Numerical simulation method of oil, gas and water three-phase at pore scale considering miscible characteristics and medium

Abstract

The invention discloses a pore-scale oil-gas-water three-phase numerical simulation method and medium considering miscible phase characteristics, which relate to the field of oil and gas field development. A contact line model, a reaction mass transfer model and a miscible phase mechanism model are coupled on the basis of a traditional three-phase color lattice Boltzmann model; the solid density fraction at the solid boundary lattice point is corrected by using the contact line model, the oil-gas miscible phase process is simulated by using the reaction mass transfer model, and the oil phase viscosity and the interfacial tension between the oil and gas phases at different positions in the calculation domain are corrected by using the miscible phase mechanism model, thereby improving the traditional three-phase color lattice Boltzmann model for the three-phase flow simulation process considering oil-gas miscible phase in the pores of the core sample, and obtaining a more accurate three-phase distribution.

Description

Technical Field

[0001] The invention relates to the field of oil and gas field development, and in particular to a pore-scale oil-gas-water three-phase numerical simulation method and medium taking miscible phase characteristics into consideration. Background Art

[0002] Gas injection is an important means to improve oil recovery. When gas enters the reservoir, the fluid in the reservoir becomes a three-phase system of oil, gas and water. In addition, the injected CO2, hydrocarbon gas and other gases will also be miscible with the formation crude oil, which leads to an extremely complex flow pattern of the three-phase system of oil, gas and water in the reservoir. Accurately understanding the flow pattern of the three-phase system of oil, gas and water at the pore scale is of great significance for revealing the mechanism of gas injection to improve oil recovery and guiding the optimization design of injection and production schemes.

[0003] The lattice Boltzmann model is often used for multiphase flow simulation at the pore scale. Currently, the commonly used multiphase lattice Boltzmann models include the color gradient model, pseudopotential model, phase field model and free energy model, among which the most widely used models are the color gradient model and pseudopotential model. Although the pseudopotential model is easy to program, it is easy to produce pseudoflow at the phase interface during multiphase simulation, that is, vortex-like non-physical velocity is generated at the phase interface. Its existence will cause the numerical simulation process to be unstable (when the pseudoflow is large, it will cause numerical divergence). At the same time, since the pseudoflow cannot be easily distinguished from the real flow velocity, it will also affect the accuracy of the flow simulation. In addition, the smaller droplets in the pseudopotential model cannot exist stably in the multiphase system, because it will gradually dissolve in other phases as the simulation proceeds, which is different from the actual physical process. Therefore, it is not suitable for oil, gas and water three-phase flow simulation. However, the color gradient model has the advantages of low pseudovelocity, stable solution under large viscosity ratio conditions, and easy adjustment of interfacial tension between phases. Therefore, it has been used to carry out pore-scale three-phase flow simulation.

[0004] Although the traditional three-phase color lattice Boltzmann model has been used for pore-scale oil, gas and water flow simulation, it still has the following problems for water flooding followed by gas flooding simulation: the phases are independent of each other and the miscibility process between oil and gas cannot be considered; it cannot reflect the changes in oil phase viscosity and interfacial tension between oil and gas after oil and gas miscibility; and it cannot accurately consider the wettability characteristics of the reservoir. Therefore, the traditional three-phase color lattice Boltzmann model is difficult to accurately describe the flow law and distribution characteristics of the three phases of oil, gas and water at the pore scale. Summary of the invention

[0005] The purpose of the present invention is to provide a pore-scale oil-gas-water three-phase numerical simulation method and medium taking into account the miscible characteristics. The multiphase color gradient lattice Boltzmann model is used as the main body, and the reaction mass transfer model, the miscible phase mechanism model and the contact line model are coupled to realize the microscopic numerical simulation of the oil-gas-water three-phase taking into account the miscible characteristics, which can accurately describe the flow law and distribution characteristics of the oil-gas-water three-phase at the pore scale.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics, characterized by comprising:

[0008] According to the pore image of the core sample, the solid area and the fluid area in the calculation domain are determined, the density and velocity of each phase of oil, gas and water in the fluid area are initialized, and a three-phase color lattice Boltzmann model corresponding to the core sample is constructed; wherein the solid area corresponds to the fixed grid unit in the three-phase color lattice Boltzmann model, and the fluid area corresponds to the flow grid unit in the improved three-phase color lattice Boltzmann model;

[0009] Fluid flow simulation is performed based on the three-phase color lattice Boltzmann model to obtain three-phase macroscopic quantities;

[0010] Based on the positions of solid boundary points and the corresponding normal vectors of the three-phase color lattice Boltzmann model, the contact line model is used to correct the density fraction of the solid boundary lattice points;

[0011] The improved reaction mass transfer model is used to simulate the oil-gas miscibility process in the oil phase region based on the three-phase macroscopic quantity to obtain the dissolved gas concentration at different positions in the oil phase; the improved reaction mass transfer model introduces a confined function and a reaction source term into the traditional reaction mass transfer model; the confined function is used to control the mass transfer process to occur only in the oil phase region; the reaction source term is used to generate a corresponding dissolved gas concentration field at the oil-gas phase interface;

[0012] Using a mixed phase mechanism model to obtain a corrected oil phase viscosity and a corrected oil-gas interfacial tension according to the dissolved gas concentration at different positions in the oil phase, so as to correct the relaxation time for setting the oil phase viscosity in the collision operator and the oil-gas interfacial tension in the disturbance operator in the three-phase color lattice Boltzmann model;

[0013] Return to step "Perform fluid flow simulation based on the three-phase color lattice Boltzmann model to obtain three-phase macroscopic quantities" until the preset simulation end condition is reached, and output the final three-phase macroscopic quantities to obtain the final three-phase distribution.

