Method for measuring the methane adsorption phase density of coal rock

Abstract

The present application relates to coal rock methane adsorption technical field, especially to a kind of measurement coal rock methane adsorption phase density method, including the calibration relationship of free phase methane amount and nuclear magnetic hydrogen nuclear relaxation signal based on low-field nuclear magnetic resonance technology, obtain the amount of adsorbed methane;Carrying out coal rock pure methane adsorption test of nuclear magnetic resonance;Establish the calibration relationship of adsorbed phase methane amount and nuclear magnetic hydrogen nuclear relaxation signal;Molecular dynamics simulation is used to construct coal rock macromolecule model;Methane molecular probe is used to detect the total free volume of macromolecule model;Based on the calibration relationship of adsorbed phase methane density and nuclear magnetic hydrogen nuclear relaxation signal, the measurement of methane adsorption phase density is carried out.The present application establishes the calibration relationship of methane adsorption phase density and nuclear magnetic hydrogen nuclear relaxation signal, solves the technical defects that current adsorbed methane density dynamic change cannot be accurately determined in real time.

Description

Technical Field

[0001] This invention relates to the field of coal methane adsorption technology, and in particular to a method for measuring the density of the adsorbed phase of coal methane. Background Technology

[0002] In coalbed methane reservoirs, methane adsorbed in the form of pores and microfractures exists. Existing experimental methods, such as volumetric and gravimetric methods, require converting excess adsorption into absolute adsorption when testing the amount of methane adsorption, but the density of the methane adsorbed phase cannot be accurately obtained through conventional experimental methods.

[0003] The patent with publication number CN119479841A uses molecular dynamics simulation to construct a three-dimensional coal macromolecule model; employs a molecular probe method to detect the free space volume of the model; conducts volumetric methane adsorption tests on coal and rock; determines the free space volume changes under a series of test pressures using helium; and replaces the free space volume of coal and rock measured by helium with the free space volume of coal and rock measured by the methane probe in the molecular simulation to correct the methane adsorption isotherm. However, this method cannot accurately determine the dynamic changes in the density of adsorbed methane. Summary of the Invention

[0004] To address the shortcomings of existing methods, this invention establishes a calibration relationship between the methane adsorption phase density and the NMR hydrogen nuclear relaxation signal, thus solving the current technical deficiency of difficulty in accurately measuring the dynamic changes in the adsorbed methane density in real time.

[0005] The technical solution adopted in this invention is: a method for measuring the density of methane adsorbed phase in coal rock, comprising the following steps:

[0006] The calibration relationship between the amount of free-phase methane and the nuclear magnetic resonance hydrogen relaxation signal was established based on low-field nuclear magnetic resonance technology, and the amount of adsorbed methane was obtained.

[0007] In 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 The amount of adsorbed methane; T free α represents the area of ​​the free methane nuclear magnetic relaxation signal; α is the linear coefficient.

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

[0011] Step 2: Perform nuclear magnetic resonance (NMR) tests on pure methane adsorption in coal and rock.

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

[0013] Step three, establishing the calibration relationship between the amount of adsorbed phase methane substance and the nuclear magnetic hydrogen relaxation signal;

[0014] As a preferred embodiment of the present application, the formula of the amount of adsorbed phase methane substance is:

[0015] n ad =C*α*T ad (3)

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

[0017] Step four, constructing a coal macromolecular model by molecular dynamics simulation;

[0018] Step five, detecting the total free volume of the macromolecular model by using a methane molecular probe;

[0019] Step six, measuring the methane adsorption phase density based on the calibration relationship between the adsorbed phase methane density and the nuclear magnetic hydrogen relaxation signal;

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

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

[0022]

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

[0024] As a preferred embodiment of the present application, the formula of the methane adsorption phase density is:

[0025]

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

[0027] As a preferred embodiment of the present application, the coal rock methane adsorption phase density measurement system comprises: a memory for storing instructions executable by a processor; and the processor is configured to execute the instructions to implement the coal rock methane adsorption phase density measurement method.

[0028] As a preferred embodiment of the present application, the computer readable medium storing computer program codes implements the coal rock methane adsorption phase density measurement method when executed by a processor.

[0029] Advantages of the present application:

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

[0031] 2. The present application solves the technical defect that the methane adsorption phase density cannot be directly obtained by experimental testing. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a flow chart of the coal rock methane adsorption phase density measurement method of the present application.

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

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

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

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

[0037] Figure 6 is the graph of coal rock adsorbed methane density changing with test pressure in the example. DETAILED DESCRIPTION

[0038] The present application will be further described below in conjunction with the drawings and examples, which are simplified schematic diagrams and only schematically illustrate the basic structure of the present application, and therefore only show the components related to the present application.

[0039] As shown in Figure 1 , a coal rock methane adsorption phase density measurement method comprises the following steps:

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

[0041] Firstly, inject helium gas with 1 MPa higher than the highest pressure of the experiment into the reference chamber of the NMR high-pressure isothermal adsorption device at room temperature to detect the gas tightness of the device;

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

[0043] Secondly, vacuumize the device, and inject pure methane gas at 0-12 MPa pressure intervals, a total of seven pressure points, without loading the sample;

[0044] Finally, carry out low-field NMR test at each pressure point to obtain the NMR 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, that is, the linear relationship between the amount of substance of free-phase methane and the NMR hydrogen nuclear relaxation signal is obtained, as shown in Figure 2 , and 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 NMR relaxation signal area of free methane, p.u; and α is the linear coefficient, α = 0.0002796.

