Coal rock methane adsorption quantity correction method based on coal macromolecular model

Abstract

The invention relates to the technical field of coal bed gas resource exploration, in particular to a coal rock methane adsorption quantity correction method based on a coal macromolecular model, which comprises the following steps: characterizing coal rock physicochemical properties; constructing a three-dimensional coal macromolecular model by utilizing molecular dynamics simulation, and verifying the coal macromolecular model; detecting the free space volume of the model by adopting a molecular probe method; performing a volumetric method coal rock methane adsorption test; determining the free space volume change under the series of test pressures through helium; and replacing the coal rock free space volume measured by helium with the coal rock free space volume measured based on a methane probe in molecular simulation so as to correct the methane adsorption isotherm. The method solves the problem that accurate determination and dynamic change of the free space volume are not considered in an existing volumetric method isothermal adsorption test.

Description

Technical Field

[0001] The invention relates to the technical field of coalbed methane resource exploration, and in particular to a method for correcting coal rock methane adsorption based on a coal macromolecular model. Background Art

[0002] Coalbed methane is mainly stored in the micro-nano pores and microcracks in coal reservoirs in an adsorbed state; therefore, accurate determination of the adsorbed gas content in coal rock is crucial for coalbed methane reserve evaluation and design of efficient production strategies; the coal rock methane adsorption determination method commonly used on a laboratory scale is mainly the volumetric isothermal adsorption test.

[0003] The volumetric method simulates the adsorption process through mass balance equations, static gas balance and pressure measurement according to the changes in pressure, volume and temperature of the adsorbate before and after adsorption, thereby calculating the amount of methane adsorbed under different pressures; the key step in determining the adsorption amount in the volumetric method is to use inert helium to determine the free space volume; the free space volume is the space for the transfer and diffusion of adsorbate molecules inherent in the adsorption system, and does not belong to the actual methane adsorption area. Therefore, the accurate determination of the free space volume is the basis for calculating the adsorption amount of coal methane.

[0004] The determination of free space volume is closely related to pressure and the selected probe. Current research generally takes free space volume as a fixed value, that is, the free space volume value measured by helium at room temperature and pressure. However, the free space volume of coal rock at different pressure points changes dynamically during the test. In addition, due to the inert nature of helium, it is often selected as a probe to determine the free space volume. However, compared with the adsorbent methane, helium with a smaller molecular diameter can detect more coal rock micro-nano space than methane, which will cause the free space volume measured by helium to be larger. Therefore, the accurate determination and dynamic changes of free space volume need to be considered in the volumetric isothermal adsorption test. Summary of the invention

[0005] In view of the shortcomings of the existing methods, the present invention solves the problem that the accurate determination and dynamic change of the free space volume are not considered in the existing volumetric isothermal adsorption test.

[0006] The technical solution adopted by the present invention is: a method for correcting the adsorption amount of coal rock methane based on a coal macromolecular model comprises the following steps:

[0007] Step 1: Construct the characterization of coal rock physical and chemical properties;

[0008] As a preferred embodiment of the present invention, step one includes: conducting vitrinite reflectance measurement, elemental analysis, solid nuclear magnetic resonance testing, low-temperature liquid nitrogen and carbon dioxide adsorption testing on the coal rock.

[0009] As a preferred embodiment of the present invention, low-temperature liquid nitrogen and carbon dioxide adsorption tests are performed to obtain specific surface area and pore volume parameters.

[0010] As a preferred embodiment of the present invention, the element composition and atomic ratio of the coal rock are determined based on elemental analysis.

[0011] As a preferred embodiment of the present invention, the type and relative content of the chemical composition of the coal rock are obtained based on the analytical results of the deconvolution of the spectrum of the solid nuclear magnetic resonance test and the elemental analysis results.

[0012] Step 2: construct a three-dimensional coal macromolecular model using molecular dynamics simulation, and verify the coal macromolecular model;

[0013] As a preferred embodiment of the present invention, step 2 specifically includes:

[0014] Firstly, an initial two-dimensional coal macromolecular model is constructed based on the physical and chemical properties of coal rocks;

[0015] Secondly, the two-dimensional coal macromolecular model was adjusted using the atomic ratio, density, and chemical composition data from coal rock experimental tests;

[0016] Finally, eight two-dimensional macromolecular model units were used to construct the initial simulation box of the amorphous structure model, and molecular dynamics simulation of structure optimization-annealing kinetics simulation-pore structure comparison-structure optimization was performed to obtain the three-dimensional coal macromolecular model with the lowest local energy.

