Method and device for calculating fluid saturation pressure in porous medium

Abstract

The application provides a method for calculating saturated pressure of fluid in porous medium, which comprises the following steps: S1, calculating saturated pressure result value of fluid at given temperature without considering the influence of porous medium; S2, considering the influence of porous medium, analyzing single-phase stability under the saturated pressure result value to obtain initial equilibrium ratio for oil-gas two-phase flash calculation; S3, based on the type of main phase, saturated pressure result value and initial equilibrium ratio, carrying out iterative calculation of fluid flash in porous medium, and recording the corresponding pressure when the stop condition is met as the saturated pressure of fluid in porous medium. The application can accurately calculate the saturated pressure of oil-gas system with any composition under given temperature condition in the condition of porous medium, and avoid the problem of iteration divergence in flash calculation when searching for saturated pressure. If the object is crude oil, the bubble point pressure is calculated, and if the object is condensate gas, the dew point pressure is calculated, which provides key technical indicators for oil and gas field development.

Description

TECHNICAL FIELD

[0001] The present application relates to the oil and gas field development field, and particularly relates to a fluid saturation pressure calculation method and device in a porous medium. BACKGROUND

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

[0003] Conventional cubic equation of state (PR, SRK, etc.) is widely used in fluid phase state calculation. The fitted equation of state can accurately represent the phase state characteristics of fluid under large space conditions (PVT still). However, the interaction between fluid molecules and the surface of porous media in the porous medium leads to obvious differences in fluid phase state compared with the conventional large space conditions.

[0004] In the dense pore, especially in the nanoscale pore medium, the critical parameters of fluid will be offset compared with the large space, and the influence of the critical parameter offset on the phase state characteristics should be considered in the calculation. The heterogeneity in the porous medium is more serious, especially in the shale, the pore size is between 1-100 nm, the span is large, and different distribution modes will lead to differences in the calculation results. In addition, due to the small pore radius and large capillary force in the dense pore medium, the vapor-liquid two-phase pressure is not equal, which will also affect the phase state characteristics. There is also adsorption in the porous medium, which will affect the composition of the fluid, at the same time, the existence of the adsorption layer leads to the decrease of the effective pore diameter, which will also lead to the difference in the phase equilibrium calculation, which needs to be considered.

[0005] Zhang (2004) found that adsorption makes the condensate gas dew point drop, while capillary force makes the condensate gas dew point slightly rise. Ma (2013) found that in porous media, the critical temperature of pure component rises, the bubble point and dew point of C1 / nC4 / nC8 mixture rise, and the two-phase region shrinks; the effect of critical parameter shift on heavy component is greater. Jin (2013) found that in porous media, the bubble point of C1-nC5 mixture drops, the lower dew point rises, and he also found that capillary force makes the bubble point and lower dew point of C1-nC5 mixture drop. Li (2015) found that when the pore throat diameter is less than 10 nm, the interaction between pore throat and fluid molecules is significant, and the smaller the pore throat, the more obvious the dew point pressure drop; in addition, capillary force makes the dew point of C1 / nC4 / nC8 mixture drop. Dong (2016) found that the adsorption layer makes the capillary pressure rise, and leads to the bubble point drop and the dew point rise of the fluid; when the pore size is greater than 100 nm, the effect can be ignored; capillary force decreases with the increase of pore size; in addition, in porous media, the dew point of Eagle Ford crude oil rises, the bubble point drops, and the closer the components of the mixture, the smaller the capillary force. Liu (2016) found that adsorption has a greater effect than capillary force; the bubble point of N2 / n-C4H10 mixture rises, and the bubble point of CH4 / n-C4H10 mixture drops; he also found that the pore size distribution has an effect on the calculation results of phase equilibrium. Lei (2017) found in the latest research that capillary force first increases and then decreases with the increase of pore size, and the dew point pressure of Wolfcamp crude oil drops in porous media.

[0006] At present, there is a relatively complete fluid phase state calculation method considering the influence of porous media, which is based on a conventional cubic state equation, and under the combined influence of capillary force, adsorption, critical parameter shift, pore size and fluid composition, flash calculation is used to calculate the phase state characteristics of fluid (oil and gas) in porous media.

[0007] If the saturation pressure of a fluid in porous media is found by flash algorithm, a large amount of calculation will be involved, the speed is slow, and the flash algorithm occasionally cannot converge in a certain temperature and pressure region. The existing fluid saturation pressure calculation method is still only applicable to conventional conditions, and does not consider the influence of porous media.