[0014] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the pore-scale oil-gas-water three-phase numerical simulation method that takes miscible phase characteristics into consideration.

[0015] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0016] The present invention provides a pore-scale oil-gas-water three-phase numerical simulation method and medium taking into account the miscible phase characteristics. On the basis of the traditional three-phase color lattice Boltzmann model, a contact line model, a reaction mass transfer model and a miscible phase mechanism model are coupled; the fluid density fraction near the solid boundary lattice provided by the contact line model based on the three-phase color lattice Boltzmann model is combined with the normal vector calculated by the position of the solid boundary lattice point to correct the density fraction at the solid boundary lattice point; the three-phase macroscopic output of the reaction mass transfer model based on the traditional three-phase color lattice Boltzmann model is corrected. The oil-gas miscibility process is simulated to obtain the dissolved gas concentration at different positions in the oil phase; the miscibility mechanism model is used to obtain the corrected values ​​of the oil phase viscosity and the interfacial tension between the oil and gas phases at different positions in the calculation domain according to the dissolved gas concentration in the oil phase output by the reaction mass transfer model, so as to correct the relaxation time for setting the oil phase viscosity in the collision operator and the interfacial tension between the oil and gas phases in the disturbance operator in the traditional three-phase color lattice Boltzmann model, thereby improving the three-phase color lattice Boltzmann model for three-phase flow simulation in the pores of the core sample and obtaining a more accurate three-phase microscopic distribution. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] 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 will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1 A schematic flow chart of a pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics provided in Example 1 of the present invention;

[0019] Figure 2 A schematic diagram of the coupling of the three-phase color lattice Boltzmann model, the contact line model, the reaction mass transfer model and the hybrid mechanism model provided in Example 1 of the present invention;

[0020] Figure 3 A schematic diagram of the classification and subclassification of different grid points in the calculation area provided in Example 1 of the present invention;

[0021] Figure 4 A distribution diagram of dissolved gas concentration during the gas phase dissolution and diffusion process in the oil phase provided in Example 1 of the present invention;

[0022] Figure 5 This is a contact angle model verification diagram provided in Example 1 of the present invention;

[0023] Figure 6The Laplace model verification diagram provided in Example 1 of the present invention;

[0024] Figure 7 This is a verification diagram of the double-hole model provided in Example 1 of the present invention;

[0025] Figure 8 A corner model verification diagram provided in Example 1 of the present invention;

[0026] Figure 9 The oil, gas and water three-phase distribution diagram of gas drive followed by water drive in a porous medium provided in Example 1 of the present invention;

[0027] Figure 10 This is a diagram of the dissolved gas concentration distribution in the porous medium provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The purpose of the present invention is to provide a pore-scale oil-gas-water three-phase numerical simulation method and medium taking into account the miscible characteristics, couple the traditional three-phase color lattice Boltzmann model with the reaction mass transfer model to consider the miscible process between oil and gas, introduce a miscible mechanism model to consider the viscosity reduction and interfacial tension reduction mechanism of oil and gas miscible phases, use a contact line model to achieve different reservoir wettability conditions, and construct a pore-scale oil-gas-water three-phase numerical simulation method taking into account the miscible characteristics to improve the accuracy of numerical simulation.

[0030] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] Example 1

[0032] like Figure 1 and Figure 2 As shown, a pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics in this embodiment specifically includes:

[0033] S1: Determine the solid area and fluid area in the calculation domain according to the pore image of the core sample, initialize the density and velocity of each phase of oil, gas and water in the fluid area, and construct a three-phase color lattice Boltzmann model corresponding to the core sample; wherein the solid area corresponds to the fixed grid unit in the three-phase color lattice Boltzmann model, and the fluid area corresponds to the flow grid unit in the three-phase color lattice Boltzmann model.

[0034] S2: Fluid flow simulation is performed based on the three-phase color lattice Boltzmann model to obtain three-phase macro quantities, including velocity and density data.

[0035] The simulation process of the three-phase color lattice Boltzmann model is mainly divided into four steps, namely collision step, perturbation step, recoloring step and migration step. The collision step and migration step are the same as the traditional lattice Boltzmann model. The perturbation step and recoloring step are used to introduce interfacial tension between different phases and to separate the three-phase fluid. In the three-phase color lattice Boltzmann model, three different colors (red r, blue b, green g) are used to describe different phases. Indicates k ( k = r, b, g) i The particle distribution function in the direction k The phase evolution equation is expressed as:

[0036] (1).

[0037] in, represents the grid coordinates; For the grid i Discrete speeds in directions; represents the time step; t express t time; is the collision operator. In the D2Q9 model It can be given by the following formula:

[0038] (2).

[0039] Collision Operator It consists of three parts:

[0040] (3).

[0041] In the formula, , and They represent the single relaxation collision operator, the perturbation operator and the recoloring operator respectively. The calculation process of the three operators is introduced below.

[0042] (1) Single relaxation collision operator

[0043] is a single relaxation collision operator with a volume force term, which is expressed as follows:

[0044] (4).

[0045] in, For the k Phase fluid in i In the calculation process, for the convenience of programming and the stability of the model, the total particle distribution function is used f i Calculate the collision process, namely:

[0046] (5).

