Isothermal adsorption-nuclear magnetic resonance combined measurement-based adsorption gas calculation method for laboratory

Abstract

The invention discloses a laboratory adsorption gas calculation method based on isothermal adsorption-nuclear magnetic resonance combined measurement, and relates to the technical field of exploration geophysics. According to the invention, on the basis of isothermal adsorption-nuclear magnetic resonance combined measurement experiment, 23MHz nuclear magnetic resonance experiment is utilized; nuclear magnetic signal conversion of bound water-adsorbed state methane is carried out through nuclear magnetic resonance T2 spectrums of coal rocks with different water saturations and T2 spectrums measured through a coal rock isothermal adsorption-nuclear magnetic resonance combined measurement experiment to obtain the hydrogen content of the adsorbed state methane, then the amount of substance of the adsorbed state methane is obtained, and finally the volume of the adsorbed state methane is obtained. The method comprises the following steps: firstly, measuring and determining the content of bound water in the coal rock by adopting a 25mm sample tube and the current minimum nuclear magnetism callback interval, then carrying out bound water-adsorbed gas nuclear magnetism signal conversion on the sample tube used in an isothermal adsorption-nuclear magnetism combined measurement experiment to obtain the hydrogen content of the adsorbed gas, further obtaining the amount of substance of the adsorbed gas, and finally obtaining the volume of the adsorbed gas.

Description

Technical Field

[0001] This invention relates to the field of exploration geophysics, and in particular to a laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement. Background Technology

[0002] The adsorbed gas and free gas signals in coal reservoirs are clearly distinguishable in the T2 NMR spectrum. Existing methods for calculating adsorbed gas all involve subtracting free methane from the total gas content of the coal. However, directly converting the T2 NMR spectrum signal into adsorbed gas quantity is not feasible for calculating adsorbed gas quantity directly using NMR signals. Indirect calculations of adsorbed gas quantity in coal reservoirs are limited by the airtightness of the experimental instruments and the determination of the compressibility coefficient of free methane, leading to calculation errors and underestimating the adsorbed gas quantity. Therefore, a direct method for calculating adsorbed gas in coal reservoirs is crucial for the application of isothermal adsorption-NMR combined analysis experiments in reservoir evaluation.

[0003] Deep coal reservoirs are mainly composed of adsorbed gas, with the amount of adsorption decreasing with increasing burial depth. Adsorbed and free gases generally exhibit a shared origin, forming a unified adsorbed-free gas coexistence system. Dynamic exchange occurs between adsorbed and free gases in the coal seam, and between the free gases in the coal seam and the free gases in the surrounding rock, creating a dynamic equilibrium system. Gao Lijun et al. and Sun Bin et al. used isothermal adsorption experiments combined with saturated NMR to infer the proportion of free and adsorbed gas content. Sun Jianmeng et al. were the first to design a simultaneous dynamic measurement experimental device for isothermal adsorption and NMR, and conducted simultaneous dynamic measurement experiments on adsorbed gas reservoir rock samples. This experiment calibrated the free adsorbed gas based on the peak separation and position of the T2 spectrum obtained by NMR measurement inversion after the sample was inflated to adsorption equilibrium. Li Jun et al. used NMR combined with isothermal adsorption to determine the content of free and adsorbed methane gas in shale. Li Yong et al. also conducted isothermal adsorption and NMR combined experiments on coal to calculate its free adsorbed gas.

[0004] Therefore, it is crucial to accurately assess reservoir gas content using NMR signals. Combining NMR signal assessment with laboratory isothermal adsorption-NMR co-detection technology can provide a better reference for reservoir evaluation. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention discloses a laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance (NMR) co-measurement. This method is based on the isothermal adsorption-NMR co-measurement experiment and utilizes a 23MHz NMR experiment. It converts the bound water-adsorbed methane NMR signal by combining the NMR T2 spectra of coal and rock with those obtained from the coal and rock isothermal adsorption-NMR co-measurement experiment. This yields the hydrogen content of the adsorbed methane, which in turn determines its amount of substance, ultimately resulting in the volume of adsorbed methane.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement includes the following steps:

[0008] s1. Use wire cutting to process the core into standard plunger samples, measure the length, diameter, and mass, and calculate the volume;

[0009] s2. Air-dry the plunger sample;

[0010] s3. Vacuum the residual gas in the pores of the sample after air drying and perform a second drying process;

[0011] s4. Determine the NMR measurement parameters: echo time, number of echoes, number of accumulations, and waiting time. Use a MesoMR23-060H-I type NMR instrument to perform isothermal adsorption-NMR combined measurement experiments on the samples, record the sample data at each pressure point, obtain the NMR T2 spectrum measured after methane adsorption in coal at different pressure points, and determine the relationship between each pressure point and the peak area of ​​adsorbed methane in the NMR T2 spectrum.