[0008] In view of the problems of the prior art, the present application provides a fluid saturation pressure calculation method and device in porous media. SUMMARY

[0009] To solve the above problems of the prior art, the present application provides a fluid saturation pressure calculation method in porous media, which comprises the following steps:

[0010] S1, calculate the saturation pressure result value of the fluid at a given temperature without considering the influence of the porous medium;

[0011] S2, analyze the single-phase stability at the saturation pressure result value considering the influence of the porous medium, and obtain an initial equilibrium ratio for oil-gas two-phase flash calculation;

[0012] S3, perform iterative calculation of the fluid flash in the porous medium based on the type of the main phase, the saturation pressure result value, and the initial equilibrium ratio, and record the corresponding pressure when the stop condition is met as the saturation pressure of the fluid in the porous medium

[0013] According to one embodiment of the present application, in step S2, the single-phase stability at the saturation pressure result value is analyzed by using a fluid phase stability analysis method in the porous medium, and an initial equilibrium ratio for oil-gas two-phase flash calculation is obtained:

[0014] S21, calculate the fugacity of the mixture in the current pore size in the porous medium and the initial value of the equilibrium ratio;

[0015] S22, when the second phase exists, calculate 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;

[0016] S23, through iterative calculation to make the main phase and the second phase pressure balanced, calculate the fugacity of the second phase and the fugacity ratio coefficient of the second phase when the pressure is balanced;

[0017] S24, if the fugacity ratio coefficient of the second phase when the pressure is balanced meets the convergence condition, judge whether it is single-phase stable under the current pore size condition based on the sum of the number of moles of the second phase.

[0018] According to one embodiment of the present application, in step S23, through iterative calculation to make the main phase and the second phase pressure balanced, the following steps are included:

[0019] a, assume that the oil phase and the gas phase have equal pressure and are equal to the main phase pressure;

[0020] b, calculate the compressibility factor of the second phase, and when there are multiple compressibility factors, select the compressibility factor corresponding to the minimum Gibbs free energy;

[0021] c, calculate the average molar mass of the second phase mixture;

[0022] d, after obtaining the average molar mass of the second phase mixture, further calculate the density of the second phase mixture;

[0023] e, based on the density of the second phase mixture, calculate the vapor-liquid two-phase liquid level interface tension;

[0024] f. Considering the scale effect of surface tension, in micropores, the influence of interface curvature on the actual interface tension is taken into account. Based on the interface tension of the vapor-liquid two-phase flat liquid surface, the actual interface tension under the current pore size is calculated.

[0025] g. Based on the actual interfacial tension, considering the capillary force and the unequal pressures of the oil and gas phases, calculate the capillary pressure, and calculate the pressure of the second phase based on the actual situation.

[0026] 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 in balance; otherwise, return to step b to continue the calculation.

[0027] According to an embodiment of the present invention, step S24 includes the following steps:

[0028] If the fugacity ratio coefficient at pressure equilibrium satisfies the convergence condition, then it is further determined whether the sum of the moles of the second phase is less than or equal to a preset value.

[0029] If the judgment result is yes, then the current pore size condition indicates single-phase stability.

[0030] If the judgment result is negative, then the current pore size condition indicates single-phase instability. The equilibrium ratio at this time is used as the initial equilibrium ratio to calculate two-phase flash evaporation.

[0031] According to an embodiment of the present invention, step S24 includes the following steps:

[0032] 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.

[0033] If the judgment result is yes, then the current pore size condition indicates single-phase stability.

[0034] If the result is negative, return to step S2 to continue the calculation until the convergence condition or the zero solution condition is met.

[0035] According to an embodiment of the present invention, the method includes the following steps:

[0036] If there are multiple pore sizes under porous medium conditions, then for each pore size, calculations are performed according to steps S21 to S24. If it is determined that the fluid in the overall porous medium is single-phase stable under each pore size condition, then it is determined that the fluid in the porous medium is single-phase stable; otherwise, it is determined that the fluid in the porous medium is single-phase unstable.

[0037] According to an embodiment of the present invention, in step S3:

[0038] If the bulk phase is oil phase, the iterative calculation of the flash of the fluid in the porous medium is performed in the first pressure range with a preset pressure difference step by step with the equilibrium ratio obtained in step S2 as the initial equilibrium ratio for the flash calculation under the saturated pressure result value;

[0039] If the bulk phase is oil phase, in the iterative calculation of the flash under each pressure condition, if the calculated molar fraction of the gas phase is greater than the first preset value, all the calculation is stopped, and the corresponding pressure at present is recorded as the saturated pressure of the fluid in the porous medium.

