A method for calculating critical properties of methane by comprehensively considering shale pore size distribution and mineral composition

Abstract

The application discloses a kind of methane critical property calculation methods considering shale pore size distribution and mineral composition comprehensively, including taking multiple shale samples of research area, respectively carrying out CO2 adsorption experiment and H2O adsorption experiment;Respectively calculate the pore fractal dimension of organic matter of multiple shale samples, clay mineral pore fractal dimension;Based on the pore fractal dimension of organic matter of shale sample, clay mineral pore fractal dimension respectively calculates the total pore quantity of organic matter and the total pore quantity of clay mineral;Respectively calculate the probability density distribution function corresponding to organic matter pore and clay mineral pore;Based on methane critical property calculation equation, the critical property of methane in shale pore is calculated by integral method.The application quantitatively characterizes the pore size distribution of shale organic matter and clay mineral by fluid adsorption experiment and fractal theory, further combines methane critical property offset calculation equation under the condition of single pore diameter of organic matter, clay mineral, and calculates the critical property of methane by integral.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of methane critical property calculation method considering shale pore size distribution and mineral composition comprehensively, belong to shale mining technical field. BACKGROUND

[0002] Shale reservoir nanometer pore develops, there is confinement effect in nanometer scale pore.There is change in critical property of methane relative to macroscopic condition (bulk phase) by the influence of confinement effect, such as bubble point pressure of methane decreases with the decrease of pore size, and pore size is smaller, and critical temperature and critical pressure are lower, and the shift of this critical property can seriously affect the phase behavior of methane.Therefore, accurate calculation of the change of critical property of methane in shale pore, for in-depth understanding of its phase behavior, optimization exploitation technology and improving resource utilization efficiency has important significance.

[0003] The existing research on the shift of methane critical property in shale reservoir, mostly based on single pore size to calculate the shift of methane molecular critical property, ignores the difference of pore size, and uses the molecular simulation results based on graphite material, so the difference of mineral composition is not considered, and the change of methane critical property under reservoir condition cannot be accurately reflected.Organic matter and clay mineral are the key minerals of shale, and organic matter pore and clay mineral pore are important space for shale gas occurrence.Methane is different in flow-solid interaction in organic matter pore and clay mineral pore, so the corresponding critical property shift is also different.In summary, there is lack of a kind of methane critical property calculation method considering shale pore size distribution and mineral composition comprehensively at present stage. SUMMARY

[0004] In order to overcome the defects in the prior art, the present application aims to provide a kind of methane critical property calculation method considering shale pore size distribution and mineral composition comprehensively.

[0005] The technical scheme provided by the present application to solve the above technical problems is: a kind of methane critical property calculation method considering shale pore size distribution and mineral composition comprehensively, comprising the following steps:

[0006] S1, taking a plurality of shale samples in research area, respectively carrying out CO2 adsorption experiment and H2O adsorption experiment;

[0007] S2, based on the data of CO2 adsorption experiment, respectively calculating the fractal dimension of organic matter pore of a plurality of shale samples;

[0008] S3, based on the data of H2O adsorption experiment, respectively calculating the fractal dimension of clay mineral pore of a plurality of shale samples;

[0009] S4, based on the fractal dimension of organic matter pore and the fractal dimension of clay mineral pore of shale sample, respectively calculating the total pore number N of organic matter 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 matter pores N1 and the total number of clay mineral pores N2 are calculated by averaging the fractal dimensions of organic matter pores and clay mineral pores of multiple 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 organic pores; f2(λ) is the probability density distribution function corresponding to clay mineral pores; 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 the critical property offset of methane 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 Flowchart for calculating critical properties of methane;

[0033] Figure 2 Graph for fitting CO2 adsorption data using V-S model;

[0034] Figure 3 Graph for fitting H2O adsorption data using FHH model. DETAILED DESCRIPTION

[0035] The technical solutions of the present application will be described clearly and completely below in conjunction with the drawings. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

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

[0037] S1, taking a plurality of shale samples in a study area, respectively performing CO2 adsorption experiments and H2O adsorption experiments;

[0038] Among them, it is assumed that the organic matter pores are all less than 2 nm, and the clay mineral pores are all greater than 2 nm, so it is considered that the CO2 adsorption experiment is used to determine the organic matter pores of shale, and the H2O adsorption experiment is used to determine the clay mineral pores of shale;

[0039] S2, calculating the fractal dimension of the organic matter pores of the plurality of shale samples based on the CO2 adsorption experiment data, respectively;

[0040] For the CO2 adsorption experiment: the fractal dimension of the organic matter pores of the shale sample is calculated using the V-S model, and the expression of the V-S model is:

[0041]

[0042] In the formula, 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 the organic matter pores; K is the fitting constant of the organic matter pores;

[0043] S3, calculating the fractal dimension of the clay mineral pores of the plurality of shale samples based on the H2O adsorption experiment data, respectively;

[0044] For the H2O adsorption experiment: the fractal dimension of the clay mineral pores of the shale sample is calculated using the FHH model, and 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 matter pore size; λ2 is the clay mineral pore size; N1 is the cumulative number of organic matter 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 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;

[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 shale pores are calculated by the integration method;

[0061] For organic pores, the critical properties of methane in organic pores were calculated using the equation for calculating the critical property shift of methane for pores <2 nm proposed by Alharthy (2013). This equation was derived based on molecular simulation results of methane in graphite slits, combined with empirical equation fitting. Here, organic matter is approximated as graphite. The equation for calculating the critical property shift of 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 molecular simulation results of methane in montmorillonite slit pores, combined with 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 deviation 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 minimum pore size measured 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 organic pores; f2(λ) is the probability density distribution function corresponding to clay mineral pores; 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 from 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 2H2O 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. Figure 2 and Figure 3 As shown in Figure 2, 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 fractal dimensions of organic matter pores and clay mineral pores 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 matter pores Fractal dimension of clay mineral pores 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 does not 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 use the technical content disclosed above to make some changes or modifications to equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modifications, 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 based on the fractal dimensions of organic pores in shale samples and the fractal dimensions of clay mineral pores. N 1t and the total pore volume of clay minerals N 2t ; S5. Total pore volume based on organic matter N 1t and the total pore volume of clay minerals N 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: V 1 is the cumulative pore volume, cm 3 / g; S is the cumulative surface area, m 2 / g; is the fractal dimension of organic matter 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: V 2 is the water vapor adsorption capacity, cm 3 / g; is the fractal dimension of clay mineral pores; C is the fitting constant of clay mineral pores; p To balance the pressure, MPa; p 0 is the saturated steam pressure, MPa.

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 is calculated by taking the average value of the organic pore fractal dimensions and the clay mineral pore fractal dimensions of multiple shale samples. N 1 and the total pore volume of clay minerals N 2.

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: 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; 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 pore volume of clay minerals; N 1 is greater than or equal to λ The cumulative number of organic pore diameters; N 2 is greater than or equal to λ The cumulative number of clay mineral pore diameters; is the probability density distribution function corresponding to organic pores; is the probability density distribution function corresponding to the 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: 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; 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 probability density distribution function corresponding to organic pores; is the probability density distribution function corresponding to the clay mineral pores; λ is the aperture; 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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