Calculation method and system for fluid phase state in porous media

Abstract

Disclosed is a method and system for calculating the phase state of a fluid in a porous medium. The method may include: step 1: establishing a porous fluid component adsorption model, and then determining multiple pore sizes; step 2: calculating the phase state parameters of each pore size; step 3: normalizing the phase state parameters of each pore size to obtain the final phase state parameters. The present invention calculates the phase state characteristics of the fluid under porous medium conditions, determines the change process of the fluid phase state with the pore size, deepens the understanding of the change of the fluid phase state under actual reservoir conditions, and provides technical support for oilfield production and decision-making.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas field development, and more specifically, to a method and system for calculating the phase state of a fluid in a porous medium. Background Art

[0002] Fluid phase calculation plays a vital role in the fields of fluid PVT analysis and fitting, reservoir numerical simulation, condensate gas reservoir development, CO2 flooding and storage.

[0003] Conventional cubic state equations (PR, SRK, etc.) are widely used in fluid phase calculations. The fitted state equations can usually accurately characterize the phase characteristics of the fluid under large space conditions (PVT reactor). However, the interaction between fluid molecules and the porous medium surface in porous media causes the fluid phase to be significantly different from that under conventional large space conditions.

[0004] In dense pores, especially nano-porous media, the critical parameters of the fluid will shift compared to large spaces, and the impact of critical parameter shift on phase characteristics should be considered during calculations. The heterogeneity in porous media is more serious, especially in shale, where the pore size is between 1-100nm, with a large span. Different distribution patterns will lead to differences in calculation results. In addition, due to the small pore radius and large capillary force in dense porous media, the pressures of the oil and gas phases are unequal, which will also affect the phase characteristics. There is also adsorption in porous media, which will affect the composition of the fluid. At the same time, the presence of the adsorption layer leads to a reduction in the effective pore diameter, which will also lead to differences in phase equilibrium calculations and need to be considered.

[0005] Although a lot of research has been done on the calculation of porous media phase state at home and abroad, due to the many influencing factors and complex mechanism of action, the research results are often contradictory. At present, there is still no systematic method for calculating the phase state of fluid under porous media conditions. Therefore, it is necessary to develop a calculation method and system for the phase state of fluid in porous media.

[0006] The information disclosed in the background technology section of the present invention is only intended to deepen the understanding of the general background technology of the present invention, and should not be regarded as acknowledging or suggesting in any form that the information constitutes the prior art already known to those skilled in the art. Summary of the invention

[0007] The present invention proposes a method and system for calculating the phase state of fluid in porous media, which can determine the change process of fluid phase state with pore size by calculating the phase state characteristics of fluid under porous media conditions, deepen the understanding of fluid phase state changes under actual reservoir conditions, and provide technical support for oilfield production and decision-making.

[0008] According to one aspect of the present invention, a method for calculating the phase state of a fluid in a porous medium is proposed. The method may include: step 1: establishing a porous fluid component adsorption model, and then determining multiple pore sizes; step 2: calculating the phase state parameters of each pore size; step 3: normalizing the phase state parameters of each pore size to obtain the final phase state parameters.

[0009] Preferably, the step 2 comprises: step 201: under given temperature and pressure conditions, calculating the amount of substance adsorbed by each component; step 202: calculating the critical temperature and critical pressure of each component according to the amount of substance adsorbed by each component; step 203: calculating the initial equilibrium ratio according to the total composition after adsorption, the critical temperature and the critical pressure, calculating the gas phase mole fraction according to the initial equilibrium ratio, and then calculating the oil phase component and the gas phase component; step 204: calculating the fugacity of each component according to the oil phase component and the gas phase component, and then calculating the fugacity error; step 205: judging the fugacity Whether the degree error is less than or equal to the error threshold, if it is the first time, proceed to step 206, if not, update the initial equilibrium ratio and proceed to step 206, if it is the second time, output the current phase parameters; Step 206: Calculate the average molar mass, volume fraction and density of the oil phase component and the gas phase component, and then calculate the plane interface tension; Step 207: Calculate the capillary pressure, replace the pressure in step 201 with the capillary pressure, and repeat steps 201-207; wherein the phase parameters include the average molar mass, the volume fraction, the density and the plane interface tension.

[0010] Preferably, the amount of the substance adsorbed by each component is calculated by formula (1):

[0011] n a,i =V a,i W s (1)

[0012] Among them, n a,i is the amount of substance adsorbed by component i, W s is the mass of the adsorbent, V a,i is the adsorption amount of component i at equilibrium.