[0012] s5. Select coal and rock, pressurize and saturate it with water to obtain water-saturated coal and rock, weigh it, and perform nuclear magnetic resonance T2 spectrum measurement using a 25 mm sample tube and a sample tube used for isothermal adsorption, respectively;

[0013] s6. The water-saturated coal and rock were air-dried to obtain coal and rock with different water saturation. The nuclear magnetic T2 spectra of the two sample tubes were measured in step s5 for each water saturation point to obtain the peak area of ​​the bound water contained in the core at different water saturation states on the T2 spectrum.

[0014] s7. The mass of bound water contained at each water saturation point was calculated by measuring the T2 NMR spectrum and weight of each water saturation point of the coal and rock using a 25mm sample tube.

[0015] s8. Based on the bound water mass and peak area at each water saturation point of coal and rock, the relationship between the bound water hydrogen content and the NMR signal is established through linear fitting;

[0016] s9. Based on the hydrogen content obtained from the conversion of the total amount of adsorbed methane signal at each pressure point in step s8, the conversion between the NMR signal of adsorbed methane at each pressure point and the volume of methane under standard conditions is completed;

[0017] s10. Based on the volume of adsorbed methane per unit mass of coal at each pressure point under standard conditions obtained in step s9, the pressure and the volume of adsorbed methane per unit mass of coal under standard conditions are fitted by a Langevin curve to obtain the evaluation parameters of the coal reservoir.

[0018] Optionally, in step s2, the air drying process is as follows: the plunger sample is placed in a drying oven and air-dried at 80°C, and weighed using a high-precision balance, and the mass after air drying is recorded.

[0019] Optionally, in step s4, the steps of the isothermal adsorption NMR assay are as follows: after placing the air-dried plunger sample into the sample cylinder, vacuum treatment is performed on it and the NMR base signal of the sample is measured. Then, methane gas is injected step by step according to the designed pressure points, and its NMR T2 signal is measured, wherein there are at least 8 pressure points.

[0020] Optionally, in step s5, the echo time is 0.06 when measuring with a 25mm sample tube, and the other parameters are the same as those used when measuring saturated coal and rock with a sample tube for isothermal adsorption, and are the same as those used in step s4.

[0021] Optionally, in step s7, the formula for the mass of bound water at each water saturation point is:

[0022] ;

[0023] In the formula, The mass of bound water contained in coal and rock. The peak area of ​​bound water in the T2 spectrum was measured for a 25mm coal and rock sample tube. The T2 peak area of ​​a 25mm sample tube was measured after the coal and rock were completely dried. The peak area of ​​movable water in the T2 spectrum of a 25mm coal and rock sample tube. The mass of coal and rock when it contains water. This refers to the quality of the coal and rock in their dried state.

[0024] Optionally, step s8 specifically includes:

[0025] ;

[0026] In the formula, The peak area of ​​bound water in the T2 NMR spectrum was measured for a 70 mm sample tube. These are the fitting coefficients. To bind water mass;

[0027] Based on the total NMR T2 spectrum signal of adsorbed methane at each pressure point measured in step s4, the conversion between the NMR signal of adsorbed methane at each pressure point and the hydrogen content of adsorbed methane is completed, including:

[0028] The hydrogen content of adsorbed methane in coal at each pressure point can be obtained using the following formula:

[0029] ;

[0030] In the formula, This refers to the hydrogen content of adsorbed methane in coal and rock. The peak area of ​​methane adsorption in coal and rock. The area of ​​the signal peak in the coal and rock basement; represents the fitting coefficient.

[0031] Optionally, in step s9, the volume of adsorbed methane per unit mass of coal rock under standard conditions at different pressure points is calculated using the following formula:

[0032] ;

[0033] In the formula, This refers to the content of adsorbed methane in coal and rock. This refers to the hydrogen content of adsorbed methane in coal and rock. This refers to the core quality.

