A method for determining methane adsorption by converting methane molar mass into nuclear magnetic signal

Abstract

A method for determining the amount of methane adsorption by converting the molar mass of methane into nuclear magnetic resonance signals, using low-field nuclear magnetic resonance technology to determine the relationship between the amount of nuclear magnetic resonance signals and the amount of methane, thereby calculating the amount of methane adsorption in coal and shale. The method uses a standard sample of known volume to measure the amount of nuclear magnetic signals under different free volumes, and without the need for a reference tank, uses the volume difference and the difference in the amount of nuclear magnetic signal to obtain the relationship between the amount of nuclear magnetic signal and the amount of methane, directly determining the relationship between the amount of methane and the amount of nuclear magnetic signal, thereby calculating the amount of methane adsorption in coal and shale.

Description

Technical Field

[0001] The invention relates to the adsorption capacity of rocks and CO2 to improve the recovery rate of coalbed methane, and in particular to a method for determining the methane adsorption amount by converting the methane molar mass through nuclear magnetic resonance signals. Background Art

[0002] At present, society pays more and more attention to environmental protection, and clean energy has received more attention. As a major coal-producing country, coal is still the main type of resource consumption in my country. In this context, coalbed methane, as a clean and efficient unconventional natural gas, has received more and more attention. Coalbed methane is mainly stored in coal reservoirs in an adsorbed state. Therefore, in the process of coalbed methane mining, the determination of the adsorbed methane content in coal is of great significance to the calculation of coalbed methane reserves in coal reservoirs and the evaluation of development effects.

[0003] At present, the measurement of adsorbed methane is basically carried out based on the volume method and the Langmuir equation isothermal adsorption experiment. However, the results of this method are easily affected by impurities, the experimental time is also long, and the results are obtained by indirect calculation, not actual measurement, and there is a certain error. With the development of nuclear magnetic resonance technology, it has been widely used in unconventional oil and gas reservoirs. Nuclear magnetic resonance technology measures the amount of methane in the fluid. 1 H is used to analyze and study the fluid. Therefore, nuclear magnetic resonance technology can be used to study the adsorbed methane in coal. However, the current method of using nuclear magnetic resonance to test the methane adsorption amount also has certain defects.

[0004] At present, the test of methane adsorption in coal or shale is mainly based on the law of conservation of mass in the adsorption process. First, the free methane is tested by nuclear magnetic resonance signal, and then a reference tank is added during the adsorption process. The pressure difference between the reference tank and the sample tank is used to calculate the reduction of free methane, thereby obtaining the adsorption amount of methane. However, this method is relatively cumbersome, and during the test, the nuclear magnetic resonance signal of free methane in the pipeline cannot be removed, which will have a certain impact on the results.

[0005] Therefore, it is necessary to find a new method to measure the amount of methane adsorption more simply, quickly and accurately. Summary of the invention

[0006] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for determining the methane adsorption amount by converting the methane molar mass into a nuclear magnetic resonance signal, solve the defects of the current method for determining the methane adsorption amount by using nuclear magnetic resonance technology, and provide a new method for obtaining nuclear magnetic resonance using low-field nuclear magnetic resonance technology. 1The method of calculating the methane adsorption amount in coal and shale by establishing the relationship between the H atom signal and the amount of methane substance. Through this method, the relationship between the nuclear magnetic resonance signal and the amount of methane substance can be directly established to calculate the methane adsorption amount in the rock, thereby realizing the real-time measurement of the methane adsorption amount under different temperature and pressure conditions.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A system for determining the amount of methane adsorption by converting methane molar mass through nuclear magnetic resonance signals comprises a methane tank and a helium tank, wherein the methane tank and the helium tank are connected to a pressure reducing valve through valves V1 and V2; a booster pump and a gas storage tank are connected between valves V1 and V2; a valve V3 is provided on the gas storage tank; the pressure reducing valve is connected to a sample tank through valves V6 and V7; a back pressure valve is connected between the pressure reducing valve and valve V6 through valve V4, and a vacuum pump is connected through valve V5, and a pressure sensor is provided between valves V6 and V7.

[0009] The sample tank is fixed on the low-field nuclear magnetic resonance device through a clamp and is provided with a pressurizing device and a constant temperature device.

[0010] The clamp is provided with an air circuit interface, which is connected with a methane tank, a helium tank, a gas storage tank, a booster pump and a vacuum pump through a closed air circuit system.

