A method and device for analyzing fluid phase stability in porous media

Abstract

The present invention provides a method for analyzing the stability of fluid phases in porous media, comprising the following steps: S1, calculating the fugacity and initial equilibrium ratio of the mixture in the current pore size in the porous medium; S2, when a second phase is present, calculating the molar number of the second phase, the sum of the molar numbers of the second phases, and the mole fraction of the second phase; S3, achieving pressure equilibrium between the main phase and the second phase through iterative calculation, calculating the fugacity of the second phase and the fugacity ratio coefficient of the second phase at pressure equilibrium; S4, if the fugacity ratio coefficient of the second phase at pressure equilibrium meets the convergence condition, then judging whether the current pore size is single-phase stable based on the sum of the molar numbers of the second phase. The present invention can accurately determine whether an oil and gas system of any composition under porous media conditions is in a single-phase stable state at a given temperature and pressure. If it is unstable, a reasonable initial equilibrium ratio for oil and gas two-phase flash calculation can be obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas reservoir engineering methods used in the field of oil and gas field development, and in particular to a method and device for analyzing the stability of fluid phases in porous media. Background Art

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

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

[0004] In dense pores, especially nanoporous media, the critical parameters of the fluid will shift compared to large spaces. The impact of the critical parameter shift on the phase characteristics should be considered during calculations. The heterogeneity in porous media is more serious, especially in shale, where the pore size ranges from 1 to 100 nm, which is a large span. Different distribution patterns will lead to differences in the calculation results. In addition, due to the small pore radius and strong capillary force in dense porous media, the pressures of the gas and liquid phases are not equal, 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] Zhang Maolin (2004) found that adsorption lowers the dew point of condensate, while capillary forces slightly increase it. Ma (2013) showed that as the critical temperature of pure components in porous media increases, the bubble and dew points of C1 / nC4 / nC8 mixtures both increase, and the two-phase region shrinks; critical parameter shifts have a greater impact on heavier components. Jin (2013) showed that in porous media, the bubble point of C1-nC5 mixtures decreases, while the lower dew point increases. He also found that capillary forces lower both the bubble and lower dew points of C1-nC5 mixtures. Li Yuansheng (2015) found that when the pore throat diameter is less than 10 nm, the interaction between the pore throat and fluid molecules is significant. The smaller the pore throat, the more pronounced the drop in dew point pressure. Furthermore, capillary forces lower the dew point of C1 / nC4 / nC8 mixtures. Dong (2016) found that the adsorption layer increases capillary pressure, leading to a decrease in the fluid's bubble point and an increase in its dew point. This effect is negligible for pore sizes greater than 100 nm. Capillary forces decrease with increasing pore size. Furthermore, in porous media, the dew point of Eagle Ford crude oil increases while the bubble point decreases. The more similar the composition of the mixture, the smaller the capillary force. Liu (2016) showed that adsorption has a greater impact than capillary forces, increasing the bubble point of N2 / n-C4H10 mixtures and decreasing that of CH4 / n-C4H10 mixtures. He also found that pore size distribution affects phase equilibrium calculations. Lei (2017), in a more recent study, found that capillary forces first increase and then decrease with increasing pore size, resulting in a decrease in the dew point pressure of Wolfcamp crude oil in porous media.

[0006] At present, there is a relatively complete method for calculating fluid phase state that takes into account the influence of porous media. Specifically, it is based on the conventional cubic equation of state. Taking into account the combined effects of capillary forces, adsorption, critical parameter offset, pore size and fluid composition, the phase characteristics of fluids (oil and gas) under porous media conditions are flash calculated.

[0007] Phase stability analysis is a crucial supporting technique for fluid phase state calculations using equations of state. It can be used to determine the single-phase stability of a fluid and provide a reasonable initial value for the equilibrium ratio in flash evaporation calculations. This can address the occasional failure of flash evaporation iterations to converge within specific temperature and pressure ranges. However, existing phase stability analysis methods are still only applicable under conventional conditions and do not consider the effects of porous media.

[0008] In response to the problems of the prior art, the present invention provides a method and device for analyzing the stability of fluid phase in porous media. Summary of the Invention

[0009] To solve the above problems of the prior art, the present invention provides a method for analyzing the stability of fluid phase in porous media, the method comprising the following steps:

[0010] S1. Calculate the fugacity and initial equilibrium ratio of the mixture in the current pore size in the porous medium;

[0011] S2. When a second phase is present, calculate the mole number of the second phase, the sum of the mole numbers of the second phases, and the mole fraction of the second phase;

[0012] S3. A pressure balance is achieved between the main phase and the second phase through iterative calculation, and the fugacity of the second phase and the fugacity ratio coefficient of the second phase at the pressure balance are calculated;

[0013] S4. If the fugacity ratio coefficient of the second phase satisfies the convergence condition when the pressure is balanced, whether the single phase is stable under the current pore size condition is determined based on the sum of the molar numbers of the second phase.

