Two-phase displacement numerical simulation method based on multi-pipe-diameter capillary model

Abstract

The invention discloses a two-phase displacement numerical simulation method based on a multi-pipe-diameter capillary model, and belongs to the field of pore scale displacement numerical simulation. The method comprises the following steps: firstly, establishing a multi-pipe-diameter capillary physical model based on flow channel size characteristics of a low-permeability reservoir; setting a capillary group as a displaced phase domain according to the number of pipe diameters required to be researched and the size of the capillary diameter, and additionally setting a rectangle I to represent a displacement phase source; and then taking the multi-pipe-diameter capillary physical model as a computational domain, further constructing a two-phase displacement numerical simulation method, and simulating the displacement process on the basis of the two-phase displacement numerical simulation method. According to the method, the single form of an existing capillary tube model is overcome, the multi-pipe-diameter capillary tube physical model is constructed, and the design purpose of the model is to determine the minimum pipe diameter threshold value capable of causing the displacement phenomenon when the capillary tube displacement behavior is explored under the multi-pipe-diameter coexistence condition; and whether the movement of fluid in a smaller pipe diameter can be effectively activated or not by adjusting injection-production parameters through a numerical simulation means is further discussed.

Description

Technical Field

[0001] The present invention belongs to the field of pore-scale displacement numerical simulation, and in particular relates to a two-phase displacement numerical simulation method based on a multi-diameter capillary model. Background Art

[0002] With the advancement of exploration and development technology in my country, low-permeability reservoirs are becoming more and more popular in both crude oil production and CO 2 The development and utilization of low-permeability reservoirs currently face many challenges: (1) The geological permeability of low-permeability reservoirs is generally low, resulting in slow flow of fluids in the reservoir and great difficulty in exploitation; (2) The porosity of low-permeability reservoirs is usually small, which means that the fluid storage space in the reservoir is limited, further increasing the difficulty of exploitation; (3) Due to the geological complexity of low-permeability reservoirs, their seepage laws are often difficult to accurately predict, which poses a great challenge to fluid management during the exploitation process.

[0003] Low permeability reservoirs are mostly developed with micron-sized pore throats, mainly medium and small pores, and the throats are mainly tubular and lamellar thin throats. Their geology generally presents low permeability, low porosity, small pore throats, and strong heterogeneity. The complex pore throat structure in the reservoir can be regarded as a connection of multiple capillaries of different radii. The capillary model can simulate the flow behavior of fluids in tiny pores, deeply study the flow mechanism between two-phase fluids, and the distribution and migration laws of fluids in complex porous media. This helps to have a deeper understanding of the fluid flow characteristics in low permeability reservoirs and provide a scientific basis for formulating effective development strategies.

[0004] In the reservoir, the original fluid is mainly concentrated in the larger pore throats, while the fluid distribution in the small pore throats is relatively small. When displacing, the displacing phase often prefers to enter and displace the pore channels with smaller seepage resistance. Since the channels of large pore throats are wider, the resistance encountered by the fluid flow is relatively small, making it easier for the displacing phase to enter the large pore throats and displace the displaced phase therein. In contrast, the channels of small pore throats are narrow, and the resistance encountered by the fluid flow is larger. In addition, the effect of capillary forces makes it difficult for the displacing phase to enter the small pore throats and displace the displaced phase therein under a small pressure difference. Therefore, it is of great practical guiding significance to explore the minimum pore size required for fluid injection under different pore sizes, and to deeply analyze whether adjusting the injection and production parameters can effectively change this minimum pore size. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the single form of the capillary model in the prior art, and to provide a two-phase displacement numerical simulation method based on a multi-diameter capillary model. In the present invention, the design purpose of the multi-diameter capillary physical model is to explore the capillary displacement behavior under the condition of coexistence of multiple diameters, determine the minimum diameter threshold that can trigger the displacement phenomenon, and further explore whether adjusting the injection and production parameters by numerical simulation can effectively activate the movement of fluids in smaller diameters. At the same time, the model is useful in studying pore-scale crude oil production and saline CO 2 It has important guiding significance and practical value in geological storage and other related fields.

