in saline layers, the liquid water evaporation dynamics is modeled by equation (7):

[0121] (7)

[0122] In formula (7), is the evaporation rate of liquid water, kg / m 2 ∙s;k v is the evaporation rate constant, 1 / s; V f,l is the volume fraction of the liquid phase; ρ w is the density of water, kg / m 3 ;p sat is the water vapor saturation pressure, Pa; p par is the water vapor partial pressure, Pa.

[0123] Therefore, the mass source phase S in equation (2) m for:

[0124] (8)

[0125] In formula (8), ρ g is the density of the gas phase, kg / m 3 .

[0126] Therefore, the phase field source term S in equation (4) is f for:

[0127] (9)

[0128] Therefore, the heat source term Q in equation (6) is:

[0129] (10)

[0130] In formula (10), L is the latent heat of evaporation, J / kg.

[0131] Therefore, the water vapor source term used to calculate the water vapor generation rate is:

[0132] (11)

[0133] In formula (11), ρ v is the density of water vapor, kg / m 3 .

[0134] The above “water transport” physics field controls the generation and transport of water vapor through equation (12):

[0135] (12)

[0136] In formula (12), M v is the molar mass of water, kg / mol, c v is the water vapor concentration, mol / m 3 ;D v is the water vapor diffusion coefficient, m 2 / s; G is the water vapor source term.

[0137] In some specific implementations, in the above-mentioned "water transport" physical field, the water vapor saturation in the model is set to 0 under the initial condition. The fluid inlet 1 of the single pore model is set as the inflow boundary condition, and the upstream water vapor saturation is 0, indicating that in this embodiment, the injected CO2 gas flow is dry. The fluid outlet 2 of the single pore model is set as the outflow boundary condition. The pore wall 3 of the single pore model is set as the insulating boundary condition, and no water vapor exchange occurs on the pore wall 3.

[0138] S304. Add the "Diluted Species Transport" physical field, and couple the "Laminar Flow" and "Diluted Species Transport" physical fields through the "Reacting Flow, Diluted Species" multi-physics field. Reasonably set the initial conditions and boundary conditions to simulate the transport and precipitation processes of NaCl in the CO2-brine two-phase displacement process.

[0139] The above “transfer of dilute species” physical field controls the transport of NaCl in brine through equation (13):

[0140] (13)

[0141] Among them, c s is the concentration of NaCl, mol / m 3 ;D s is the diffusion coefficient of NaCl in salt water, m 2 / s.

[0142] In some specific implementations, in the above-mentioned “transfer of dilute species” physical field, the initial concentration of NaCl in the brine is set to 5341.9 mol / m3 . Set the fluid inlet 1 of the single pore model as the inflow boundary condition, and the concentration constraint is 0. Set the fluid outlet 2 of the single pore model as the outflow boundary condition. Set the pore wall 3 of the single pore model as the flux boundary condition, which represents the process of dissolved salt precipitation from brine, and calculate it through equation (14):

[0143] (14)

[0144] Where n is the normal vector; b is the pore diameter, m; r m is the sedimentation rate coefficient, 1 / s; X NaCl(aq) is the mass fraction of NaCl, kg / kg; X L is the solubility limit of NaCl, kg / kg; M m is the molar mass of NaCl, kg / mol;

[0145] S305. Add the "Deformed Geometry" physical field and set the initial and boundary conditions appropriately to simulate the accumulation of salt precipitation on the pore walls during the CO2-brine two-phase displacement process.

[0146] In some specific implementations, in the above-mentioned "deformed geometry" physical field, the mesh smoothing type is set to Yeoh, the hardening factor is set to 10, and the initial deformation of the mesh is set to 0 m. The normal mesh displacement of the fluid inlet 1 and the fluid outlet 2 of the single pore model is specified to be 0 m. The accumulation process of salt precipitation on the pore wall surface is simulated by specifying the normal mesh displacement velocity of the pore wall 3 of the single pore model. The normal mesh displacement velocity of the pore wall 3 of the single pore model is calculated by formula (15):

[0147] (15)

[0148] In formula (15), ρ m is the density of the salt precipitate, kg / m 3 .