[0014] According to one embodiment of the present invention, in step S1:

[0015] The fugacity of the mixture in the current pore size is calculated using the following formula:

[0016]

[0017] The compressibility factor is calculated using the following formula. When multiple compressibility factors exist, the one with the smallest Gibbs free energy is selected:

[0018]

[0019] The initial value of the balance ratio is calculated using the following formula:

[0020]

[0021] Among them, f zi represents the fugacity of the mixture in the current pore size; z i represents the fluid composition under the current pore size conditions, z i (i=1,2,…,N c ); p represents the system pressure; b i Represents the repulsion coefficient of each component; b m represents the average repulsion coefficient of the mixture; Z represents the compression factor; A m 、B m Calculate parameters for phase equilibrium; represents the 0.5 power of the gravitational coefficient of each component, represents the average gravitational coefficient of the mixture; K i represents the balance ratio; p ci represents the critical pressure of each component; ω i represents the eccentricity factor of each component; T ci represents the critical temperature of each component; T represents the system temperature.

[0022] According to one embodiment of the present invention, in step S2:

[0023] The molar number of the second phase was calculated using the following formula:

[0024] (Y i ) g =z i (K i ) g

[0025] (Y i ) o =z i / (K i ) o (i=1,2,…,N c )

[0026] The sum of the molar numbers of the second phase is calculated using the following formula:

[0027]

[0028]

[0029] The mole fraction of the second phase was calculated using the following formula:

[0030]

[0031]

[0032] Wherein, the subscripts o and g indicate that the second phase is oil phase and gas phase respectively; Y i Indicates the number of moles; z i represents the fluid composition under the current pore size conditions, z i (i=1,2,…,N c );K i represents the equilibrium ratio; S represents the sum of the molar numbers; y i represents the mole fraction.

[0033] According to one embodiment of the present invention, step S3 includes the following steps:

[0034] a. Assume that the oil phase and gas phase pressures are equal and equal to the bulk phase pressure;

[0035] b. Calculate the compressibility factor of the second phase. If there are multiple compressibility factors, choose the one that minimizes the Gibbs free energy.

[0036] c. Calculate the average molar mass of the second phase mixture;

[0037] d. After obtaining the average molar mass of the second phase mixture, further calculating the density of the second phase mixture;

[0038] e. Calculate the interfacial tension between the vapor and liquid phases based on the density of the second phase mixture;

[0039] f. Considering the scale effect of surface tension, in tiny pores, the influence of interface curvature on the actual interfacial tension is considered, and the actual interfacial tension under the current pore size conditions is calculated based on the interfacial tension of the vapor-liquid two-phase plane surface;

[0040] g. Based on the actual interfacial tension, taking into account the capillary force and the unequal pressures of the oil and gas phases, calculate the capillary pressure, and then calculate the pressure of the second phase based on the actual situation;

[0041] h. Determine whether the difference between the current capillary pressure and the capillary pressure calculated in the previous iteration is within the error range. If it is within the error range, it indicates that the pressure of the main phase and the second phase is balanced. Otherwise, return to step b to continue the calculation.

[0042] According to one embodiment of the present invention, in step S3:

[0043] The fugacity ratio coefficient of the second phase at pressure equilibrium is calculated using the following formula:

[0044]

[0045]

[0046] Wherein, the subscripts o and g indicate that the second phase is oil phase and gas phase respectively; R i represents the fugacity ratio coefficient; f zi represents the fugacity of the mixture in the current pore size; f yi represents the fugacity of the second phase mixture; S represents the sum of the molar numbers.

[0047] According to one embodiment of the present invention, step S4 includes the following steps:

[0048] If the fugacity ratio coefficient at pressure equilibrium satisfies the convergence condition, further determining whether the sum of the molar numbers of the second phase is less than or equal to a preset value;

[0049] If the judgment result is yes, the current pore size condition is single-phase stable;

[0050] If the judgment result is no, the single-phase is unstable under the current pore size conditions, and the equilibrium ratio at this time is used as the initial equilibrium ratio to calculate the two-phase flash evaporation.

[0051] According to one embodiment of the present invention, step S4 includes the following steps:

[0052] If the fugacity ratio coefficient at pressure equilibrium does not meet the convergence condition, then after updating the equilibrium ratio, determine whether the updated equilibrium ratio meets the zero solution condition;

[0053] If the judgment result is yes, the current pore size condition is single-phase stable;

[0054] If the judgment result is no, return to step S2 to continue the calculation until the convergence condition or the zero solution condition is met.

[0055] According to one embodiment of the present invention, the method comprises the following steps:

[0056] If there are multiple pore sizes under the porous medium condition, calculations are performed according to steps S1 to S4 for each pore size. If each pore size condition is judged to be single-phase stable, the fluid in the entire porous medium is judged to be single-phase stable. Otherwise, the fluid in the porous medium is judged to be single-phase unstable.

[0057] According to another aspect of the present invention, a storage medium is provided, which contains a series of instructions for executing the method steps described in any one of the above.

