Method for determining reasonable injection pressure of coal seam gas by phase change of liquid carbon dioxide

Abstract

The application discloses a method for determining a reasonable displacement pressure of liquid carbon dioxide phase change displacement of coal seam gas, and specifically comprises the following steps: S1, performing an isothermal adsorption experiment on a coal sample of a target coal seam; S2, respectively calculating virtual saturated vapor pressures of CO2 and CH4 gas; S3, establishing a relationship between gas adsorption potential and gas adsorption equilibrium pressure; S4, respectively calculating densities of CO2 and CH4; S5, respectively calculating adsorption spaces of CO2 and CH4 in a coal body; S6, establishing a relationship formula between adsorption potentials of the two kinds of gas and the adsorption spaces; S7, determining a reasonable displacement pressure; S8, determining a reasonable displacement pressure; S9, calculating a CO2 percolation coefficient; S10, determining a reasonable percolation pressure; and S11, determining a reasonable injection pressure. The method provides scientific guidance for popularization and application of the liquid CO2 phase change displacement technology of CH4 in a coal seam.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of coal mine gas control, and particularly relates to a method for determining reasonable pressure injection pressure of liquid carbon dioxide phase change displacement of coal seam gas. BACKGROUND

[0002] More than 95% of the coal seams mined in China belong to low-permeability coal seams, and the permeability is mostly 10 -6 ~ 10 -7 μm 2 . In order to improve the coal seam gas extraction rate and shorten the pre-extraction time, artificial permeability improvement must be implemented. Since the 1970s, various coal seam pressure relief and permeability improvement and gas enhanced extraction technologies have been tested in many mining areas in China, including hydraulic fracturing, water pressure cutting, gas pressure pulse cracking, liquid CO2 phase change blasting, directional energy-gathering blasting, high-voltage electric pulse controllable shock wave blasting, etc., and certain application effects have been achieved. However, compared with the above coal seam permeability improvement technologies, liquid CO2 phase change displacement of coal seam CH4 has the dual gas enhanced extraction effects of fracturing permeability improvement of coal and rock and phase change displacement of gas, and is a research hotspot of current low-permeability coal seam fracturing reconstruction technology. However, compared with the mature application of liquid CO2 in oil and gas production, the application of liquid CO2 in coal mine underground is still in the experimental exploration stage, and relatively few engineering cases are available, and relevant standards and specifications have not been formed. In particular, the determination of reasonable liquid CO2 pressure injection pressure of a single hole has become the primary problem that puzzles technical personnel on site. If the pressure injection pressure is too low, the coal seam CH4 extraction efficiency is affected, and if the pressure injection pressure is too high, an accident may occur, and even personnel and property losses may be caused. Therefore, the determination of reasonable liquid CO2 pressure injection pressure of a single hole has become a key factor restricting whether the liquid CO2 phase change displacement of coal seam CH4 technology can enter industrial application. SUMMARY

[0003] The purpose of the application is to provide a method for determining reasonable pressure injection pressure of liquid carbon dioxide phase change displacement of coal seam gas, which can accurately control the reasonable pressure injection pressure of liquid CO2, save pressure injection cost, improve the coal seam CH4 extraction efficiency, and provide scientific guidance for the popularization and application of the liquid CO2 phase change displacement of coal seam CH4 technology.

[0004] The technical scheme adopted by the application is that the method for determining reasonable pressure injection pressure of liquid carbon dioxide phase change displacement of coal seam gas comprises the following specific steps:

[0005] S1, performing isothermal adsorption experiment on the target coal seam coal sample;

[0006] S2, calculating the virtual saturation vapor pressure of CO2 and CH4 gas respectively

[0007] S3, establish CO2 gas adsorption potential relationship with CO2 gas adsorption equilibrium pressure ; establish CH4 gas adsorption potential relationship with CH4 gas adsorption equilibrium pressure ;

[0008] S4, respectively calculate CO2, CH4 density

[0009] S5, respectively calculate CO2, CH4 in the coal body inside the adsorption space

[0010] S6, establish CO2 gas adsorption potential relationship with adsorption space ; establish CH4 gas adsorption potential relationship with adsorption space ;

[0011] S7, determine reasonable displacement pressure P 置 ;

[0012] S8, determine reasonable displacement pressure P 驱 ;

[0013] S9, calculate CO2 permeation coefficient

[0014] S10, determine reasonable permeation pressure P 渗 ;

[0015] S11, determine reasonable injection pressure P 注 .