[0047] in, is the total relaxation time of the three-phase fluid, Use the following formula to improve the calculation accuracy:

[0048] (6).

[0049] in, r k Representative k The density of the phase fluid, k = r , b , g , r Represents the total density of the three-phase fluid. Representative k The relaxation time of the phase fluid and the kinematic viscosity of the fluid u k Related, it can be expressed as:

[0050] (7).

[0051] For fluid i The total equilibrium distribution function in direction:

[0052] (8).

[0053] in, oh i are weights in different directions. For the D2Q9 model, oh0=4 / 9, oh 1-4 =1 / 9, oh 5-8 =1 / 36.

[0054] Fluid velocity It can be calculated by the following formula:

[0055] (9).

[0056] in, F For the additional volume force.

[0057] The external force term in formula (4) F i The external force format proposed in the prior art (Guo Z, Zheng C, Shi B. Discretelattice effects on the forcing term in the lattice Boltzmann method [J]. Physical Review E, 2002, 65 (4): 046308) is used for calculation:

[0058] (10).

[0059] In addition, k The pressure of the phase fluid is defined as: ,in, c s is the lattice sound speed, and its value is , represents the side length of the grid, and the total pressure is .

[0060] (2) Perturbation operator

[0061] is a perturbation operator used to consider the influence of interfacial tension between different phases. This operator uses continuous interfacial tension to introduce the influence of interphase interaction to ensure the conservation of mass and momentum. It can reduce pseudo-flow at the interface and improve the isotropy of the interface. At the same time, it can be seen from the formula (12) below x that this form of perturbation step can directly introduce the interfacial tension as a parameter without additional differences and approximations. It can be calculated by the following formula:

[0062] (11).

[0063] in, B i Defined as the parameter for model solution stability, B0=-2 / 9, B 1-4 =1 / 9, B 5-8 =1 / 36; It is used to adjust k Phase fluid and The parameters of the interfacial tension between the phase fluids, k- Interfacial tension for:

[0064] (12).

[0065] In the simulation, without loss of generality, it is assumed that = = a certain value.

[0066] is a concentration factor that ensures that interfacial tension is added only at k Phase fluid and At the phase-fluid interface:

[0067] (13).

[0068] in, for k The initial density of the phase fluid is usually taken as 1; Representative Density of the phase fluid; for The initial density of the phase fluid.

[0069] for k Phase fluid and The color gradient at the interface of the phase fluid introduces the interaction between different fluids. The following color gradient format is used for calculation to obtain numerically stable results:

[0070] (14).

[0071] Among them, the gradient term is solved using a fourth-order isotropic discrete method, and the specific formula is as follows:

[0072] (15).

[0073] in, The expression of is to replace the “ k " is replaced by " ”. The fourth-order weight function is:

[0074] (16).

[0075] (3) Recoloring operator

[0076] It is a recoloring operator used to update the three-phase fluid distribution, reduce the mixing between different phases and ensure that the fluids are not miscible. k The recoloring operator for the phase fluid is:

[0077] (17).

[0078] in, is the total particle distribution function after the perturbation step; It is a separation parameter used to adjust the phase interface thickness and needs to satisfy 0≤ ≤1 and = to ensure conservation of mass and momentum. Is the color gradient and grid speed The angle between ) is calculated using the following formula:

[0079] (18).

[0080] in, The specific calculation formula is as follows:

[0081] (19).

[0082] in, β 0 yes The initial value of is set to 0.7. Determined by the interfacial tension between the three phases:

[0083] (20).

[0084] in, Respectively m Phase fluid and k Interfacial tension between phase fluids, m Phase fluid and The interfacial tension between the phase fluids and k Phase fluid and Interfacial tension between phase fluids.

[0085] In formula (19), g ( ) is expressed as:

[0086] (twenty one).

[0087] The basic theory of the three-phase color lattice Boltzmann model was introduced above. The following is its specific simulation process.

[0088] 1. Unit vectors in all directions , weight coefficient oh i , the viscosity of each phase is the relaxation time , interfacial tension between the three phases and external forces The basic parameters such as are defined, and the solid and fluid regions in the calculation domain are defined according to the actual problem being simulated, and the density at each location in the fluid region is initialized.

[0089] 2. Calculate the collision operator. First, calculate the external force terms in different directions and bring them into the single relaxation collision operator to calculate the total particle distribution function after the collision step update. Then calculate the color gradient at the phase interface based on the fluid distribution in the calculation domain at this time, and bring it into the perturbation operator to calculate the total particle distribution function after the perturbation step update. Calculate the separation parameters at different positions and the angle between the color gradient and the grid velocity , and bring it into the recoloring operator to calculate the particle distribution function of each phase.

[0090] 3. Carry out the calculation of the migration step, that is, move the particle distribution functions of different directions of the grid points along the direction of the corresponding unit vector. After the migration step, calculate the particle distribution function at the boundary according to the actual situation simulated (that is, set the boundary conditions). At this time, the distribution functions in the calculation domain (particle distribution functions in the calculation domain and at the boundary) are updated. Then, the density, velocity and other macro quantities of each phase are calculated based on the particle distribution function in the calculation domain.