[0040] According to one embodiment of the present application, in step S3:

[0041] If the bulk phase is gas phase, the iterative calculation of the flash of the fluid in the porous medium is performed in the second pressure range with a preset pressure difference step by step with the equilibrium ratio obtained in step S2 as the initial equilibrium ratio for the flash calculation under the saturated pressure result value;

[0042] If the bulk phase is gas phase, in the iterative calculation of the flash under each pressure condition, if the calculated molar fraction of the gas phase is equal to the second preset value, all the calculation is stopped, and the corresponding pressure at present is recorded as the saturated pressure of the fluid in the porous medium.

[0043] According to another aspect of the present application, there is also provided a storage medium containing a series of instructions for performing the method steps as claimed in any one of the above.

[0044] According to another aspect of the present application, there is also provided a device for calculating the saturated pressure of the fluid in the porous medium, which performs the method as claimed in any one of the above, and the device comprises:

[0045] a saturated pressure result module for calculating the saturated pressure result value of the fluid at a given temperature without considering the influence of the porous medium;

[0046] an equilibrium ratio module for analyzing the single-phase stability under the saturated pressure result value considering the influence of the porous medium, and obtaining the initial equilibrium ratio for the flash calculation of the oil-gas two-phase;

[0047] a saturated pressure module for performing the iterative calculation of the flash of the fluid in the porous medium based on the type of the bulk phase, the saturated pressure result value and the initial equilibrium ratio, and recording the corresponding pressure when the stop condition is met as the saturated pressure of the fluid in the porous medium.

[0048] The application provides a porous medium fluid saturation pressure calculation method and device, which can accurately calculate the saturation pressure of an oil-gas system with any composition under a given temperature condition under the condition of a porous medium, and avoids the problem of iteration non-convergence when searching for the saturation pressure in flash calculation.

[0049] Other features and advantages of the present application will be set forth in the following specification, and in part will become apparent to those skilled in the art upon exercise of the disclosure hereof, or by practice of the application hereof. The objects and other advantages of the present application will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. BRIEF DESCRIPTION OF DRAWINGS

[0050] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present application and, together with the description, further serve to explain the principles of the application and to enable a person skilled in the pertinent art to make and use the application.

[0051] Figure 1 A porous medium fluid saturation pressure calculation method flow chart according to one embodiment of the present application is shown.

[0052] Figure 2 A porous medium fluid saturation pressure calculation device structure block diagram according to one embodiment of the present application is shown.

[0053] Figure 3 A H oilfield actual core pore radius distribution according to one embodiment of the present application is shown. DETAILED DESCRIPTION

[0054] To make the objects, technical solutions and advantages of the present application clearer, the following further describes the embodiments of the present application with reference to the drawings.

[0055] Figure 1 A porous medium fluid saturation pressure calculation method flow chart according to one embodiment of the present application is shown.

[0056] The present application aims to establish a set of fluid saturation pressure calculation methods suitable for the condition of a porous medium, and is a calculation method of saturation pressure applied to simulated calculation of oil-gas phase equilibrium considering the influence of a porous medium.

[0057] On the basis of a conventional phase stability analysis method, the influence of pore size distribution, capillary pressure and critical parameter deviation on fluid phase state is considered, and a porous medium fluid phase stability analysis method is established.

[0058] Suppose that the mixture system has N c components, and is distributed in pore radii rj (j = 1, 2,..., N b ) of N b pores, the composition of the fluid in each pore is uniform, and is z i (i = 1, 2,..., N c ). The temperature and pressure of the system are T and p, respectively.

[0059] At this time, the equation of state of the fluid in the porous medium is established as follows:

[0060]

[0061]

[0062]

[0063]

[0064]

[0065]

[0066]

[0067]

[0068]

[0069] wherein A m,j represents a phase equilibrium calculation parameter; a m,j and b m,j are the average attractive and repulsive 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 a phase equilibrium calculation parameter; X i,j and X k,j are the i component and the k component in the jth capillary bundle mixture, respectively; and are the 0.5th power of the attractive coefficient of the i component and the k component in the jth capillary bundle, respectively; a ci,j represents the attractive coefficient of each component in the jth capillary bundle mixture; b i,j represents the repulsive coefficient of each component in the mixture in the jth capillary bundle; m i,j represents an intermediate parameter in the phase equilibrium calculation; ω i,j is the eccentric factor of the i component in the jth capillary bundle; T ci,j and p ci,j represent the critical temperature and critical pressure of the i component in the jth capillary bundle, respectively; For a binary interaction coefficient, in a hydrocarbon-hydrocarbon system:

[0070] The critical parameters of closed fluids in dense porous media deviate from those under conventional conditions. Zarragoicoechea and Kuz (2004) proposed a relationship between the critical parameters and pore size, but this formula only applies to... This is not applicable to macromolecules and extremely small pores. Therefore, considering the different pore sizes, the present invention adopts the following relationship:

[0071]

[0072] In the formula, σ eff D represents the effective diameter of the molecule. eff Indicates the effective diameter of the pores; T cp and T cb These represent the critical temperatures of the fluid in porous media and large spaces, respectively; p cp and p cb These represent the critical pressures of the fluid in porous media and large spaces, respectively.

