Methane critical property calculation method comprehensively considering shale pore size distribution and mineral composition

Abstract

The invention discloses a methane critical property calculation method comprehensively considering shale pore size distribution and mineral composition, which comprises the following steps: taking a plurality of shale samples in a research area, and respectively carrying out CO2 adsorption experiment and H2O adsorption experiment; respectively calculating organic matter pore fractal dimensions and clay mineral pore fractal dimensions of the plurality of shale samples; respectively calculating the total pore number of organic matters and the total pore number of clay minerals based on the organic matter pore fractal dimension and the clay mineral pore fractal dimension of the shale sample; respectively calculating probability density distribution functions corresponding to the organic matter holes and the clay mineral holes; on the basis of the methane critical property calculation equation, the critical property of methane in shale pores is calculated through an integral method. The pore size distribution of shale organic matters and clay minerals is quantitatively represented through a fluid adsorption experiment and a fractal theory, and the critical property of methane is calculated through integration by further combining a methane critical property offset calculation equation under the condition of single pore size of the organic matters and the clay minerals.

Description

Technical Field

[0001] The invention relates to a method for calculating critical properties of methane by comprehensively considering shale pore size distribution and mineral composition, and belongs to the technical field of shale mining. Background Art

[0002] Nanopores are well developed in shale reservoirs, and there is a confinement effect in nanoscale pores. Due to the influence of the confinement effect of methane, the critical properties will change relative to the macroscopic conditions (bulk phase). For example, the bubble point pressure of methane decreases with the decrease of pore size, and the smaller the pore size, the lower the critical temperature and critical pressure. This shift in critical properties will seriously affect the phase behavior of methane. Therefore, accurately calculating the changes in critical properties of methane in shale pores is of great significance for a deep understanding of its phase behavior, optimizing mining technology, and improving resource utilization efficiency.

[0003] Most of the existing studies on the shift of critical properties of methane in shale reservoirs are based on a single pore size to calculate the shift of critical properties of methane molecules, ignoring the difference in pore size, and using the molecular simulation results based on graphite materials. Therefore, the difference in mineral composition is not taken into account, and the change of critical properties of methane under reservoir conditions cannot be accurately reflected. Organic matter and clay minerals are the key minerals that make up shale, and organic pores and clay mineral pores are important spaces for the storage of shale gas. There are differences in the fluid-solid interactions of methane in organic pores and clay mineral pores, so the corresponding critical property shifts are also different. In summary, at this stage, there is a lack of a method for calculating the critical properties of methane that comprehensively considers the pore size distribution and mineral composition of shale. Summary of the invention

[0004] In order to overcome the defects in the prior art, the present invention aims to provide a method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale.

[0005] The technical solution provided by the present invention to solve the above technical problems is: a method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale, comprising the following steps:

[0006] S1. Take multiple shale samples from the study area and conduct CO2 adsorption experiments and H2O adsorption experiments respectively;

[0007] S2. Calculate the fractal dimensions of organic pores of multiple shale samples based on CO2 adsorption experimental data;

[0008] S3. Calculate the fractal dimensions of clay mineral pores of multiple shale samples based on H2O adsorption experimental data;

[0009] S4. Calculate the total number of organic pores N based on the fractal dimensions of organic pores in shale samples and clay mineral pores 1tand the total pore number N of clay minerals 2t ;

[0010] S5. Total pore volume N based on organic matter 1t and the total pore number N of clay minerals 2t Calculate the probability density distribution functions corresponding to organic matter pores and clay mineral pores respectively;

[0011] S6. Based on the calculation equation of the critical properties of methane, the critical properties of methane in shale pores are calculated by the integration method.

[0012] A further technical solution is that the calculation formula in step S2 is:

[0013]

[0014] Where: V1 is the cumulative pore volume, cm 3 / g; S is the cumulative surface area, m 2 / g;D f1 is the fractal dimension of organic pores; K is the fitting constant of organic pores.

[0015] A further technical solution is that the calculation formula in step S3 is:

[0016]

[0017] Where: V2 is the amount of water vapor adsorption, cm 3 / g;D f2 is the fractal dimension of clay mineral pores; C is the fitting constant of clay mineral pores.

[0018] A further technical solution is that in step S4, the total number of organic pores N1 and the total number of clay mineral pores N2 are calculated by taking the average values ​​of the organic pore fractal dimensions and the clay mineral pore fractal dimensions of a plurality of shale samples.