[0058] According to another aspect of the present invention, there is also provided a device for analyzing the phase stability of fluid in a porous medium, for performing any of the above methods, the device comprising:

[0059] A calculation module, which is used to calculate the fugacity and initial equilibrium ratio of the mixture in the current pore size in the porous medium;

[0060] a second phase calculation module, configured to calculate, when a second phase exists, the number of moles of the second phase, the sum of the number of moles of the second phase, and the mole fraction of the second phase;

[0061] an iterative calculation module, which is used to achieve pressure equilibrium between the main phase and the second phase through iterative calculation, and calculate the fugacity of the second phase and the fugacity ratio coefficient of the second phase when the pressure is balanced;

[0062] The single-phase stability judgment module is used to judge whether the single-phase is stable under the current pore size conditions based on the sum of the molar numbers of the second phase if the fugacity ratio coefficient of the second phase meets the convergence condition when the pressure is balanced.

[0063] The present invention provides a method and device for analyzing the stability of fluid phases in porous media. These methods can accurately determine whether an oil and gas system of any composition under porous media conditions is in a single-phase stable state at a given temperature and pressure. If it is unstable, a reasonable initial equilibrium ratio for oil and gas two-phase flash calculation can be obtained.

[0064] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purposes and other advantages of the present invention can be realized and obtained through the structures particularly pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0066] Figure 1 A flow chart of a method for analyzing fluid phase stability in porous media according to an embodiment of the present invention is shown;

[0067] Figure 2 shows a structural block diagram of a device for analyzing phase stability of fluid in porous media according to one embodiment of the present invention; and

[0068] Figure 3 The figure shows the actual core pore radius distribution diagram of the H oil field according to one embodiment of the present invention. DETAILED DESCRIPTION

[0069] To make the objectives, technical solutions and advantages of the present invention more clear, embodiments of the present invention are described in further detail below with reference to the accompanying drawings.

[0070] Figure 1 A flow chart of a method for analyzing fluid phase stability in porous media according to an embodiment of the present invention is shown.

[0071] The present invention aims to establish a set of fluid single-phase stability analysis methods applicable to porous media conditions, and is a phase stability analysis method that considers the influence of porous media when simulating and calculating oil and gas phase states.

[0072] Based on the conventional phase stability analysis method, the influence of factors such as pore size distribution, capillary pressure and critical parameter deviation on the fluid phase state is considered, and a phase stability analysis method for fluids in porous media is established.

[0073] Assume that the mixture system has a total of N c components, distributed in pore radii of r j (j=1,2,…,N b ) b The composition of the fluid in each pore is the same, which is z i (i=1,2,…,N c ). The system temperature and pressure are T and p respectively.

[0074] At this point, the state equation of the fluid in the porous medium is established as follows:

[0075]

[0076]

[0077]

[0078] a i,j (T) = a ci,j α i,j (T) (4)

[0079]

[0080]

[0081]

[0082]

[0083]

[0084] Where A m,j Represents the phase equilibrium calculation parameter; a m,j and b m,j are the average attraction and repulsion constants of the mixture system in the jth capillary bundle, respectively; p j represents the system pressure in the jth capillary bundle; R represents the gas constant; B m,j represents the phase equilibrium calculation parameter; X i,j and X k,j are the i component and k component in the mixture of the j-th capillary bundle respectively; and are the 0.5 power of the gravitational coefficients of component i and component k in the jth capillary bundle respectively; a ci,j represents the gravitational coefficient of each component in the mixture of the jth capillary bundle; b i,j represents the repulsion coefficient of each component in the mixture in the jth capillary bundle; m i,j Represents the intermediate parameter of phase equilibrium calculation; 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 j-th capillary bundle respectively; is the binary interaction coefficient, for the hydrocarbon-hydrocarbon system:

[0085] The critical parameters of confined fluids in dense porous media will deviate from those under conventional conditions. Zarragoicoechea and Kuz (2004) proposed a relationship between the critical parameters and pore size, but this formula is only applicable to This is not applicable to large molecules and very small pores. Therefore, considering the different pore sizes, the present invention adopts the following relationship:

[0086]

[0087] Where σ eff Denotes the effective molecular diameter, D eff represents the effective pore diameter; T cp and T cb represent the critical temperature of the fluid in porous media and large space respectively; p cp and p cb represent the critical pressures of fluid in porous media and large space, respectively.

[0088] like Figure 1 As shown, in step S1, the fugacity and initial value of the equilibrium ratio of the mixture in the current pore size in the porous medium are calculated.

[0089] In one embodiment, in step S1:

[0090] The fugacity of each component of the mixture in each pore is the same. The fugacity of the mixture in the current pore size is calculated using the following formula:

[0091]

[0092] The compressibility factor is calculated using the following formula. When multiple compressibility factors exist, the one with the smallest Gibbs free energy is selected:

[0093]

[0094] The initial value of the balance ratio is calculated using the following formula:

[0095]

[0096] Among them, f zi represents the fugacity of the mixture in the current pore size; z i represents the fluid composition under the current pore size conditions, z i (i=1,2,…,N c ); p represents the system pressure; b i Represents the repulsion coefficient of each component; b m represents the average repulsion coefficient of the mixture; Z represents the compression factor; A m 、B mCalculate parameters for phase equilibrium; represents the 0.5 power of the gravitational coefficient of each component, represents the average gravitational coefficient of the mixture; K i represents the balance ratio; p ci represents the critical pressure of each component; ω i represents the eccentricity factor of each component; T ci represents the critical temperature of each component; T represents the system temperature.