[0073] like Figure 1 As shown, in step S1, the saturation pressure of the fluid at a given temperature, without considering the influence of porous media, is calculated. Specifically, the saturation pressure of the fluid at a given temperature, without considering the influence of porous media, is calculated according to conventional fluid saturation pressure calculation methods, and denoted as...

[0074] like Figure 1 As shown, in step S2, the influence of porous media is considered, and the single-phase stability under the saturation pressure result is analyzed to obtain the initial equilibrium ratio for oil-gas two-phase flash evaporation calculation.

[0075] In one embodiment, in step S2, the single-phase stability under saturated pressure is analyzed using a fluid phase stability analysis method within a porous medium to obtain the initial equilibrium ratio for oil-gas two-phase flash evaporation calculations. The fluid phase stability analysis method within a porous medium is as follows:

[0076] S21. Calculate the fugacity of the mixture in the porous medium at the current pore size and the initial value of the equilibrium ratio;

[0077] S22. When a second phase exists, calculate 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.

[0078] S23. Through iterative calculation, the pressure of the main phase and the second phase is balanced, and the fugacity of the second phase and the fugacity ratio coefficient of the second phase are calculated when the pressure is balanced.

[0079] S24, if the fugacity ratio coefficient of the second phase satisfies the convergence condition when the pressure is balanced, judging whether it is single-phase stable under the current pore size condition based on the sum of the mole number of the second phase.

[0080] In one embodiment, in step S21: the fugacity of each component of the mixture in each pore is the same, and the fugacity of the mixture in the current pore size is calculated by the following formula:

[0081]

[0082] In one embodiment, in step S21: the compressibility factor is calculated by the following formula, and when there are multiple compressibility factors, the one corresponding to the minimum Gibbs free energy is selected:

[0083]

[0084] In one embodiment, in step S21: the initial value of the equilibrium ratio is calculated by the following formula:

[0085]

[0086] wherein f zi represents the fugacity of the mixture in the current pore size; z i represents the fluid composition under the condition of the current pore size, 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 compressibility factor; A m , B m are phase equilibrium calculation parameters; represents the 0.5th power of the attraction coefficient of each component, represents the average attraction coefficient of the mixture; K i represents the equilibrium ratio; p ci represents the critical pressure of each component; ω i represents the eccentric factor of each component; T ci represents the critical temperature of each component; T represents the system temperature.

[0087] And the phase equilibrium calculation parameters A m , B m are calculated by the following formulas, respectively:

[0088]

[0089]

[0090] In one embodiment, in step S22: when the main phase is oil, test whether there is a gas phase (i.e. the second phase is a gas phase), and when the main phase is gas, then test whether there is an oil phase (at this time the second phase is an oil phase), and the molar number of the second phase is calculated according to the actual situation by the following formula:

[0091]

[0092] In one embodiment, in step S22: the sum of the molar number of the second phase is calculated by the following formula:

[0093]

[0094] In one embodiment, in step S22: the mole fraction of the second phase is calculated by the following formula:

[0095]

[0096] wherein the subscripts o and g respectively represent that the second phase is an oil phase and a gas phase; Y i represents the molar number; z i represents the fluid composition under the current pore size condition, z i (i = 1, 2, …, N c ); K i represents the equilibrium ratio; S represents the sum of the molar number; y i represents the mole fraction.

[0097] In one embodiment, in step S23, the pressure equilibrium between the main phase and the second phase by iterative calculation comprises the following steps:

[0098] a. Assume that the oil phase pressure p o is equal to the gas phase pressure p g , and both are equal to the main phase pressure; that is, p o = p g , at this time the capillary pressure is p c = 0.

[0099] b. Calculate the compressibility factor of the second phase by formula (12), and when there are multiple compressibility factors, select the one corresponding to the minimum Gibbs free energy.

