Method for measuring coal rock methane adsorption phase density

Abstract

The invention relates to the technical field of coal rock methane adsorption, in particular to a method for measuring coal rock methane adsorption phase density, which comprises the following steps: establishing a calibration relationship between the amount of free-phase methane substance and nuclear magnetic hydrogen nuclear relaxation signals based on a low-field nuclear magnetic resonance technology to obtain the amount of adsorbed-state methane; carrying out nuclear magnetic resonance coal rock pure methane adsorption test; establishing a calibration relationship between the amount of the methane substance in the adsorption phase and the nuclear magnetic hydrogen nuclear relaxation signal; constructing a coal rock macromolecular model by utilizing molecular dynamics simulation; detecting the total free volume of the macromolecular model by adopting a methane molecular probe; and measuring the methane adsorption phase density based on the calibration relationship between the adsorption phase methane density and the nuclear magnetic hydrogen nuclear relaxation signal. The calibration relation between the methane adsorption phase density and the nuclear magnetic hydrogen relaxation signal is established, and the technical defect that the dynamic change of the adsorption state methane density is difficult to accurately measure in real time at present is overcome.

Description

Technical Field

[0001] The present invention relates to the technical field of coal-rock methane adsorption, and in particular to a method for measuring the density of methane adsorbed phase in coal-rock. Background Art

[0002] In the coalbed methane reservoir, methane in the adsorbed state exists in pores and microfractures; in existing experimental methods, such as the volumetric method and the gravimetric method, when testing the methane adsorption amount, the excess adsorption amount needs to be converted into the absolute adsorption amount, but the density of the methane adsorbed phase cannot be accurately obtained by conventional experimental means.

[0003] For the patent with the publication number CN119479841A, a three-dimensional coal macromolecular model is constructed by using molecular dynamics simulation; the free space volume of the model is detected by using the molecular probe method; the coal-rock methane adsorption test by the volumetric method is carried out; the change of the free space volume under a series of test pressures is calibrated by helium; the free space volume of the coal-rock measured by helium is replaced by the free space volume of the coal-rock measured based on the methane probe in the molecular simulation, so as to realize the correction of the methane adsorption isotherm; however, this method cannot accurately measure the dynamic change problem of the density of adsorbed methane. Summary of the Invention

[0004] Aiming at the deficiencies of the existing methods, the present invention establishes the calibration relationship between the density of the methane adsorbed phase and the nuclear magnetic hydrogen nucleus relaxation signal, and solves the technical defect that it is difficult to accurately measure the dynamic change of the density of adsorbed methane in real time.

[0005] The technical solution adopted by the present invention is: a method for measuring the density of methane adsorbed phase in coal-rock includes the following steps:

[0006] Based on the low-field nuclear magnetic resonance technology, establish the calibration relationship between the amount of substance of free-phase methane and the nuclear magnetic hydrogen nucleus relaxation signal to obtain the amount of adsorbed methane;

[0007] As a preferred embodiment of the present invention, the formula for the amount of adsorbed methane is:

[0008] n free =α*T free (1)

[0009] In the formula, n free is the amount of adsorbed methane; T free is the area of the nuclear magnetic relaxation signal of free-phase methane; α is the linear coefficient.

[0010] As a preferred embodiment of the present invention, the low-field nuclear magnetic resonance is tested by using a nuclear magnetic high-pressure isothermal adsorption device.

[0011] Step Two: Carry out the pure methane adsorption test of coal-rock by nuclear magnetic resonance;

[0012] As a preferred embodiment of the present invention, the adsorbed methane spectrum and the free methane spectrum are divided based on the spectral peaks of the nuclear magnetic resonance T2 spectrum.

[0013] Step 3: Establish the calibration relationship between the amount of methane in the adsorbed phase and the nuclear magnetic hydrogen nucleus relaxation signal;

[0014] As a preferred embodiment of the present invention, the formula for the amount of methane in the adsorbed phase is:

[0015] n ad = C * α * T ad (3)

[0016] In the formula, T ad is the area of the nuclear magnetic relaxation signal of the adsorbed methane, and C is the conversion coefficient.