[0013] Preferably, the critical temperature of each component is calculated by formula (2):

[0014]

[0015] The critical pressure of each component is calculated by formula (3):

[0016]

[0017] Where, ΔT cis the critical temperature offset, σ eff Denotes the effective molecular diameter, D eff represents the effective pore diameter, T cp and T cb denote the critical temperature of the fluid in the porous medium and in the large space, Δp c is the critical pressure offset, p cp and p cb They represent the critical pressures of fluids in porous media and large spaces, respectively.

[0018] Preferably, the fugacity of each component of the oil phase is calculated by formula (4):

[0019]

[0020] The fugacity of each gas phase component is calculated by formula (5):

[0021]

[0022] Among them, Z L,j and Z V,j denote the compressibility factors of the oil phase and vapor phase mixture in the jth capillary bundle, a m,j and b m,j are the average attraction and repulsion constants of the mixture system in the jth capillary bundle, a i,j (T) = a ci,j α i,j (T), ω i,j is the eccentricity factor of component i in the jth capillary bundle, T ci,j and p ci,j denote the critical temperature and critical pressure of component i in the jth capillary bundle, respectively. is the binary interaction coefficient.

[0023] Preferably, the average molar mass of the oil phase component and the gas phase component is calculated by formula (6):

[0024]

[0025] The density of the oil phase component and the gas phase component is calculated by formula (7):

[0026]

[0027] Among them, M L is the average molar mass of the oil phase components, M V is the average molar mass of the gas phase components, M irepresents the molar mass of component i, ρ L is the density of the oil phase component, ρ V is the density of the gas phase component.

[0028] Preferably, the planar interfacial tension is calculated by formula (8):

[0029]

[0030] Among them, γ ∞ is the plane interfacial tension, [P i ] is the isotonic specific volume of component i.

[0031] Preferably, the capillary pressure is calculated by formula (9):

[0032]

[0033] Among them, p ca,j is the capillary pressure in the jth capillary bundle, θ is the angle between the oil-gas interface and the pore surface, γ j Represents the actual interfacial tension of the jth capillary bundle.

[0034] Preferably, the method further comprises: calculating the dew point pressure according to the final phase state parameter.

[0035] According to another aspect of the present invention, a system for calculating the phase state of fluid in porous media is proposed, characterized in that the system comprises: a memory storing computer executable instructions; a processor running the computer executable instructions in the memory to perform the following steps: Step 1: Establishing a porous fluid component adsorption model, and then determining multiple pore sizes; Step 2: Calculating the phase state parameters of each pore size; Step 3: Normalizing the phase state parameters of each pore size to obtain the final phase state parameters.

[0036] Preferably, the step 2 comprises: step 201: under given temperature and pressure conditions, calculating the amount of substance adsorbed by each component; step 202: calculating the critical temperature and critical pressure of each component according to the amount of substance adsorbed by each component; step 203: calculating the initial equilibrium ratio according to the total composition after adsorption, the critical temperature and the critical pressure, calculating the gas phase mole fraction according to the initial equilibrium ratio, and then calculating the oil phase component and the gas phase component; step 204: calculating the fugacity of each component according to the oil phase component and the gas phase component, and then calculating the fugacity error; step 205: judging the fugacity Whether the degree error is less than or equal to the error threshold, if it is the first time, proceed to step 206, if not, update the initial equilibrium ratio and proceed to step 206, if it is the second time, output the current phase parameters; Step 206: Calculate the average molar mass, volume fraction and density of the oil phase component and the gas phase component, and then calculate the plane interface tension; Step 207: Calculate the capillary pressure, replace the pressure in step 201 with the capillary pressure, and repeat steps 201-207; wherein the phase parameters include the average molar mass, the volume fraction, the density and the plane interface tension.

[0037] Preferably, the amount of the substance adsorbed by each component is calculated by formula (1):

[0038] n a,i =V a,i W s (1)

[0039] Among them, n a,i is the amount of substance adsorbed by component i, W s is the mass of the adsorbent, V a,i is the adsorption amount of component i at equilibrium.

[0040] Preferably, the critical temperature of each component is calculated by formula (2):

[0041]

[0042] The critical pressure of each component is calculated by formula (3):

[0043]

[0044] Where, ΔT c is the critical temperature offset, σ eff Denotes the effective molecular diameter, D eff represents the effective pore diameter, T cp and T cb denote the critical temperature of the fluid in the porous medium and in the large space, Δp c is the critical pressure offset, p cp and pcb They represent the critical pressures of fluids in porous media and large spaces, respectively.

[0045] Preferably, the fugacity of each component of the oil phase is calculated by formula (4):

[0046]

[0047] The fugacity of each gas phase component is calculated by formula (5):

[0048]

[0049] Among them, Z L,j and Z V,j denote the compressibility factors of the oil phase and vapor phase mixture in the jth capillary bundle, a m,j and b m,j are the average attraction and repulsion constants of the mixture system in the jth capillary bundle, a i,j (T) = a ci,j α i,j (T), ω i,j is the eccentricity factor of component i in the jth capillary bundle, T ci,j and p ci,j denote the critical temperature and critical pressure of component i in the jth capillary bundle, respectively. is the binary interaction coefficient.