[0100] c. Calculate the average molar mass of the second phase mixture:

[0101]

[0102] wherein the subscripts o and g respectively represent that the second phase is an oil phase and a gas phase; M represents the molar mass; y i represents the mole fraction; M i represents the molar mass of the i component.

[0103] d. After obtaining the average molar mass of the second phase mixture, further calculate the density of the second phase mixture:

[0104]

[0105] In the formula, the subscripts o and g represent the second phase as oil phase and gas phase respectively; p represents density; M represents molar mass; p represents pressure; Z represents compressibility factor; R represents gas constant.

[0106] e. Based on the density of the second phase mixture, calculate the vapor-liquid two-phase liquid level interfacial tension:

[0107]

[0108] In the formula, γ ∞ is the liquid level interfacial tension, [P i ] is the isochoric specific volume of component i.

[0109] f. Considering the scale effect of surface tension, considering the influence of interface curvature on actual interfacial tension in micro-pore, based on the vapor-liquid two-phase liquid level interfacial tension, calculate the actual interfacial tension under the condition of current pore size:

[0110]

[0111] In the formula, γ represents the actual interfacial tension in the micro-pore, δ represents the Tolman length, and r represents the pore radius.

[0112] The relationship for calculating the Tolman length is:

[0113]

[0114] In the formula, v s represents the molar volume of solid, N A represents Avogadro constant.

[0115] g. According to the actual interfacial tension, considering the capillary force action and the inequality of oil and gas two-phase pressure, the capillary pressure is calculated, and the pressure of the second phase is calculated according to the actual situation:

[0116]

[0117] In the formula, p c is the capillary pressure, θ is the included angle between the oil-gas interface and the pore surface; r eff represents the effective pore radius.

[0118] h. determining whether the difference between the current capillary pressure calculated by formula (22) and the capillary pressure calculated by the previous iteration is within an error range, and if so, indicating that the primary phase and the second phase are in pressure equilibrium, otherwise returning to step b to continue the calculation. In one embodiment, the error range is 10 -8 .

[0119] In one embodiment, in step S23, the fugacity ratio coefficient of the second phase at pressure equilibrium is calculated by formula (11): yi ) o or (f yi ) g .

[0120] In one embodiment, in step S23, the fugacity ratio coefficient of the second phase at pressure equilibrium is calculated by formula (11):

[0121]

[0122] wherein the subscripts o and g represent the second phase being an oil phase and a 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; and S represents the sum of the mole numbers.

[0123] In one embodiment, in step S24, formula (24) is used to determine whether the convergence condition is met:

[0124]

[0125] In one embodiment, in step S24, if the fugacity ratio coefficient at pressure equilibrium meets the convergence condition, it is further determined whether the sum of the mole numbers (S o or S g ) of the second phase is less than or equal to a preset value (the preset value can be set to 1). If the result is yes, the current pore size condition is single-phase stable, and the single-phase stable state is that the system remains single-phase, and there is no two-phase and no need to continue to calculate two-phase flash; if the 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 two-phase flash.

[0126] In one embodiment, in step S24, if the fugacity ratio coefficient at pressure equilibrium does not meet the convergence condition, then after updating the equilibrium ratio, it is determined whether the updated equilibrium ratio meets the zero solution condition; if the determination result is yes, then under the current pore size condition, it is a single-phase stable state, at this time the single-phase stable state is that the system remains a single phase, there are no two phases and there is no need to continue calculating two-phase flash evaporation; if the determination result is no, then return to step S22 to continue the calculation until the convergence condition or the zero solution condition is met, at this time the single-phase stable state is that the system remains a single phase, there are no two phases and there is no need to continue calculating two-phase flash evaporation.

[0127] In one embodiment, the balance ratio is updated according to equation (25):

[0128] K i (n+1) =K i (n) R i (n) (25)

[0129] Where represents the nth iteration.

[0130] In one embodiment, equation (26) is used to determine whether the zero solution condition is satisfied:

[0131]

[0132] In one embodiment, if there are multiple pore sizes under porous medium conditions, calculations are performed for each pore size according to steps S21 to S24. If it is determined that the fluid in the overall porous medium is single-phase stable under each pore size condition, it is determined that the fluid in the porous medium is single-phase stable; otherwise, it is determined that the fluid in the porous medium is single-phase unstable.

[0133] like Figure 1 As shown, in step S3, if the main phase is oil, the fluid flashing calculation in the porous medium is performed by gradually reducing the pressure within the first pressure range using a preset pressure difference. The equilibrium ratio obtained in step S2 is used as the initial equilibrium ratio for the flashing calculation under the saturated pressure result value.

