Method for testing content of adsorbed gas in coal bed gas diffusion process

Abstract

The invention belongs to the technical field of oil-gas field development, and particularly relates to a method for testing the content of adsorbed gas in a coal bed gas diffusion process. The problem that the desorption degree of coal bed gas cannot be clearly known at present is solved. According to the technical scheme, high-temperature and high-pressure isothermal adsorption experiments under the condition of different gas proportions are carried out, methane adsorption-desorption capacity of mixed gas of different proportions under different partial pressures is determined, gas diffusion experiments are carried out to evaluate gas migration capacity in the diffusion process and methane gas partial pressure distribution in a sample, and different experiment results are synthesized to determine the methane gas content of the sample. The desorbed gas content in the coal bed gas diffusion process can be calculated; according to the new method, the proportion of the methane desorption gas amount in the coal bed gas development process can be calculated, and the desorption degree of the adsorbed gas of the coal bed reservoir is evaluated.

Description

Technical Field

[0001] The present invention belongs to the field of oil and gas field development, and particularly relates to a method for testing the content of adsorbed gas during the diffusion process of coalbed methane. Background Art

[0002] With the increasing depletion of conventional energy sources, the efficient development of unconventional energy sources has become the key to ensuring China's strategic energy security. As a typical unconventional energy source, the main difference between coalbed methane and conventional gas reservoirs lies in that the gas occurrence modes in coal rocks are divided into adsorbed gas and free gas, resulting in a long development cycle and low recovery rate of coalbed methane wells. Currently, isothermal adsorption experiments (volumetric method, gravimetric method) are usually used to restore the adsorption-desorption process of gas in coal rocks, and gas diffusion experiments are used to analyze the migration of gas in coal rocks. However, in the actual production process, gas desorption and diffusion are continuous processes, and evaluating gas desorption and diffusion separately cannot directly reflect the actual reservoir development status. Therefore, it is particularly important to design a method for testing the content of adsorbed gas during the diffusion process of coalbed methane. While clarifying the gas desorption-diffusion law in coal rocks, it can directly reflect the development status of the actual reservoir, formulate targeted production increase measures, shorten the development cycle of coalbed methane, and improve the recovery rate of coalbed methane. Summary of the Invention

[0003] The object of the present invention is to provide a method for testing the content of adsorbed gas during the diffusion process of coalbed methane to solve the problem of unclear understanding of the development stage in coalbed methane development. By carrying out high-temperature and high-pressure isothermal adsorption experiments and gas diffusion experiments under different gas ratios, and comprehensively analyzing different experimental results, the content of adsorbed gas during the diffusion process of coalbed methane can be calculated; in the isothermal adsorption experiment, the methane desorption amount under the same pressure and different methane partial pressures can be obtained, and combined with the flow rate test in the diffusion experiment, the proportion of the content of adsorbed gas in methane diffusion can be determined; the new method in the present invention can calculate the proportion of methane desorbed gas volume during the development process of coalbed methane and evaluate the desorption degree of adsorbed gas in coalbed reservoirs.

[0004] To achieve the above object, a method for testing the content of adsorbed gas during the diffusion process of coalbed methane according to the present invention includes the following steps:

[0005] First step, prepare a coal rock powder sample, carry out high-temperature and high-pressure isothermal adsorption-desorption experiments to obtain the characteristic curve of methane desorption amount changing with pressure;

[0006] Second step, configure methane and helium mixed gases with different ratios, carry out high-temperature and high-pressure isothermal adsorption-desorption experiments to obtain the curve of methane desorption amount changing with gas ratio under the same pressure;

[0007] In the third step, conduct methane diffusion experiments on columnar coal and rock samples, monitor the variation law of methane concentration at both ends of the sample cylinder over time, analyze the change in methane diffusion flow rate and methane partial pressure per unit time, and calculate the proportion of desorbed gas volume in the methane diffusion flow rate per unit time;