[0050] Preferably, the average molar mass of the oil phase component and the gas phase component is calculated by formula (6):

[0051]

[0052] The density of the oil phase component and the gas phase component is calculated by formula (7):

[0053]

[0054] Among them, M L is the average molar mass of the oil phase components, M V is the average molar mass of the gas phase components, M i represents the molar mass of component i, ρ L is the density of the oil phase component, ρ V is the density of the gas phase component.

[0055] Preferably, the planar interfacial tension is calculated by formula (8):

[0056]

[0057] Among them, γ ∞ is the plane interfacial tension, [P i ] is the isotonic specific volume of component i.

[0058] Preferably, the capillary pressure is calculated by formula (9):

[0059]

[0060] Among them, p ca,j is the capillary pressure in the jth capillary bundle, θ is the angle between the oil-gas interface and the pore surface, γ j Represents the actual interfacial tension of the jth capillary bundle.

[0061] Preferably, the method further comprises: calculating the dew point pressure according to the final phase state parameter.

[0062] The methods and apparatus of the present invention have other features and advantages that will be apparent from, or will be described in detail in, the accompanying drawings and subsequent detailed descriptions incorporated herein, which together serve to explain the specific principles of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention in conjunction with the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present invention.

[0064] Figure 1 A flow chart showing the steps of a method for calculating the phase state of a fluid in a porous medium according to the present invention.

[0065] Figure 2 A schematic diagram showing the dew point pressures of pores of different sizes according to an embodiment of the present invention.

[0066] Figure 3 A schematic diagram showing dew point pressures for different critical parameters according to an embodiment of the present invention.

[0067] Figure 4 A schematic diagram showing the distribution of pores of three sizes according to an embodiment of the present invention is shown.

[0068] Figure 5 Shown according to Figure 4 Schematic diagram of the dew point pressure of three pore size distributions. DETAILED DESCRIPTION

[0069] The present invention will be described in more detail below with reference to the accompanying drawings. Although preferred embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0070] Figure 1 A flow chart showing the steps of a method for calculating the phase state of a fluid in a porous medium according to the present invention.

[0071] In this embodiment, the method for calculating the phase state of fluid in a porous medium according to the present invention may include: Step 1: establishing a porous fluid component adsorption model, and then determining multiple pore sizes; Step 2: calculating the phase state parameters of each pore size; Step 3: normalizing the phase state parameters of each pore size to obtain the final phase state parameters.

[0072] In one example, step 2 includes: step 201: under given temperature and pressure conditions, calculating the amount of substance adsorbed by each component; step 202: calculating the critical temperature and critical pressure of each component according to the amount of substance adsorbed by each component; step 203: calculating the initial equilibrium ratio according to the total composition after adsorption, the critical temperature and the critical pressure, calculating the gas phase mole fraction according to the initial equilibrium ratio, and then calculating the oil phase component and the gas phase component; step 204: calculating the fugacity of each component according to the oil phase component and the gas phase component, and then calculating the fugacity error; step 205: Determine whether the fugacity error is less than or equal to the error threshold. If it is the first time, proceed to step 206. If not, update the initial equilibrium ratio and proceed to step 206. If it is the second time, output the current phase parameters. Step 206: Calculate the average molar mass, volume fraction and density of the oil phase component and the gas phase component, and then calculate the plane interface tension. Step 207: Calculate the capillary pressure, replace the pressure in step 201 with the capillary pressure, and repeat steps 201-207. The phase parameters include the average molar mass, volume fraction, density and plane interface tension.

[0073] In one example, the amount of substance adsorbed by each component is calculated by formula (1):

[0074] n a,i =V a,i W s (1)

[0075] Among them, n a,i is the amount of substance adsorbed by component i, W s is the mass of the adsorbent, V a,i is the adsorption amount of component i at equilibrium.

[0076] In one example, the critical temperature of each component is calculated by formula (2):

[0077]

[0078] The critical pressure of each component is calculated by formula (3):

[0079]

[0080] Where, ΔT c is the critical temperature offset, σ eff Denotes the effective molecular diameter, D eff represents the effective pore diameter, T cp and T cb denote the critical temperature of the fluid in the porous medium and in the large space, Δp c is the critical pressure offset, p cp and p cb They represent the critical pressures of fluids in porous media and large spaces, respectively.