[0134] Specifically, if the main phase is the oil phase, then at a pressure of Within a certain range, iterative calculations of fluid flashing within porous media were performed by gradually decreasing the pressure by a differential pressure of 0.01 MPa, using the equilibrium ratio K obtained in step S2. i As The initial equilibrium ratio calculated under pressure, and the equilibrium ratio K obtained from the last iteration after the flash calculation under the current pressure. i This serves as the initial equilibrium ratio for subsequent flash calculations under first-stage pressure conditions, preventing the flash calculation iterations from failing to converge.

[0135] like Figure 1As shown, in step S3, if the main phase is the oil phase, during the flash evaporation iteration calculation under each pressure condition, if the calculated gas phase mole fraction is greater than a first preset value, then all calculations are stopped, and the current corresponding pressure is recorded as the fluid saturation pressure within the porous medium. Specifically, the first preset value is 0.

[0136] like Figure 1 As shown, in step S3, if the main phase is gas, the fluid flashing calculation in the porous medium is performed by gradually increasing the pressure with a preset pressure difference within the second pressure range. The equilibrium ratio obtained in step S2 is used as the initial equilibrium ratio for flashing calculation under the saturated pressure result value.

[0137] Specifically, if the main phase is gas, then at a pressure of Within a certain range, the pressure difference is gradually increased by 0.01 MPa to perform iterative calculations of fluid flashing in porous media, using the equilibrium ratio K obtained in step S2. i As The initial equilibrium ratio calculated under pressure, and the equilibrium ratio K obtained from the last iteration after the flash calculation under the current pressure. i This serves as the initial equilibrium ratio for subsequent flash calculations under first-stage pressure conditions, preventing the flash calculation iterations from failing to converge.

[0138] like Figure 1 As shown, in step S3, if the main phase is gas, during the flash evaporation iteration calculation for each pressure level, if the calculated gas phase mole fraction equals the second preset value, then all calculations are stopped, and the current corresponding pressure is recorded as the fluid saturation pressure within the porous medium. Specifically, the second preset value is set to 1.

[0139] In one embodiment, because the variation in saturation pressure within a porous medium increases as the pore size decreases—meaning that during the depressurization 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—it is only necessary to calculate the saturation pressure of the fluid within the overall porous medium for a multi-porous medium using only one pore size: for crude oil, the largest pores are considered; for condensate gas, the smallest pores are considered.

[0140] The present invention provides a method and apparatus for calculating the saturation pressure of fluid in a porous medium, which can also be used in conjunction with a computer-readable storage medium storing a computer program. Executing the computer program runs the method for calculating the saturation pressure of fluid in a porous medium. The computer program is capable of executing computer instructions, which include computer program code. The computer program code can be in the form of source code, object code, executable file, or some intermediate form.

[0141] The computer readable storage medium can include any entity or device capable of carrying computer program code, recording medium, U disk, mobile hard disk, magnetic disk, optical disk, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signal, telecommunication signal, software distribution medium, etc.

[0142] It should be noted that the content contained in the computer readable storage medium can be appropriately increased or decreased according to the requirements of legislation and patent practice in the jurisdiction, for example, in some jurisdictions, according to legislation and patent practice, the computer readable storage medium does not include electrical carrier signals and telecommunication signals.

[0143] Figure 2 A structural block diagram of a porous medium fluid saturation pressure calculation device according to an embodiment of the present application is shown.

[0144] As Figure 2 shown, a porous medium fluid saturation pressure calculation device 200 includes a saturation pressure result module 201, a balance ratio module 202, and a saturation pressure module 203.

[0145] In an embodiment, the saturation pressure result module 201 is configured to calculate a saturation pressure result value of a fluid at a given temperature without considering the influence of the porous medium.

[0146] In an embodiment, the balance ratio module 202 is configured to analyze the single-phase stability of the saturation pressure result value considering the influence of the porous medium to obtain an initial balance ratio for oil-gas two-phase flash calculation.

[0147] In an embodiment, the saturation pressure module 203 is configured to perform iterative calculation of the fluid flash in the porous medium based on the type of the main phase, the saturation pressure result value, and the initial balance ratio, and record the corresponding pressure when the stop condition is met as the porous medium fluid saturation pressure.

[0148] Figure 3 A H oilfield actual core pore radius distribution according to an embodiment of the present application is shown.