[0008] In the above method for testing the adsorbed gas content during the coalbed methane diffusion process, the step of obtaining the characteristic curve of methane desorption volume varying with pressure is as follows:

[0009] In the first step, use a crusher and sieve to make the coal and rock sample into coal and rock powder with a mesh size of 40 - 60 as the test sample;

[0010] In the second step, for the physical experiment, use a high-temperature and high-pressure adsorption - permeation combined testing instrument for testing. The known volume of the reference cylinder of the equipment is V r , and set the experimental conditions through similarity criterion conversion according to the actual situation in the formation;

[0011] In the third step, load the powdered coal and rock sample into the sample cylinder. After evacuating the sample cylinder and the reference cylinder to vacuum, introduce helium gas into the reference cylinder, and record the first equilibrium pressure of the reference cylinder as p h1 , and the first equilibrium pressure of the sample cylinder as p h2 . After the pressure stabilizes, open the connecting valve to connect the sample cylinder and the reference cylinder. After the pressure stabilizes again, record the pressure of the reference cylinder p h3 and the pressure of the sample cylinder p h4 after the second equilibrium;

[0012] In the fourth step, based on the known volume of the reference cylinder and the pressures before and after equilibrium, use the material balance principle combined with the state equation to obtain the free space volume in the sample cylinder,

[0013]

[0014] where V f is the free space volume, cm 3 ; V r is the volume of the reference cylinder, cm 3 ; p h1 is the first equilibrium pressure of the reference cylinder, MPa; p h2 is the first equilibrium pressure of the sample cylinder, MPa; p h3 is the second equilibrium pressure of the reference cylinder, MPa; p h4 is the second equilibrium pressure of the sample cylinder, MPa; T1 is the first equilibrium temperature, K; T2 is the second equilibrium temperature, K; Z h1 is the compression factor of the gas in the reference cylinder at the first equilibrium, dimensionless; Z h2 is the compression factor of the gas in the sample cylinder at the first equilibrium, dimensionless; Z h3 is the compression factor of the gas in the reference cylinder at the second equilibrium, dimensionless;h4 is the compression factor of the gas in the sample cylinder at the second equilibrium, dimensionless;

[0015] Step 5: Evacuate the sample cylinder and the reference cylinder to vacuum again. Introduce methane into the reference cylinder until its pressure reaches the initial predetermined pressure, and perform the first pressure stabilization. After the pressure stabilization, open the connecting valve to connect the sample cylinder and the reference cylinder, and perform the second pressure stabilization. Record the current pressure P1 after the second pressure stabilization;

[0016] Step 6: Close the connecting valve, open the valve of the reference cylinder, slowly release some gas, then close the valve of the reference cylinder. After the pressure is balanced, open the connecting valve. After the pressure in the sample cylinder reaches the predetermined pressure, close the connecting valve. Record the current pressure P2 after the pressure stabilization;

[0017] Step 7: Repeat Step 6 until the measurement of the last pressure point is completed. Use the Langmuir theoretical model for non-linear fitting calculation to obtain V L , P L and ρ a , and calculate the excess adsorption amount and the absolute adsorption amount accordingly.

[0018]

[0019] In the formula, is the excess adsorption amount, cm 3 / g; V L is the Langmir volume, cm 3 / g; n abs is the absolute adsorption amount, cm 3 / g; P i is the pressure at the i-th adsorption point, MPa; P L is the Langmir pressure, MPa; ρ g，i is the density of free-phase methane at the i-th pressure point, g / cm 3 ; ρ a is the density of the adsorbed phase, g / cm 3 ;

[0020] In the above method for testing the adsorbed gas content during the diffusion process of coalbed methane, the steps for obtaining the curve of methane desorption amount varying with gas ratio at the same pressure are as follows:

[0021] Step 1: Select the same test sample as used in the methane adsorption-desorption experiment. Use a crusher and a sieve to make the coal rock sample into coal rock powder with a mesh size of 40 - 60 as the test sample;