[0034] Optionally, in step s10, the Langmuir volume and Langmuir pressure of the core are obtained by nonlinear fitting using the Langmuir equation:

[0035] ;

[0036] In the formula, This represents the volume of adsorbed methane per unit mass of coal under standard conditions. For gas pressure, For the Langevin volume, This is the pressure of the Langevin.

[0037] The beneficial effect of this invention is that it can directly determine the hydrogen content of adsorbed methane and further obtain the gas content of adsorbed methane through gas and water NMR experiments in the same sample tube. This can effectively reduce the experimental errors caused by traditional indirect calculation of adsorbed methane. For example, when calculating the free methane content by subtracting the total gas content of the core from the free methane content in the indirect method, errors will be introduced in determining the porosity of the core methane and the methane compressibility coefficient. Attached Figure Description

[0038] Figure 1 This is a flowchart of a laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement;

[0039] Figure 2 Isothermal adsorption-NMR combined measurement of T2 spectra of dried coal and rock samples;

[0040] Figure 3 NMR T2 spectra of coal and petrite at different water saturation levels - 70mm sample tube;

[0041] Figure 4 NMR T2 spectra of coal and petrite at different water saturation levels - 25mm sample tubes;

[0042] Figure 5A graph showing the relationship between the hydrogen content of bound water in coal and rock and the peak area of ​​the T2 NMR spectrum;

[0043] Figure 6 The results of the Landau simulation are presented to evaluate the adsorbed gas volume calculated using the volumetric method and the new method of this invention under different pressures. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] This invention uses standard coal plunger samples and dry standard plunger coal samples as examples to illustrate the specific implementation of the method of this invention.

[0046] Specific steps are as follows: Figure 1 As shown:

[0047] s1. Core samples were collected from the study area, and standard plunger samples were fabricated using wire cutting. The length was measured to be 50.53 mm, the diameter 25.06 mm, and the sample volume was calculated to be 25.05 cm³. 3 .

[0048] s2. Based on the plunger sample described in step s1, complete the air drying process, including:

[0049] The plunger sample was placed in a drying oven and air-dried at 80°C. It was then weighed using a high-precision balance, and the mass after air drying was recorded as 33.32g.

[0050] s3. After completing step s2 of drying and weighing the sample, the sample is evacuated and dried a second time. Specifically, a vacuum pump is used to remove the residual gas in the pores of the sample and the sample is dried a second time.

[0051] s4. Determine the NMR measurement parameters: echo time TE 0.2 ms, number of echoes NECH 13000, number of accumulations NS 16, and waiting time TW 10000 ms. Perform isothermal adsorption NMR measurements on the sample using a MesoMR23-060H-I NMR spectrometer, and record the sample isothermal adsorption NMR measurement data at at least 8 pressure points, including:

[0052] After the air-dried plunger sample was placed into the sample cylinder, it was evacuated and the NMR base signal of the sample was measured. Then, methane gas was gradually injected at the designed pressure points (2.355 MPa, 4.08 MPa, 5.76 MPa, 7.862 MPa, 10.811 MPa, 13.42 MPa, 17.818 MPa, 20.34 MPa), and isothermal adsorption was performed for 24 hours, and the NMR T2 signal was measured.

[0053] Figure 2 To obtain all NMR T2 spectra at each pressure point and to divide the adsorption peak regions.

[0054] s5. Select coal and rock samples, pressurize and saturate them with water to obtain a water-saturated core, and weigh it. Measure the water content using a 25mm sample tube and a sample tube used for isothermal adsorption, respectively. Figure 4 The NMR T2 spectrum measurement is shown. For this NMR measurement, the echo time is required to be 0.06 when measuring with a 25mm sample tube. Other parameters are the same as those used when measuring saturated coal and rock with the sample tube used for isothermal adsorption and are basically the same as those in step s4.

[0055] s6. The saturated coal and rock were air-dried to obtain coal and rock with different water saturation levels. Step s5 was repeated for each different water saturation point, and the measurements were as follows: Figure 3 , Figure 4 The T2 spectra corresponding to the 25mm and 70mm sample tubes are shown, and the peak areas of bound water in the T2 spectra of cores with different water saturation states are obtained.