[0091] 4. Repeat the collision migration process in steps 2 and 3 above. In each iteration, the present invention couples the contact line model to correct the density of the solid boundary grid points in the three-phase color lattice Boltzmann model. For oil-gas miscibility, the coupled reaction mass transfer model reflects the miscibility process between the injected gas and crude oil to determine the dissolved gas concentration field in the oil phase region. At the same time, the coupled miscibility mechanism model corrects the oil phase viscosity in the single relaxation collision operator and the oil-gas interfacial tension in the perturbation operator in the three-phase color lattice Boltzmann model based on the dissolved gas concentration field output by the reaction mass transfer model. Until the simulation reaches stability (for example, for water drive simulation, the preset water injection volume or the preset cutoff time is reached; for gas drive, the preset gas injection volume or the preset cutoff time is reached), and the density and velocity fields in the calculation domain are output once every certain time step.

[0092] The following is a brief introduction to the grid unit conversion. In order to facilitate calculation and programming, the present invention does not use the actual physical unit system, but uses the grid unit system. The two can be converted based on similar criteria. The most basic parameter in the grid Boltzmann method simulation is the length of the grid. L ,density r and viscosity u , which correspond to the length in actual physical units ,density and viscosity , the calculation method of the similarity coefficient corresponding to each basic parameter is as follows:

[0093] (twenty two).

[0094] Among them, the length L ,density r and viscosity u The parameters include the dimensions of length, mass and time. Taking pressure as an example, its dimensions can be expressed by three basic parameters, namely:

[0095] (twenty three).

[0096] in,[ P ]、[ r ]、[ u ]and[ L ] represent the dimensions of pressure, density, viscosity, and length respectively. According to the dimensional relationship, the calculation formula of the pressure similarity coefficient can be obtained as follows:

[0097] (twenty four).

[0098] Therefore, the similarity coefficients of any other actual physical quantities can be calculated through the conversion relationship between dimensions, thereby converting grid units and actual physical units.

[0099] S3: Based on the solid boundary point positions and corresponding normal vectors of the three-phase color lattice Boltzmann model, the contact line model is applied to correct the density fraction of the solid boundary lattice points.

[0100] The density fraction on the boundary solid grid point is calculated using the characteristic line model (contact line model) x k , thereby obtaining the density fraction of the boundary grid points (i.e., the virtual density fraction). After setting the virtual density fraction, different three-phase contact angles on the solid surface are achieved, that is, different reservoir wettabilities are achieved.

[0101] In the characteristic line model, the grid points in the computational domain are divided into C F andC S There are two categories, corresponding to the grid points in the fluid and solid regions. Each part corresponds to two subcategories. C F , can be divided into and , corresponding to the grid points at the boundary and inside of the fluid, respectively; for the grid points in the solid region C S , can be divided into and , corresponding to the grid points at the solid boundary and inside, respectively, as shown in the schematic diagram Figure 3 The black line in the figure represents the boundary of the solid region, and the circle represents the fluid grid point. C F , solid and hollow represent and ; Squares represent solid grid points C S , solid and hollow represent and .

[0102] The implementation process of the characteristic line model can be divided into the following three steps. First, calculate each solid boundary grid point The unit normal vector at In order to reduce the generation of false velocity in the simulation, especially under curved boundary conditions, the eighth-order isotropic discrete method is used to calculate it. The specific formula is as follows:

[0103] (25).

[0104] in, s ( ) is a solid phase signature function, which takes 1 at the solid phase grid point and 0 at the fluid grid point; is an eighth-order weight function:

[0105] (26).

[0106] The second step is to calculate the contact line angle. In order to achieve a smooth transition of the contact angle at the junction of the three phases, the density fraction of each phase is used as a weighting coefficient to determine the contact line angle at different solid boundary lattice positions:

[0107] (27).

[0108] in, x r , x b , x gdenote the virtual density fractions of the red, blue and green phases at the solid boundary lattice points, respectively; i rb , i rg , i br , i bg These are the contact angles between the red-blue, red-green, blue-red, and blue-green phase fluids, respectively. i r and i b are the contact line angles of the red phase and the blue phase at the solid boundary lattice points, respectively.

[0109] because x r + x b + x g =1, so only the wetting boundary needs to be taken for any two-phase fluid. In the present invention, the red phase fluid and the blue phase fluid are selected. At the initial time step, the density fraction at the solid boundary grid is unknown, and it is initialized using the density fraction around the grid. The formula is as follows:

[0110] (28).

[0111] The third step is to obtain the contact line angle i k To calculate the density fraction on the boundary solid grid points x k The density of solid grid points is also divided into three types: r, g, and b. Figure 3 Taking point P in as an example, first obtain two characteristic lines passing through this point ( and ), which are related to the normal vector at point P The angle between , and about symmetry, Figure 3 middle, represents the tangent vector at point P. Then the characteristic line is extended outward Until it intersects the grid line at point D 1. If the point D 1 is on a horizontal grid line, then the point D The density fraction at 1 grid can be calculated by linear interpolation:

[0112] (29).

[0113] The subscripts lef and rig represent the points D 1 The adjacent grids on the left and right; and Represents the grid intersection points D 1: The density fraction of the adjacent grids on the left and right sides; x represents the horizontal coordinate of the grid. In addition, if the point D 1 is on a vertical grid line, then the point D The density fraction at 1 grid can be calculated by linear interpolation:

[0114] (30).