[0022] Step 2: Select helium-methane mixed gases with different ratios. Refer to the test steps of the methane adsorption-desorption experiment during the test process to obtain the curve of methane desorption amount varying with methane partial pressure n c (Pc )

[0023] In the above method for testing the adsorbed gas content during the coalbed methane diffusion process, the step of obtaining the proportion of the desorbed gas amount in the methane diffusion flow rate per unit time is as follows:

[0024] First step: After loading the columnar coal rock into the sample cylinder, evacuate it, and then introduce methane. After reaching the test pressure, close the reference cylinder to saturate the columnar core with methane. At the same time, introduce helium into the helium chamber until the test pressure is reached and then close the helium valve;

[0025] Second step: Open the connecting valve between the sample cylinder and the helium chamber, and use a gas chromatograph to monitor the change law of the methane concentration in the helium chamber over time in real time. Calculate the methane diffusion flow rate per unit time (Equation 4) and the partial pressures of methane in the methane chamber and the helium chamber (Equations 5 and 6) accordingly.

[0026]

[0027] In the formula, Q—the methane flow rate per unit time, cm 3 ·s -1 ; C1, C2—the methane concentrations in the methane chamber and the helium chamber, mol·cm -3 ; H1, H2—the helium concentrations in the methane chamber and the helium chamber, mol·cm -3 ; v—the volume of the helium chamber, cm 3 ; t—time, s; P, P 1,C , P 2,C —the system pressure, the partial pressures of methane in the methane chamber and the helium chamber per unit time, MPa;

[0028] Third step: Combining the curve of the methane desorption amount versus the methane partial pressure V c (P c ) under the same pressure, the methane desorbed gas amount during the gas diffusion process per unit time can be expressed by Equation (7), and the proportion of the methane desorbed gas amount is expressed by Equation (8).

[0029]

[0030] In the formula, V—the methane desorbed gas amount per unit time, cm 3 ·s -1 ; V c —the methane desorption amounts under different pressure differences, cm 3 ; a—the proportion of the methane desorbed gas amount,

[0031] Compared with the prior art, the present invention has the following beneficial effects: (1) The method can simultaneously complete the tests of the gas adsorption amount and the gas desorbed gas amount; (2) The method is convenient, effective, and has high working efficiency; (3) The method can clarify the proportion of the adsorbed gas amount during the methane diffusion process. Description of the Drawings

[0032] In the drawings:

[0033] Figure 1 is the overall technical roadmap of the method.

[0034] Figure 2 is the equipment flowchart of the method.

[0035] Figure 3 is a schematic diagram of the methane diffusion amount and the proportion of desorbed gas within different diffusion times. Detailed Embodiment

[0036] The present invention will be further described below in conjunction with the embodiments and the drawings;

[0037] The present invention provides a method for testing the adsorbed gas content during the diffusion process of coalbed methane, Figure 1 is the overall technical roadmap of the method, Figure 2 is the equipment flowchart of the method. This method includes the following steps:

[0038] First step, prepare a coal-rock powder sample, conduct a high-temperature and high-pressure isothermal adsorption-desorption experiment on methane, and draw a curve of the methane desorption amount versus pressure;

[0039] Second step, prepare a coal-rock powder sample, configure methane and helium mixed gases with different ratios, conduct a high-temperature and high-pressure isothermal adsorption experiment, and draw a curve of the methane desorption amount versus the gas ratio at the same pressure;

[0040] Third step, prepare a columnar coal-rock sample, conduct a methane diffusion experiment, monitor the variation law of the methane concentration at both ends of the sample cylinder over time, and combine with the curve of the methane desorption amount versus the gas ratio to calculate the proportion of the desorbed gas amount in the gas diffusion amount per unit time;

[0041] Furthermore, the steps of the high-temperature and high-pressure isothermal adsorption-desorption experiment are as follows:

[0042] First step, use a crusher and a sieve to make a 40-60 mesh coal-rock powder from the same type of columnar core as the gas adsorption test sample;