[0011] A method for determining the amount of methane adsorption by converting methane molar mass using nuclear magnetic resonance signals, characterized in that it comprises the following steps:

[0012] First, inject 3MPa helium into the experimental system at room temperature, check the air tightness, and evacuate after passing the inspection;

[0013] Then the whole system is evacuated, pure methane gas is injected into the empty sample tank, the nuclear magnetic T2 spectrum of methane at different pressure points is tested, and the nuclear magnetic signal amplitude at different pressures in the empty tank state is obtained; then a known volume of standard sample is placed in the sample tank, pure methane gas is injected, and the nuclear magnetic T2 spectrum of methane at the same pressure as in the empty tank state is tested to obtain the nuclear magnetic signal amplitude at different pressures in the state of placing the standard sample. The difference in the nuclear magnetic signal amplitude at the same pressure in the empty tank state and after the standard sample is placed is used in combination with the ideal gas state equation to determine the relationship between the nuclear magnetic signal amplitude and the amount of methane substance.

[0014] 20g of coal or shale sample is added to the sample tank, and a methane isothermal adsorption experiment is carried out under low-field nuclear magnetic resonance monitoring; the nuclear magnetic resonance T2 spectrum after adsorption equilibrium at different pressure points is tested, and the nuclear magnetic resonance signal amplitude value is inverted; based on the established relationship between the amount of methane substance and the nuclear magnetic resonance signal amplitude, the standard state volume of adsorbed methane is obtained.

[0015] Further, the specific operating steps for determining the methane adsorption amount are as follows:

[0016] (1) Air tightness inspection and base signal measurement:

[0017] Set the temperature of the thermostat to 25 degrees Celsius, open the V2, V6, and V7 valves, and inject helium into the entire system until the inlet pressure reaches 3 MPa. After the pressure stabilizes for 2 hours, it proves that the airtightness is good;

[0018] (2) Open valves V2 and V4 to discharge helium from all pipelines;

[0019] (3) Then, open valves V5 and V7 to evacuate the entire system to an inlet pressure of -0.05 MPa, and measure the T2 spectrum under vacuum as the base signal;

[0020] (4) Methane calibration process:

[0021] Place the empty sample can in a holder, place the holder in the NMR device, and then connect the gas pipeline, pressurizing device and constant temperature device;

[0022] Open V2, V6, and V7, and inject methane into the sample tank. When the pressure in the sample tank reaches 0.5 MPa, close V7 and measure the T2 spectrum of methane at 0.5 MPa.

[0023] Open V7 and continue to inject methane until the pressure reaches 0.75 MPa, and test the T2 spectrum of methane at this pressure;

[0024] Repeat the above steps and continue to measure the NMR signal of methane in the empty tank at pressures of 1.00 MPa, 1.25 MPa, and 1.50 MPa, denoted as Qi, where i is 0.50, 0.75, 1.00, 1.25, and 1.50, respectively;

[0025] After obtaining the methane NMR signal when the tank is empty, remove the holder and the sample tank, put the standard sample with a known volume of Vs into the sample tank, repeat the operation steps when the tank is empty, and obtain the T2 spectrum and NMR signal of methane at pressures of 0.05 MPa, 0.75 MPa, 1.00 MPa, 1.25 MPa, and 1.50 MPa after the standard sample is placed, which are recorded as Qi´;

[0026] (5) Calibration of methane quantity and NMR signal:

[0027] According to the ideal gas state property: in the empty tank state:

[0028] PiV0 = niZiRT(1);

[0029] When the standard sample is added:

[0030] PiV0´=ni´ZiRT(2);

[0031] In formulas (1)-(2), Pi is the pressure at which the T2 spectrum is measured each time, V0 is the volume of the sample tank, ni is the amount of methane at different pressures in the empty tank state, Zi is the compressibility factor of methane at different pressures, R is a thermodynamic constant, T is the experimental temperature, V0´ is the volume of the sample tank after the standard sample is placed, and ni´ is the amount of methane at different pressures when the standard sample is placed

[0032] Since V0´=V0-Vs, substituting into formula (2) we can get:

[0033] Pi(V0-Vs)=niZiRT=PiV0-PiVs(3);

[0034] Combining equations (3) and (1), we can obtain:

[0035] PiVs=(ni-ni´)ZiRT=△niZiRT (4);

[0036] Since in each NMR T2 spectrum test, the NMR signals Qi and Qi' of methane completely correspond to the amounts ni and ni' of methane, the difference △Qi of the NMR signals of methane in the empty tank and when the standard sample is placed in the tank also completely corresponds to the difference △ni of the amount of methane under the same pressure. The relationship between the NMR signal of methane and the amount of adsorbed methane can be obtained.