[0115] The subscripts low and up represent the points respectively. D 1 The lower and upper adjacent grids; and Represents the grid intersection points D 1 Density fraction of the adjacent grids on the upper and lower sides; y Represents the vertical coordinate of the grid. The same method can be used to It is worth noting that point D 1 and point D 2 should satisfy that the grid points adjacent to them all belong to C F If this condition is not met, the contact line should continue to be extended outward until the intersection with the grid line meets this condition. If there is still no intersection that meets this condition after extending outward for three grid widths, or if it exceeds the calculation area of ​​each core in parallel computing, the density score of the grid point is approximately calculated using formula (28).

[0116] After calculating and (point D 2 grids), one of them needs to be selected as the density score of point P. In order to ensure the continuity of the density score along the tangent direction of the solid surface, the density score of point P is selected according to the following formula:

[0117] (31).

[0118] When the density fractions of the grid points at all solid boundaries are set using the above method, the set contact angle can be accurately achieved.

[0119] S4: Using an improved reaction mass transfer model to simulate the oil-gas mixing process in the oil phase region based on the three-phase macroscopic quantity, the dissolved gas concentration at different positions in the oil phase is obtained; the improved reaction mass transfer model introduces a confined function and a reaction source term into the traditional reaction mass transfer model; the confined function is used to control the mass transfer process to occur only in the oil phase region; the reaction source term is used to generate a corresponding dissolved gas concentration field at the oil-gas phase interface.

[0120] The traditional three-phase color lattice Boltzmann model is coupled with the reaction mass transfer model to consider the miscible process between oil and gas.

[0121] The oil-gas miscibility process is essentially the diffusion process of the injected gas (which can be hydrocarbon gas and CO2, etc.) in the oil phase. When the injected gas comes into contact with the oil phase, the injected gas dissolves at the oil-gas interface and generates a corresponding dissolved gas concentration field at the oil-gas interface; then the dissolved gas begins to diffuse in the oil phase. If the injected gas is hydrocarbon gas, since hydrocarbon gas is only soluble in oil but not in water, the diffusion process only occurs in the oil phase.

[0122] In order to accurately reflect the miscibility process between the injected gas and crude oil, this paper first adopts the mass transfer model and adds confined functions and reaction source terms to characterize it. The mass transfer model describes the movement process of the injected gas and is used to characterize how the dissolved gas diffuses in the oil phase; the confined function is used to limit the dissolved gas diffusion process to the oil phase (corresponding to Formula 35); the reaction source term is used to realize the dissolution process of the gas at the oil-gas interface (corresponding to Formula 37). The following mainly introduces the model and couples it with the previous three-phase color lattice Boltzmann model. The specific coupling methods include: the velocity in the equilibrium distribution function is provided by the previous three-phase model , the density of each phase used in the confined function r and the region over which the reaction source term is applied.

[0123] The concentration distribution function evolution equation in the reaction mass transfer model is:

[0124] (32).

[0125] in, g i for i The concentration distribution function of dissolved gas in the oil phase in the direction; is the relaxation time related to the macroscopic diffusion coefficient, and the relationship between the two is D g =(1 / 3)( -0.5). is the equilibrium distribution function of dissolved gas concentration:

[0126] (33).

[0127] in, oh i is the weight coefficient. The D2Q5 model is selected in the diffusion model. It can reduce the amount of calculation of the model while ensuring the calculation accuracy. Therefore, the weights of different directions are oh 0=1 / 3, oh 1-4 =1 / 6; is the velocity at different locations in the computational domain, given by formula (9); C g It represents the concentration of dissolved gas, and the calculation formula is as follows:

[0128] (34).

[0129] In formula (32), ψ is a confined function, which is used to control the mass transfer process to occur only in the oil phase ( k Phase) within:

[0130] (35).

[0131] in, , is the equilibrium distribution function when the velocity is 0; β g For adjusting the oil phase ( k phase), and the concentration distribution state in the oil phase ( k Phase) is evenly distributed; Is the color gradient and grid speed The angle between It is the particle driving condition used to prevent dissolved gas from diffusing into other phases besides the oil phase:

[0132] (36).

[0133] When in the oil phase ( k Phase) , , restricted domain function ψ When the dissolved gas is 0, it can diffuse freely; when it is in the other two phases, , , the dissolved gas is pulled back into the oil phase by the negative source term (opposite to the color gradient) k In the phase), the outward diffusion process is restricted, so the confined mass transfer process is realized.

[0134] In formula (32), S i is the reaction source term, used in the oil and gas phase ( k - The corresponding dissolved gas concentration field is generated at the interface of the phase:

[0135] (37).

[0136] in, S k is the chemical reaction source term, defined as Sk = K ( C s - C g ) A ; K is the reaction rate at the interface; A It is the two-phase contact area where the reaction occurs, namely the oil-gas contact area; C s is the maximum dissolved gas concentration that can be achieved by the reaction. To ensure the mass conservation of the model, it is necessary to Subtract the corresponding mass source term, that is, the reaction source term S i Multiply by the molar mass of the dissolved gas.

[0137] Through the Chapman-Enskog expansion, the above concentration field evolution equation can be restored to the macroscopic mass transfer equation (Equation 38), thus proving the accuracy of the above lattice Boltzmann model.

[0138] (38).

[0139] Among them, the two terms on the left are transient phase and convection terms respectively; the first term on the right is the diffusion term, and the second term is used to realize the confined mass transfer process, where n Is the color gradient G kl The unit vector of the direction, the third term on the right side is the reaction source term. represents the particle driving condition; Represents the gradient.

[0140] S5: Using a mixed phase mechanism model to obtain a corrected oil phase viscosity and a corrected oil-gas interfacial tension according to the dissolved gas concentration at different positions in the oil phase, so as to correct the relaxation time for setting the oil phase viscosity in the collision operator in the three-phase color lattice Boltzmann model and the oil-gas interfacial tension of the corrected disturbance operator.