[0017] Step 4: Use molecular dynamics simulation to construct a macromolecular model of coal and rock;

[0018] Step 5: Use methane molecule probes to detect the total free volume of the macromolecular model;

[0019] Step 6: Based on the calibration relationship between the density of methane in the adsorbed phase and the nuclear magnetic hydrogen nucleus relaxation signal, measure the density of methane in the adsorbed phase;

[0020] As a preferred embodiment of the present invention, the measurement of the density of methane in the adsorbed phase is calculated based on the volume of methane in the adsorbed phase.

[0021] As a preferred embodiment of the present invention, the formula for the volume of methane in the adsorbed phase is:

[0022]

[0023] In the formula, V bulk is the total free space; ρ free is the density of free-phase methane, and n free is the amount of free-phase methane.

[0024] As a preferred embodiment of the present invention, the formula for the density of methane in the adsorbed phase is:

[0025]

[0026] In the formula, V ad is the volume of methane in the adsorbed phase; V bulk is the total free space; ρ free is the density of free-phase methane; n free is the amount of free-phase methane.

[0027] As a preferred embodiment of the present invention, a system for measuring the density of adsorbed methane in coal and rock includes: a memory for storing instructions executable by a processor; and a processor for executing the instructions to implement a method for measuring the density of adsorbed methane in coal and rock.

[0028] As a preferred embodiment of the present invention, a computer-readable medium storing computer program code, the computer program code implementing a method for measuring the density of adsorbed methane in coal and rock when executed by a processor.

[0029] Advantages of the present invention:

[0030] 1. The present invention provides a new method for real-time and dynamic measurement of the density of adsorbed methane in coal;

[0031] 2. The present invention solves the technical defect that the density of the methane adsorption phase cannot be directly obtained through experimental testing. Description of the Drawings

[0032] Figure 1 is a flowchart of the method for measuring the density of adsorbed methane in coal and rock of the present invention;

[0033] Figure 2 is the nuclear magnetic resonance spectrum and calibration relationship of free methane in the embodiment;

[0034] Figure 3 is the nuclear magnetic resonance spectrum of methane adsorption test in the embodiment;

[0035] Figure 4 is the calibration relationship diagram of adsorbed methane in the embodiment;

[0036] Figure 5 is the methane adsorption curve of nuclear magnetic resonance test in the embodiment;

[0037] Figure 6 is the diagram of the density of adsorbed methane in coal and rock changing with the test pressure in the embodiment. Detailed Embodiments

[0038] The present invention will be further described below with reference to the drawings and embodiments. This figure is a simplified schematic diagram, which only illustrates the basic structure of the present invention in a schematic manner, so it only shows the components related to the present invention.

[0039] As Figure 1 shown, a method for measuring the density of adsorbed methane in coal and rock includes the following steps:

[0040] Step 1: Establish a calibration relationship between the amount of substance of free-phase methane and the nuclear magnetic resonance hydrogen nucleus relaxation signal based on low-field nuclear magnetic resonance technology;

[0041] First, at room temperature, helium gas with a pressure 1 MPa higher than the highest experimental pressure is injected into the reference chamber of the nuclear magnetic high-pressure isothermal adsorption device for airtightness detection of the device;

[0042] The nuclear magnetic high-pressure isothermal adsorption device refers to the patent with the publication number CN104713894A.

[0043] Secondly, evacuate the device. Under the condition of not loading samples, pure methane gas is injected successively at a total of seven pressure points in the pressure range of 0 - 12 MPa;

[0044] Finally, low-field nuclear magnetic resonance tests are carried out at each pressure point to obtain the nuclear magnetic T2 spectrum of pure methane at a series of pressure points. According to the gas state equation, the pressure is converted into the amount of substance of methane, and the linear relationship between the amount of substance of free-phase methane and the nuclear magnetic hydrogen nucleus relaxation signal can be obtained. As Figure 2 shown, the formula is:

[0045] n free =α*T free (1)

[0046] In the formula, n free is the amount of adsorbed methane, g; T free is the area of the nuclear magnetic relaxation signal of free methane, p.u; α is the linear coefficient; α = 0.0002796.