[0019] A further technical solution is that the calculation formula in step S4 is:

[0020]

[0021]

[0022] Where: max1 is the maximum pore size measured in the CO2 adsorption experiment, nm; λ max2 is the maximum pore size measured in the H2O adsorption experiment, nm; λ min1 is the minimum pore size measured in the CO2 adsorption experiment, nm; λ min2 is the minimum pore size measured in the H2O adsorption experiment, nm; D f1is the fractal dimension of organic pores; D f2 is the fractal dimension of clay mineral pores; N 1t is the total pore volume of organic matter; N 2t is the total pore volume of clay minerals.

[0023] A further technical solution is that the calculation formula in step S5 is:

[0024]

[0025]

[0026] Where: N 1t is the total pore volume of organic matter; N 2t is the total number of pores in clay minerals; N1 is the cumulative number of organic pores with a diameter greater than or equal to λ; N2 is the cumulative number of clay mineral pores with a diameter greater than or equal to λ; f1(λ) is the probability density distribution function corresponding to organic pores; f2(λ) is the probability density distribution function corresponding to clay mineral pores.

[0027] A further technical solution is that the calculation formula in step S6 is:

[0028]

[0029]

[0030] Where: max1 is the maximum pore size measured in the CO2 adsorption experiment, nm; λ max2 is the maximum pore size measured in the H2O adsorption experiment, nm; λ min1 is the minimum pore size measured in the CO2 adsorption experiment, nm; λ min2 is the minimum pore size measured in the H2O adsorption experiment, nm; f1(λ) is the probability density distribution function corresponding to the organic pore; f2(λ) is the probability density distribution function corresponding to the clay mineral pore; λ is the pore size; T cb is the critical temperature of methane molecules under bulk conditions; P cb is the critical pressure of methane molecules under bulk conditions; σ is the molecular size; T cp is the critical temperature of methane in shale pores; P cp is the critical pressure of methane in shale pores.

[0031] The present invention has the following beneficial effects: the present invention quantitatively characterizes the pore size distribution of shale organic matter and clay minerals through fluid adsorption experiments and fractal theory, further combines the calculation equation of methane critical property offset under the single pore size conditions of organic matter and clay minerals, and calculates the critical properties of methane by integration. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the calculation process of the critical properties of methane;

[0033] Figure 2 To fit the CO2 adsorption data graph using the VS model;

[0034] Figure 3 This is the H2O adsorption data fitted using the FHH model. DETAILED DESCRIPTION

[0035] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. 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.

[0036] like Figure 1 As shown, a method for testing the mechanical strength of deep shale natural fractures after hydration of the present invention specifically comprises the following steps:

[0037] S1. Take multiple shale samples from the study area and conduct CO2 adsorption experiments and H2O adsorption experiments respectively;

[0038] It is assumed that the pores of organic matter are all smaller than 2nm, and the pores of clay minerals are all larger than 2nm. Therefore, it is considered that the CO2 adsorption experiment is used to determine the organic pores of shale, and the H2O adsorption experiment is used to determine the clay mineral pores of shale.

[0039] S2. Calculate the fractal dimensions of organic pores of multiple shale samples based on CO2 adsorption experimental data;

[0040] For CO2 adsorption experiment: the VS model is used to calculate the fractal dimension of organic pores in shale samples. The VS model expression is:

[0041]

[0042] Where: V1 is the cumulative pore volume, cm 3 / g; S is the cumulative surface area, m 2 / g;D f1 is the fractal dimension of organic pores; K is the fitting constant of organic pores;

[0043] S3. Calculate the fractal dimensions of clay mineral pores of multiple shale samples based on H2O adsorption experimental data;

[0044] For H2O adsorption experiment: the fractal dimension of clay mineral pores in shale samples was calculated using the FHH model. The expression of the FHH model is:

[0045]

[0046] Where: V2 is the amount of water vapor adsorption, cm 3 / g;D f2 is the fractal dimension of clay mineral pores; C is the fitting constant of clay mineral pores;

[0047] S4. Calculate the total number of organic pores N based on the fractal dimensions of organic pores in shale samples and clay mineral pores 1t and the total pore number N of clay minerals 2t ;

[0048] Assuming that both organic pores and clay mineral pores satisfy the fractal scaling law, the expression is:

[0049]

[0050]

[0051] Where: ε is the pore size; λ1 is the organic pore size; λ2 is the clay mineral pore size; N1 is the cumulative number of organic pores greater than or equal to λ; N2 is the cumulative number of clay mineral pores greater than or equal to λ; λ max1 is the maximum pore size measured in the CO2 adsorption experiment, nm; λ max2 is the maximum pore size measured in the H2O adsorption experiment, nm;