[0081] In one example, the fugacity of each component of the oil phase is calculated by formula (4):

[0082]

[0083] The fugacity of each gas phase component is calculated by formula (5):

[0084]

[0085] Among them, Z L,j and Z V,j denote the compressibility factors of the oil phase and vapor phase mixture in the jth capillary bundle, a m,j and b m,j are the average attraction and repulsion constants of the mixture system in the jth capillary bundle, a i,j (T) = a ci,j α i,j (T), ω i,j is the eccentricity factor of component i in the jth capillary bundle, T ci,j and p ci,j denote the critical temperature and critical pressure of component i in the jth capillary bundle, respectively. is the binary interaction coefficient, for the hydrocarbon-hydrocarbon system,

[0086] In one example, the average molar mass of the oil phase component and the gas phase component is calculated by formula (6):

[0087]

[0088] The density of the oil phase component and the gas phase component is calculated by formula (7):

[0089]

[0090] Among them, M L is the average molar mass of the oil phase components, M V is the average molar mass of the gas phase components, M i represents the molar mass of component i, ρ L is the density of the oil phase component, ρ V is the density of the gas phase component.

[0091] In one example, the planar interfacial tension is calculated by equation (8):

[0092]

[0093] Among them, γ ∞ is the plane interfacial tension, [P i ] is the isotonic specific volume of component i.

[0094] In one example, the capillary pressure is calculated by equation (9):

[0095]

[0096] Among them, p ca,j is the capillary pressure in the jth capillary bundle, θ is the angle between the oil-gas interface and the pore surface. When the adsorption layer is considered, θ is zero, and γ j Represents the actual interfacial tension of the jth capillary bundle.

[0097] In one example, the method further includes: calculating the dew point pressure according to the final phase state parameters.

[0098] Specifically, the method for calculating the phase state of a fluid in a porous medium according to the present invention may include:

[0099] Step 1: Establish a porous fluid component adsorption model and then determine multiple pore sizes. The model assumptions include: (1) the oil-gas two-phase system is a closed system; (2) the phase equilibrium process between the oil and gas phases is completed instantaneously; (3) the effect of capillary force on the pressure of the oil-gas two-phase system is considered; (4) the effect of adsorption on the components and the effect of adsorption layer thickness on the effective pore diameter are considered; (5) the deviation of the critical parameters of the components in the porous medium is considered; (6) the effect of pore size distribution is considered; it is assumed that the oil-gas system has a total of N c components, the total composition of the system in the jth capillary bundle is Z i,j (i=1,2,…,Nc ; j = 1, 2, ..., N b ), the total amount of substance is F j (j=1,2,…,N b ), the oil phase mole fraction at equilibrium is L j , the oil phase composition is X i,j , the gas phase mole fraction is V j , the gas phase composition is Y i,j .

[0100] Step 2: Calculate the phase parameters of each pore size, including:

[0101] Step 201: In order to study the interaction between the fluid in the porous medium and the surface of the porous medium, the adsorption effect is considered. Since different hydrocarbons have different adsorption characteristics in dense pores, two adsorption models are established to characterize: light hydrocarbons (C 4 and below), using the Langmuir monolayer adsorption model; heavy hydrocarbons (C 5 And above), the BET multilayer adsorption model was adopted.

[0102] The Langmuir monolayer adsorption model can simulate the adsorption process of light hydrocarbons well:

[0103]

[0104] Where V a and V m They represent the adsorption capacity at equilibrium and the maximum adsorption capacity, p represents the equilibrium pressure, b L is the isothermal adsorption constant.

[0105] When considering multi-component competitive adsorption, the improved Langmuir model is used:

[0106]

[0107] In the formula, C 1,i is the adsorption coefficient of the component, which is related to the component, temperature and the physical properties of the porous medium surface. i Represents the partial pressure of component i:

[0108] p i = p·Y i (12)

[0109] For hydrocarbons with larger molecules (C 5 And above) are calculated using the BET multilayer adsorption model:

[0110]

[0111] In the formula, p 0is the saturated vapor pressure of the adsorbate at a given temperature, C 2 is a constant.

[0112] Therefore, under given temperature and pressure conditions, the amount of substance adsorbed by each component is calculated by formula (1).

[0113] Step 202: The thickness of the adsorption layer produced by adsorption can be calculated according to the equation proposed by Dong (2016):

[0114]

[0115] In the formula, r a and r m They represent the adsorption layer thickness and the maximum adsorption layer thickness at a given pressure, respectively.

[0116] When considering the case where the adsorption completely covers the surface, the maximum adsorption layer thickness can be calculated by the following formula:

[0117]

[0118] In the formula, v l Indicates the molar volume of the liquid, N A Represents Avogadro's constant.