[0149] Taking the H oilfield crude oil as an example, the application of the porous medium fluid saturation pressure calculation method is carried out. The H oilfield saturated CO2 crude oil composition is shown in Table 1, and the actual core pore distribution is shown in Figure 3

[0150] Table 1 H oilfield saturated CO2 crude oil composition and equation of state parameters

[0151] Component Z i (mol%)]] M i (g / mol)]]> p cb (MPa) T cb (K)]]> w i ]]> CO2 41.177 44.01 7.376 304.2 0.225 [C1] 7.503 16.043 4.600 190.6 0.008 [C2] 2.631 30.07 4.884 305.4 0.098 [C3] 3.321 44.097 4.246 369.8 0.152 iC4 0.830 58.124 3.648 408.1 0.176 [nC4] 1.180 58.124 3.800 425.2 0.193 iC5 0.820 72.151 3.384 460.4 0.227 [nC5] 0.470 72.151 3.374 469.6 0.251 [C6] 1.621 86 3.289 507.5 0.275 [C7 + _1]]> 19.637 119.4151 2.318 606.4 0.273 [C7 + _2]]> 17.233 252.3154 1.158 776.9 0.484 [C7 + _3]]> 3.575 535 0.584 938.9 1.160

[0152] ​Because the variation in saturation pressure within a porous medium increases as the pore size decreases—meaning that the crude oil bubble point pressure first appears in the largest pore during the depressurization process—the overall fluid saturation pressure within a porous medium can be obtained by calculating only the case with the largest pore size for a multi-porous medium. For the porous medium in the H oilfield core, only the case with a pore radius of 1 μm needs to be calculated.

[0153] The saturation pressure of H-saturated CO2 crude oil at 65℃ was calculated to be 18.02 MPa using conventional fluid saturation pressure calculation methods. Using the porous media fluid saturation pressure calculation method provided in this invention, the saturation pressure of H-saturated CO2 crude oil under the same conditions was calculated to be 17.92 MPa. Since the fluid phase in large pores is less affected by the porous media, the actual bubble point pressure in the core will be slightly lower than the saturation pressure obtained under conventional conditions.

[0154] In summary, the present invention provides a method and apparatus for calculating the saturation pressure of fluids in porous media. This method can accurately calculate the saturation pressure of any oil and gas system with different compositions under porous media conditions at a given temperature, avoiding the problem of iterative non-convergence in flash evaporation calculations when searching for saturation pressure. If the object is crude oil, the bubble point pressure is calculated; if the object is condensate gas, the dew point pressure is calculated, providing key technical indicators for oil and gas field development.

[0155] It should be understood that the embodiments disclosed herein are not limited to the specific structures, processing steps, or materials disclosed herein, but should be extended to equivalent substitutions of these features as understood by those skilled in the art. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0156] In the description of this invention, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," "tail," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0157] In the description of the application, it is necessary to point out that, unless otherwise explicitly specified and limited, the terms "connected", "connected" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0158] The phrase "one embodiment" or "an embodiment" appearing in the specification is intended to mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the application. The appearances of the phrase "one embodiment" or "an embodiment" throughout the specification are not necessarily all referring to the same embodiment.

[0159] The embodiments of the present application are given for the purpose of illustration and description, and are not exhaustive or limiting of the present application. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are chosen and described in order to best explain the principles of the application and its practical application, and to enable others skilled in the art to understand the application for various embodiments with various modifications as are suited to the particular use contemplated.

[0160] Although the embodiments disclosed by the present application are as above, the content described is only the embodiments adopted for the purpose of facilitating the understanding of the present application, and is not intended to limit the present application. Any person skilled in the art of the present application can make any modification and change in the form of implementation and details without departing from the spirit and scope of the present application, but the patent protection scope of the present application shall be subject to the scope defined by the appended claims.

Claims

1. A method for calculating fluid saturation pressure in a porous medium, characterized by, The method comprises the following steps: S1, calculating a saturation pressure result value of a fluid at a given temperature without considering the influence of a porous medium; S2, analyzing single-phase stability at the saturation pressure result value considering the influence of the porous medium to obtain an initial equilibrium ratio for oil-gas two-phase flash calculation; S3, based on the type of the main phase, the saturation pressure result value and the initial equilibrium ratio, performing iterative calculation of fluid flashing in the porous medium, and recording the corresponding pressure when the stop condition is met as the saturation pressure of the fluid in the porous medium; In step S2, the single-phase stability at the saturation pressure result value is analyzed by using a fluid phase stability analysis method in the porous medium to obtain an initial equilibrium ratio for oil-gas two-phase flash calculation: S21, calculating the fugacity of the mixture in the current pore size in the porous medium and the initial value of the equilibrium ratio; S22, when the second phase exists, calculating 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; S23, through iterative calculation to make the main phase and the second phase pressure balanced, calculating the fugacity of the second phase and the fugacity ratio coefficient of the second phase when the pressure is balanced; S24, if the fugacity ratio coefficient of the second phase when the pressure is balanced meets the convergence condition, judging whether it is single-phase stable under the current pore size condition based on the sum of the number of moles of the second phase.