[0052] The total pore volume of organic matter and clay minerals is:

[0053]

[0054]

[0055] Where: max1 is the maximum pore size measured in the CO2 adsorption experiment, nm; λ max2 is the maximum pore size measured in the H2O adsorption experiment, nm; λ min1 is the minimum pore size measured in the CO2 adsorption experiment, nm; λ min2 is the minimum pore size measured in the H2O adsorption experiment, nm; D f1 is the fractal dimension of organic pores; D f2 is the fractal dimension of clay mineral pores; N 1t is the total pore volume of organic matter; N 2t is the total pore volume of clay minerals;

[0056] S5. Total number of pores based on organic matter N 1t and the total pore number N of clay minerals 2t Calculate the probability density distribution functions corresponding to organic matter pores and clay mineral pores respectively;

[0057]

[0058]

[0059] Where: N 1t is the total pore volume of organic matter; N 2t is the total number of pores in clay minerals; N1 is the cumulative number of organic pores with diameters greater than or equal to λ; N2 is the cumulative number of clay mineral pores with diameters greater than or equal to λ; f1(λ) is the probability density distribution function corresponding to organic pores; f2(λ) is the probability density distribution function corresponding to clay mineral pores;

[0060] S6. Based on the calculation equation of the critical properties of methane, the critical properties of methane in the shale pores are calculated by the integration method;

[0061] For organic pores, the critical properties of methane in organic pores are calculated using the critical property shift calculation equation for pores <2 nm proposed by Alharthy (2013). The critical property shift calculation equation for methane proposed by Alharthy (2013) is based on the molecular simulation results of methane in graphite slit pores and is obtained by fitting the empirical equation. Here, organic matter is approximated as graphite. The critical property shift calculation equation for methane is expressed as:

[0062] T cp =T cb ×[1-1.0983×e -0.929λ ]

[0063] P cp =P cb ×[0.2622+0.4083λ 2 -0.5466λ]

[0064] For clay mineral pores, the critical properties of methane in clay mineral pores are calculated using the methane critical property shift calculation equation proposed by Xing et al. (2022). The methane critical property shift calculation equation proposed by Xing et al. (2022) is based on the molecular simulation results of methane in montmorillonite slit pores, combined with the empirical equation fitting. Here, the clay mineral is approximated as montmorillonite. The methane critical property shift calculation equation is expressed as:

[0065]

[0066]

[0067] Based on the calculation equation of methane critical property shift under the condition of single pore size of organic matter and clay minerals, combined with the probability density distribution function corresponding to organic pores and clay mineral pores, the critical properties of methane are calculated by integration. The integral equations of critical temperature and critical pressure can be expressed as:

[0068]

[0069]

[0070] Where: max1 is the maximum pore size measured in the CO2 adsorption experiment, nm; λ max2 is the maximum pore size measured in the H2O adsorption experiment, nm; λ min1 is the minimum pore size measured in the CO2 adsorption experiment, nm. If the measured minimum pore size is smaller than the methane molecule size, the methane molecule size is used; λ min2 is the minimum pore size measured in the H2O adsorption experiment, nm; f1(λ) is the probability density distribution function corresponding to the organic pore; f2(λ) is the probability density distribution function corresponding to the clay mineral pore; λ is the pore size; T cb is the critical temperature of methane molecules under bulk conditions; P cb is the critical pressure of methane molecules under bulk conditions; σ is the molecular size; T cp is the critical temperature of methane in shale pores; P cp is the critical pressure of methane in shale pores.

[0071] Example

[0072] CO2 adsorption experiments and H2O adsorption experiments were carried out on ten shale samples of the Jurassic Ziliujing Formation in the Sichuan Basin. The experimental data corresponding to one of the samples are shown in Tables 1 and 2.

[0073] Table 1 CO2 adsorption experimental data

[0074]

[0075] Table 2 H2O adsorption experimental data

[0076]

[0077]

[0078] The VS model and FHH model were used to fit the CO2 adsorption data and H2O adsorption data, respectively, to obtain the fractal dimensions of organic matter pores and clay mineral pores. The fitting curves are shown in Figure 2 and Figure 3 As shown, the water vapor adsorption experimental data uses a fitting curve with p / p0>0.45 to represent the fractal dimension of clay mineral pores.

[0079] The fitted organic matter pore fractal dimensions and clay mineral pore fractal dimensions of the ten shale samples are shown in Table 3.