[0056] s7. Using the NMR T2 spectra measured at each water saturation point of the coal and rock at 25mm and the weight of each water saturation point, the mass of bound water contained at each water saturation point is calculated; the formula is:

[0057] ;

[0058] In the formula, The mass of bound water contained in coal and rock. The peak area of ​​bound water in the T2 spectrum was measured for a 25mm coal and rock sample tube. The T2 peak area of ​​a 25mm sample tube was measured after the coal and rock were completely dried. The peak area of ​​movable water in the T2 spectrum of a 25mm coal and rock sample tube. The mass of coal and rock when it contains water. This refers to the quality of the coal and rock in their dried state.

[0059] s8. Based on the mass of bound water and the peak area at each water saturation point of coal and rock, through methods such as Figure 5 The linear fitting shown establishes the relationship between the hydrogen content of bound water and the NMR signal; including:

[0060] ;

[0061] In the formula, The peak area of ​​bound water in the T2 NMR spectrum was measured for a 70 mm sample tube. These are the fitting coefficients. To bind water mass;

[0062] Based on the total NMR T2 spectrum signal obtained from step 4 for the adsorbed gas at each pressure point, the conversion between the NMR signal of adsorbed methane and the hydrogen content of adsorbed methane at each pressure point is completed, including:

[0063] The hydrogen content of adsorbed methane in coal at different pressure points can be obtained using the following formula:

[0064] ;

[0065] In the formula, This refers to the hydrogen content of adsorbed methane in coal and rock. This represents the area of ​​the methane adsorption peak in coal and rock. These are the fitting coefficients. This represents the area of ​​the signal peak in the coal and rock basement.

[0066] s9. Based on the hydrogen content obtained from the conversion of the total adsorbed methane signal at each pressure point in step s8, the conversion between the adsorbed methane NMR signal at each pressure point and the methane volume under standard conditions is completed, including:

[0067] The volume of adsorbed methane per unit mass of coal under standard conditions at different pressures can be calculated using the following formula:

[0068] ;

[0069] In the formula, This refers to the content of adsorbed methane in coal and rock. This refers to the hydrogen content of adsorbed methane in coal and rock. This refers to the core quality.

[0070] s10. Based on the volume of adsorbed methane per unit mass of coal rock under standard conditions obtained from step s9, the reservoir evaluation parameters of the coal rock are obtained by fitting the pressure and the volume of adsorbed methane per unit mass of coal rock under standard conditions using the Randolph curve.

[0071] The Langmuir volume of the core was determined to be 30.04 cm³ using nonlinear fitting of the Langmuir equation. 3 / g, the Landau pressure is 8.87 MPa:

[0072] ;

[0073] In the formula, This represents the volume of adsorbed methane per unit mass of coal under standard conditions. This represents the gas pressure.

[0074] The volume of adsorbed methane per unit mass of coal at each pressure point obtained by the traditional volumetric method under standard conditions is used to fit the pressure-volume volume of adsorbed methane per unit mass of coal under standard conditions, thereby obtaining the reservoir evaluation parameters of the coal-rock ...

[0075] The Langmuir volume of the core was determined to be 28.46 cm³ using nonlinear fitting of the Langmuir equation. 3 / g, the Landau pressure is 7.81 MPa:

[0076] ;

[0077] The difference between the two methods in the Langevin law can be intuitively seen from... Figure 6 The variation pattern shown indicates that the new method measures a larger volume of adsorbed gas, and the calculated Langevin body is also larger, which to some extent makes up for the lack of traditional volumetric methods in calculating adsorbed gas.

[0078] In the formula, This represents the volume of adsorbed methane per unit mass of coal under standard conditions. This represents the gas pressure.