[0047] Step 2. Carry out the pure methane adsorption test on coal and rock based on nuclear magnetic resonance;

[0048] Inject methane gas according to the set pressure range of 0 - 30 MPa at a series of pressure points. At each pressure point, low-field nuclear magnetic resonance tests are carried out at an interval of 30 minutes. When the signal amplitudes of two consecutive low-field nuclear magnetic resonance tests are almost the same, it is regarded as the adsorption equilibrium state, and then the nuclear magnetic adsorption test at the next pressure point is carried out; The nuclear magnetic resonance T2 spectrograms at a series of pressure points are as Figure 3 shown. The left spectral peak with a relaxation time range of 0.1 - 10 ms in the T2 spectrum is the spectrum of adsorbed methane, and the right spectral peak with a relaxation time range of 10 - 10 4 ms in the T2 spectrum is the spectrum of free methane.

[0049] Step 3. Establish the calibration relationship between the amount of substance of adsorbed-phase methane and the nuclear magnetic hydrogen nucleus relaxation signal;

[0050] First, after the test at the maximum pressure point of 30 MPa in Step 2 is completed, open the valve of the reference chamber to release a part of methane gas to reduce the pressure, then close the valve of the reference chamber and open the valve connected to the sample chamber to create a new equilibrium state. When the two chambers reach the equilibrium state, measure the nuclear magnetic signals of the two chambers;

[0051] Secondly, after testing a pressure point, continue to release a part of the methane gas in the reference chamber to reduce the pressure and create a new equilibrium state, and repeat the above measurement steps; at least five equilibrium points need to be tested for the nuclear magnetic resonance signals;

[0052] Finally, according to the law of conservation of mass, the change in the amount of adsorbed methane in the closed space is equal to the change in the amount of free methane. Therefore, the calibration relationship between the amount of methane in the adsorbed phase and the nuclear magnetic resonance signal can be established as Figure 4 shown, and the formula is:

[0053] Δn ad =C*Δn free (2)

[0054] In the formula, Δn ad is the change in the amount of methane in the adsorbed phase, g; C is the conversion coefficient, dimensionless, C = 0.00027429; Δn free is the change in the amount of methane in the free phase, g.

[0055] Therefore, the adsorbed methane can be expressed as:

[0056] n ad =C*α*T ad (3)

[0057] In the formula, n ad is the amount of methane in the adsorbed phase, g; T ad is the area of the nuclear magnetic relaxation signal of the adsorbed methane, p.u.

[0058] According to Equation (3), the nuclear magnetic relaxation signal of the adsorbed methane T2 spectrum in Figure 3 can be converted into the methane adsorption amount, as Figure 5 shown.

[0059] Step 4: Use molecular dynamics simulation to construct a macromolecular model of coal and rock and verify its reliability;

[0060] According to the physical and chemical properties of coal and rock, use molecular dynamics simulation method to construct a three-dimensional macromolecular model with low potential energy and stable structure, and adjust the structural parameters of the three-dimensional macromolecular model by comparing with the elemental composition, density, chemical structure ratio and pore structure of experimental tests; the three-dimensional macromolecular model is detailed in the patent with the publication number CN119479841A.

[0061] Step 5: Use methane molecular probes to detect the total free volume of the macromolecular model;

[0062] To avoid the interaction between the probe and coal and rock, inert helium gas is usually used as the probe in the experimental method for testing the total free volume of coal and rock. However, compared with the adsorbate methane, helium gas with a smaller molecular diameter can detect more micro-nano spaces in coal and rock, which will lead to an overestimated free space volume measured by helium gas. Therefore, the molecular simulation method is used to detect the total free volume V that can be entered by methane in the macromolecular model bulk 。

[0063] Step 6: Combine nuclear magnetic resonance experiments and simulations to establish a calibration relationship between the density of methane in the adsorbed phase and the nuclear magnetic hydrogen nucleus relaxation signal, and realize the real-time measurement of the density of methane in the adsorbed phase;

[0064] The density of the adsorbed phase can be expressed as:

[0065]

[0066] In the formula, ρ ad is the density of methane in the adsorbed phase, g / cm 3 ; V ad is the volume of methane in the adsorbed phase, cm 3 。

[0067] Among them, the amount of methane in the adsorbed phase can be obtained by conversion according to the calibration equation in Step 3. Therefore, only the volume of methane in the adsorbed phase needs to be obtained.