[0017] Step 3, using a molecular probe method to detect the free space volume of the model;

[0018] As a preferred embodiment of the present invention, step three specifically includes: randomly inserting a probe molecule with a fixed Connolly radius into the macromolecular model, rolling along the van der Waals surface of the macromolecular model skeleton, scanning point by point, and identifying the free space volume based on the gaps wrapped by the skeleton surface.

[0019] Step 4: Conduct volumetric coal methane adsorption test;

[0020] Step 5: Determine the free space volume change under the series of test pressures by using helium; replace the free space volume of coal rock measured by helium with the free space volume of coal rock measured by the methane probe in the molecular simulation, so as to correct the methane adsorption isotherm;

[0021] As a preferred embodiment of the present invention, the formula for correcting the methane adsorption isotherm is:

[0022]

[0023] Among them, n e_Mis the corrected methane adsorption capacity, n e is the methane adsorption capacity of the experimental test, n Total is the total amount of methane adsorption, P is the test pressure, Z is the gas compressibility factor, R is the ideal gas constant, T is the test temperature, V f_P is the difference in free space volume between helium and methane detection at different pressures, V He_P is the volume of free space measured at a series of equilibrium pressure points, is the free space volume of methane detection under series equilibrium pressure.

[0024] Beneficial effects of the present invention:

[0025] 1. The change of free space volume under different pressures during the adsorption process is comprehensively considered, and the change of free space volume during the adsorption process is determined;

[0026] 2. The present invention accurately reveals the difference in free space volume detected by different probes at the molecular level, and obtains more accurate methane adsorption test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow chart of the coal rock methane adsorption correction method based on the coal macromolecular model of the present invention;

[0028] Figure 2 It is a two-dimensional molecular structure model diagram of coal rock in the embodiment;

[0029] Figure 3 This is a three-dimensional macromolecular structure model diagram of coal rock in the embodiment;

[0030] Figure 4 The free space volumes of coal rocks simulated and detected by helium and methane probes in the embodiment are compared respectively;

[0031] Figure 5 3 is a comparison diagram of the corrected methane adsorption isotherm and the original isotherm adsorption curve in the embodiment. DETAILED DESCRIPTION

[0032] The present invention is further described below in conjunction with the accompanying drawings and embodiments. This figure is a simplified schematic diagram, which only illustrates the basic structure of the present invention in a schematic manner, and therefore it only shows the components related to the present invention.

[0033] like Figure 1 As shown, a method for correcting the adsorption of coal rock methane based on a coal macromolecular model comprises the following steps:

[0034] Step 1: Characterization of physical and chemical properties of coal and rock;

[0035] Conduct vitrinite reflectance measurement, elemental analysis, solid nuclear magnetic resonance test, low-temperature liquid nitrogen and carbon dioxide adsorption test on coal rock; determine the maturity of organic matter in coal rock based on vitrinite reflectance; determine the elemental composition and atomic ratio of coal rock based on elemental analysis; determine the carbon skeleton structure parameters of coal rock based on solid nuclear magnetic resonance; determine the micro-nano pore structure characteristics of coal rock based on low-temperature liquid nitrogen and carbon dioxide adsorption experiments;

[0036] Firstly, based on the analytical results of solid NMR spectrum deconvolution and the results of elemental analysis, the type and relative content of the chemical composition of the coal rock are calculated; then, according to the low-temperature liquid nitrogen and carbon dioxide adsorption experiments, the mesopore and micropore pore structure information of the coal rock, including the specific surface area and pore volume parameters, are determined respectively; that is, the four parameters of specific surface area, pore volume parameters, type and relative content of the chemical composition of the coal rock are obtained through step one.

[0037] Step 2: Construction and verification of real coal macromolecular model;

[0038] According to the four parameters of the experimental test in step 1, the number of molecular structures and the relative relationship between molecules of the coal macromolecular model are determined, and a three-dimensional coal macromolecular model is constructed by molecular dynamics simulation. The material composition, thermodynamic properties and physical parameters of the three-dimensional coal macromolecular model are compared with the experimental test results to verify the reliability of the coal macromolecular model;

[0039] First, the initial two-dimensional coal macromolecular model is constructed according to the physical and chemical properties of coal rock in step 1. Then, the two-dimensional coal macromolecular model is adjusted by continuously comparing it with the atomic ratio, density, and chemical composition data of coal rock experimental tests. The final two-dimensional structural model is as follows Figure 2 As shown; 8 two-dimensional macromolecular model units were used as amorphous structure models to construct the initial simulation box, and molecular dynamics simulations of structure optimization-annealing dynamics simulation-pore structure comparison-structure optimization were carried out. Finally, a three-dimensional coal macromolecular model with the lowest local energy and consistent with the experimental test density and pore structure parameters was obtained, as shown Figure 3 As shown; compared with the existing macromolecular model construction methods, which usually only focus on the comparative verification of the density and chemical structure characteristics of the macromolecular model with experimental data, the coal rock type, maturity and physical properties are not taken into account in the modeling process.