[0006] The specific technical solutions adopted by the present invention are as follows:

[0007] The present invention provides a two-phase displacement numerical simulation method based on a multi-diameter capillary model, which is as follows:

[0008] S1: According to the target tube diameter number i and the diameter size D of each capillary tube i , set up a two-dimensional multi-diameter capillary model;

[0009] S2: Rectangle I is used to represent the displacement phase source and is placed on one side of the two-dimensional multi-diameter capillary model, so that one side of the displacement phase source coincides with one end of each capillary, thereby constructing a multi-diameter capillary physical model;

[0010] S3: Using the multi-diameter capillary physical model as the calculation domain, a two-phase displacement numerical model based on the phase field method is established;

[0011] S4: According to the two-phase displacement numerical model, the displacement process is simulated by numerical simulation method to obtain the minimum mobilization aperture of multi-diameter capillaries and the flow state change of the two-phase fluid; on this basis, the injection and production parameters are adjusted, and the displacement effect under different parameters is observed to improve the recovery rate or storage efficiency.

[0012] Preferably, in the two-dimensional multi-diameter capillary model of S1, the capillaries are represented by rectangles and the fluid inside them is the displaced phase source, and the capillaries are arranged horizontally from top to bottom.

[0013] Furthermore, in S2, rectangle I is placed on the left side of the two-dimensional multi-diameter capillary model so that the right boundary of the displacement phase source coincides with the left boundaries of all capillaries.

[0014] Preferably, the S1 is as follows:

[0015] S11: Use wide value W i Different rectangles represent each capillary, so that W i =D i ;

[0016] S12: Length L of all capillaries i Set to the same value, place all capillaries in a way that their long sides are parallel to the X-axis of the rectangular coordinate system and do not overlap each other, and make the left vertical boundaries of all capillaries correspond to the same value of the X-axis of the rectangular coordinate system;

[0017] S13: setting the vertical distance between two adjacent capillaries according to the target flow channel distance.

[0018] Furthermore, the S2 is specifically as follows:

[0019] S21: Place the displacement phase source represented by rectangle Ⅰ on the left side of the multi-diameter capillary; one of the two adjacent sides of rectangle Ⅰ is parallel to the X-axis of the rectangular coordinate system, and the other is parallel to the Y-axis of the rectangular coordinate system;

[0020] S22: The bottom side of rectangle I parallel to the X-axis of the rectangular coordinate system has the same y value as the bottom side of the capillary located at the bottom, and the top side of rectangle I parallel to the X-axis has the same y value as the top side of the capillary located at the top, that is, the side length of rectangle I parallel to the Y-axis is exactly the same as the interval occupied by the multi-diameter capillary on the Y-axis.

[0021] Preferably, S3 is as follows:

[0022] S31: In the two-phase displacement process, the laminar flow model is adopted, and the mass conservation in the fluid control equation is expressed by the continuity equation:

[0023]

[0024] Where u is the fluid velocity, m / s; ρ is the fluid density, kg·m -3 ;

[0025] The momentum equation of the fluid is expressed by the Navier-Stokes equation, which is used to solve its velocity field and pressure field:

[0026]

[0027] Where t is time, s; I is the unit matrix; μ is the fluid viscosity, Pa·s; F s is the interfacial tension term, Pa·m -1 ; p is the fluid pressure, Pa;

[0028] S32: The phase field method is used to track the interface between the displacing phase and the displaced phase. The convection-diffusion Cahn-Hilliard equation that controls the evolution of the interface profile is:

[0029]

[0030] Where φ is the phase field variable; γ is the mobility, m 3 ·s·kg -1 ;λ is the mixing energy density, N;ε is the interface thickness, m;ψ is the phase field auxiliary variable;

[0031] Interfacial tension term F s Defined as:

[0032]

[0033] Where G is the chemical potential, J·m -3 ;

[0034] Subscript 1 represents the displaced phase, subscript 2 represents the displacing phase, and the volume fraction of the single-phase fluid is calculated as follows:

[0035]

[0036] V f,1 +V f,2 =1 (9)

[0037] Where V f,1 is the volume fraction of the displaced phase, V f,2 is the volume fraction of the displacing phase;

[0038] The dynamic viscosity and density of the fluid physical parameters on the interface between the two phases during the displacement process are also defined as functions of the phase field variables, so that they change continuously on the interface;

[0039] The density at the interface between the two phases is:

