Method for calculating phase-phase equilibrium of trihydrocarbon in nanopore

Abstract

The invention relates to the technical field of chemical engineering, and discloses a method for calculating phase-phase equilibrium of trihydrocarbon in nanopores, which comprises the following steps: S1, determining critical parameters of each component; s2, constructing a pressure balance model; s3, constructing a stability test model; s4, constructing a two-phase flash evaporation calculation model considering the influence of the nanopores; and S5, constructing a three-phase VL1L2 flash evaporation calculation model considering the influence of nanopores. According to the method, the offset of the critical property and the capillary effect can be combined into a phase equilibrium calculation model, a calculation model suitable for representing the phase state behavior of the trihydrocarbon phase in the nanopore is constructed, the model is verified through experimental data, and the established model is used for analyzing the influence of the nanopore on the phase state behavior of the trihydrocarbon phase; the method can be used for accurately representing the phase state behavior of the fluid in the shale nanopores, provides technical support for shale oil and gas reservoir component simulation, CO2 flooding key parameter calculation and the like, and supports efficient and economical exploitation of shale oil and gas reservoirs.

Description

Technical Field

[0001] The invention relates to the technical field of chemical engineering, in particular to a method for calculating phase equilibrium of three hydrocarbon phases in a nanopore. Background Art

[0002] As global energy demand grows, carbon capture, utilization and storage (CCUS) technology has received increasing attention. 2 Flooding is an important means to achieve CCUS. 2 Injecting it underground can significantly reduce the viscosity of crude oil, thereby increasing the recovery rate of crude oil and achieving CO 2 By storing the captured CO 2 Injecting CO deep underground can prevent it from being released into the atmosphere, thereby slowing down global warming. This is a win-win option for the oil industry that can both improve economic benefits and reduce environmental pollution. 2 It can lead to three hydrocarbon phases (i.e., gas phase, oil-rich liquid phase and CO-rich liquid phase). 2 liquid phase) coexists, changing the relative permeability curve, thereby affecting the oil and gas recovery rate or CO 2 In addition, there are abundant nanopores in shale reservoirs, and the phase behavior of formation fluids in nanopores is very different from that in the bulk phase, which makes conventional fluid phase behavior calculation models inapplicable in shale reservoirs, affecting CO 2 Currently, there is no model for calculating the three-hydrocarbon phase equilibrium in nanopores.

[0003] Therefore, it is necessary to design a practical method for calculating the phase equilibrium of three hydrocarbon phases in nanopores. Summary of the invention

[0004] The purpose of the present invention is to provide a method for calculating the phase equilibrium of three hydrocarbon phases in nanopores to solve the problems raised in the above background technology.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: a method for calculating the phase equilibrium of three hydrocarbon phases in a nanopore, comprising the following steps: S1: Determination of critical parameters of each component; S2: Construction of pressure balance model; S3: Stability test model construction; S4: Construction of two-phase flash evaporation computational model considering the influence of nanopores; S5: Three-phase VL considering the effect of nanopores 1 L 2 Flash evaporation calculation model construction; S6: Three-phase VL in nanopores 1 L2 Construction of phase equilibrium calculation algorithm; S7: Model verification and analysis of the influence of nanopores on the three-phase behavior.

[0006] According to the above technical solution, the content of step S1 is: calculating the critical temperature and critical pressure of each component in the confined space according to the known critical temperature and critical pressure of the bulk fluid.

[0007] According to the above technical solution, the content of step S2 is: construct two-phase and three-phase VL 1 L 2 Pressure balance model.

[0008] According to the above technical solution, step S3 includes: at a given temperature, pressure and mixture composition Z i Calculate the TPD function of the confined fluid and determine whether TPD is non-negative. If it is non-negative, the component is stable; if it is negative, the component will split into two or more phases.