[0068] The volume of methane in the adsorbed phase can be obtained by subtracting the volume occupied by free-phase methane from the total free space:

[0069]

[0070] In the formula, V ad is the volume of methane in the adsorbed phase, cm 3 ; V bulk is the total free space, cm 3 ; ρ free is the density of free-phase methane, which can be calculated by the ideal gas state equation, g / cm 3 ; n free is the amount of free-phase methane.

[0071] Substituting Equation (5) into (4) can obtain the density of methane in the adsorbed phase, and the formula is:

[0072]

[0073] Combining Equations (3) and (6) can obtain the change in the density of the adsorbed phase at different pressures; as Figure 6 shown, the real-time monitoring of the density of methane in the adsorbed phase during the adsorption process is realized based on this method.

[0074] Taking the above ideal embodiments of the present invention as an inspiration, through the above description, relevant staff can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for measuring the adsorption density of coal methane, characterized in that: The following steps are involved: Step 1: Based on low-field nuclear magnetic resonance technology, a calibration relationship between the amount of free-phase methane and the nuclear magnetic hydrogen nuclear relaxation signal is established to obtain the amount of adsorbed methane; Step 2: Conducting a nuclear magnetic resonance test on pure methane adsorption on coal rock; Step 3: Establish the calibration relationship between the amount of adsorbed methane and the nuclear magnetic resonance hydrogen nuclear relaxation signal; Step 4: Use molecular dynamics simulation to construct a coal rock macromolecular model; Step 5, using a methane molecular probe to detect the total free volume of the macromolecular model; Step 6: Based on the calibration relationship between the adsorption phase methane density and the nuclear magnetic resonance hydrogen nuclear relaxation signal, measure the methane adsorption phase density.

2. The method for measuring the adsorption phase density of coal rock methane according to claim 1, characterized in that: The formula for the methane adsorption phase density is: Where V ad is the volume of methane adsorption phase; V bulk is the total free space; ρ free is the methane free phase density; n free is the amount of free phase methane.

3. The method for measuring the adsorption phase density of coal rock methane according to claim 2, characterized in that: The formula for the amount of adsorbed methane is: n free =α*T free (1) Where n free is the amount of adsorbed methane; T free is the NMR relaxation signal area of ​​free methane; α is the linear coefficient.

4. The method for measuring the adsorption phase density of coal rock methane according to claim 3, characterized in that: The formula for the amount of methane in the adsorbed phase is: n ad =C*α*T ad (3) Where, T ad is the NMR relaxation signal area of ​​adsorbed methane, and C is the conversion coefficient.

5. The method for measuring the adsorption phase density of coal methane according to claim 2, characterized in that: The formula for the volume of methane in the adsorbed phase is: Where V bulk is the total free space; ρ free is the methane free phase density, n free is the amount of free phase methane.

6. The method for measuring the adsorption phase density of coal methane according to claim 1, characterized in that: Low-field nuclear magnetic resonance is tested using a nuclear magnetic high-pressure isothermal adsorption device.

7. The method for measuring the adsorption phase density of coal methane according to claim 1, characterized in that: The adsorbed methane spectrum and free methane spectrum are divided based on the spectral peaks of the nuclear magnetic resonance T2 spectrum.

8. A system for measuring the adsorption density of coal methane, characterized in that: include: a memory for storing instructions executable by a processor; A processor, used to execute instructions to implement the method for measuring the adsorption phase density of coal methane as described in any one of claims 1-7.

9. A computer readable medium storing a computer program code, characterized in that: The computer program code, when executed by a processor, implements the method for measuring the adsorption phase density of coal methane as described in any one of claims 1 to 7.

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