[0037] na=kQa;

[0038] According to the state equation of ideal gas, the standard state volume of methane adsorption is obtained:

[0039] Va=naRT / P (5);

[0040] Where Va is the standard state volume of the adsorbed amount of methane, Qa is the nuclear magnetic signal of methane, na is the amount of adsorbed methane, and k is a parameter;

[0041] (6) Methane isothermal adsorption experiment:

[0042] Weigh 20g of sample and put it into the sample tank, fix it in the low-field NMR device through the clamp, and connect the gas system; evacuate to -0.05MPa and test the air tightness of the device; close the V7 valve, open the V1, V2, and V6 valves, inject methane into the gas system, stop injecting after the set pressure, open V7 to start adsorption, and test the NMR T2 spectrum after adsorption equilibrium.

[0043] Record the equilibrium pressure; then close V7, continue to inject methane, and test the NMR T2 spectrum after adsorption equilibrium at different pressures.

[0044] The beneficial effects of the present invention are:

[0045] 1. The test process is simpler and faster, eliminating the need for a reference tank in existing methods;

[0046] 2. It is beneficial to the principle of drainage method. The relationship between the amount of methane and the amplitude of nuclear magnetic signal is directly determined by volume difference and signal difference, avoiding the influence of free methane in the gas pipeline on the nuclear magnetic signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 The NMR T2 spectra of methane at different pressures in empty tank and standard state;

[0048] Figure 2 is the relationship between the NMR T2 spectrum peak area and the amount of methane substance;

[0049] Figure 3 This is the NMR T2 spectrum of methane after adsorption under different pressures;

[0050] Figure 4 is the isotherm adsorption curve of methane;

[0051] Figure 5 This is a schematic diagram of the structure of this system. DETAILED DESCRIPTION

[0052] The present invention is further described below in conjunction with the accompanying drawings and embodiments, but the present invention is not limited to the following embodiments.

[0053] like Figure 5 The system shown: The device for testing the methane adsorption using low-field nuclear magnetic resonance includes helium and methane gas sources, a booster pump, a vacuum pump, a sample tank and a low-field nuclear magnetic resonance device. The sample tank can be fixed in the low-field nuclear magnetic resonance device through a clamp. The clamp is provided with a gas path interface, which is connected to the gas source, gas storage tank, booster pump and vacuum pump through a closed gas path system. The helium and methane gas sources are connected to the gas source through valve V1 and connected to the gas storage tank. The gas storage tank is used to store the gas required for the experiment and is connected to the booster pump. The booster pump pressurizes the gas in the gas storage tank to provide gas with the pressure required for the experiment. The gas tank is provided with a valve V3 for exhausting the gas in the gas tank; valve V2 and a pressure reducing valve are provided between the booster pump and the vacuum pump for controlling the amount of gas entering the gas system, and valve V4 and a back pressure valve are also provided for exhausting the gas in the gas system; the vacuum pump is connected to the gas system through valve V5 for evacuating the entire system; valves V6 and V7 and a pressure sensor provided between the two valves are provided after the vacuum pump for controlling the gas entering the sample tank.

[0054] Embodiment: The specific operation steps of the method for determining the methane adsorption amount are as follows:

[0055] 1. First check the air tightness of the device:

[0056] (1) First open V1, V2, V5, V6, and V7 to inject helium into the gas tank, and then close all valves;

[0057] (2) Open the V2, V6, and V7 valves, and inject helium into the entire system until the inlet pressure reaches 3 MPa. Observe whether the inlet pressure is stable and confirm its air tightness. After confirmation, open V2 and V4 to discharge the gas in the entire system and prepare for the methane calibration test;

[0058] 2. Methane NMR calibration experiment:

[0059] (1) Open V1 and V2 to inject methane into the gas tank;

[0060] (2) Open V5, V6, and V7, evacuate the entire system to an inlet pressure of -0.05 MPa, and measure the T2 spectrum under vacuum as the base signal;