[0040] ρ=ρ 1 +(ρ 2 -ρ 1 )V f，2 (10)

[0041] The viscosity at the interface between the two phases is:

[0042] μ=μ 1 +(μ 2 -μ 1 )V f，2 (11)

[0043] S33: setting the density, viscosity and interfacial tension of the selected displacement phase and displaced phase;

[0044] S34: setting boundary conditions and initial conditions of the displacement process; the boundary conditions include inlet pressure or inlet velocity, wetting angle, outlet pressure or outlet velocity, and the initial conditions include initial temperature and initial pressure;

[0045] S35: Determine the initial distribution state of the two-phase fluid in the calculation domain and perform displacement simulation.

[0046] Preferably, the S4 is as follows:

[0047] S41: As the occurrence state of the fluid in the multi-diameter capillary physical model tends to be stable, when the displacement simulation is completed, the output content includes: the migration distribution cloud map of the two-phase fluid at different times, and the volume fraction of the single-phase fluid in different flow domains at different times; the mobilization of the fluid in each capillary can be intuitively observed based on the cloud map of the output results; at the same time, by analyzing and processing the output results, the change curve of the recovery rate or storage rate is obtained;

[0048] S42: According to the principle of the single variable method, adjust the key displacement parameters such as interfacial tension, wetting angle, inlet pressure, inlet velocity and two-phase viscosity ratio, and repeat the steps of S3; when the displacement process reaches the end point, a cloud map and a curve map containing the result information can also be generated; through the analysis of these output results, it is possible to intuitively observe the evolution of the flow pattern during the fluid migration process after the displacement conditions change, the difference in the occurrence state, and the change in the degree of fluid mobilization in each capillary channel.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] (1) Based on the numerical simulation of two-phase flow displacement, this paper innovatively proposes a multi-diameter capillary model (i.e., a multi-diameter capillary physical model). The design of this model takes into account the complex geological conditions of the coexistence of large pore throats and small pore throats in the same formation. The proposed model provides an effective tool for two-phase displacement in low-permeability reservoirs to explore the approximate range of the minimum pore diameter used in the fluid displacement process under the condition of simultaneous displacement of different pore diameters. The determination of this range is of certain significance for optimizing pressure-driven strategies, improving fluid utilization efficiency, reducing resource waste, and reducing environmental risks.

[0051] (2) In the process of exploring the minimum mobilization aperture range, the above-mentioned exploration process was further improved by relying on the two-phase flow numerical simulation method. Through this method, the flow characteristics of fluids in capillaries with different apertures can be deeply analyzed at the microscopic scale, and a more accurate and comprehensive analysis of fluid mobilization efficiency and displacement effect can be made, which can also provide a reference for production practice at the macroscopic level. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 It is a schematic diagram of the model of the present invention.

[0053] Figure 2 Schematic diagram of fluid distribution at the initial moment in the embodiment.

[0054] Figure 3 It is a cloud diagram of the displacement process in the embodiment.

[0055] Figure 4 It is a curve diagram of the recovery rate changing with time in the embodiment.

[0056] Figure 5 This is a cloud diagram of the displacement process when the inlet displacement pressure is changed in the embodiment.

[0057] Figure 6 This is a cloud diagram of the displacement process when the capillary wettability is changed in the example. DETAILED DESCRIPTION

[0058] The present invention is further described and illustrated below in conjunction with the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0059] The present invention provides a two-phase displacement numerical simulation method based on a multi-diameter capillary model. The method first establishes a capillary model with multiple diameters based on the flow channel size characteristics of a low-permeability reservoir: a capillary group is set as a displaced phase domain according to the number of diameters to be studied and the size of the capillary diameters, and a rectangle I is set to represent a displacing phase source. The capillary group and rectangle I constitute a multi-diameter capillary model; then, the capillary model is used as a calculation domain to further construct a numerical simulation method for two-phase displacement, and the displacement process is simulated on this basis.

[0060] Each step of the two-phase displacement numerical simulation method of the present invention will be described in detail below.

[0061] S1: According to the number of tube diameters i and the size of each capillary tube diameter D i , set different capillaries to obtain a two-dimensional multi-diameter capillary model.