[0040] Step 3: Coal rock free space volume simulation;

[0041] For the constructed three-dimensional coal macromolecular model, the molecular probe method is used to detect the free space volume of the model. A probe molecule with a fixed Connolly radius is randomly inserted into the macromolecular model and rolled along the van der Waals surface of the macromolecular model skeleton, scanning point by point, so as to identify the free space volume according to the gaps wrapped on the skeleton surface.

[0042] The free space volume simulation method of coal rock of the present invention can visualize the free space volume of coal rock at the molecular level, and is not affected by the interaction between the probe and the detection object, so it is possible to accurately determine the free space volume of the three-dimensional macromolecular model of coal using a methane molecular probe; Figure 4 As shown, helium and methane molecular probes were used to detect the free space volume of the three-dimensional coal macromolecular model.

[0043] Step 4: Conduct volumetric coal methane adsorption test;

[0044] According to the national standard GB / T 19560-2008 "High-pressure isothermal adsorption test method for coal", a volumetric methane adsorption experiment was carried out on coal rock at 303K to obtain the methane adsorption isotherm of the volumetric experimental test;

[0045] About 10 g of coal rock sample was dried and degassed in a vacuum environment at 110°C for 24 hours to remove impurities, and a leakage test of the experimental device was carried out. The free space volume of the coal rock was determined using inert helium gas, and then the methane adsorption isotherm was determined at 0 MPa to 25 MPa using a pressurization-equilibrium-pressurization procedure.

[0046] Step 5: Correction of methane adsorption;

[0047] First, the free space volume change under the series of test pressures is determined by helium; then the free space volume of the coal rock measured by helium in the volumetric method test is replaced by the free space volume of the coal rock measured based on the methane probe in the molecular simulation, and then the methane adsorption isotherm in step 4 is corrected;

[0048] In the volumetric methane adsorption experiment, the volume of free space is fixed and the probe is helium; however, the volume of free space will change with the test pressure, and the free space that helium can enter is larger than that of methane, which will cause the methane adsorption amount tested by the volumetric method to be inaccurate.

[0049] The methane adsorption capacity tested by volume method is:

[0050]

[0051] Where n e is the methane adsorption capacity of the experimental test, cm 3 / g;n Total is the total amount of methane adsorption; P is the test pressure, MPa; V He is the free space volume measured by helium, cm 3 ; Z is the gas compressibility factor; R is the ideal gas constant; T is the test temperature, 303.15K.

[0052] First, taking helium as the adsorbent, the free space volume in the sample chamber measured by helium at a series of pressure points is tested and can be expressed as:

[0053]

[0054] Where V He_P is the free space volume measured at a series of equilibrium pressure points; Formula 2 is the dynamic free volume measured at different pressures, V He_P That is the distinguishing point.

[0055] Due to the difference in accessible pore space between methane and helium, the free space volume measured by helium needs to be converted to the free space volume measured by methane. The difference in free space volume between helium and methane detection needs to be calculated using the formula:

[0056]

[0057] Where V f The difference in free space volume detected for helium and methane; is the free space volume for methane detection, which is obtained by molecular probe method.

[0058] Calculate the free space volume difference at different pressure points, the formula is:

[0059]

[0060] Where V f_P The difference in free space volume detected for helium and methane at different pressures; is the free space volume of methane detection under a series of equilibrium pressures, which is detected by the molecular probe method.

[0061] The corrected methane adsorption can be expressed as:

[0062]

[0063] Where n e_M is the corrected methane adsorption amount.

[0064] Substituting equations (1) and (4) into equation (5), we obtain:

[0065]

[0066] Figure 5 This is a comparison chart of the corrected coal rock methane adsorption capacity and the original test curve. After considering the change in free space volume under different pressures and the difference in free space volume measured by methane and helium, the methane adsorption capacity is greater than the original test adsorption capacity, which shows that the existing volumetric test will underestimate the methane adsorption capacity of coal rock, and the error will increase with the increase of test pressure.