[0043] Second step, in a physical experiment, it is necessary to truly reflect the actual formation situation. For the physical simulation experiment, a high-temperature and high-pressure adsorption-permeation combined test instrument is selected for testing, and the experimental conditions are set by converting through similarity criteria according to the actual situation in the formation;

[0044] Third step, put the coal-rock powder into the sample cylinder, evacuate it, introduce 1 MPa helium into the reference cylinder and stabilize the pressure. After the pressure is stabilized, record the first equilibrium pressure of the reference cylinder as p h1 , and the first equilibrium pressure of the sample cylinder as p h2, open the connecting valve to connect the sample cylinder and the reference cylinder. After the pressure stabilizes again, record the pressure p of the reference cylinder after the second equilibrium h3 and the pressure p of the sample cylinder h4 ;

[0045] Step 4: Based on the known volume of the reference cylinder and the pressures before and after equilibrium, use the material balance principle combined with the state equation to obtain the free space volume in the sample cylinder. The calculation results are shown in Table 1;

[0046] Table 1 Calculation results of the free space volume of the sample cylinder

[0047]

[0048] Step 5: Evacuate the sample cylinder and reference cylinder system again, heat it up to 100 °C, introduce 60 MPa of methane into the reference cylinder, and perform the first pressure stabilization. After recording the stable pressure, open the connecting valve and perform the second pressure stabilization. After stabilization, record the test pressure again. This pressure is the test pressure P1;

[0049] Step 6: Close the connecting valve, slowly release some gas from the reference cylinder, then close the reference cylinder. After the pressure stabilizes, open the connecting valve to make the pressure of the sample cylinder reach the predetermined test pressure and stabilize. After the pressure stabilizes, record the test pressure P2;

[0050] Step 7: Repeat Step 6 until the determination of the last pressure point is completed. Use the Langmuir theoretical model for non-linear fitting calculation to obtain V L , P L and ρ a , and calculate the excess adsorption amount and the absolute adsorption amount based on this. The calculation results are shown in Table 2;

[0051] Table 2 Calculation results of the excess adsorption amount and the absolute adsorption amount

[0052]

[0053] Furthermore, the experimental steps for the high-temperature and high-pressure isothermal adsorption of the methane and helium mixed gas are as follows:

[0054] Step 1: Select methane-helium mixed gases with different ratios, repeat the gas adsorption amount test process, and obtain the curve of methane desorption amount versus methane partial pressure n c (P c ). Taking 30 MPa as an example, the test results are shown in Table 3;

[0055] Table 3 Curve of methane desorption amount versus methane partial pressure at 30 MPa

[0056]

[0057] Further, the steps of the methane diffusion experiment and the calculation of the proportion of desorbed gas are as follows:

[0058] In the first step, measure the length and diameter of the columnar coal rock sample. After replacing the sample in the sample cylinder with the columnar coal rock sample, evacuate the sample cylinder, and then introduce 30 MPa of methane into the sample cylinder to saturate the columnar coal rock sample. Close the reference cylinder, introduce 30 MPa of helium into the helium chamber, and introduce 30 MPa of methane into the methane chamber.

[0059] In the second step, connect the sample cylinder with the methane chamber and the helium chamber, and monitor the change of methane concentration in the helium chamber in real time, so as to calculate the methane diffusion flow rate per unit time and the methane partial pressures in the methane chamber and the helium chamber. The test results of methane concentration are shown in Table 4.