[0061] (3) Open V2, V6, and V7, and inject methane into the sample tank. When the inlet pressure reaches 0.50 MPa, close V7 and measure the nuclear magnetic resonance signal of methane at 0.50 MPa, which is recorded as Q. 0.50 ;

[0062] (4) Open V7 and inject methane into the sample tank. When the inlet pressure reaches 0.75 MPa, close V7 and measure the NMR signal Q of methane at 0.75 MPa. 0.70 ;

[0063] (5) Repeat the above step (4) to test the methane signal quantities Q1.00, Q1.25, and Q1.50 at 1.00 MPa, 1.25 MPa, and 1.50 MPa respectively;

[0064] (6) After the methane test at different pressures in the empty tank is completed, open V4, exhaust the gas in the system, and take out the sample tank;

[0065] (7) Place a known volume V in the sample tank. s The standard sample is then placed, and steps (3) to (5) are repeated to obtain the NMR signal quantities Q´0.50, Q´0.70, Q´1.00, Q´1.25, and Q´1.50 of methane at different pressures after the standard sample is placed;

[0066] 3. Methane adsorption experiment:

[0067] (1) Weigh 20g of 60-80 mesh coal sample and dry it completely in a vacuum drying oven to avoid affecting the subsequent experimental results. Put the sample into a sample tank, put it into the nuclear magnetic resonance instrument, and install the constant temperature device and pressurizing device;

[0068] (2) Open V5, V6, and V7, evacuate to an inlet pressure of -0.05 MPa, and measure the NMR T2 spectrum under vacuum as the background signal;

[0069] (3) Open V2 and V6, close V7, and inject methane until the inlet pressure reaches 2.0 MPa. After stabilization, open V7 and start adsorption. During the adsorption process, measure the NMR T2 spectrum every 5 minutes until two consecutive T2 spectra are basically consistent. The adsorption is considered complete, and the inlet pressure after adsorption equilibrium is recorded.

[0070] (4) Close V7, open V2, continue to inject methane, and stop injecting when the inlet pressure reaches 4 MPa. After the pressure stabilizes, open V7 and start adsorption. After the adsorption is completed, measure the T2 spectrum and record the equilibrium pressure.

[0071] (5) Repeat step 6 to measure the NMR T2 spectra and equilibrium pressures after adsorption equilibrium at 6 MPa, 8 MPa, 10 MPa, and 12 MPa, respectively;

[0072] 4. Calculation part:

[0073] (1) Calibration of methane NMR signal:

[0074] like Figure 1 As shown in the figure, the NMR T2 spectrum of methane is in the form of a single peak. As the pressure of injected methane increases, the NMR signal amplitude of methane increases and the peak area increases. The difference in peak area is calculated based on the peak area in the empty tank state and the peak area in the standard sample state, and the relationship between the NMR signal and the amount of methane is calculated. The NMR T2 peak area of ​​methane under different pressures is shown in Table 1.

[0075] Table 1

[0076]

[0077] According to formula (4), △ni is calculated:

[0078] Experimental pressure, temperature, standard sample volume (standard sample volume is 3.03453E-05m 3 As an example, the amount of methane under different pressures is shown in Table 2 and Table 3. The relationship between the peak area of ​​the NMR T2 spectrum and the amount of methane is shown in Table 3. Figure 2 As shown;

[0079] Table 2

[0080]

[0081] (2) Methane adsorption equilibrium calculation:

[0082] The T2 spectra of adsorption equilibrium at different pressures are shown in Figure 2. Figure 3 As shown in Table 3, the NMR T2 spectrum of methane adsorbed in coal shows a double peak. The peaks from left to right represent adsorbed methane and free methane respectively. The T2 spectrum peak area after adsorption equilibrium is shown in Table 3, the adsorption amount is calculated as shown in Table 4, and the isothermal adsorption curve of methane is shown in Table 4. Figure 4 As shown,

[0083] Table 3

[0084]

[0085] Table 4

[0086]

[0087] In summary: Based on the principle of the water displacement method, the amount of methane at different pressures with the same volume as the standard sample is obtained. Combined with the difference in the nuclear magnetic resonance signal under the two states of the sample tank, the relationship between the nuclear magnetic resonance signal and the amount of methane is directly determined, thereby calculating the content of adsorbed methane.