[0149] The CO2 material and the brine material are established, and the two-dimensional single-pore geometric model is meshed after adding the physical field. Specifically, the single-pore geometric model is meshed by the physical field, and the mesh unit size is set to "fine".

[0150] After meshing, the parameters are calculated according to the two-dimensional single-pore geometric model, CO2 material, brine material and physical field, and the microscopic mechanism of salt precipitation caused by CO2 injection into the saline layer is obtained according to the calculated results. For example, the present application can add research steps, set the output time step, and calculate the solution. Specifically, a "transient with phase initialization" study is added. The "transient with phase initialization" study includes two research steps, step 1 is "phase initialization" and step 2 is "transient"; setting the output time step means setting the output time step to 0.1 ms in step 2. The solution is as follows: Figures 4 to 7 As shown, Figure 4 The distribution diagram of CO2, brine and salt precipitation after 6.1 ms of gas injection is provided. Figure 5 The temperature distribution diagram after 6.1 ms of gas injection is provided. Figure 6 The water vapor saturation distribution diagram after 6.1 ms of gas injection is provided. Figure 7 The NaCl concentration profile after 6.1 ms of gas injection is provided.

[0151] After obtaining the solution results, the microscopic mechanism of salt precipitation caused by CO2 injection is analyzed according to the solution results. Specifically, according to the solution results of the pore-scale multi-field coupling model of salt precipitation caused by CO2 injection in saline water layers, the change law of the research variables of each physical field is examined, and the microscopic control mechanism of salt precipitation caused by CO2 injection is analyzed. Figure 4 As shown, Figure 4 The distribution diagram of CO2, brine and salt precipitation after 6.1 ms of gas injection is provided. According to the salt precipitation position 01, it can be concluded that salt precipitation mainly occurs at the intersection of CO2-brine-pore wall. Figure 5 As shown, Figure 5 The temperature distribution diagram after 6.1 ms of gas injection is provided. Figure 5 It can be concluded that the evaporation of liquid water mainly occurs at the CO2-brine two-phase interface, and the evaporation of liquid water will take away heat and reduce the temperature of CO2 and brine. Figure 6 As shown, Figure 6 The water vapor saturation distribution diagram after 6.1 ms of gas injection is provided. Figure 6 It can be concluded that the water vapor generated by the evaporation of liquid water will accumulate near the CO2-brine two-phase interface. With the continuous injection of dry gas, in the single pore model, the water vapor saturation shows a gradually increasing trend from the fluid inlet 1 to the two-phase interface. Figure 7 As shown, Figure 7 The NaCl concentration distribution after 6.1 ms of gas injection is provided. Figure 7 It can be concluded that the precipitation of NaCl mainly occurs at the CO2-brine two-phase interface. In the single pore model, the concentration of NaCl shows a gradually increasing trend from the two-phase interface to the fluid outlet 2.

[0152] This application establishes a pore-scale two-dimensional single-pore geometric model. The numerical simulation at the pore scale can easily obtain the real-time distribution of relevant variables such as flow rate, temperature, and concentration in the domain. At the same time, it can also consider the properties of the CO2-brine interface, and model the water evaporation dynamics and salt precipitation dynamics, which can deeply study the microscopic mechanism of salt precipitation caused by injecting CO2 into the saline layer; further, this application performs fully coupled numerical simulations on multi-phase fluid flow, gas-liquid phase change, heat transfer, material transport and other multi-physical processes, and examines the influence of relevant variables on salt precipitation, as well as the interaction between various physical fields, to provide technical means for studying the mechanism of salt precipitation in the process of underground CO2 injection. Therefore, the method provided in this application constructs a multi-physical field coupling model at the pore scale, which can numerically simulate the CO2-brine two-phase displacement, liquid water evaporation, heat transfer, dissolved salt transport and accumulation of salt precipitation on the pore wall in the process of CO2 saline layer storage, so as to realize in-depth research on the microscopic mechanism of salt precipitation caused by injecting CO2 into the saline layer.

[0153] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form as well.