[0009] According to the above technical solution, the content of step S4 is: input known system pressure, temperature, phase composition, critical parameters, BIP and other parameters, correct the critical temperature and critical pressure of each component in the nanopore according to step S1, initialize the equilibrium constant through the stability test calculation result, calculate the molar proportion of the two phases and the proportion of each component in the two phases based on the material balance, and judge whether the result satisfies the fugacity balance. When calculating the fugacity, the influence of capillary force on the pressure of each phase needs to be considered. If satisfied, output the fluid phase behavior characterization parameters such as the molar proportion of the two phases, the proportion of each component, the pressure of the two phases, and the capillary force; if not satisfied, update the molar proportion of the two phases and the equilibrium constant, and calculate again until the fugacity balance is satisfied.

[0010] According to the above technical solution, the content of step S5 is: input the known system pressure, temperature, phase composition, critical parameters, BIP, correct the critical temperature and critical pressure of each component in the nanopore according to step S1, initialize the equilibrium constant through the two-phase flash calculation results, and calculate VL based on the material balance. 1 L 2 Molar proportions of the three phases and their VL 1 L 2 The proportion of each component in the three phases determines whether the result satisfies the fugacity balance. When calculating the fugacity, the influence of capillary force on the pressure of each phase needs to be considered. If it satisfies, VL is output. 1 L 2 Molar proportion of three phases, proportion of each component, VL 1 L 2 The three-phase pressure, capillary force and other fluid phase behavior characterization parameters; if not satisfied, the three-phase molar proportions and equilibrium constants are updated and calculated again until the fugacity balance is satisfied.

[0011] According to the above technical solution, the content of step S6 is: coupling the stability test model in the nanopore built in the above steps S1 to S5 with the flash evaporation calculation model to construct a three-phase phase equilibrium calculation algorithm.

[0012] According to the above technical solution, the content of step S7 is: draw the Px and PT phase diagrams, and verify the accuracy of the constructed model in combination with the experimental test results; by calculating the Px phase diagram and PT phase diagram under different nanopore and bulk conditions, analyze the influence of confined space and different pore sizes on the phase behavior of the fluid.

[0013] Compared with the prior art, the beneficial effects achieved by the present invention are: 0. The present invention can combine the shift of critical properties and the capillary effect into the phase equilibrium calculation model, construct a calculation model suitable for characterizing the phase behavior of three hydrocarbon phases in nanopores, verify the model through experimental data, and use the constructed model to analyze the influence of nanopores on the phase behavior of three hydrocarbon phases.

[0014] 1. The present invention can be used to accurately characterize the phase behavior of fluids in shale nanopores, and provide a reference for shale oil and gas reservoir component simulation, CO 2 It provides technical support for the calculation of key drive parameters, and supports the efficient and economical exploitation of shale oil and gas reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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:

[0016] Figure 1 It is a schematic diagram of the pressure balance process between two phases in the nanopore of the present invention; Figure 2 The three-phase VL in the nanopore of the present invention 1 L 2 Schematic diagram of pressure balance between the two chambers; Figure 3 is the calculated P of the oil sample of the present invention x Schematic diagram of the comparison between the phase diagram and experimental data; Figure 4 The present invention injects 70 mol% CO into the bulk phase and confined nanopores. 2 PT phase diagram of Oil-G: (a) 10nm; (b) 100nm schematic diagram; Figure 5 The present invention injects 70 mol% CO into the bulk phase and confined nanopores. 2 VL in the PT phase diagram of Oil-G 1 L 2 Comparison of three-phase regions: (a) 10nm; (b) 100nm schematic diagram; Figure 6 The present invention injects 70 mol% CO into the bulk phase and confined nanopores. 2 PT phase diagram of Oil-G: (a) 10nm; (b) schematic diagram of 100nm. DETAILED DESCRIPTION

[0017] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0018] See also Figure 1-6 The present invention provides a technical solution: a method for calculating the phase equilibrium of three hydrocarbon phases in a nanopore, comprising the following steps: S1: Determination of critical parameters of each component According to the nanopore radius r and the fluid size parameter σ LJ (See formula (1)) to calculate the critical temperature (T cp ) and critical pressure (P cp ) and the critical temperature of the bulk fluid (T cb ) and critical pressure (P cb ) (see Eqs. (2) and (3)), and then calculate T according to the critical temperature and critical pressure of the bulk fluid (parameters well known to researchers in related fields): cp and P cp ; (1) (2) (3).