[0080] Table 3 Fractal dimensions of organic matter pores and clay mineral pores

[0081] sample Fractal dimension of organic pores Fractal dimension of pores in clay minerals 1 2.552 2.736 2 2.502 2.780 3 2.679 2.744 4 2.485 2.730 5 2.577 2.713 6 2.645 2.556 7 2.655 2.745 8 2.521 2.639 9 2.570 2.538 10 2.526 2.736 average 2.571 2.692

[0082] The critical temperature T of methane molecules under bulk conditions cb =190.6K, critical pressure P cb =4.6MPa and molecular size σ=0.3565 are substituted into the critical property calculation equation of methane to carry out calculations.

[0083] The critical temperature of methane in the shale pores in the study area was calculated as T cp =154.3K, critical pressure P cp =2.97MPa.

[0084] The above description is not intended to limit the present invention in any form. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are within the scope of the technical solution of the present invention.

Claims

1. A method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale, characterized in that: The following steps are involved: S1. Take multiple shale samples from the study area and conduct CO2 adsorption experiments and H2O adsorption experiments respectively; S2. Calculate the fractal dimensions of organic pores of multiple shale samples based on CO2 adsorption experimental data; S3. Calculate the fractal dimensions of clay mineral pores of multiple shale samples based on H2O adsorption experimental data; S4. Calculate the total number of organic pores N based on the fractal dimensions of organic pores in shale samples and clay mineral pores 1t and the total pore number N of clay minerals 2t ; S5. Total number of pores based on organic matter N 1t and the total pore number N of clay minerals 2t Calculate the probability density distribution functions corresponding to organic matter pores and clay mineral pores respectively; S6. Based on the calculation equation of the critical properties of methane, the critical properties of methane in shale pores are calculated by the integration method.

2. The method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale according to claim 1, characterized in that: The calculation formula in step S2 is: Where: V1 is the cumulative pore volume, cm 3 / g; S is the cumulative surface area, m 2 / g; is the fractal dimension of organic pores; K is the fitting constant of organic pores.

3. The method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale according to claim 1, characterized in that: The calculation formula in step S3 is: Where: V2 is the amount of water vapor adsorption, cm 3 / g; is the fractal dimension of clay mineral pores; C is the fitting constant of clay mineral pores.

4. The method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale according to claim 1, characterized in that: In step S4, the total number of organic pores N1 and the total number of clay mineral pores N2 are calculated by taking the average values ​​of the organic pore fractal dimensions and the clay mineral pore fractal dimensions of the plurality of shale samples.

5. The method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale according to claim 4, characterized in that: The calculation formula in step S4 is: Where: max1 is the maximum pore size measured in the CO2 adsorption experiment, nm; λ max2 is the maximum pore size measured in the H2O adsorption experiment, nm; λ min1 is the minimum pore size measured in the CO2 adsorption experiment, nm; λ min2 is the minimum pore size measured in the H2O adsorption experiment, nm; is the fractal dimension of organic matter pores; is the fractal dimension of clay mineral pores; N 1t is the total pore volume of organic matter; N 2t is the total pore volume of clay minerals.

6. The method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale according to claim 1, characterized in that: The calculation formula in step S5 is: Where: N 1t is the total pore volume of organic matter; N 2t is the total number of pores in clay minerals; N1 is the cumulative number of organic pores with a diameter greater than or equal to λ; N2 is the cumulative number of clay mineral pores with a diameter greater than or equal to λ; f1(λ) is the probability density distribution function corresponding to organic pores; f2(λ) is the probability density distribution function corresponding to clay mineral pores.

7. The method for calculating the critical properties of methane by comprehensively considering the pore size distribution and mineral composition of shale according to claim 1, characterized in that: The calculation formula in step S6 is: Where: max1 is the maximum pore size measured in the CO2 adsorption experiment, nm; λ max2 is the maximum pore size measured in the H2O adsorption experiment, nm; λ min1 is the minimum pore size measured in the CO2 adsorption experiment, nm; λ min2 is the minimum pore size measured in the H2O adsorption experiment, nm; f1(λ) is the probability density distribution function corresponding to the organic pore; f2(λ) is the probability density distribution function corresponding to the clay mineral pore; λ is the pore size; T cb is the critical temperature of methane molecules under bulk conditions; P cb is the critical pressure of methane molecules under bulk conditions; σ is the molecular size; T cp is the critical temperature of methane in shale pores; P cp is the critical pressure of methane in shale pores.

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