Claims

1. A method for determining the amount of methane adsorption by converting methane molar mass into nuclear magnetic resonance signals, characterized in that: The system includes a methane tank and a helium tank, which are connected to the pressure reducing valve through valve V1 and valve V2; the booster pump and the gas storage tank are connected between valve V1 and valve V2; the gas storage tank is provided with valve V3; the pressure reducing valve is connected to the sample tank through valve V6 and valve V7; the pressure reducing valve and valve V6 are connected to the back pressure valve through valve V4, and are connected to the vacuum pump through valve V5, and a pressure sensor is provided between the valve V6 and valve V7; The sample tank is fixed on the low-field nuclear magnetic resonance device through a clamp, and is provided with a pressurizing device and a constant temperature device; The clamp is provided with a gas circuit interface, which is connected to a methane tank, a helium tank, a gas storage tank, a booster pump, and a vacuum pump through a closed gas circuit system; The specific steps for determining the methane adsorption amount are as follows: (1) Air tightness inspection and base signal measurement: The temperature of the thermostat was set to 25 ˚C, and helium was injected into the entire system until the inlet pressure reached 3 MPa. After the pressure was stable for 2 hours, it was proved that the airtightness was good; (2) Exhaust helium from all pipelines; (3) Evacuate the entire system to an inlet pressure of -0.05 MPa, and measure the T2 spectrum under vacuum as the base signal; (4) Methane calibration process: Place the empty sample can in a holder, place the holder in the NMR device, and then connect the gas pipeline, pressurizing device and constant temperature device; Inject methane into the sample tank. When the pressure in the sample tank reaches 0.5 MPa, close valve V7 and measure the T2 spectrum of methane at 0.5 MPa. Open valve V7 and continue to inject methane until the pressure reaches 0.75 MPa, and test the T2 spectrum of methane at this pressure; Repeat the above steps and continue to measure the NMR signal of methane in the empty tank at pressures of 1.00 MPa, 1.25 MPa, and 1.50 MPa, denoted as Qi, where i is 0.50, 0.75, 1.00, 1.25, and 1.50, respectively; After obtaining the methane NMR signal when the tank is empty, remove the holder and the sample tank, put the standard sample with a known volume of Vs into the sample tank, repeat the operation steps when the tank is empty, and obtain the T2 spectrum and NMR signal of methane at pressures of 0.05 MPa, 0.75 MPa, 1.00 MPa, 1.25 MPa, and 1.50 MPa after the standard sample is placed, which are recorded as Qi´; (5) Calibration of methane quantity and NMR signal: According to the ideal gas state equation: in the empty tank state: PiV0 = niZiRT(1); When the standard sample is added: PiV0´=ni´ZiRT(2); In formulas (1)-(2), Pi is the pressure at which the T2 spectrum is measured each time, V0 is the volume of the sample tank, ni is the amount of methane at different pressures in the empty tank state, Zi is the compressibility factor of methane at different pressures, R is a thermodynamic constant, T is the experimental temperature, V0´ is the volume of the sample tank after the standard sample is placed, and ni´ is the amount of methane at different pressures when the standard sample is placed Since V0´=V0-Vs, substituting into formula (2) we can get: Pi(V0-Vs)=ni´ZiRT=PiV0-PiVs(3); Combining equations (3) and (1), we can obtain: PiVs=(ni-ni´)ZiRT=△niZiRT (4); Since in each NMR T2 spectrum test, the NMR signals Qi and Qi' of methane completely correspond to the amounts ni and ni' of methane, the difference △Qi of the NMR signals of methane in the empty tank and when the standard sample is placed in the tank also completely corresponds to the difference △ni of the amount of methane under the same pressure. The relationship between the NMR signal of methane and the amount of adsorbed methane can be obtained. na=kQa; According to the state equation of ideal gas, the standard state volume of methane adsorption is obtained: Va=naRT / P (5); Where Va is the standard state volume of the adsorbed amount of methane, Qa is the nuclear magnetic signal of methane, na is the amount of adsorbed methane, and k is a parameter; (6) Methane isothermal adsorption experiment: 20g of coal or shale sample is added to the sample tank, and a methane isothermal adsorption experiment is carried out under low-field nuclear magnetic resonance monitoring; the nuclear magnetic resonance T2 spectrum after adsorption equilibrium at different pressure points is tested, and the nuclear magnetic resonance signal amplitude value is inverted; based on the established relationship between the amount of methane substance and the nuclear magnetic resonance signal amplitude, the standard state volume of adsorbed methane is obtained.

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