[0062] As a preferred embodiment of the present invention, Figure 1 As shown, in the two-dimensional multi-diameter capillary model, the capillaries are represented by slender rectangles and the fluid inside them is the displaced phase source, and the capillaries are arranged horizontally from top to bottom.

[0063] As a preferred embodiment of the present invention, the steps are as follows when used:

[0064] S11: Use wide value W i Different rectangles represent each capillary, so that W i =D i ;

[0065] S12: Length L of all capillaries i Set to the same value, place all capillaries in a way that their long sides are parallel to the X-axis of the rectangular coordinate system and do not overlap each other, and make the left vertical boundaries of all capillaries correspond to the same value of the X-axis of the rectangular coordinate system;

[0066] S13: The vertical distance between two adjacent capillaries is set according to the flow channel distance to be studied.

[0067] S2: A rectangle I is used to represent a displacement phase source and is placed on one side of the two-dimensional multi-diameter capillary model, so that one side of the displacement phase source coincides with one end of each capillary, thereby constructing a multi-diameter capillary physical model.

[0068] As a preferred embodiment of the present invention, Figure 1 As shown, in the multi-diameter capillary physical model, rectangle I is placed on the left side of the two-dimensional multi-diameter capillary model so that the right boundary of the displacement phase source coincides with the left boundaries of all capillaries.

[0069] As a preferred embodiment of the present invention, the steps are as follows when used:

[0070] S21: Place the displacement phase source represented by rectangle Ⅰ on the left side of the multi-diameter capillary; one of the two adjacent sides of rectangle Ⅰ is parallel to the X-axis of the rectangular coordinate system, and the other is parallel to the Y-axis of the rectangular coordinate system; there is no specific requirement for the length of the side parallel to the X-axis, which is only used as a symbol of the displacement phase source;

[0071] S22: The bottom side of rectangle I parallel to the X-axis of the rectangular coordinate system has the same y value as the bottom side of the capillary located at the bottom, and the top side of rectangle I parallel to the X-axis has the same y value as the top side of the capillary located at the top, that is, the side length of rectangle I parallel to the Y-axis is exactly the same as the interval occupied by the multi-diameter capillary on the Y-axis.

[0072] S23: Rectangle Ⅰ and all the capillaries on its right side are combined to form a multi-diameter capillary physical model, such as Figure 1 On this basis, the numerical simulation of two-phase displacement is carried out.

[0073] S3: Using the multi-diameter capillary physical model established in S2 as the calculation domain, a two-phase displacement numerical model based on the phase field method is established.

[0074] As a preferred embodiment of the present invention, the steps are as follows when used:

[0075] S31: In the two-phase displacement process, due to the small Reynolds number of the fluid, the laminar flow model is adopted, and the mass conservation in the fluid control equation is expressed by the continuity equation:

[0076]

[0077] Where u is the fluid velocity, m / s; ρ is the fluid density, kg·m -3 ;

[0078] The momentum equation of the fluid is expressed by the Navier-Stokes equation, which is used to solve its velocity field and pressure field:

[0079]

[0080] Where t is time, s; I is the unit matrix; μ is the fluid viscosity, Pa·s; F s is the interfacial tension term, Pa·m -1 ; p is the fluid pressure, Pa;

[0081] S32: The phase field method is used to track the interface between the displacing phase and the displaced phase. The convection-diffusion Cahn-Hilliard equation that controls the evolution of the interface profile is:

[0082]

[0083] Where φ is the phase field variable; γ is the mobility, m 3 ·s·kg -1 ;λ is the mixing energy density, N;ε is the interface thickness, m;ψ is the phase field auxiliary variable;

[0084] Interfacial tension term F s Defined as:

[0085]

[0086] Where G is the chemical potential, J·m -3 ;

[0087] Subscript 1 represents the displaced phase, subscript 2 represents the displacing phase, and the volume fraction of the single-phase fluid is calculated as follows:

[0088]

[0089] V f,1 +V f,2 =1 (9)

[0090] Where V f,1 is the volume fraction of the displaced phase, V f,2 is the volume fraction of the displacing phase;

[0091] The dynamic viscosity and density of the fluid physical parameters on the interface between the two phases during the displacement process are also defined as functions of the phase field variables, so that they change continuously on the interface;

[0092] The density at the interface between the two phases is:

[0093] ρ=ρ 1 +(ρ 2 -ρ 1 )Vf，2 (10)