[0097] Furthermore, the phase equilibrium calculation parameter A m 、B m , respectively calculated by the following formula:

[0098]

[0099]

[0100] like Figure 1 As shown, in step S2, when the second phase exists, the mole number of the second phase, the sum of the mole numbers of the second phase, and the mole fraction of the second phase are calculated.

[0101] Specifically, when the main phase is oil, the presence of a gas phase is tested (i.e., the second phase is a gas phase), and when the main phase is a gas phase, the presence of an oil phase is tested (in this case, the second phase is an oil phase). The number of moles of the second phase is calculated according to the actual situation using the following formula:

[0102]

[0103] The sum of the molar numbers of the second phase is calculated using the following formula:

[0104]

[0105] The mole fraction of the second phase is calculated by normalization using the following formula:

[0106]

[0107] Wherein, the subscripts o and g indicate that the second phase is oil phase and gas phase respectively; Y i Indicates the number of moles; z i represents the fluid composition under the current pore size conditions, z i (i=1,2,…,N c );K i represents the equilibrium ratio; S represents the sum of the molar numbers; y i represents the mole fraction.

[0108] like Figure 1As shown, in step S3, the pressure of the main phase and the second phase is balanced by iterative calculation, and the fugacity of the second phase and the fugacity ratio coefficient of the second phase when the pressure is balanced are calculated.

[0109] In one embodiment, in step S3, achieving pressure equilibrium between the main phase and the second phase through iterative calculation comprises the following steps:

[0110] a. Assume that the oil phase pressure p o and gas phase pressure p g are equal and equal to the main phase pressure; that is, p o =p g , at this time the capillary pressure is p c =0.

[0111] b. Use equation (12) to calculate the compressibility factor of the second phase. When there are multiple compressibility factors, select the one that corresponds to the smallest Gibbs free energy.

[0112] c. Calculate the average molar mass of the second phase mixture:

[0113]

[0114] Wherein, subscripts o and g indicate that the second phase is oil phase and gas phase respectively; M represents the molar mass; y i Indicates the mole fraction; M i represents the molar mass of component i.

[0115] d. After obtaining the average molar mass of the second phase mixture, further calculate the density of the second phase mixture:

[0116]

[0117] Wherein, the subscripts o and g indicate that the second phase is oil phase and gas phase, respectively; ρ represents density; M represents molar mass; p represents pressure; Z represents compressibility factor; and R represents gas constant.

[0118] e. Calculate the interfacial tension between the vapor and liquid phases based on the density of the second phase mixture:

[0119]

[0120] Where, γ ∞ is the interfacial tension of the flat liquid surface, [P i ] is the isotonic specific volume of component i.

[0121] f. Considering the scale effect of surface tension, in tiny pores, the influence of interface curvature on the actual interfacial tension is considered. Based on the interfacial tension of the vapor-liquid two-phase plane, the actual interfacial tension under the current pore size conditions is calculated:

[0122]

[0123] Where γ represents the actual interfacial tension in the tiny pores, δ represents the Tolman length, and r represents the pore radius.

[0124] The relationship for calculating the Tolman length is:

[0125]

[0126] Where, v s represents the molar volume of the solid, N A Represents Avogadro's constant.

[0127] g. Based on the actual interfacial tension, taking into account the capillary force and the unequal pressures of the oil and gas phases, the capillary pressure is calculated, and the pressure of the second phase is calculated based on the actual situation:

[0128]

[0129] Where p c is the capillary pressure, θ is the angle between the oil-gas interface and the pore surface; r eff represents the effective pore radius.

[0130] h. Determine whether the difference between the current capillary pressure calculated by equation (22) and the capillary pressure calculated in the previous iteration is within the error range. If it is within the error range, it indicates that the pressure of the main phase and the second phase is balanced. Otherwise, return to step b to continue the calculation. In one embodiment, the error range is 10 -8 .

[0131] In one embodiment, in step S3, the second phase fugacity (f yi ) o or (f yi ) g .

[0132] In one embodiment, in step S3, the fugacity ratio coefficient of the second phase at pressure equilibrium is calculated using the following formula:

[0133]

[0134] Wherein, the subscripts o and g indicate that the second phase is oil phase and gas phase respectively; R i represents the fugacity ratio coefficient; f zi represents the fugacity of the mixture in the current pore size; f yi represents the fugacity of the second phase mixture; S represents the sum of the molar numbers.

[0135] like Figure 1As shown, in step S4, if the fugacity ratio coefficient of the second phase meets the convergence condition when the pressure is balanced, it is determined whether the single phase is stable under the current pore size condition based on the sum of the molar numbers of the second phase.