[0019] S2: Pressure balance model construction ① Construction of two-phase pressure balance model The two-phase equilibrium that may occur is the gas phase and the oil-rich liquid phase equilibrium (VL 1 equilibrium), gas phase and CO-rich 2 Liquid phase equilibrium (VL 2 balance), oil-rich liquid phase and CO-rich 2 Liquid phase equilibrium (L 1 -L 2 Considering the influence of capillary force, the three two-phase pressure balances in the nanopore are as follows: Figure 1 As shown in equations (4) to (6).

[0020] (4) (5) (6) Among them, PV and PL 1 and PL 2 They are gas phase, oil-rich liquid phase and CO-rich liquid phase. 2 Liquid phase pressure; PCV-L 1 、PC VL 2 and PCL 2 -L 1 They are between the gas phase and the oil-rich liquid phase, the gas phase and the CO-rich liquid phase. 2 Between the liquid phases and between the oil-rich liquid phase and the CO-rich liquid phase 2 Capillary forces between liquid phases; The Young-Laplace equation is used to calculate the capillary force, as shown in Eq. (7).

[0021] (7) Where σ represents the interfacial tension (IFT); θ is the contact angle. In this study, θ is assumed to be zero. In order to characterize the effect of fluid composition on capillary force, the interfacial tension can be calculated according to formula (8); (8) Where nc is the number of components; ρ w and ρ nw are the densities of the wetting and non-wetting phases, respectively; x i and i represents the mole fraction of the i-th component of the wetting phase and the non-wetting phase respectively; P chi represents the Parachor constant of the ith component, which can be calculated by formula (9): (9) Where Mi is the molecular mass of the i-th component; ② Three-phase VL 1 L 2 Pressure balance model construction Taking into account the influence of capillary force, the pressure balance among the three phases in the nanopore is as follows: Figure 2 And formula (10) and (11) show: (10) (11) Where P C1 and P C2 The oil-rich liquid phase and the CO-rich liquid phase are 2 Between the liquid phase and the gas phase and CO-rich 2 The capillary force and capillary pressure between liquid phases are calculated according to equations (7) to (9).

[0022] S3: Stability test model construction At a given temperature, pressure and mixture composition z i Under this condition, the TPD function of the confined fluid is calculated (Eq. (12)). When w i When the TPD value is non-negative across all values, the mixture is stable; otherwise, the mixture will split into two or more phases at a given temperature and pressure. (12) Among them, w i represents the composition of the test phase, z i represents the component of a given mixture (i.e., the test phase); φ is the fugacity coefficient, which can be calculated using the state equation (the calculation method is well known in the field of phase behavior); P represents the phase pressure; the subscripts w and z represent the test phase and the experimental phase, respectively; when solving the TPD function, the compressibility factors of the test phase and the experimental phase (which can be determined by the state equation and are well-known parameters in the field of phase behavior) must be compared in each iteration to determine Pw and Pz (if the compressibility factor of the test phase is low, P z is a given pressure, P w =P z +P c Otherwise, P w is a given pressure, P z =P w +P c ; Traverse w i It is time-consuming, so we can determine whether the mixture is stable by calculating all the saddle points of the g(w) function and judging whether the value corresponding to the saddle point of the g(w) function is non-negative. We have given a variety of equilibrium constants (K i =w i / z i ) is used to find all saddle points of the g(w) function to ensure the accuracy of the stability test, as shown in formula (13):

[0023] (13) in: represents the equilibrium coefficient of the ith component calculated by the Wilson equation, as shown in Eq. (14).