[0047] Step two, carry out pure methane adsorption test of coal rock based on NMR;

[0048] According to the series of pressure points, set the interval 0-30 MPa to inject methane gas, and carry out low-field NMR test at each pressure point with 30-minute interval. When the signal amplitude of two consecutive low-field NMR tests is almost the same, it is considered to be in adsorption equilibrium state, and then the NMR adsorption test of the next pressure point is carried out. The NMR T2 spectrum at a series of pressure points is shown in Figure 3 , the left spectrum peak of T2 spectrum relaxation time range 0.1-10 ms is the adsorbed methane spectrum, and the right spectrum peak of T2 spectrum relaxation time range 10-10 4 ms is the free methane spectrum.

[0049] Step three, establish the calibration relationship between the amount of substance of adsorbed methane and the NMR hydrogen nuclear relaxation signal.

[0050] Firstly, after the test at the maximum pressure point 30 MPa in step two is completed, open the reference chamber valve to release part of the methane gas to reduce the pressure, then close the reference chamber valve and open the valve connected with the sample chamber to create a new equilibrium state. When the two chambers reach the equilibrium state, the NMR signals of the two chambers are measured.

[0051] Secondly, after completing the test of 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, repeat the above measurement steps; at least five equilibrium points need to be tested.

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

[0053] Δn ad =C*Δn free (2)

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

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

[0056] n ad =C*α*T ad (3)

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

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

[0059] Step four, use molecular dynamics simulation to construct a coal macromolecular model and verify its reliability.

[0060] According to the physicochemical properties of coal, a three-dimensional macromolecular model with low potential energy and stable structure is constructed by molecular dynamics simulation method, and the structure parameters of the three-dimensional macromolecular model are adjusted by comparing the element composition, density, chemical structure ratio and pore structure with experimental test; the three-dimensional macromolecular model is disclosed in patent CN119479841A.

[0061] Step five, use a methane molecular probe to detect the total free volume of the macromolecular model.

[0062] To avoid the interaction between the probe and coal, inert helium is usually used as the probe to measure the total free volume of coal. However, compared with methane, the molecular diameter of helium is smaller, which can detect more micro-nano space in coal. This will lead to the free volume of helium to be larger than that of methane. Therefore, the total free volume of methane in macromolecular model is detected by molecular simulation. bulk .

[0063] Step six, the calibration relationship between the adsorbed phase methane density and the nuclear magnetic hydrogen relaxation signal is established by combining the nuclear magnetic experiment and simulation, and the real-time measurement of the adsorbed phase density of methane is realized.

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

[0065]

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

[0067] The amount of substance of the adsorbed phase methane can be converted according to the calibration equation in step three, so only the adsorbed phase volume of methane needs to be obtained.

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

[0069]

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

[0071] The adsorbed phase density of methane can be obtained by substituting equation (5) into equation (4), and the formula is:

[0072]

[0073] The adsorbed phase density change under different pressures can be obtained by combining equations (3) and (6). As shown in Figure 6 , the real-time monitoring of the adsorbed phase density of methane in the adsorption process is realized based on this method.

[0074] With the above ideal embodiments according to the present application as the inspiration, through the above description, relevant staff can make various changes and modifications without deviating from the technical idea of the present application. The technical scope of the present application is not limited to the content of the specification, and must be determined according to the scope of the claims.

Claims

1. A method of measuring the density of a coal rock methane adsorption phase, characterized in that, The method comprises the following steps: Step one, establishing the calibration relationship between the amount of free-phase methane and the nuclear magnetic hydrogen relaxation signal based on low-field nuclear magnetic resonance technology to obtain the amount of adsorbed methane; Step two, performing pure-methane adsorption testing of coal rock by nuclear magnetic resonance; Step three, establishing the calibration relationship between the amount of adsorbed-phase methane and the nuclear magnetic hydrogen relaxation signal; Step four, constructing a macromolecular model of coal rock by molecular dynamics simulation; Step five, detecting the total free volume of the macromolecular model by using a methane molecular probe; Step six, measuring the density of the methane adsorption phase based on the calibration relationship between the adsorbed-phase methane density and the nuclear magnetic hydrogen relaxation signal; The formula of the methane adsorption phase density is: （6） wherein is the volume of the methane adsorption phase; is the total free space; is the density of the free phase methane; is the amount of substance of the free phase methane; （1） wherein is the area of the free-state methane nuclear magnetic relaxation signal; is a linear coefficient; （3） wherein is the area of the adsorbed methane nuclear magnetic relaxation signal, is the conversion factor.

2. The method of measuring the density of the methane adsorption phase of coal according to claim 1, characterized in that, The formula of the methane adsorption phase volume is: （5） wherein is the total free space; is the methane free phase density, is the amount of substance of free phase methane.

3. The method of measuring the density of the methane adsorption phase of coal according to claim 1, wherein, The low-field nuclear magnetic resonance is tested by using a nuclear magnetic high-pressure isothermal adsorption device.

4. The method of measuring the density of the methane adsorption phase of coal according to claim 1, wherein, The adsorbed methane spectrum and the free methane spectrum are divided based on the T2 spectrum of the nuclear magnetic resonance.

5. A coal petrography methane adsorption phase density system characterized by, The method comprises: a memory for storing instructions executable by a processor; a processor for executing the instructions to implement the method for measuring the density of the methane adsorption phase of coal rock according to any one of claims 1-4.

6. A computer readable medium having stored thereon a computer program code, characterized in that, The computer program code, when executed by the processor, implements the method for measuring the density of the methane adsorption phase of coal rock according to any one of claims 1-4.

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