[0119] The effective diameter of the pore can be further obtained based on the calculated adsorption layer thickness:

[0120] D eff =2(rr a ) (15)

[0121] The critical parameters of confined fluids in dense porous media will be offset compared to conventional conditions. Zarragoicoechea and Kuz (2004) proposed the relationship between the critical parameters and pore size, but this formula is only applicable to This is not applicable to large molecules and extremely small pores. Therefore, considering the different sizes of pores, the relationship proposed by Ma and Jin (2013) is used, that is, according to the amount of adsorbed substances of each component, the critical temperature of each component is calculated by formula (2), and the critical pressure of each component is calculated by formula (3).

[0122] Step 203: Calculate the initial equilibrium ratio according to the total composition, critical temperature and critical pressure after adsorption using formula (16):

[0123]

[0124] According to the law of conservation of matter:

[0125] L j +Vj =1 (17)

[0126] X i,j L j +Y i,j V j =Z i,j (18)

[0127] Among them, the composition of each phase satisfies the following normalization conditions:

[0128]

[0129] The equilibrium ratio of component i in the jth capillary bundle at equilibrium is:

[0130]

[0131] Combining equations (17), (18), (19) and (20), the oil phase composition is:

[0132]

[0133] Substituting equation (21) into equation (20), the gas phase composition is obtained as follows:

[0134]

[0135] Combining equations (19), (21) and (22), we get:

[0136]

[0137] According to the given K i,j The vapor phase mole fraction V can be calculated by the Newton-Raphson iteration method using formula (23). j Based on this, the liquid phase mole fraction L is calculated by equations (17), (21), and (22) respectively. j And the vapor-liquid two-phase composition X i,j and Y i,j .

[0138] Step 204: Based on the oil phase components and the gas phase components, the fugacity of each component in the oil phase is calculated by formula (4), and the fugacity of each component in the gas phase is calculated by formula (5), where Z L,j and Z V,j They represent the compressibility factors of the oil phase and gas phase mixture in the jth capillary bundle, respectively, and can be calculated by the following formula:

[0139]

[0140] Formula (24) has three roots, the largest positive root of which is the compressibility factor Z of the gas phase mixture: V,j, the smallest positive root is the compressibility factor Z of the oil phase mixture L,j .

[0141] Then the fugacity error is calculated as:

[0142]

[0143] Step 205: Determine whether the fugacity error is less than or equal to the error threshold ε 0 If it is the first time, then proceed to step 206, if not, then update the initial balance ratio to Go to step 206, if it is the second time, then output the current phase state parameters.

[0144] Step 206: Calculate the average molar mass of the oil phase component and the gas phase component by formula (6), calculate the density of the oil phase component and the gas phase component by formula (7), and after determining the oil and gas two-phase composition, calculate the volume of the oil and gas two-phase mixture:

[0145]

[0146] The volume fractions of the oil and gas phases are obtained as follows:

[0147]

[0148] The plane interfacial tension is then calculated using formula (8), where the phase parameters include average molar mass, volume fraction, density and plane interfacial tension.

[0149] Step 207: Considering the scale effect of surface tension, in tiny pores, the influence of interface curvature on the actual interface tension cannot be ignored:

[0150]

[0151] Where γ represents the actual interfacial tension in the micropores, and δ represents the Tolman length.

[0152] Ahn (1972) proposed a relationship for calculating the Tolman length:

[0153]

[0154] In the formula, v s Represents the molar volume of a solid.

[0155] Then, the capillary pressure is calculated by formula (9), the pressure in step 201 is replaced by the capillary pressure, and steps 201-207 are repeated.

[0156] Step 3: Normalize the phase parameters of each pore size to obtain the final phase parameters. According to the final phase parameters, calculate the dew point pressure: for the same temperature, calculate the gas phase mole fraction of the system at different pressure levels. When the pressure is high, the gas phase mole fraction is 1 (that is, all gas phase). When the pressure gradually decreases, the gas phase mole fraction will suddenly drop below 1 at a certain pressure point (that is, the oil phase appears). The pressure point at this moment is the dew point pressure at the current system and temperature.

[0157] This method calculates the phase characteristics of the fluid under porous media conditions and determines the change process of the fluid phase with the size of pores, which deepens the understanding of the change of fluid phase under actual reservoir conditions and provides technical support for oilfield production and decision-making.

[0158] Application Examples

[0159] To facilitate understanding of the solution and effects of the embodiments of the present invention, a specific application example is given below. Those skilled in the art should understand that this example is only for facilitating understanding of the present invention, and any specific details thereof are not intended to limit the present invention in any way.

[0160] The method for calculating the phase state of a fluid in a porous medium according to the present invention comprises:

[0161] Step 1: Establish a porous fluid component adsorption model to determine pores of multiple sizes.