[0024] (14) Where T ci represents the critical temperature of the ith component, P ci is the critical pressure of the ith component, ω iis the eccentricity factor of the i-th component (a known empirical parameter in the phase field); the critical temperature and pressure need to be the critical temperature and critical pressure in the nanopore calculated by step S1.

[0025] S4: Construction of two-phase flash evaporation computational model considering the effect of nanopores ① Input the known pressure (P), temperature (T), mole fraction of each component in the mixture (z i ), critical temperature, critical pressure, eccentricity factor, binary interaction coefficient (BIP) between components, where the critical temperature and pressure need to be calculated using the critical temperature and critical pressure in the nanopore in step S1, the BIP between hydrocarbon components is set to 0, CO 2 The BIP between the hydrocarbon components is obtained by fitting the experimental points (see step (6)).

[0026] ② The w corresponding to the minimum value of the g(w) function calculated by the stability test i , using K i =w i / z i Initialize the equilibrium constant.

[0027] ③ Calculate the molar proportion of the two phases and the molar fraction of each component in the two phases based on the material balance. The two-phase material balance equation is shown in formula (15):

[0028] (15) Among them, β y represents the molar proportion of the y phase. According to the newly calculated β y Calculate x i and i :

[0029] (16) Among them, x i and i They represent the mole fractions of component i in the x-phase and the y-phase after phase separation, respectively.

[0030] ⑤According to the calculated x i and i , calculate the fugacity of each component in phase x and phase y (this is a known method in the field of phase behavior. Fugacity calculation requires input of pressure, temperature, component, critical temperature, critical pressure, eccentricity factor, BIP, where pressure needs to be calculated based on input pressure P and capillary force). According to the compressibility factor and material composition, determine whether phase x and phase y are gas phase, oil-rich liquid phase or CO-rich phase. 2 Liquid phase. If it is gas phase and oil-rich liquid phase or CO-rich 2 Liquid phase, then oil-rich liquid phase or CO-rich liquid phase 2The pressure of the liquid phase is the input pressure P, and the pressure of the gas phase is calculated by equation (4) or equation (5); if it is an oil-rich liquid phase or a CO-rich liquid phase 2 liquid phase, the pressure of the oil-rich liquid phase is the input pressure P, and the CO-rich 2 The pressure of the liquid phase is calculated by equation (6).

[0031] ⑥ If the fugacity of the corresponding components in the two phases is equal, the fugacity balance required for phase equilibrium is satisfied and the calculation ends. If the fugacity balance is not satisfied, β needs to be updated y and K i , repeat steps ③~⑤ until the fugacity balance is satisfied. To ensure the convergence of the model, βy and K i The update of can be done by trying different methods such as Newton iteration and trust region.

[0032] ⑦ Output the calculated molar ratio of phases x and y (β x =1-β y and β y ), the proportion of each component in the two phases (x i and i ), two-phase pressure, capillary force and other fluid phase behavior characterization parameters.

[0033] S5: Three-phase VL considering the effect of nanopores 1 L 2 Flash evaporation calculation model construction ① Input known pressure (P), temperature (T), composition (z i ), critical temperature, critical pressure, eccentricity factor, binary interaction coefficient (BIP) between components, where the critical temperature and pressure need to be the critical temperature and critical pressure in the nanopore calculated in step (1), the BIP between hydrocarbon components is set to 0, CO 2 The BIP between the hydrocarbon components is obtained by fitting the experimental points (see step (6)).