[0060] Table 4 Test results of methane concentration at different times

[0061] Time (s) <![CDATA[Helium chamber methane concentration (mol·cm -3 )]]> <![CDATA[Indoor methane concentration of methane (mol·cm -3 )]]> 1 <![CDATA[4.46×10 -7 > <![CDATA[4.46×10 -3 > 15 <![CDATA[5.35×10 -6 > <![CDATA[4.45×10 -3 > 29 <![CDATA[1.56×10 -5 > <![CDATA[4.44×10 -3 > 39 <![CDATA[2.59×10 -5 > <![CDATA[4.43×10 -3 > 54 <![CDATA[4.10×10 -5 > <![CDATA[4.42×10 -3 > 76 <![CDATA[6.47×10 -5 > <![CDATA[4.40×10 -3 > 90 <![CDATA[8.03×10 -5 > <![CDATA[4.38×10 -3 > 105 <![CDATA[9.81×10 -5 > <![CDATA[4.36×10 -3 > 117 <![CDATA[1.12×10 -4 > <![CDATA[4.35×10 -3 >

[0062] In the third step, draw the curve of the proportion of gas desorption amount changing with time during the gas diffusion process. The calculation results of the proportion of methane gas desorption amount at different times are shown in Table 5.

[0063] Table 5 Methane desorbed gas volume and proportion of desorbed gas at different times

[0064] Serial number Time (s) <![CDATA[Gas diffusion amount (g / cm 3 )]]> Proportion of desorbed gas during diffusion process (%) 1 1 0.05 0.8 2 15 0.62 1.5 3 29 1.78 2.9 4 39 2.95 4.1 5 54 4.68 6.3 6 76 7.40 9.2 7 90 9.12 11.5 8 105 11.05 13.8 9 117 12.50 15.0

[0065] Compared with the prior art, the present invention has the following beneficial effects: Compared with the prior art, the present invention has the following beneficial effects: (1) The method has strong adaptability and a wider application range; (2) The method is convenient and effective, and has high working efficiency; (3) Comprehensive calculation of multiple factors, and the calculation results have high accuracy.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: The present invention can still be modified or equivalently replaced, and any modification or partial replacement without departing from the spirit and scope of the present invention should be covered by the scope of the claims of the present invention.