[0067] In order to verify the reliability of the methane adsorption curve corrected by the present invention, the correction curve calculated by the present invention and the methane adsorption curve measured based on low-field nuclear magnetic resonance were compared and verified; the low-field nuclear magnetic resonance adsorption method has the characteristics of stable data and high accuracy because it directly calibrates the amount of adsorbed methane by real-time monitoring of the relaxation state of hydrogen nuclei in methane, and is often used for comparative analysis with the results of volumetric isothermal adsorption experiments; Figure 5 The corrected coal methane adsorption amount is highly consistent with the methane adsorption amount measured by low-field nuclear magnetic resonance, which verifies the reliability of the coal methane adsorption correction method based on the coal macromolecular model proposed in the present invention.

[0068] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for correcting the adsorption of coal methane based on a coal macromolecular model, characterized in that: The following steps are involved: Step 1: Construct the characterization of coal rock physical and chemical properties; Step 2: construct a three-dimensional coal macromolecular model using molecular dynamics simulation, and verify the coal macromolecular model; Step 3, using a molecular probe method to detect the free space volume of the model; Step 4: Conduct volumetric coal methane adsorption test; Step 5. Determine the change in free space volume under a series of test pressures using helium; replace the free space volume of coal rock measured by helium with the free space volume of coal rock measured based on a methane probe in molecular simulation to achieve correction of the methane adsorption isotherm.

2. The method for correcting the adsorption of coal rock methane based on the coal macromolecular model according to claim 1 is characterized in that: The formula for correcting the methane adsorption isotherm is: Among them, n e_M is the corrected methane adsorption capacity, n e is the methane adsorption capacity of the experimental test, n Total is the total amount of methane adsorption, P is the test pressure, Z is the gas compressibility factor, R is the ideal gas constant, T is the test temperature, V f_P is the difference in free space volume between helium and methane detection at different pressures, V He_P is the volume of free space measured at a series of equilibrium pressure points, is the free space volume of methane detection under series equilibrium pressure.

3. The method for correcting the adsorption of coal methane based on the coal macromolecular model according to claim 1 is characterized in that: Step one includes: conducting vitrinite reflectance measurement, elemental analysis, solid nuclear magnetic resonance testing, low-temperature liquid nitrogen and carbon dioxide adsorption testing on coal rock.

4. The method for correcting the adsorption of coal rock methane based on the coal macromolecular model according to claim 3 is characterized in that: Low-temperature liquid nitrogen and carbon dioxide adsorption tests were performed to obtain specific surface area and pore volume parameters.

5. The method for correcting the adsorption of coal rock methane based on the coal macromolecular model according to claim 3 is characterized in that: The element composition and atomic ratio of coal rock are determined based on elemental analysis.

6. The method for correcting the adsorption of coal rock methane based on the coal macromolecular model according to claim 3 is characterized in that: Based on the analytical results of spectral deconvolution of solid-state nuclear magnetic resonance test and the results of elemental analysis, the type and relative content of the chemical composition of the coal rock were obtained.

7. The method for correcting the adsorption of coal rock methane based on the coal macromolecular model according to claim 1, characterized in that: Step 2 specifically includes: Firstly, an initial two-dimensional coal macromolecular model is constructed based on the physical and chemical properties of coal rocks; Secondly, the two-dimensional coal macromolecular model was adjusted using the atomic ratio, density, and chemical composition data from coal rock experimental tests; Finally, eight two-dimensional macromolecular model units were used to construct the initial simulation box of the amorphous structure model, and molecular dynamics simulation of structure optimization-annealing kinetics simulation-pore structure comparison-structure optimization was performed to obtain the three-dimensional coal macromolecular model with the lowest local energy.

8. The method for correcting the adsorption of coal rock methane based on the coal macromolecular model according to claim 1, characterized in that: Step three specifically includes: randomly inserting a probe molecule with a fixed Connolly radius into the macromolecular model, rolling along the van der Waals surface of the macromolecular model skeleton, scanning point by point, and identifying the free space volume based on the gaps wrapped by the skeleton surface.

9. A coal rock methane adsorption correction system based on a coal macromolecular model, characterized in that: include: a memory for storing instructions executable by a processor; A processor, used to execute instructions to implement the coal rock methane adsorption correction method based on the coal macromolecular model as described in any one of claims 1-8.

10. A computer readable medium storing computer program code, characterized in that: The computer program code, when executed by a processor, implements the method for correcting the adsorption of coal rock methane based on a coal macromolecular model as described in any one of claims 1 to 8.

Images