[0094] The viscosity at the interface between the two phases is:

[0095] μ=μ 1 +(μ 2 -μ 1 )V f，2 (11)

[0096] In the formula, ρ 1 is the density of the displaced phase; ρ 2 is the density of the displacement phase; μ 1 is the viscosity of the displaced phase; μ 2 is the viscosity of the displacing phase;

[0097] S33: setting the physical property parameters such as density and viscosity of the selected displacement phase and displaced phase;

[0098] S34: setting boundary conditions and initial conditions of the displacement process; wherein the boundary conditions include displacement pressure, wetting angle, injection speed, etc., and the initial conditions include initial temperature, pressure, etc.;

[0099] S35: Determine the initial distribution state of the two-phase fluid in the calculation domain and perform displacement simulation.

[0100] S4: Based on the two-phase displacement numerical model obtained in S3, the displacement process is simulated by numerical simulation to obtain the minimum mobilization aperture of multi-diameter capillaries (i.e., the minimum mobilization aperture of the capillary where the displaced phase appears) and the flow state change of the two-phase fluid; on this basis, the injection and production parameters are adjusted, and the displacement effect under different parameters is observed to improve the recovery factor or storage efficiency.

[0101] As a preferred embodiment of the present invention, the steps are as follows when used:

[0102] S41: As the occurrence state of the fluid in the multi-diameter capillary physical model tends to be stable, when the displacement simulation is completed, the output content includes: the migration distribution cloud map of the two-phase fluid at different times, the volume fraction value of the single-phase fluid at different times, etc.; the mobilization status of the fluid in each capillary can be intuitively observed based on the cloud map of the output results; at the same time, by analyzing and processing the output results, the change curve of the recovery factor or the sealing factor can be obtained;

[0103] S42: According to the principle of the single variable method, adjust the key displacement parameters such as interfacial tension, wetting angle and inlet pressure, and repeat the steps of S3; when the displacement process reaches the end point, a cloud map and a curve map containing the result information can also be generated; through the analysis of these output results, it is possible to intuitively observe the evolution of the flow pattern during the fluid migration process after the displacement conditions change, the difference in the occurrence state, and the change in the degree of fluid mobilization in each capillary channel.

[0104] The method of the present invention and the corresponding effects will be specifically described below through embodiments.

[0105] Example

[0106] This example is based on a low permeability reservoir and explores the capillary diameters of D 1 =0.5μm, D 2 =1μm, D 3 =2μm, D 4 =3μm, D 5 =4μm, D 6 = The minimum pore size for water displacement in a channel of 5 μm. The implementation steps are as follows:

[0107] Step 1: According to the number of tube diameters to be studied i = 6 and the capillary diameter D i (i=1, 2, 3, 4, 5, 6) Set different capillaries to obtain a two-dimensional multi-diameter capillary model. The two-dimensional multi-diameter capillary model is represented by a long and thin rectangle, and the fluid in the capillary is the source of the displaced phase.

[0108] This step specifically includes:

[0109] (1) Use the width value W 1 =0.5μm, W 2 =1μm, W 3 =2μm, W 4 =3μm, W 5 =4μm, W 6 =5μm represents different capillary diameters, making W 1 =D 1 , W 2 =D 2 , W 3 =D 3 , W 4 =D 4 , W 5 =D 5 , W 6 =D 6 ;

[0110] (2) The length of all capillaries is L i Set to the same value, i.e. L 1=L 2 =L 3 =L 4 =L 5 =L 6 = 200 μm, place all capillaries in a way that their long sides are parallel to the X-axis of the rectangular coordinate system and do not overlap with each other, and make the left vertical boundaries of all capillaries correspond to the same value of the X-axis;

[0111] (3) The vertical spacing between two adjacent capillaries is set to 4 μm.

[0112] Step 2: Rectangle I is used to represent the displacement phase source and is placed on the left side of the two-dimensional multi-diameter capillary model. A parallel two-dimensional multi-diameter capillary model is placed on the right side so that the right boundary of the displacement phase source coincides with the left boundary of all capillaries. A multi-diameter capillary physical model is constructed and the entire model is used as the calculation domain.