[0141] The miscibility mechanism model is introduced to reflect the mechanism of oil-gas miscibility reduction and interfacial tension reduction. Gas injection miscibility drive can significantly increase the reservoir recovery rate, mainly due to the two mechanisms of miscibility reduction and interfacial tension reduction. Miscibility reduction refers to the miscibility between the injected gas and the formation crude oil, the increase of the dissolved gas concentration in the oil phase, the significant decrease in the viscosity of the crude oil, and the reduction of the displacement resistance, which makes the crude oil easier to be produced; the reduction of interfacial tension is because after the oil and gas are miscible, the phase interface between the oil and gas disappears, and the interfacial tension between the two phases approaches 0, which increases the capillary number of the injected gas, improves the oil displacement efficiency of the injected gas, and thus improves the reservoir recovery rate. The above-mentioned reaction mass transfer model only considers the dissolution and diffusion process of the injected gas in the oil phase, and does not consider the influence of the dissolved gas on the properties of the oil phase. Therefore, the present invention establishes a mathematical model considering the miscibility mechanism to reflect the influence of the dissolved gas on the properties of the oil phase.

[0142] In view of the miscible viscosity reduction mechanism, the oil phase viscosity at different positions in the calculation domain is dynamically corrected according to the dissolved gas concentration in the oil phase obtained from the reaction mass transfer model:

[0143] (39).

[0144] in, C and C max are the concentration of dissolved gas in the oil phase and the maximum concentration of dissolved gas in the oil phase. C max Take 0.1. and The dissolved gas concentration is C The oil phase viscosity at represents the modified oil phase viscosity; R is the viscosity of the oil phase without dissolved gas concentration and gas phase viscosity m g The logarithm of the ratio, R =ln( / m g ). When the dissolved gas concentration reaches the maximum dissolved gas concentration in the oil phase, the oil phase viscosity decreases to the gas phase viscosity. The oil phase viscosity divided by the oil phase density gives the oil phase kinematic viscosity, which is the kinematic viscosity of the fluid in the modified formula (7): u k .

[0145] In view of the mechanism that miscibility reduces interfacial tension, the mathematical model also dynamically corrects the interfacial tension between the two phases based on the dissolved gas concentration in the oil phase at the oil-gas interface:

[0146] (40).

[0147] in, and No dissolved gas and dissolved gas concentration C The interfacial tension between oil and gas phases is It indicates the corrected interfacial tension between oil and gas phases. When the dissolved gas concentration in the oil phase at the oil-gas interface reaches the maximum dissolved gas concentration, the oil and gas are completely miscible and the interfacial tension is reduced to 0.

[0148] S6: Return to step “perform fluid flow simulation based on the three-phase color lattice Boltzmann model to obtain three-phase macroscopic quantities” until the preset simulation end condition is reached, and output the final three-phase macroscopic quantities to obtain the final three-phase distribution.

[0149] Before executing the return step, for the fluid flow simulation process of the three-phase color lattice Boltzmann model, the density fraction of the solid boundary lattice points is corrected, the dissolved gas concentration at different positions in the oil phase is obtained, the corrected oil phase viscosity and the corrected oil-gas interfacial tension are obtained, the relaxation time used to set the oil phase viscosity in the collision operator in the three-phase color lattice Boltzmann model and the oil-gas interfacial tension in the corrected disturbance operator are corrected, so the flow simulation is re-performed based on the corrected data until the simulation end conditions are reached.

[0150] In this embodiment, the three-phase color gradient model is used as the main body. By coupling the reaction mass transfer model, the miscibility mechanics model and the contact line model, a pore-scale oil-gas-water three-phase numerical simulation method that takes into account the miscibility characteristics is formed, thereby realizing the pore-scale oil-gas-water three-phase flow simulation under different reservoir wettability conditions.

[0151] Among them, the coupling process of the three-phase color lattice Boltzmann model, the reaction mass transfer model and the mixed phase mechanism model is mainly reflected in: the reaction mass transfer model calculates the concentration equilibrium distribution function Required macro speed , calculate the confined mass transfer function ψ Desired density fraction and color gradient for each phase and the area where the reaction process occurs A Generated during the iterative calculation of the three-phase model; the dissolved gas concentration at different locations in the oil phase required by the mixed phase mechanism model C g Produced by iterative calculation of reaction mass transfer model; oil phase viscosity calculated by mixed phase mechanism model Interfacial tension between oil and gas It is used in the iteration process of the three-phase color lattice Boltzmann model to correct some parameters.

[0152] The coupling process between the three-phase color lattice Boltzmann model and the contact line model is mainly reflected in: the three-phase color lattice Boltzmann model provides the contact line model with the positions of the solid boundary lattice points and the corresponding normal vectors , the contact line model provides the density fraction of the solid at the solid boundary grid point for the three-phase color lattice Boltzmann model x k .