[0136] In one embodiment, in step S4, equation (24) is used to determine whether the convergence condition is met:

[0137]

[0138] In one embodiment, in step S4, if the fugacity ratio coefficient at the time of pressure equilibrium satisfies the convergence condition, the sum of the molar number of the second phase (S o or S g ) is less than or equal to a preset value (the preset value can be set to 1). If the judgment result is yes, the current pore size condition is single-phase stable. In this case, the single-phase stable state means that the system remains single-phase, there is no two phases, and there is no need to continue calculating the two-phase flash. If the judgment result is no, the current pore size condition is single-phase unstable, and the equilibrium ratio at this time is used as the initial equilibrium ratio for calculating the two-phase flash.

[0139] In one embodiment, in step S4, if the fugacity ratio coefficient at pressure equilibrium does not meet the convergence condition, the equilibrium ratio is updated, and then it is determined whether the updated equilibrium ratio meets the zero solution condition. If the determination result is yes, the system is single-phase stable under the current pore size condition. At this time, the single-phase stable state means that the system remains single-phase, and there is no two-phase and no need to continue calculating the two-phase flash. If the determination result is no, the calculation is returned to step S2 to continue until the convergence condition or the zero solution condition is met. At this time, the single-phase stable state means that the system remains single-phase, and there is no two-phase and no need to continue calculating the two-phase flash.

[0140] In one embodiment, the balance ratio is updated according to formula (25):

[0141] K i (n+1) =K i (n) R i (n) (25)

[0142] Where, represents the nth iteration.

[0143] In one embodiment, equation (26) is used to determine whether the zero solution condition is satisfied:

[0144]

[0145] In one embodiment, if there are multiple pore sizes under the porous medium condition, calculations are performed according to steps S1 to S4 for each pore size. If each pore size condition is judged to be single-phase stable, the fluid in the entire porous medium is judged to be single-phase stable; otherwise, the fluid in the porous medium is judged to be single-phase unstable.

[0146] Because the magnitude of the saturation pressure in porous media increases with decreasing pore size, that is, during the pressure reduction process, the bubble point pressure of crude oil first appears in the largest pores, while the dew point pressure of condensate gas first appears in the smallest pores. Therefore, for multi-porosity media, only one pore size needs to be calculated to obtain the saturation pressure of the fluid in the entire porous medium: for crude oil, the largest pore is considered; for condensate gas, the smallest pore is considered. Based on the analysis method of fluid phase stability in porous media, the steps for calculating the saturation pressure of fluid in porous media are given as follows:

[0147] Step 1: Calculate the saturation pressure of the fluid at a given temperature without considering the influence of porous media.

[0148] Specifically, the saturation pressure of the fluid at a given temperature without considering the influence of porous media is calculated according to the conventional fluid saturation pressure calculation method, which is recorded as

[0149] Step 2: Use the fluid phase stability analysis method in porous media to analyze the single-phase stability under the saturation pressure result value, and obtain the initial equilibrium ratio for oil and gas two-phase flash calculation.

[0150] Step 3: Based on the type of the main phase, the saturation pressure result value, and the initial equilibrium ratio, perform iterative calculation of the flash evaporation of the fluid in the porous medium. The corresponding pressure when the stop condition is met is recorded as the saturation pressure of the fluid in the porous medium.

[0151] Specifically, if the main phase is the oil phase, the flash evaporation iterative calculation of the fluid in the porous medium is performed by stepwise pressure reduction with a preset pressure difference within the first pressure range, wherein the equilibrium ratio obtained in step 2 is used as the initial equilibrium ratio of the flash evaporation calculation under the saturation pressure result value.

[0152] Specifically, if the main phase is oil phase, then at a pressure of The flash evaporation of the fluid in the porous medium is iteratively calculated by stepwise pressure reduction with a pressure difference of 0.01 MPa within the range, and the equilibrium ratio K obtained in step 2 is used. i As The initial equilibrium ratio of the flash calculation under the current pressure, and the equilibrium ratio K obtained by the last iteration after the flash calculation under the current pressure is completed i It is used as the initial equilibrium ratio for subsequent flash calculations under the first-level pressure conditions to avoid non-convergence of flash calculation iterations.

[0153] Specifically, if the main phase is oil, during the flash evaporation iterations at each pressure level, if the calculated gas phase mole fraction is greater than a first preset value, all calculations are terminated and the current corresponding pressure is recorded as the saturation pressure of the fluid in the porous medium. Specifically, the first preset value is 0.

[0154] Specifically, if the main phase is a gas phase, the flash evaporation iterative calculation of the fluid in the porous medium is performed by stepwise pressurization with a preset pressure difference within the second pressure range, wherein the equilibrium ratio obtained in step 2 is used as the initial equilibrium ratio of the flash evaporation calculation under the saturation pressure result value.