[0034] ② x calculated by two-phase flash evaporation i and i , using K iu =x i / z i and K iy =y i / z i Initialize the equilibrium constant. (As shown in step (6), two-phase flash evaporation is the precursor of three-phase flash evaporation)

[0035] ③ Calculate VL based on material balance 1 L 2 The molar proportion of each phase and the proportion of each component in the three phases. The three-phase material balance equation is shown in formula (17):

[0036] (17) Among them, β y and β u represents the molar ratio of y phase and u phase. y and β u Calculate x i ,y i and u i :

[0037] (18) Among them, x i ,y i and u i They represent the mole fractions of component i in the x-phase, y-phase and u-phase after phase separation, respectively.

[0038] ④According to the calculated x i ,y i and u i Calculate the fugacity of each component in phase x, phase y, and phase u. According to the compressibility factor and material composition, determine whether phase x, phase y, and phase u are gas phase, oil-rich liquid phase, or CO-rich liquid phase. 2 Liquid phase. The pressure of the oil-rich liquid phase is the input pressure P, and the gas phase and CO-rich 2 The pressure of the liquid phase is calculated by equations (10) and (11).

[0039] ⑤ If the fugacity of the corresponding components in the three phases are equal, the fugacity balance required for phase equilibrium is satisfied and the calculation ends. If the fugacity balance is not satisfied, β needs to be updated y , β u , K iu and K iy , repeat steps ③~⑤ until the fugacity balance is met. To ensure the convergence of the model, β y , β u , K iu and K iy The update of can be done by trying different methods such as Newton iteration and trust region.

[0040] ⑦ Output the calculated molar ratio of x, y, and u phases (β x =1-β y -β u , β y and β u ), the proportion of each component in the three phases (x i ,y i and u i ), three-phase pressure, capillary force and other fluid phase behavior characterization parameters.

[0041] S6: Three-phase VL in nanopores 1 L 2 Phase equilibrium calculation algorithm construction ① Perform stability test on a given mixture at specified pressure and temperature. If it is stable, output the single-phase fluid phase behavior characterization parameters; if it is found to be unstable, proceed to the following step ②.

[0042] ② Perform two-phase flash calculation and perform stability tests on the x and y phases obtained by flash evaporation respectively. If both are stable, output the characterization parameters of the two-phase fluid phase behavior; if either phase is unstable, proceed to step S3.

[0043] ③ Perform three-phase flash calculation and output the parameters characterizing the phase behavior of the three-phase fluid.

[0044] S7: Model verification and analysis of the influence of nanopores on the three-phase behavior ①Px phase diagram drawing Given a temperature, applying the three-phase VL in the nanopore 1 L 2 Phase equilibrium calculation algorithm, calculates the phase equilibrium of a very small pressure and a certain component (such as CO 2 ) changes (such as setting CO 2 The change interval of the percentage is 0.1mol% and the change interval of the pressure is 0.1bar). When the molar percentage of a phase is zero, the corresponding values ​​of P and x are the points on the envelope of the Px phase diagram. x The values ​​of P and x at each point on the phase diagram envelope are plotted as the Px phase diagram.

[0045] ②PT phase diagram drawing Given the molar fraction of a substance, the three-phase VL in the nanopore is applied 1 L 2 The phase equilibrium calculation algorithm calculates the phase behavior of the fluid when the pressure and temperature change is very small (such as setting the temperature change interval to 0.1K and the pressure change interval to 0.1bar). When the molar fraction of a phase is zero, the corresponding P and T values ​​are the points on the PT phase diagram envelope. The PT phase diagram is drawn based on the P and T values ​​of each point on the PT phase diagram envelope.

[0046] ③Model verification Compare the agreement between the points on the phase diagram and the experimental test points, and adjust the CO 2 The binary interaction parameter BIP between the hydrocarbon components makes the agreement rate exceed 90% ( Figure 3 ).

[0047] ④ Analysis of the influence of confined space on PT phase diagram and Px phase diagram Calculate the PT phase diagram of the bulk phase and the fluid in different nanopores ( Figure 4 , 5 ), analyze the changes in the PT phase diagram; calculate the P of the fluid in the bulk phase and different nanopores xPhase diagram ( Figure 6 ), analysis P x Phase diagram changes.