Claims

1. A method for testing the content of adsorbed gas during the diffusion process of coalbed methane, characterized in that, It includes the following steps: S100. Conduct methane adsorption - desorption experiments on powdered coal - rock samples to obtain the characteristics of methane gas desorption volume varying with pressure. The test steps are as follows: S101. Prepare the test coal - seam samples into 40 - 60 - mesh sample powder as the test samples. S102. Put the test samples into the sample cylinder, open connecting valves 1 and 6, close the rest of the connecting valves, then evacuate to vacuum. Close connecting valves 1 and 6, open connecting valve 4 and the helium control valve, introduce helium into the reference cylinder. After reaching the predetermined pressure, close connecting valve 4 and the helium control valve, open connecting valve 1, and calculate the free - space volume of the sample cylinder according to equation (1). where, V f is the free space volume, cm 3 ; V r is the reference cylinder volume, cm 3 ; p h1 is the first equilibrium pressure of the reference cylinder, MPa; p h2 is the first equilibrium pressure of the sample cylinder, MPa; p h3 is the second equilibrium pressure of the reference cylinder, MPa; p h4 is the second equilibrium pressure of the sample cylinder, MPa; T1 is the first equilibrium temperature, K; T2 is the second equilibrium temperature, K; Z h1 is the compressibility factor of the gas in the reference cylinder at the first equilibrium, dimensionless; Z h2 is the compressibility factor of the gas in the sample cylinder at the first equilibrium, dimensionless; Z h3 is the compressibility factor of the gas in the reference cylinder at the second equilibrium, dimensionless; Z h4 is the compressibility factor of the gas in the sample cylinder at the second equilibrium, dimensionless; S103. Close all connecting valves, then open connecting valves 1 and 6. After evacuating the sample cylinder and the reference cylinder to vacuum, close connecting valves 1 and 6, open connecting valves 2 and the methane control valve, introduce methane into the reference cylinder. After the pressure reaches the target test pressure, close all connecting valves, open connecting valve 1. When the pressure in the sample cylinder is stable, calculate the methane adsorption amount at the current pressure. Then open the vent valve, reduce the pressure in the sample cylinder to the next predetermined value, close the vent valve, and calculate the methane adsorption amount at the next pressure. The difference in methane adsorption amounts at adjacent test pressures is the methane desorption amount. S200. Conduct adsorption - desorption experiments under different - proportion mixed gases (methane, helium) to obtain the variation law of the maximum adsorption amount of different - proportion mixed gases with the gas proportion, and clarify the variation law of methane desorption amount with pressure under different partial pressures. The steps are as follows: S201. Prepare the test coal - seam samples into 40 - 60 - mesh sample powder as the test samples. S202. Close all connecting valves, then open connecting valves 1 and 6. After evacuating the reference cylinder and the sample cylinder to vacuum, close all connecting valves, open the methane control valve and the helium control valve. After adjusting the gas - pressure proportion, close the methane control valve and the helium control valve, open connecting valves 2 and 4. After the pressure is stable, close connecting valves 2 and 4. At this time, the gas in the reference cylinder is a mixed gas of methane and helium. Open connecting valves 2 and 4, configure different mixed gases (methane, helium) in the reference cylinder, and record the stable pressure after the pressure is stable. S203. Repeat steps S102 and S103 to obtain the curve nc(Pc) of methane desorption amount varying with methane partial pressure. S300. After evacuating the sample cylinder to a vacuum state, saturate it with methane at the test pressure, conduct a methane diffusion experiment, monitor the variation law of methane concentration at both ends of the sample cylinder with time, analyze the variation of methane diffusion flow rate and methane partial pressure per unit time, and calculate the proportion of desorbed gas amount in the methane diffusion flow rate per unit time. The steps are as follows: S301. Prepare a plunger - shaped sample. After measuring the length L and diameter r of the coal - rock sample, put it into the sample cylinder. S302. Close all connecting valves, open connecting valve 6, evacuate the sample cylinder to a vacuum state, then close connecting valve 6, open the methane control valve and connecting valve 3, make the pressure in the sample cylinder reach the test pressure, and let it stand until the pressure in the sample cylinder is stable. S303. Close all connecting valves, open the helium control valve, and after the pressure in the helium chamber reaches the pressure of the sample cylinder, close all connecting valves, open connecting valves 3 and 5, and use a gas chromatograph to test the methane gas content in the helium chamber and the methane chamber at different diffusion times. Record the methane concentration C1 in the methane chamber, the methane concentration C2 in the helium chamber, the helium concentration H1 in the methane chamber, and the helium concentration H2 in the helium chamber. Use the ratio of the methane concentration to the helium concentration to characterize the methane partial pressures at both ends of the sample cylinder. The volumes of the methane chamber and the helium chamber are known to be the same, V. The calculation method for the methane diffusion flow rate per unit time is given by Equation (2), and the expression methods for the methane partial pressures at both ends of the sample cylinder are given by Equation (3) and Equation (4). where Q is the methane flow rate per unit time, cm 3 ·s -1 ; C1 and C2 are the methane concentrations in the methane chamber and the helium chamber, respectively, mol·cm -3 ; H1 and H2 are the helium concentrations in the methane chamber and the helium chamber, respectively, mol·cm -3 ; v is the volume of the helium chamber, m 3 ; t is the time, s; P, P 1,C and P 2,C are the system pressure, the partial pressure of methane in the methane chamber per unit time, and the partial pressure of methane in the helium chamber, respectively, MPa; S304, combined with the methane desorption amount curve V varying with methane partial pressure under the same pressure c (P c ), during the gas diffusion process per unit time, the methane desorbed gas amount can be expressed by Equation (5), and the proportion of the methane desorbed gas amount is expressed by Equation (6). where V is the methane desorbed gas volume per unit time, cm 3 ·s -1 ; V c is the methane desorbed volume under different pressure differences, cm 3 ; a is the proportion of the methane desorbed gas volume.

2. The testing method for the adsorbed gas content during the coalbed methane diffusion process according to claim 1, wherein The calculation method for the amount of desorbed gas during the methane diffusion process is the integral of the methane partial pressures at both ends of the sample cylinder in the methane desorption amount curve under different methane partial pressures.

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