[0155] Specifically, if the main phase is gas, then at a pressure of The flash evaporation of the fluid in the porous medium is iteratively calculated by increasing the pressure step by step with a pressure difference of 0.01 MPa. The equilibrium ratio K obtained in step 2 is used. i As The initial equilibrium ratio of the flash calculation under the current pressure, and the equilibrium ratio K obtained by the last iteration after the flash calculation under the current pressure is completed i It is used as the initial equilibrium ratio for subsequent flash calculations under the first-level pressure conditions to avoid non-convergence of flash calculation iterations.

[0156] Specifically, if the main phase is a gas phase, during the flash evaporation iterative calculation at each pressure level, if the calculated gas phase mole fraction is equal to the second preset value, all calculations are stopped and the current corresponding pressure is recorded as the saturation pressure of the fluid in the porous medium. Specifically, the second preset value is 1.

[0157] In one embodiment, because the magnitude of the saturation pressure in a porous medium increases with decreasing pore size, that is, during the pressure reduction process, the bubble point pressure of crude oil first appears in the largest pores, while the dew point pressure of condensate gas first appears in the smallest pores. Therefore, for a multi-porosity medium, only one pore size needs to be calculated to obtain the saturation pressure of the fluid in the entire porous medium: for crude oil, the largest pore size is considered; for condensate gas, the smallest pore size is considered.

[0158] The method and apparatus for analyzing the phase stability of a fluid in a porous medium provided by the present invention may also be used in conjunction with a computer-readable storage medium having a computer program stored thereon. The computer program is executed to implement the method for analyzing the phase stability of a fluid in a porous medium. The computer program is capable of executing computer instructions, which include computer program code. The computer program code may be in source code form, object code form, executable file, or some intermediate form.

[0159] Computer-readable storage media may include: any entity or device that can carry computer program code, recording media, USB flash drives, mobile hard drives, magnetic disks, optical disks, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc.

[0160] It should be noted that the content contained in computer-readable storage media can be appropriately increased or decreased according to the requirements of legislation and patent practices in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practices, computer-readable storage media do not include electrical carrier signals and telecommunications signals.

[0161] Figure 2 The figure shows a structural block diagram of a device for analyzing phase stability of fluid in porous media according to an embodiment of the present invention.

[0162] like Figure 2 As shown, a device 200 for analyzing the phase stability of fluid in a porous medium includes: a calculation module 201 , a second phase calculation module 202 , an iterative calculation module 203 , and a single-phase stability judgment module 204 .

[0163] In one embodiment, the calculation module 201 is used to calculate the fugacity and initial equilibrium ratio of the mixture in the current pore size in the porous medium.

[0164] In one embodiment, the second phase calculation module 202 is configured to calculate the mole number of the second phase, the sum of the mole numbers of the second phases, and the mole fraction of the second phase when the second phase exists.

[0165] In one embodiment, the iterative calculation module 203 is used to achieve pressure balance between the main phase and the second phase through iterative calculation, and calculate the fugacity of the second phase and the fugacity ratio coefficient of the second phase when the pressure is balanced.

[0166] In one embodiment, the single-phase stability determination module 204 is configured to determine whether the single-phase is stable under the current pore size condition based on the sum of the molar numbers of the second phase if the fugacity ratio coefficient of the second phase meets the convergence condition when the pressure is balanced.

[0167] Figure 3 The figure shows the actual core pore radius distribution diagram of the H oil field according to one embodiment of the present invention.

[0168] Taking H oilfield crude oil as an example, the application of fluid phase stability analysis method in porous media is carried out. The fluid composition of H oilfield is shown in Table 1, and the actual core pore distribution is shown in Figure 3 shown.

[0169] Table 1H oilfield well flow composition and state equation parameters

[0170] Actual components pseudo-components <![CDATA[Z i (mol%)]]> <![CDATA[M i (g / mol)]]> <![CDATA[p cb (kPa)]]> <![CDATA[T cb (K)]]> <![CDATA[w i ]]> <![CDATA[CO2]]> psu0 0.640 44.01 7376.460 304.2 0.225 <![CDATA[C1]]> psu1 14.627 16.043 4600.155 190.6 0.008 <![CDATA[C2]]> psu2 4.769 30.07 4883.865 305.4 0.098 <![CDATA[C3]]> PSU3 8.358 44.097 4245.518 369.8 0.152 <![CDATA[iC4]]> PSU4 1.620 58.124 3647.700 408.1 0.176 <![CDATA[nC4]]> psu5 1.950 58.124 3799.688 425.2 0.193 <![CDATA[iC5]]> psu6 1.080 72.151 3384.255 460.4 0.227 <![CDATA[nC5]]> psu7 1.180 72.151 3374.123 469.6 0.251 <![CDATA[C6]]> psu8 1.140 86 3289.010 507.5 0.275 <![CDATA[C7]]> PSU9 4.419 96 3138.035 543.2 0.308 <![CDATA[C8]]> psu10 3.049 107 2950.584 570.5 0.351 <![CDATA[C9]]> psu11 2.879 121 2729.696 598.5 0.391 <![CDATA[C 10 ]]> PSU12 4.589 134 2534.138 622.1 0.444 <![CDATA[C 11 + ]]> PSU13 49.700 263 2003.499 667.9 0.790

[0171] According to existing methods for calculating oil and gas flash evaporation in porous media, under reservoir temperature (65°C), the gas mole fractions at pressures of 6.40, 6.35, 5.22, and 4.26 MPa are 0%, 0%, 2%, and 5%, respectively. This means that the crude oil system in the core is a single-phase oil system at pressures of 6.40 and 6.35 MPa, and a two-phase oil and gas system at pressures of 5.22 and 4.26 MPa.