[0079] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement, characterized in that, Includes the following steps: s1. Use wire cutting to process the core into standard plunger samples, measure the length, diameter, and mass, and calculate the volume; s2. Air-dry the plunger sample; s3. Vacuum the air-dried sample and perform a second drying process; s4. Determine the NMR measurement parameters: echo time, number of echoes, number of accumulations, and waiting time. Use an NMR instrument to perform isothermal adsorption-NMR combined measurement experiments on the sample, record the sample data at each pressure point, obtain the NMR T2 spectrum measured after methane adsorption in coal at different pressure points, and determine the relationship between each pressure point and the peak area of ​​adsorbed methane in the NMR T2 spectrum. s5. Select coal and rock, pressurize and saturate with water to obtain water-saturated coal and rock, weigh it, and perform nuclear magnetic resonance T2 spectrum measurement using a 25 mm sample tube and a sample tube used for isothermal adsorption, respectively; s6. The water-saturated coal and rock were air-dried to obtain coal and rock with different water saturation. The nuclear magnetic T2 spectra of the two sample tubes were measured in step s5 for each water saturation point to obtain the peak area of ​​the bound water contained in the core at different water saturation states on the T2 spectrum. s7. The mass of bound water contained at each water saturation point was calculated by measuring the T2 NMR spectrum and weight of each water saturation point of the coal and rock using a 25mm sample tube. s8. Based on the bound water mass and peak area at each water saturation point in coal and rock, the relationship between the bound water hydrogen content and the NMR signal is established through linear fitting; s9. Based on the hydrogen content obtained from the conversion of the total amount of adsorbed methane signal at each pressure point in step s8, the conversion between the NMR signal of adsorbed methane at each pressure point and the volume of methane under standard conditions is completed; s10. Based on the volume of adsorbed methane per unit mass of coal at each pressure point under standard conditions obtained in step s9, the pressure and the volume of adsorbed methane per unit mass of coal under standard conditions are fitted by a Langevin curve to obtain the evaluation parameters of the coal reservoir.

2. The laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement as described in claim 1, characterized in that, In step s2, the air drying process is as follows: the plunger sample is placed in a drying oven and air-dried at 80°C, and weighed using a high-precision balance, and the mass after air drying is recorded.

3. The laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement as described in claim 2, characterized in that, In step s4, the steps of the isothermal adsorption NMR assay are as follows: after the air-dried plunger sample is placed into the sample cylinder, it is evacuated and the NMR base signal of the sample is measured. Then, methane gas is injected step by step according to the designed pressure points, and its NMR T2 signal is measured, with at least 8 pressure points.

4. The laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement as described in claim 3, characterized in that, In step s5, the echo time is 0.06 when measuring with a 25mm sample tube. Other parameters are the same as those used when measuring saturated coal and rock with the sample tube used for isothermal adsorption, and are the same as those used in step s4.

5. The laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement as described in claim 4, characterized in that, In step s7, the formula for the mass of bound water at each water saturation point is: ； In the formula, The mass of bound water contained in coal and rock. The peak area of ​​bound water in the T2 spectrum was measured for a 25mm coal and rock sample tube. The T2 peak area of ​​a 25mm sample tube was measured after the coal and rock were completely dried. The peak area of ​​movable water in the T2 spectrum of a 25mm coal and rock sample tube. The mass of coal and rock when it contains water. This refers to the quality of the coal and rock in their dried state.

6. The laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement as described in claim 5, characterized in that, Step s8 specifically includes: ； In the formula, The peak area of ​​bound water in the T2 NMR spectrum was measured for a 70 mm sample tube. These are the fitting coefficients. To bind water mass; Based on the total NMR T2 spectrum signal of adsorbed methane at each pressure point measured in step s4, the conversion between the NMR signal of adsorbed methane at each pressure point and the hydrogen content of adsorbed methane is completed, including: The hydrogen content of adsorbed methane in coal at each pressure point can be obtained using the following formula: ； In the formula, This refers to the hydrogen content of adsorbed methane in coal and rock. The peak area of ​​methane adsorption in coal and rock. The area of ​​the signal peak in the coal and rock basement; represents the fitting coefficient.

7. The laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement as described in claim 6, characterized in that, In step s9, the volume of adsorbed methane per unit mass of coal rock under standard conditions is calculated using the following formula: ； In the formula, This refers to the content of adsorbed methane in coal and rock. This refers to the hydrogen content of adsorbed methane in coal and rock. This refers to the core quality.

8. The laboratory method for calculating adsorbed gas based on isothermal adsorption-nuclear magnetic resonance co-measurement as described in claim 7, characterized in that, In step s10, the Langmuir volume and Langmuir pressure of the core are obtained by nonlinear fitting using the Langmuir equation: ； In the formula, This represents the volume of adsorbed methane per unit mass of coal under standard conditions. For gas pressure, For the Langevin volume, This is the pressure of the Langevin.

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