2. The method of claim 1, wherein, In step S23, through iterative calculation to make the main phase and the second phase pressure balanced, the following steps are included: a, assuming that the oil phase and the gas phase are equal in pressure and are equal to the main phase pressure; b, calculating the compressibility factor of the second phase, when there are multiple compressibility factors, selecting the one corresponding to the minimum Gibbs free energy; c, calculating 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, based on the density of the second phase mixture, calculating the vapor-liquid two-phase liquid level interface tension; f, considering the scale effect of the surface tension, in a small pore, considering the influence of the interface curvature on the actual interface tension, based on the vapor-liquid two-phase liquid level interface tension, calculating the actual interface tension under the current pore size condition; g, according to the actual interface tension, considering the capillary force action and the inequality of the oil-gas two-phase pressure, calculating the capillary pressure, and calculating the pressure of the second phase according to the actual situation; h, judging whether the difference between the current capillary pressure and the capillary pressure of the previous iterative calculation is within the error range, if it meets the error range, it represents that the main phase and the second phase are pressure balanced, otherwise, return to step b to continue calculation.

3. The method of claim 1, wherein, Step S24 comprises the following steps: If the fugacity ratio coefficient when the pressure is balanced meets the convergence condition, further judging whether the sum of the number of moles of the second phase is less than or equal to a preset value; If the judgment result is yes, it is single-phase stable under the current pore size condition; If the judgment result is no, it is single-phase unstable under the current pore size condition, and the equilibrium ratio at this time is used as the initial equilibrium ratio for two-phase flash calculation.

4. The method of claim 1, wherein, Step S24 comprises the following steps: If the fugacity ratio coefficient when the pressure is balanced does not meet the convergence condition, after updating the equilibrium ratio, judging whether the updated equilibrium ratio meets the zero solution condition; If the result is yes, the current pore size condition is single-phase stable; If the result is no, return to step S2 to continue the calculation until the convergence condition or zero solution condition is met.

5. The method of calculating fluid saturation pressure in a porous medium according to any one of claims 1 to 4, wherein, The method comprises the following steps: If there are multiple pore sizes in the porous medium, the calculation is performed according to steps S21 to S24 for each pore size, and if it is judged that the fluid is single-phase stable under each pore size condition, it is judged that the fluid is single-phase stable in the overall porous medium, otherwise it is judged that the fluid is single-phase unstable in the porous medium.

6. The method of claim 1, wherein, In step S3: If the main phase is the oil phase, the fluid flash iteration calculation in the porous medium is performed in the first pressure range with a preset pressure difference step by step, wherein the equilibrium ratio obtained in step S2 is used as the initial equilibrium ratio for the flash calculation under the saturation pressure result value. If the main phase is the oil phase, in the flash iteration calculation in each pressure condition, if the calculated gas phase mole fraction is greater than the first preset value, stop all calculations, and record the current corresponding pressure as the saturation pressure of the fluid in the porous medium.

7. The method of claim 1, wherein, In step S3: If the main phase is the gas phase, the fluid flash iteration calculation in the porous medium is performed in the second pressure range with a preset pressure difference step by step, wherein the equilibrium ratio obtained in step S2 is used as the initial equilibrium ratio for the flash calculation under the saturation pressure result value. If the main phase is the gas phase, in the flash iteration calculation in each pressure condition, if the calculated gas phase mole fraction is equal to the second preset value, stop all calculations, and record the current corresponding pressure as the saturation pressure of the fluid in the porous medium.

8. A storage medium, characterized by It comprises a series of instructions for performing the method steps of any one of claims 1-7.

9. An apparatus for calculating fluid saturation pressure in a porous medium, comprising: The device comprises: a saturation pressure result module for calculating the saturation pressure result value of the fluid at a given temperature without considering the influence of the porous medium; an equilibrium ratio module for analyzing the single-phase stability under the saturation pressure result value considering the influence of the porous medium, and obtaining the initial equilibrium ratio for the oil-gas two-phase flash calculation; a saturation pressure module for performing the fluid flash iteration calculation in the porous medium based on the type of the main phase, the saturation pressure result value, and the initial equilibrium ratio, and recording the corresponding pressure when the stop condition is met as the saturation pressure of the fluid in the porous medium.

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