[0113] This step specifically includes:

[0114] (1) The displacement phase source represented by rectangle I is placed on the left side of the capillary group (i.e., multi-diameter capillary). One of the two adjacent sides of rectangle I is parallel to the X-axis of the rectangular coordinate system, and the other is parallel to the Y-axis. The length of the side parallel to the X-axis is set to 40 μm, which serves as a symbol of the displacement phase source.

[0115] (2) The bottom side of rectangle I parallel to the X-axis and the bottom side of the capillary tube have the same y value in the rectangular coordinate system, which is 0; the top side of rectangle I parallel to the X-axis and the top side of the capillary tube have the same y value in the rectangular coordinate system, which is 35.5 μm; that is, the side length of rectangle I parallel to the Y-axis just occupies the same interval on the Y-axis as the capillary tube group;

[0116] (3) Rectangle I and all the capillaries on its right side combine to form a multi-diameter capillary model, such as Figure 1 As shown, a numerical simulation of water-to-oil displacement was carried out on this basis.

[0117] Step 3: Based on the multi-diameter capillary physical model constructed in steps 1 and 2, a two-phase displacement numerical model based on the phase field method is established according to the specific research content.

[0118] This step specifically includes:

[0119] (1) In the two-phase displacement process, since the Reynolds number of the fluid is small, the laminar flow model is adopted, and the mass conservation in the fluid control equation is expressed by the continuity equation:

[0120]

[0121] Where u is the fluid velocity, m / s; ρ is the fluid density, kg·m -3 .

[0122] The momentum equation of the fluid is expressed by the Navier-Stokes equation, which is used to solve its velocity field and pressure field:

[0123]

[0124] Where t is time, s; I is the unit matrix; μ is the fluid viscosity, Pa·s; F s is the interfacial tension term, Pa·m -1 ; p is the fluid pressure, Pa.

[0125] (2) The phase field method is used to track the interface between the displacing water phase and the displaced oil phase. The convection-diffusion Cahn-Hilliard equation that controls the evolution of the interface profile is:

[0126]

[0127] In the formula,

[0128] In the above formula, φ is the phase field variable; γ is the mobility, m 3 ·s·kg -1 ; λ is the mixing energy density, N; ε is the interface thickness, m; ψ is the phase field auxiliary variable.

[0129] The interfacial tension term can be defined as:

[0130]

[0131] Further,

[0132] Where G is the chemical potential, J·m -3 .

[0133] Subscript 1 represents the displaced oil phase, subscript 2 represents the displaced water phase, and the volume fraction of the single-phase fluid can be calculated as follows:

[0134]

[0135] V f,1 +V f,2 =1 (9)

[0136] Where V f,1 is the volume fraction of the displaced phase, V f,2 is the volume fraction of the displacing phase.

[0137] The dynamic viscosity and density of the fluid physical parameters at the interface between the two phases during the displacement process are also defined as functions of the phase field variables, so that they change continuously at the interface.

[0138] The density at the interface between the two phases is:

[0139] ρ=ρ 1 +(ρ 2 -ρ 1 )V f，2 (10)

[0140] The viscosity at the interface between the two phases is:

[0141] μ=μ 1 +(μ 2 -μ 1 )V f，2 (11)

[0142] (3) Set the temperature, pressure, density, viscosity and interfacial tension of the selected displacing phase and the displaced phase as shown in Table 1 below:

[0143] Table 1. Physical parameters of fluids

[0144]

[0145] (4) Set the boundary conditions and initial conditions of the displacement process, as shown in Table 2. The boundary conditions include the wetting angle, inlet pressure, and outlet pressure, and the initial conditions include the initial temperature and initial pressure.

[0146] Table 2. Boundary conditions and initial conditions

[0147]

[0148] (5) Determine the initial distribution state of the two-phase fluid in the calculation domain:

[0149] At the initial moment, rectangle Ⅰ is filled with water phase, and the six capillaries are filled with oil phase, such as Figure 2 shown.

[0150] Step 4: Use numerical simulation to simulate the displacement process and obtain the minimum mobilization aperture of the set capillary (i.e., the minimum mobilization aperture of the capillary at which the displaced phase appears) and the flow state changes of the two-phase fluid; on this basis, the injection and production parameters can be further adjusted, and the displacement effect under different parameters can be observed to achieve an increase in the recovery rate or storage efficiency.