[0153] This embodiment solves the defect that existing research focuses on two-phase seepage and does not consider the oil-gas miscibility process. First, an immiscible oil-gas-water three-phase model is constructed based on the multiphase color gradient model. Because the oil-gas miscibility process is essentially a process in which the gas phase dissolves at the oil-gas interface and diffuses in the oil phase, the mass transfer model is used to describe the gas diffusion law and couple it with the three-phase color lattice Boltzmann model. However, since the injected gas can only be miscible with the oil phase and cannot be dissolved in the water phase, and the traditional mass transfer model cannot characterize this phenomenon, a restricted function is introduced into the mass transfer model to reflect this process, and a reaction source term is introduced to realize the dissolution of the gas phase at the oil-gas interface. A miscibility mechanism model is added to the above-mentioned coupling model, and the dissolved gas concentration in the oil phase is used to locally correct the oil phase viscosity and the interfacial tension between oil and gas, so as to realize the consideration of viscosity reduction and interfacial tension reduction after oil-gas miscibility. The contact line model is used to calculate the virtual density at the solid boundary, so as to realize different oil-gas-water three-phase contact angles, so that the reservoir wettability can be accurately characterized. The constructed model was verified by using examples such as contact angle model, Young-Laplace law, prism coupling reaction mass transfer model, double pore model and corner model. Figure 4-Figure 8 As shown, the simulation results are in line with expectations and have good consistency with the theoretical results, which proves the accuracy of the model. Figure 7 Among them, (a) is the three-phase displacement process at different time steps without considering the miscible viscosity reduction mechanism, (b) is the three-phase displacement process at different time steps considering the miscible viscosity reduction mechanism, and (c) is the concentration distribution of dissolved gas in the oil phase at different time steps considering the miscible viscosity reduction mechanism. Figure 6 In the figure, Δp represents the pressure difference, and the LBM simulation result represents the simulation result based on the three-phase color lattice Boltzmann model in the present invention. Figure 6 In the figure, the six points in the LBM simulation results can be fitted into a straight line (the straight line corresponding to the fitting result), which indicates that the simulation method of the present invention is effective. Figure 8Among them, (a) is the three-phase displacement process at different time steps without considering the miscible phase reduction viscosity and interfacial tension mechanism, (b) is the three-phase displacement process at different time steps considering only the miscible phase reduction interfacial tension mechanism, (c) is the distribution diagram of dissolved gas concentration in the oil phase at different time steps considering only the miscible phase reduction interfacial tension mechanism, (d) is the three-phase displacement process at different time steps considering both the miscible phase reduction viscosity and interfacial tension mechanism, (e) is the distribution diagram of dissolved gas concentration in the oil phase at different time steps considering both the miscible phase reduction viscosity and interfacial tension mechanism.

[0154] The model constructed in the present invention is suitable for pore-scale oil-gas-water three-phase flow simulation considering the mixing mechanism. The simulation obtains the oil-gas-water three-phase distribution in the porous medium and the dissolved gas concentration distribution in the oil phase as shown in the figure. Figure 9-Figure 10 As shown, it provides an effective means for understanding the mechanism of gas injection to enhance oil recovery and optimizing the design of injection-production schemes.

[0155] Example 2

[0156] This embodiment provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the method for numerical simulation of oil, gas and water three-phase at a pore scale taking into account miscible characteristics in Embodiment 1 is implemented.

[0157] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0158] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics, characterized in that: include: According to the pore image of the core sample, the solid area and the fluid area in the calculation domain are determined, the density and velocity of each phase of oil, gas and water in the fluid area are initialized, and a three-phase color lattice Boltzmann model corresponding to the core sample is constructed; wherein the solid area corresponds to the fixed grid unit in the three-phase color lattice Boltzmann model, and the fluid area corresponds to the flow grid unit in the three-phase color lattice Boltzmann model; Fluid flow simulation is performed based on the three-phase color lattice Boltzmann model to obtain three-phase macroscopic quantities; Based on the positions of solid boundary points and the corresponding normal vectors of the three-phase color lattice Boltzmann model, the contact line model is used to correct the density fraction of the solid boundary lattice points; The improved reaction mass transfer model is used to simulate the oil-gas miscibility process in the oil phase region based on the three-phase macroscopic quantity to obtain the dissolved gas concentration at different positions in the oil phase; the improved reaction mass transfer model introduces a confined function and a reaction source term into the traditional reaction mass transfer model; the confined function is used to control the mass transfer process to occur only in the oil phase region; the reaction source term is used to generate a corresponding dissolved gas concentration field at the oil-gas phase interface; Using a mixed phase mechanism model to obtain a corrected oil phase viscosity and a corrected oil-gas interfacial tension according to the dissolved gas concentration at different positions in the oil phase, so as to correct the relaxation time for setting the oil phase viscosity in the collision operator and the oil-gas interfacial tension in the disturbance operator in the three-phase color lattice Boltzmann model; Return to step "Perform fluid flow simulation based on the three-phase color lattice Boltzmann model to obtain three-phase macroscopic quantities" until the preset simulation end condition is reached, and output the final three-phase macroscopic quantities to obtain the final three-phase distribution.

2. The pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics according to claim 1 is characterized in that: The three-phase color lattice Boltzmann model k The phase evolution equation is expressed as: ； in, ; ； ； ； In the formula, Indicates k Phase fluid in i The particle distribution function in the direction, k = r , b , g , r represents the red phase fluid, b represents the blue phase fluid, g represents green phase fluid; , and Respectively represent k Single relaxation collision operators, perturbation operators and recoloring operators for phase fluids; represents the coordinates of the grid points; Indicates that the grid is i Discrete speeds in directions; i =0, 2, ..., 8; represents the time step; Representative k The relaxation time of the phase fluid is related to the kinematic viscosity of the fluid; For the k Phase fluid in i equilibrium distribution function in direction; F i represents the external force term; It is used to adjust Phase fluid and The parameters of the interfacial tension between the phase fluids, = r , b , g , ; is a concentration factor; for k Phase fluid and Color gradient at the phase-fluid interface; ω i are weights in different directions; B i Defined as the parameter for model solution stability; is the total particle distribution function after the perturbation step; is the separation parameter; Is the color gradient and the lattice discrete velocity Angle between represents the fluid velocity; Representative k Density of the phase fluid; Representative Density of the phase fluid; ρ represents the total density of the three-phase fluid; For fluid i The total equilibrium distribution function in direction; Represents the collision operator.