[0154] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for constructing a pore-scale multi-field coupling model for salt precipitation caused by CO2 injection in saline water layers, characterized in that: The following steps are involved: S1. establishing a two-dimensional single pore geometric model, wherein the two-dimensional single pore geometric model includes a parallel fluid inlet and a fluid outlet, and a pore wall connecting the fluid inlet and the fluid outlet; S2, respectively establishing CO2 material and brine material, adding physical fields, setting boundary conditions and initial conditions of the two-dimensional single-pore geometric model, and meshing the two-dimensional single-pore geometric model; S3, calculating parameters according to the two-dimensional single pore geometric model, CO2 material, salt water material and physical field, and obtaining the microscopic mechanism of salt precipitation caused by CO2 injection into the saline layer according to the calculated results; In step S2, adding a physical field specifically includes: S301, add the "laminar flow" physical field and the "fluid phase field" physical field, couple the "laminar flow" and "fluid phase field" physical fields through the multi-physics field "two-phase flow, phase field", set initial conditions and boundary conditions, and simulate the two-phase displacement process of CO2-brine in a single pore; S302, add the "fluid heat transfer" physical field, couple the "laminar flow" physical field and the "fluid heat transfer" physical field through the multi-physics field "non-isothermal flow", set initial conditions and boundary conditions to simulate the heat transfer occurring during the CO2-brine two-phase displacement process in a single pore; S303, add the "Water Transport" physical field, and couple the "Laminar Flow" physical field and the "Water Transport" physical field through the "Water Flow" multi-physics field, set initial conditions and boundary conditions, and simulate the generation and transport of water vapor in the CO2-brine two-phase displacement process in a single pore; S304, add the "Diluted Species Transport" physics field, and couple the "Laminar Flow" physics field and the "Diluted Species Transport" physics field through the "Reacting Flow, Diluted Species" multiphysics field, set initial conditions and boundary conditions, and simulate the transport and precipitation process of NaCl in the CO2-brine two-phase displacement process in a single pore; S305. Add the "Deformed Geometry" physics field and set the initial and boundary conditions to simulate the accumulation of salt precipitation on the pore wall during the CO2-brine two-phase displacement process in a single pore. There is no order restriction between steps S302 to S305.

2. The method according to claim 1, characterized in that In the step S2, a CO2 material is established as a gas-added thermodynamic system, the substance CO2 is added, and the density of CO2 is calculated using the Peng-Robinson model, and the viscosity of CO2 is calculated using the Brokaw model; The step S2 of establishing the salt water material specifically involves adding the material "water" to approximately calculate the viscosity of the salt water, and using formula (1) to calculate the density of the salt water at different temperatures and pressures: Formula (1); In formula (1), ρ l is the density of the salt water, kg / m 3 ; ρ ref is the reference density of salt water, kg / m 3 ; α is the coefficient of thermal expansion, 1 / K; β is the compressibility factor, 1 / Pa; T is the temperature, K; P is the pressure, Pa; T ref is the reference temperature, K; P ref is the reference pressure, Pa.

3. The method according to claim 2, characterized in that In step S301, the "laminar flow" physical field controls the flow of the fluid through equations (2) and (3): Formula (2); In formula (2), u is the fluid velocity, m / s; S m is the mass source term, 1 / s, calculated according to formula (8): Formula (8); in, ρ g is the density of the gas phase, kg / m 3 ; is a dimensionless phase field variable; is the evaporation rate of liquid water; Formula (3); In formula (3), ρ is the fluid density, kg / m 3 ; t is the simulation time, s; p is the fluid pressure, Pa; I is the unit tensor; μ is the fluid dynamic viscosity, ; F st is the surface tension acting on the CO2 / brine interface, N / m; In the "laminar flow" physical field, the fluid inlet of the two-dimensional single pore geometric model is a velocity boundary condition, the fluid outlet of the two-dimensional single pore geometric model is a pressure boundary condition, and the pore wall of the two-dimensional single pore geometric model is a wall condition; The "fluid phase field" physical field tracks the movement of the CO2-brine two-phase interface through equation (4): Formula (4); In formula (4), is a dimensionless phase field variable; γ is the mobility, m 3 ·s / kg; λ is the mixing energy density, N; ε is the interface thickness parameter, m; S f is the phase field source term, 1 / s, calculated according to formula (9): Formula (9); In the "fluid phase field" physical field, the fluid inlet of the two-dimensional single pore geometric model is the inlet boundary condition; the fluid outlet of the two-dimensional single pore geometric model is the outlet boundary condition; and the pore wall of the two-dimensional single pore geometric model is the wetting wall boundary condition.