[0151] This step specifically includes:

[0152] (1) As the state of the fluid in the multi-diameter capillary model becomes stable, when the simulation is completed, the output content includes: the migration distribution diagram of the water phase and the oil phase at different times, the volume fraction of the single-phase fluid at different times, etc. The mobilization of the fluid in each capillary can be intuitively observed based on the result cloud map, such as Figure 3As shown in the figure, it can be seen that under the specified boundary conditions, the minimum operating aperture of this multi-diameter capillary model is D 3 =2μm. At the same time, by analyzing and processing the output results, the recovery rate change curve can be obtained, such as Figure 4 As shown in the figure, it can be seen that the recovery rate increases rapidly in the early stage of displacement. As the displacement continues, different capillaries are filled with water phase one after another, and the recovery rate curve also has an inflection point and tends to be flat as a whole. 1 = 0.5 μm and D 2 The crude oil in the capillary with a diameter of =1μm was not mobilized, and the crude oil recovery factor remained almost unchanged in the later stage of displacement.

[0153] (2) According to the principle of the single variable method, adjust the two key displacement parameters, the wetting angle and the inlet pressure, and repeat the process of step 3. When the displacement process reaches the end point, a cloud map and a curve graph containing the result information can also be generated. By analyzing these output results, it is possible to intuitively observe the evolution of the flow pattern during the fluid migration process, the difference in the occurrence state, and the change in the degree of fluid mobilization in each capillary channel after the displacement conditions change.

[0154] By changing the inlet pressure, Figure 5 It can be observed from the result cloud that the displacement process has accelerated, but even if the displacement time is extended, the minimum mobilization pore size remains unchanged. Figure 6 It can be observed from the result cloud diagram that water-wet wettability facilitates displacement, and when the displacement time is extended, crude oil is mobilized in the capillaries where no crude oil was mobilized originally when the wettability is water-wet.

[0155] The present invention overcomes the single form of the existing capillary model and constructs a multi-diameter capillary physical model. The design purpose of this model is to explore the capillary displacement behavior under the condition of coexistence of multiple diameters, determine the minimum diameter threshold that can trigger the displacement phenomenon, and further explore whether adjusting the injection and production parameters through numerical simulation can effectively activate the movement of fluids in smaller diameters.

[0156] The above-described embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A two-phase displacement numerical simulation method based on a multi-diameter capillary model, characterized in that: The details are as follows: S1: According to the target tube diameter number i and the diameter size D of each capillary tube i , set up a two-dimensional multi-diameter capillary model; S2: Rectangle I is used to represent the displacement phase source and is placed on one side of the two-dimensional multi-diameter capillary model, so that one side of the displacement phase source coincides with one end of each capillary, thereby constructing a multi-diameter capillary physical model; S3: Using the multi-diameter capillary physical model as the calculation domain, a two-phase displacement numerical model based on the phase field method is established; S4: According to the two-phase displacement numerical model, the displacement process is simulated by numerical simulation method to obtain the minimum mobilization aperture of multi-diameter capillaries and the flow state change of the two-phase fluid; on this basis, the injection and production parameters are adjusted, and the displacement effect under different parameters is observed to improve the recovery rate or storage efficiency.

2. The two-phase displacement numerical simulation method based on a multi-diameter capillary model according to claim 1 is characterized in that: In the two-dimensional multi-diameter capillary model of S1, the capillaries are represented by rectangles and the fluid inside them is the displaced phase source, and the capillaries are arranged horizontally from top to bottom.

3. The two-phase displacement numerical simulation method based on a multi-diameter capillary model according to claim 2 is characterized in that: In S2, rectangle I is placed on the left side of the two-dimensional multi-diameter capillary model so that the right boundary of the displacement phase source coincides with the left boundaries of all capillaries.

4. The two-phase displacement numerical simulation method based on a multi-diameter capillary model according to claim 1 is characterized in that: The S1 is specifically as follows: S11: Use wide value W i Different rectangles represent each capillary, so that W i =D i ; S12: Length L of all capillaries i Set to the same value, place all capillaries in a way that their long sides are parallel to the X-axis of the rectangular coordinate system and do not overlap each other, and make the left vertical boundaries of all capillaries correspond to the same value of the X-axis of the rectangular coordinate system; S13: setting the vertical distance between two adjacent capillaries according to the target flow channel distance.