3. The pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics according to claim 2 is characterized in that: Based on the solid boundary point positions and corresponding normal vectors of the three-phase color lattice Boltzmann model, the contact line model is used to correct the density fraction of the solid boundary lattice points, including: Calculate the unit normal vector at each solid boundary grid point in the three-phase color grid Boltzmann model; Using the density fraction of each phase at the solid boundary grid point as a weighting coefficient, and combining the contact angle between each two-phase fluid at the solid boundary grid point to determine the contact line angle at different solid boundary grid points; The density fraction correction value of the solid boundary grid point is calculated according to the unit normal vector and the contact line angle in combination with the density fraction of the fluid at the solid boundary grid.

4. The pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics according to claim 3 is characterized in that: The calculation expression based on which the contact line angle at different solid boundary grid points is determined is: ； In the formula, θ rb , θ rg , θ br , θ bg They are the contact angles between red-blue, red-green, blue-red, and blue-green phase fluids, respectively; θ r and θ b are the contact line angles of the red phase and the blue phase at the solid boundary lattice points, respectively; ξ r , ξ b , ξ g are the density fractions of the red, blue, and green phases at the solid boundary grid points, respectively. The density fractions of different phases at the solid boundary grid points at the initial time step are set using the following formula: ； In the formula, ξ k is the solid boundary grid point k The density fraction of the phase, w i is the weight coefficient in different directions; C F represents the fluid grid region; represents the grid points at the solid boundary; Indicates the number of fluid grid points within a preset range at the solid boundary grid point. k Density fraction of the phase fluid.

5. The pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics according to claim 3 is characterized in that: The density fraction correction value of the solid boundary grid point is calculated according to the unit normal vector and the contact line angle in combination with the density fraction of the fluid at the solid boundary grid, specifically including: Two characteristic lines at each solid boundary grid point are determined according to the unit normal vector and the contact line angle; the two characteristic lines are symmetrical about the unit normal vector, and the angle between the characteristic line and the unit normal vector is ; θ k is the contact line angle; Extend each characteristic line outward to obtain a target grid intersection point; the grid points adjacent to both sides of the target grid intersection point are all fluid grid points; When the grid width of the characteristic line extension is not greater than the preset width or the characteristic line extension does not exceed the calculation domain, the density fraction of the fluid at the target grid intersection is determined according to the density fractions of the fluids at the adjacent grid points on both sides of the target grid intersection; the density fraction correction value at the corresponding solid boundary grid point is calculated according to the density fraction of the fluids at the two target grid intersections; when the grid width of the characteristic line extension is greater than the preset width or the characteristic line extends beyond the calculation domain, the density fraction correction value at the corresponding solid boundary grid point is calculated according to the density fraction of the grid within a preset range at the solid boundary grid point.

6. The pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics according to claim 4 is characterized in that: The calculation expression of the unit normal vector at each solid boundary grid point is: ； In the formula, is a solid phase signature function, which takes 1 at the solid phase lattice point and 0 at the fluid lattice; is an eighth-order weight function.

7. The pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics according to claim 4 is characterized in that: The evolution equation of the dissolved gas concentration distribution function corresponding to the improved reaction mass transfer model is: ； in, ; ； In the formula, g i For dissolved gas i Concentration distribution function in direction; τ g is the relaxation time related to the macroscopic diffusion coefficient; is the equilibrium distribution function of dissolved gas concentration; ψ is a restricted domain function; S i is the reaction source term; is the equilibrium distribution function when the velocity is 0; Parameters for adjusting the concentration distribution of dissolved gas in the oil phase; It is a particle driving condition used to prevent dissolved gas from diffusing into other phases except the oil phase; S k is the chemical reaction source term, defined as S k = K ( C s - C g ) A ; K is the reaction rate at the interface; A It is the two-phase contact area where the reaction occurs, namely the oil-gas contact area; C s is the maximum dissolved gas concentration that can be achieved by the reaction; C g Indicates the concentration of dissolved gas; , Represents the side length of the grid.

8. The pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics according to claim 2 is characterized in that: The expression of the corrected oil phase viscosity is: ； The expression of the modified interfacial tension between oil and gas phases is: ； In the formula, The dissolved gas concentration is C Corrected oil phase viscosity at ; It is the viscosity of the oil phase when there is no dissolved gas; C and C max are the concentration of dissolved gas in the oil phase and the maximum concentration that can be achieved by dissolved gas in the oil phase; R It is the logarithm of the ratio of the viscosity of the oil phase excluding the dissolved gas concentration to the viscosity of the gas phase; The dissolved gas concentration is C The corrected interfacial tension between oil and gas phases at ; It is the interfacial tension between oil and gas phases when there is no dissolved gas.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the pore-scale oil-gas-water three-phase numerical simulation method considering miscible phase characteristics described in any one of claims 1 to 8 is implemented.

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