[0162] Step 2: Calculate the phase parameters of each pore size, including:

[0163] Step 201: Under given temperature and pressure conditions, the amount of substance adsorbed by each component is calculated by formula (1); Step 202: According to the amount of substance adsorbed by each component, the critical temperature of each component is calculated by formula (2), and the critical pressure of each component is calculated by formula (3); Step 203: According to the total composition, critical temperature and critical pressure after adsorption, the initial equilibrium ratio is calculated, and the gas phase mole fraction is calculated according to the initial equilibrium ratio, and then the oil phase component and the gas phase component are calculated; Step 204: According to the oil phase component and the gas phase component, the fugacity of each component in the oil phase is calculated by formula (4), and the fugacity of each component in the gas phase is calculated by formula (5), and then the fugacity error is calculated; Step 205: Determine the fugacity error Is the difference less than or equal to the error threshold? If it is the first time, proceed to step 206; if not, update the initial equilibrium ratio and proceed to step 206; if it is the second time, output the current phase parameters; Step 206: Calculate the volume fraction of the oil phase component and the gas phase component, calculate the average molar mass of the oil phase component and the gas phase component by formula (6), calculate the density of the oil phase component and the gas phase component by formula (7), and then calculate the plane interface tension by formula (8); Step 207: Calculate the capillary pressure by formula (9), replace the pressure in step 201 with the capillary pressure, and repeat steps 201-207; wherein the phase parameters include the average molar mass, volume fraction, density and plane interface tension.

[0164] Step 3: Normalize the phase parameters of each pore size to obtain the final phase parameters.

[0165] The original gas phase components and critical parameters of the fluid samples obtained from a condensate gas reservoir after conventional PVT physical simulation experiments and phase state fitting are shown in Table 1.

[0166] Table 1

[0167]

[0168] Experimental measurements show that under normal conditions, the dew point pressure at 110.4°C is 26.3MPa.

[0169] Figure 2 A schematic diagram showing the dew point pressures of pores of different sizes according to an embodiment of the present invention.

[0170] Calculate the capillary pressure under different pore diameter conditions, and calculate the phase state results when considering the capillary pressure, such as Figure 2 As shown, it can be seen that the capillary force decreases with the increase of pore diameter; the dew point pressure of condensate gas is increased when the capillary force is considered; the capillary force has a more obvious effect when the pore diameter is less than 10nm, but it can be ignored when the pore diameter is greater than 100nm.

[0171] Figure 3A schematic diagram showing dew point pressures for different critical parameters according to an embodiment of the present invention.

[0172] The influence of critical parameter deviation on the calculation results of condensate gas phase state under different pore diameter conditions is calculated, such as Figure 3 As shown, the results show that when the critical parameter offset is considered, the dew point pressure of the condensate gas decreases; the degree of influence on the phase state calculation results is: critical parameter offset > capillary pressure; when the pore diameter is greater than 100nm, the critical parameter offset has little effect on the phase state results, and when the pore diameter is less than 10nm, the impact of the critical parameter offset is very significant.

[0173] Figure 4 A schematic diagram showing the distribution of pores of three sizes according to an embodiment of the present invention is shown.

[0174] Figure 5 Shown according to Figure 4 Schematic diagram of the dew point pressure of three pore size distributions.

[0175] The condensate gas phase characteristics under three different pore size distribution modes (average pore diameter is 10nm) are simulated and calculated (only capillary force is considered). Figure 4 The phase calculation results are shown in Figure 5 The results show that the heterogeneity of pore size distribution will further lead to the increase of condensate gas dew point pressure; the effect of pore size distribution on condensate gas dew point mainly depends on the smallest pore size and has nothing to do with the distribution pattern.

[0176] In summary, the present invention calculates the phase characteristics of the fluid under porous media conditions and determines the change process of the fluid phase with the size of the pores, which deepens the understanding of the change of fluid phase under actual reservoir conditions and provides technical support for oilfield production and decision-making.

[0177] Those skilled in the art should understand that the purpose of the above description of the embodiments of the present invention is only to exemplarily illustrate the beneficial effects of the embodiments of the present invention, and is not intended to limit the embodiments of the present invention to any given examples.

[0178] According to an embodiment of the present invention, a system for calculating the phase state of fluid in a porous medium is provided, characterized in that the system comprises: a memory storing computer executable instructions; a processor running the computer executable instructions in the memory to perform the following steps: Step 1: establishing a porous fluid component adsorption model, and then determining multiple pore sizes; Step 2: calculating the phase state parameters of each pore size; Step 3: normalizing the phase state parameters of each pore size to obtain the final phase state parameters.