4. The method according to claim 3, characterized in that In step S302, the "fluid heat transfer" physical field calculates the change in fluid temperature through equation (6): Formula (6); In formula (6), C p is the heat capacity, J / (kg·K); k is the thermal conductivity, W / (m·K); Q is the heat source, W / (m 3 s), Q is the heat source term, calculated according to formula (10): Formula (10); In formula (10), L is the latent heat of vaporization, J / kg; In the "fluid heat transfer" physical field, the fluid inlet of the two-dimensional single pore geometric model is an inflow boundary condition, the fluid outlet of the two-dimensional single pore geometric model is an outflow boundary condition, and the pore wall of the two-dimensional single pore geometric model is an adiabatic boundary condition.

5. The method according to claim 4, characterized in that In step S303, the liquid water evaporation dynamics is modeled in the "water transport" physical field using equation (7): Formula (7) in, is the evaporation rate of liquid water, kg / m 2 ·s; k v is the evaporation rate constant, 1 / s; V f,l is the volume fraction of the liquid phase; ρ w is the density of water, kg / m 3 ; p sat is the water vapor saturation pressure, Pa; p par is the water vapor partial pressure, Pa; The "water transport" physical field controls the generation and transport of water vapor through equation (12): Formula (12); In formula (12), M v is the molar mass of water, kg / mol, c v is the water vapor concentration, mol / m 3 ; D v is the water vapor diffusion coefficient, m 2 / s; G is the water vapor source term, calculated according to formula (11): Formula (11) In formula (11), ρ v is the density of water vapor, kg / m 3 ; In the "water transport" physical field, the fluid inlet of the two-dimensional single pore geometric model is an inflow boundary condition, the fluid outlet of the two-dimensional single pore geometric model is an outflow boundary condition, and the pore wall of the two-dimensional single pore geometric model is an insulating boundary condition.

6. The method according to claim 5, characterized in that In step S304, the "diluted species transfer" physical field controls the transport of NaCl in the brine through equation (13): Formula (13) In formula (13), c s is the concentration of NaCl, mol / m 3 ; D s is the diffusion coefficient of NaCl in salt water, m 2 / s; In the "Transfer of Diluted Species" physical field, the fluid inlet of the two-dimensional single pore geometric model is the inflow boundary condition, the fluid outlet of the two-dimensional single pore geometric model is the outflow boundary condition, and the pore wall of the two-dimensional single pore geometric model is the flux boundary condition, which represents the process of dissolved salt precipitation from brine and is calculated by formula (14): Formula (14) In formula (14), n is the normal vector; b is the pore diameter, m; r m is the sedimentation rate coefficient, 1 / s; X NaCl(aq) is the mass fraction of NaCl, kg / kg; X L is the solubility limit of NaCl, kg / kg; M m is the molar mass of NaCl, kg / mol.

7. The method according to claim 5, characterized in that In step S305, in the "deformed geometry" physical field, the normal mesh displacement of the fluid inlet and fluid outlet of the two-dimensional single pore geometric model is 0 m, and the accumulation process of salt precipitation on the pore wall is simulated by specifying the normal mesh displacement speed of the pore wall of the two-dimensional single pore geometric model. The normal mesh displacement speed of the pore wall of the two-dimensional single pore geometric model is calculated by formula (15): Formula (15); In formula (15), ρ m is the density of the salt precipitate, kg / m 3 .

8. The method according to claim 7, characterized in that The step S3 comprises: Add a study step, set the output time step to solve, and obtain the CO2 / brine and salt precipitation distribution map, temperature distribution map, water vapor saturation distribution map and / or NaCl concentration distribution map; The microscopic mechanism of salt precipitation caused by CO2 injection into saline layers is analyzed based on the CO2 / brine and salt precipitation distribution map, temperature distribution map, water vapor saturation distribution map and / or NaCl concentration distribution map.

9. The method according to any one of claims 1 to 8, characterized in that: The height of the two-dimensional single pore geometric model is 40 μm to 120 μm, and the width is 100 μm to 200 μm.

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