[0172] Using the method provided in this invention, the single-phase stability of crude oil under the same conditions was calculated. The results showed that the single-phase stability of the porous medium system at pressures of 6.40, 6.35, 5.22, and 4.26 MPa was stable, stable, unstable, and unstable, respectively. This result is consistent with the calculation method for oil and gas flash vaporization in porous media.

[0173] Furthermore, existing methods for calculating oil and gas flash evaporation in porous media failed to converge at temperatures of 405°C and pressures of 5.68 MPa and 5.67 MPa, resulting in an inability to calculate results. However, the method provided in this invention calculated the single-phase stability of crude oil under the same conditions, finding the system's single-phase stability to be stable at 5.68 MPa and unstable at 5.67 MPa, respectively. Substituting the initial equilibrium ratio obtained by the stability analysis method at 5.67 MPa into the flash evaporation calculation yielded a converged result (gas phase mole fraction of 0.014%).

[0174] In summary, the present invention provides a method and device for analyzing the stability of fluid phases in porous media. This method can accurately determine whether an oil and gas system of any composition under porous media conditions is in a single-phase stable state at a given temperature and pressure. If it is unstable, a reasonable initial equilibrium ratio for oil and gas two-phase flash calculation can be obtained.

[0175] It should be understood that the embodiments disclosed herein are not limited to the specific structures, processing steps, or materials disclosed herein, but should extend to equivalent substitutions of these features understood by those skilled in the relevant art. It should also be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to be limiting.

[0176] In the description of the present invention, unless otherwise specified, "plurality" means two or more; terms such as "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," and "tail" indicate positions or relationships based on those shown in the accompanying drawings. These terms are intended solely to facilitate the description of the present invention and simplify the description. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limiting the present invention. Furthermore, terms such as "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0177] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "connected" and "connection" should be understood in a broad sense. For example, they can refer to fixed connection, detachable connection, or integral connection; mechanical connection, electrical connection; direct connection, or indirect connection through an intermediary. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0178] References in this specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Therefore, appearances of the phrases "one embodiment" or "an embodiment" in various places throughout this specification do not necessarily refer to the same embodiment.

[0179] The embodiments of the present invention are presented for purposes of illustration and description and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those skilled in the art. The embodiments are chosen and described in order to better illustrate the principles of the invention and its practical application and to enable those skilled in the art to understand the invention and design various embodiments with various modifications as suited for specific applications.

[0180] Although the embodiments disclosed herein are as described above, the contents described herein are merely embodiments for facilitating understanding of the present invention and are not intended to limit the present invention. Any person skilled in the art of the present invention may make any modifications and changes in the form and details of the implementation without departing from the spirit and scope disclosed herein. However, the scope of patent protection of the present invention shall still be subject to the scope defined by the appended claims.

Claims

1. A method for analyzing the stability of fluid phase in porous media, characterized in that: Considering the effects of pore size distribution, capillary pressure, and critical parameter deviation on fluid phase behavior, the method comprises the following steps: S1. Calculate the fugacity and initial equilibrium ratio of the mixture in the current pore size in the porous medium; S2. When a second phase is present, calculate the mole number of the second phase, the sum of the mole numbers of the second phases, and the mole fraction of the second phase; S3. A pressure balance is achieved between the main phase and the second phase through iterative calculation, and the fugacity of the second phase and the fugacity ratio coefficient of the second phase at the pressure balance are calculated; S4. If the fugacity ratio coefficient of the second phase satisfies the convergence condition when the pressure is balanced, determining whether the single phase is stable under the current pore size condition based on the sum of the molar numbers of the second phase; Step S3 includes the following steps: a. Assume that the oil phase and gas phase pressures are equal and equal to the bulk phase pressure; b. Calculate the compressibility factor of the second phase. If there are multiple compressibility factors, choose the one that minimizes the Gibbs free energy. c. Calculate the average molar mass of the second phase mixture; d. After obtaining the average molar mass of the second phase mixture, further calculating the density of the second phase mixture; e. Calculate the interfacial tension between the vapor and liquid phases based on the density of the second phase mixture; f. Considering the scale effect of surface tension, in tiny pores, the influence of interface curvature on the actual interfacial tension is considered, and the actual interfacial tension under the current pore size conditions is calculated based on the interfacial tension of the vapor-liquid two-phase plane surface; g. Based on the actual interfacial tension, taking into account the capillary force and the unequal pressures of the oil and gas phases, calculate the capillary pressure, and then calculate the pressure of the second phase based on the actual situation; h. Determine whether the difference between the current capillary pressure and the capillary pressure calculated in the previous iteration is within the error range. If it is within the error range, it indicates that the pressure of the main phase and the second phase is balanced. Otherwise, return to step b to continue calculation; Taking into account the different pore sizes, the following relationship is used: Where: σ eff Denotes the effective molecular diameter, D eff represents the effective pore diameter; T cp and T cb represent the critical temperature of the fluid in porous media and large space respectively; p cp and p cb represent the critical pressures of fluid in porous media and large space, respectively.