[0179] In one example, step 2 includes: step 201: under given temperature and pressure conditions, calculating the amount of substance adsorbed by each component; step 202: calculating the critical temperature and critical pressure of each component according to the amount of substance adsorbed by each component; step 203: calculating the initial equilibrium ratio according to the total composition after adsorption, the critical temperature and the critical pressure, calculating the gas phase mole fraction according to the initial equilibrium ratio, and then calculating the oil phase component and the gas phase component; step 204: calculating the fugacity of each component according to the oil phase component and the gas phase component, and then calculating the fugacity error; step 205: Determine whether the fugacity error is less than or equal to the error threshold. If it is the first time, proceed to step 206. If not, update the initial equilibrium ratio and proceed to step 206. If it is the second time, output the current phase parameters. Step 206: Calculate the average molar mass, volume fraction and density of the oil phase component and the gas phase component, and then calculate the plane interface tension. Step 207: Calculate the capillary pressure, replace the pressure in step 201 with the capillary pressure, and repeat steps 201-207. The phase parameters include the average molar mass, volume fraction, density and plane interface tension.

[0180] In one example, the amount of substance adsorbed by each component is calculated by formula (1):

[0181] n a,i =V a,i W s (1)

[0182] Among them, n a,i is the amount of substance adsorbed by component i, W s is the mass of the adsorbent, V a,i is the adsorption amount of component i at equilibrium.

[0183] In one example, the critical temperature of each component is calculated by formula (2):

[0184]

[0185] The critical pressure of each component is calculated by formula (3):

[0186]

[0187] Where, ΔT c is the critical temperature offset, σ eff Denotes the effective molecular diameter, D eff represents the effective pore diameter, T cp and T cb denote the critical temperature of the fluid in the porous medium and in the large space, Δp c is the critical pressure offset, p cp and p cbThey represent the critical pressures of fluids in porous media and large spaces, respectively.

[0188] In one example, the fugacity of each component of the oil phase is calculated by formula (4):

[0189]

[0190] The fugacity of each gas phase component is calculated by formula (5):

[0191]

[0192] Among them, Z L,j and Z V,j denote the compressibility factors of the oil phase and vapor phase mixture in the jth capillary bundle, a m,j and b m,j are the average attraction and repulsion constants of the mixture system in the jth capillary bundle, a i,j (T) = a ci,j α i,j (T), ω i,j is the eccentricity factor of component i in the jth capillary bundle, T ci,j and p ci,j denote the critical temperature and critical pressure of component i in the jth capillary bundle, respectively. is the binary interaction coefficient.

[0193] In one example, the average molar mass of the oil phase component and the gas phase component is calculated by formula (6):

[0194]

[0195] The density of the oil phase component and the gas phase component is calculated by formula (7):

[0196]

[0197] Among them, M L is the average molar mass of the oil phase components, M V is the average molar mass of the gas phase components, M i represents the molar mass of component i, ρ L is the density of the oil phase component, ρ V is the density of the gas phase component.

[0198] In one example, the planar interfacial tension is calculated by equation (8):

[0199]

[0200] Among them, γ ∞ is the plane interfacial tension, [P i ] is the isotonic specific volume of component i.

[0201] In one example, the capillary pressure is calculated by equation (9):

[0202]

[0203] Among them, p ca,j is the capillary pressure in the jth capillary bundle, θ is the angle between the oil-gas interface and the pore surface, γ j Represents the actual interfacial tension of the jth capillary bundle.

[0204] In one example, the method further includes: calculating the dew point pressure according to the final phase state parameters.

[0205] This system calculates the phase characteristics of fluids under porous media conditions and determines how the fluid phase changes with pore size, thereby deepening the understanding of fluid phase changes under actual reservoir conditions and providing technical support for oilfield production and decision-making.

[0206] Those skilled in the art should understand that the purpose of the above description of the embodiments of the present invention is only to exemplarily illustrate the beneficial effects of the embodiments of the present invention, and is not intended to limit the embodiments of the present invention to any given examples.

[0207] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for calculating the phase state of a fluid in a porous medium, characterized in that: include: Step 1: Establish a porous fluid component adsorption model to determine multiple pore sizes; Step 2: Calculate the phase parameters of each pore size; Step 3: normalizing the phase parameters of each pore size to obtain the final phase parameters; Wherein, the step 2 comprises: Step 201: Under given temperature and pressure conditions, calculate the amount of substance adsorbed by each component; Step 202: Calculate the critical temperature and critical pressure of each component according to the amount of the substance adsorbed by each component; Step 203: calculating the initial equilibrium ratio according to the total composition after adsorption, the critical temperature and the critical pressure, calculating the gas phase mole fraction according to the initial equilibrium ratio, and then calculating the oil phase component and the gas phase component; Step 204: Calculate the fugacity of each component according to the oil phase component and the gas phase component, and then calculate the fugacity error; Step 205: Determine whether the fugacity error is less than or equal to the error threshold. If it is the first time, proceed to step 206. If not, update the initial equilibrium ratio and proceed to step 206. If it is the second time, output the current phase state parameters. Step 206: Calculate the average molar mass, volume fraction and density of the oil phase component and the gas phase component, and then calculate the plane interfacial tension; Step 207: Calculate the capillary pressure, replace the pressure in step 201 with the capillary pressure, and repeat steps 201-207; Wherein, the phase parameters include the average molar mass, the volume fraction, the density and the planar interface tension.