5. The two-phase displacement numerical simulation method based on a multi-diameter capillary model according to claim 4 is characterized in that: The S2 is specifically as follows: S21: Place the displacement phase source represented by rectangle Ⅰ on the left side of the multi-diameter capillary; one of the two adjacent sides of rectangle Ⅰ is parallel to the X-axis of the rectangular coordinate system, and the other is parallel to the Y-axis of the rectangular coordinate system; S22: The bottom side of rectangle I parallel to the X-axis of the rectangular coordinate system has the same y value as the bottom side of the capillary located at the bottom, and the top side of rectangle I parallel to the X-axis has the same y value as the top side of the capillary located at the top, that is, the side length of rectangle I parallel to the Y-axis is exactly the same as the interval occupied by the multi-diameter capillary on the Y-axis.

6. The two-phase displacement numerical simulation method based on a multi-diameter capillary model according to claim 1 is characterized in that: The S3 is as follows: S31: In the two-phase displacement process, the laminar flow model is adopted, and the mass conservation in the fluid control equation is expressed by the continuity equation: ρ▽·u=0 (1) Where u is the fluid velocity, m / s; ρ is the fluid density, kg·m -3 ; The momentum equation of the fluid is expressed by the Navier-Stokes equation, which is used to solve its velocity field and pressure field: Where, t is time, s; I is the unit matrix; μ is the fluid viscosity, Pa·s; F s is the interfacial tension term, Pa·m -1 ; p is the fluid pressure, Pa; S32: The phase field method is used to track the interface between the displacing phase and the displaced phase. The convection-diffusion Cahn-Hilliard equation that controls the evolution of the interface profile is: ψ=-▽·e 2 ▽φ+(φ 2 -1)φ (4) Where φ is the phase field variable; γ is the mobility, m 3 ·s·kg -1 ;λ is the mixing energy density, N;ε is the interface thickness, m;ψ is the phase field auxiliary variable; Interfacial tension term F s Defined as: F s =G▽φ (5) Where G is the chemical potential, J·m -3 ; Subscript 1 represents the displaced phase, subscript 2 represents the displacing phase, and the volume fraction of the single-phase fluid is calculated as follows: V f,1 +V f,2 =1 (9) Where V f,1 is the volume fraction of the displaced phase, V f,2 is the volume fraction of the displacing phase; The dynamic viscosity and density of the fluid physical parameters on the interface between the two phases during the displacement process are also defined as functions of the phase field variables, so that they change continuously on the interface; The density at the interface between the two phases is: p=p1+(p2-p1)V f，2 (10) The viscosity at the interface between the two phases is: μ=μ1+(μ2-μ1)V f，2 (11) S33: setting the density, viscosity and interfacial tension of the selected displacement phase and displaced phase; S34: setting boundary conditions and initial conditions of the displacement process; the boundary conditions include inlet pressure or inlet velocity, wetting angle, outlet pressure or outlet velocity, and the initial conditions include initial temperature and initial pressure; S35: Determine the initial distribution state of the two-phase fluid in the calculation domain and perform displacement simulation.

7. The two-phase displacement numerical simulation method based on a multi-diameter capillary model according to claim 1 is characterized in that: The S4 is specifically as follows: S41: As the occurrence state of the fluid in the multi-diameter capillary physical model tends to be stable, when the displacement simulation is completed, the output content includes: the migration distribution cloud map of the two-phase fluid at different times, and the volume fraction of the single-phase fluid in different flow domains at different times; the mobilization of the fluid in each capillary can be intuitively observed based on the cloud map of the output results; at the same time, by analyzing and processing the output results, the change curve of the recovery rate or storage rate is obtained; S42: According to the principle of the single variable method, adjust the key displacement parameters such as interfacial tension, wetting angle, inlet pressure, inlet velocity and two-phase viscosity ratio, and repeat the steps of S3; when the displacement process reaches the end point, a cloud map and a curve map containing the result information can also be generated; through the analysis of these output results, it is possible to intuitively observe the evolution of the flow pattern during the fluid migration process after the displacement conditions change, the difference in the occurrence state, and the change in the degree of fluid mobilization in each capillary channel.

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