2. A method for analyzing fluid phase stability in porous media according to claim 1, characterized in that: In step S1: The fugacity of the mixture in the current pore size is calculated using the following formula: The compressibility factor is calculated using the following formula. When multiple compressibility factors exist, the one with the smallest Gibbs free energy is selected: The initial value of the balance ratio is calculated using the following formula: Among them, f zi represents the fugacity of the mixture in the current pore size; z i represents the fluid composition under the current pore size conditions, z i (i=1,2,…,N c ); p represents the system pressure; b i Represents the repulsion coefficient of each component; b m represents the average repulsion coefficient of the mixture; Z represents the compression factor; A m 、B m Calculate parameters for phase equilibrium; represents the 0.5 power of the gravitational coefficient of each component, represents the average gravitational coefficient of the mixture; K i represents the balance ratio; p ci represents the critical pressure of each component; ω i represents the eccentricity factor of each component; T ci represents the critical temperature of each component; T represents the system temperature.

3. The method for analyzing the stability of fluid phase in porous media according to claim 1, wherein: In step S2: The molar number of the second phase was calculated using the following formula: (Y i ) g =z i (K i ) g (Y i ) o =z i / (K i ) o (i=1,2,…,N c ) The sum of the molar numbers of the second phase is calculated using the following formula: The mole fraction of the second phase was calculated using the following formula: Wherein, the subscripts o and g indicate that the second phase is oil phase and gas phase respectively; Y i Indicates the number of moles; z i represents the fluid composition under the current pore size conditions, z i (i=1,2,…,N c );K i represents the equilibrium ratio; S represents the sum of the moles; y i represents the mole fraction.

4. A method for analyzing fluid phase stability in porous media according to claim 1, characterized in that: In step S3: The fugacity ratio coefficient of the second phase at pressure equilibrium is calculated using the following formula: Wherein, the subscripts o and g indicate that the second phase is oil phase and gas phase respectively; R i represents the fugacity ratio coefficient; f zi represents the fugacity of the mixture in the current pore size; f yi represents the fugacity of the second phase mixture; S represents the sum of the molar numbers.

5. The method for analyzing the stability of fluid phase in porous media according to claim 1, wherein: Step S4 includes the following steps: If the fugacity ratio coefficient at pressure equilibrium satisfies the convergence condition, further determining whether the sum of the molar numbers of the second phase is less than or equal to a preset value; If the judgment result is yes, the current pore size condition is single-phase stable; If the judgment result is no, the single-phase is unstable under the current pore size conditions, and the equilibrium ratio at this time is used as the initial equilibrium ratio to calculate the two-phase flash evaporation.

6. The method for analyzing the stability of fluid phase in porous media according to claim 1, wherein: Step S4 includes the following steps: If the fugacity ratio coefficient at pressure equilibrium does not meet the convergence condition, then after updating the equilibrium ratio, determine whether the updated equilibrium ratio meets the zero solution condition; If the judgment result is yes, the current pore size condition is single-phase stable; If the judgment result is no, return to step S2 to continue the calculation until the convergence condition or the zero solution condition is met.

7. A method for analyzing fluid phase stability in porous media according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: If there are multiple pore sizes under the porous medium condition, calculations are performed according to steps S1 to S4 for each pore size. If each pore size condition is judged to be single-phase stable, the fluid in the entire porous medium is judged to be single-phase stable. Otherwise, the fluid in the porous medium is judged to be single-phase unstable.

8. A storage medium, characterized in that: It contains a series of instructions for executing the method steps according to any one of claims 1 to 7.

9. A device for analyzing the stability of fluid phase in porous media, characterized in that: Executing the method according to any one of claims 1 to 7, the device comprises: A calculation module, which is used to calculate the fugacity and initial equilibrium ratio of the mixture in the current pore size in the porous medium; a second phase calculation module, configured to calculate, when a second phase exists, the number of moles of the second phase, the sum of the number of moles of the second phase, and the mole fraction of the second phase; an iterative calculation module, which is used to achieve pressure equilibrium between the main phase and the second phase through iterative calculation, and calculate the fugacity of the second phase and the fugacity ratio coefficient of the second phase when the pressure is balanced; The single-phase stability judgment module is used to judge whether the single-phase is stable under the current pore size conditions based on the sum of the molar numbers of the second phase if the fugacity ratio coefficient of the second phase meets the convergence condition when the pressure is balanced.

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