2. The method for calculating the phase state of a fluid in a porous medium according to claim 1, wherein: The amount of the substance adsorbed by each component is calculated by formula (1): n a,i =V a,i W s (1) Among them, n a,i is the amount of substance adsorbed by component i, W s is the mass of the adsorbent, V a,i is the adsorption amount of component i at equilibrium.

3. The method for calculating the phase state of a fluid in a porous medium according to claim 1, wherein: The critical temperature of each component is calculated by formula (2): The critical pressure of each component is calculated by formula (3): Where, ΔT c is the critical temperature offset, σ eff Denotes the effective molecular diameter, D eff represents the effective pore diameter, T cp and T cb denote the critical temperature of the fluid in the porous medium and in the large space, Δp c is the critical pressure offset, p cp and p cb They represent the critical pressures of fluids in porous media and large spaces, respectively.

4. The method for calculating the phase state of a fluid in a porous medium according to claim 1, wherein: The fugacity of each component in the oil phase is calculated by formula (4): The fugacity of each gas phase component is calculated by formula (5): Among them, Z L,j and Z V,j denote the compressibility factors of the oil phase and vapor phase mixture in the jth capillary bundle, a m,j and b m,j are the average attraction and repulsion constants of the mixture system in the jth capillary bundle, a i,j (T) = a ci,j α i,j (T), ω i,j is the eccentricity factor of component i in the jth capillary bundle, T ci,j and p ci,j denote the critical temperature and critical pressure of component i in the jth capillary bundle, respectively. is the binary interaction coefficient, and the total number of oil and gas systems is N c Components, X i,j It is composed of oil phase.

5. The method for calculating the phase state of a fluid in a porous medium according to claim 4, wherein: The average molar mass of the oil phase component and the gas phase component is calculated by formula (6): The density of the oil phase component and the gas phase component is calculated by formula (7): Among them, M L is the average molar mass of the oil phase components, M V is the average molar mass of the gas phase components, M i represents the molar mass of component i, ρ L is the density of the oil phase component, ρ V is the density of the gas phase component.

6. The method for calculating the phase state of a fluid in a porous medium according to claim 5, wherein: The plane interfacial tension is calculated by formula (8): Among them, γ ∞ is the plane interfacial tension, [P i ] is the isotonic specific volume of component i.

7. The method for calculating the phase state of a fluid in a porous medium according to claim 1, wherein: The capillary pressure is calculated by formula (9): Among them, p ca,j is the capillary pressure in the jth capillary bundle, θ is the angle between the oil-gas interface and the pore surface, γ j Represents the actual interfacial tension of the jth capillary bundle.

8. The method for calculating the phase state of a fluid in a porous medium according to claim 1, wherein: Also includes: The dew point pressure is calculated according to the final phase parameters.

9. A system for calculating the phase state of a fluid in a porous medium, characterized in that: The system includes: a memory storing computer executable instructions; A processor, wherein the processor runs the computer executable instructions in the memory to perform the following steps: Step 1: Establish a porous fluid component adsorption model to determine multiple pore sizes; Step 2: Calculate the phase parameters of each pore size; Step 3: normalizing the phase parameters of each pore size to obtain the final phase parameters; Wherein, the step 2 comprises: Step 201: Under given temperature and pressure conditions, calculate the amount of substance adsorbed by each component; Step 202: Calculate the critical temperature and critical pressure of each component according to the amount of the substance adsorbed by each component; Step 203: calculating the initial equilibrium ratio according to the total composition after adsorption, the critical temperature and the critical pressure, calculating the gas phase mole fraction according to the initial equilibrium ratio, and then calculating the oil phase component and the gas phase component; Step 204: Calculate the fugacity of each component according to the oil phase component and the gas phase component, and then calculate the fugacity error; Step 205: Determine whether the fugacity error is less than or equal to the error threshold. If it is the first time, proceed to step 206. If not, update the initial equilibrium ratio and proceed to step 206. If it is the second time, output the current phase state parameters. Step 206: Calculate the average molar mass, volume fraction and density of the oil phase component and the gas phase component, and then calculate the plane interfacial tension; Step 207: Calculate the capillary pressure, replace the pressure in step 201 with the capillary pressure, and repeat steps 201-207; Wherein, the phase parameters include the average molar mass, the volume fraction, the density and the planar interface tension.

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