[0048] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0049] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for calculating phase equilibrium of three hydrocarbon phases in a nanopore, characterized by: The following steps are involved: S1: Determination of critical parameters of each component; S2: Construction of pressure balance model; S3: Stability test model construction; S4: Construction of two-phase flash evaporation computational model considering the influence of nanopores; S5: Construction of three-phase VL1L2 flash calculation model considering the effect of nanopores; S6: Construction of the calculation algorithm for the three-phase VL1L2 phase equilibrium in nanopores; S7: Model verification and analysis of the influence of nanopores on the three-phase behavior.

2. The method for calculating phase equilibrium of three hydrocarbon phases in a nanopore according to claim 1, characterized in that: The content of step S1 is: calculating the critical temperature and critical pressure of each component in the confined space according to the known critical temperature and critical pressure of the bulk fluid.

3. The method for calculating the phase equilibrium of three hydrocarbon phases in a nanopore according to claim 2, characterized in that: The content of step S2 is: construct two-phase and three-phase VL1L2 pressure balance models respectively.

4. The method for calculating phase equilibrium of three hydrocarbon phases in a nanopore according to claim 3, characterized in that: Step S3 is as follows: at a given temperature, pressure and mixture composition Z i Calculate the TPD function of the confined fluid and determine whether TPD is non-negative. If it is non-negative, the component is stable; if it is negative, the component will split into two or more phases.

5. The method for calculating phase equilibrium of three hydrocarbon phases in a nanopore according to claim 4, characterized in that: The content of step S4 is: input known system pressure, temperature, phase composition, critical parameters, BIP and other parameters, correct the critical temperature and critical pressure of each component in the nanopore according to step S1, initialize the equilibrium constant through the stability test calculation result, calculate the molar proportion of the two phases and the proportion of each component in the two phases based on the material balance, and judge whether the result satisfies the fugacity balance. When calculating the fugacity, the influence of capillary force on the pressure of each phase needs to be considered. If satisfied, output the fluid phase behavior characterization parameters such as the molar proportion of the two phases, the proportion of each component, the pressure of the two phases, and the capillary force; if not satisfied, update the molar proportion of the two phases and the equilibrium constant, and calculate again until the fugacity balance is satisfied.

6. The method for calculating phase equilibrium of three hydrocarbon phases in a nanopore according to claim 5, characterized in that: The content of step S5 is: input the known system pressure, temperature, phase composition, critical parameters, and BIP, correct the critical temperature and critical pressure of each component in the nanopore according to step S1, initialize the equilibrium constant through the two-phase flash evaporation calculation result, calculate the molar proportion of the three phases VL1L2 and the proportion of each component in the three phases VL1L2 based on the material balance, and judge whether the result satisfies the fugacity balance. When calculating the fugacity, the influence of capillary force on the pressure of each phase needs to be considered. If satisfied, output the fluid phase behavior characterization parameters such as the molar proportion of the three phases VL1L2, the proportion of each component, the three-phase pressure of VL1L2, and the capillary force; if not satisfied, update the molar proportion of the three phases and the equilibrium constant, and calculate again until the fugacity balance is satisfied.

7. The method for calculating phase equilibrium of three hydrocarbon phases in a nanopore according to claim 6, characterized in that: The content of step S6 is: coupling the stability test model in the nanopore built in the above steps S1 to S5 with the flash evaporation calculation model to construct a three-phase phase equilibrium calculation algorithm.

8. The method for calculating phase equilibrium of three hydrocarbon phases in a nanopore according to claim 7, characterized in that: The content of step S7 is: draw the Px and PT phase diagrams, and verify the accuracy of the constructed model in combination with the experimental test results; by calculating the Px phase diagram and PT phase diagram under different nanopore and bulk conditions, analyze the influence of confined space and different pore sizes on the phase behavior of the fluid.

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