[0016] The features of the present application are also characterized by:

[0017] The coal sample is made into a standard coal column with a size of ;

[0018] S1.2, according to the "Coal High Pressure Isothermal Adsorption Test Method GB / T 19560-2008", single-component CO2 and CH4 gas isothermal adsorption experiment is carried out on the target coal seam coal sample in the laboratory by using static capacity method, and the adsorption amount of the coal sample to single-component CO2 and CH4 gas under different injection pressures can be obtained respectively

[0019] S2 is as follows:

[0020] The virtual saturation vapor pressure of CO2 and CH4 gas is calculated under supercritical conditions Specifically as shown in formula (1), formula (2):

[0021]

[0022]

[0023] wherein: —CO2 virtual saturated vapor pressure;

[0024] —CH4 virtual saturated vapor pressure;

[0025] T — equilibrium temperature, K;

[0026] —CO2 gas critical pressure, MPa;

[0027] —CH4 gas critical pressure, MPa;

[0028] —CO2 critical temperature, K;

[0029] —CH4 critical temperature, K.

[0030] S3, CO2, CH4 gas adsorption potential and CO2, CH4 gas adsorption equilibrium pressure The relationship is shown in the following formula (3), formula (4):

[0031]

[0032]

[0033] wherein: —CO2 gas adsorption potential, J / mol;

[0034] —CH4 gas adsorption potential, J / mol;

[0035] —CO2 adsorption equilibrium pressure, MPa;

[0036] —CH4 adsorption equilibrium pressure, MPa;

[0037] —CO2 virtual saturated vapor pressure, MPa;

[0038] —CH4 virtual saturated vapor pressure, MPa;

[0039] —CO2 gas equilibrium pressure at constant temperature, MPa;

[0040] Equilibrium pressure of CH4 gas at constant temperature, MPa

[0041] Absolute temperature of CO2, K

[0042] Absolute temperature of CH4, K

[0043] In S4:

[0044] Density of CO2, CH4 Calculated from formula (5), formula (6):

[0045]

[0046]

[0047] In the formula: Density of CO2, g / cm 3 ;

[0048] Density of CH4, g / cm 3 ;

[0049] Molecular weight of CO2 gas molecule, g / mol

[0050] Molecular weight of CH4 gas molecule, g / mol

[0051] Critical pressure of CO2 gas, MPa

[0052] Critical pressure of CH4 gas, MPa

[0053] Critical temperature of CO2 gas, K

[0054] Critical temperature of CH4 gas, K

[0055] R - Gas molar constant, take 8.314 J·mol -1 ·K -1 ;

[0056] In S5, first, calculate the adsorption space of CO2 and CH4 in the coal body from formula (7), formula (8)

[0057]

[0058]

[0059] In the formula: Adsorption space of CO2, cm 3 / g;

[0060] Adsorption space of CH4, cm 3 / g;

[0061] Absolute adsorption amount of CO2, mol / g;

[0062] Absolute adsorption amount of CH4, mol / g;

[0063] Molecular weight of CO2 gas molecules, g / mol;

[0064] Molecular weight of CH4 gas molecules, g / mol;

[0065] Density of CO2, g / cm 3 ;

[0066] Density of CH4, g / cm 3 .

[0067] S6: CO2 gas adsorption potential and adsorption space CH4 gas adsorption potential and adsorption space , as shown in formula (11), formula (12):

[0068]

[0069]

[0070] In the formula: is a constant.

[0071] S7 is specifically: by comparing step S6, the adsorption characteristic curve of the displacement gas CO2 and the displaced gas CH4 is obtained, and when the adsorption potential of the displacement gas is higher than that of the displaced gas, the injection pressure interval is obtained. The pressure interval is a reasonable displacement pressure interval;

[0072] S8 is specifically: CO2 displacement coal rock CH4 physical simulation experiment is carried out on the target coal sample, the total injection amount of single-component CH4 gas under different injection pressures and the total injection amount of single-component CO2 gas under different pressures are obtained. The total flow rate q of the outlet mixed gas, the total injection amount of CO2 gas CO2 production CH4 production and displacement time t; the displacement efficiency η of CO2 displacing CH4 is calculated according to formula (13), (14) q and displacement replacement ratio μ q Formula (13), (14) are as follows:

[0073]

[0074]

[0075] In the formula: η q -CO2 displacement CH4 efficiency, %;

[0076] -CH4 production, mL;

[0077] -CH4 total injection amount, mL;

[0078] -outlet CH4 concentration, %;

[0079] q-outlet mixed gas total flow, mL;

[0080] μ q -displacement replacement ratio;

[0081] -CO2 total injection amount, mL;

[0082] -CO2 production, mL;

[0083] t-displacement time, min.

[0084] According to formula (13), formula (14), the displacement efficiency and the displacement replacement ratio can be calculated, according to the displacement pressure, the displacement efficiency can be divided into three intervals, the displacement efficiency < 50% is the displacement low efficiency stage; the displacement efficiency is 50% ~ 60% is the displacement medium efficiency stage; the displacement efficiency > 60% is the displacement high efficiency stage; the injection pressure in the displacement medium efficiency stage and the displacement high efficiency stage is selected as the reasonable displacement pressure interval.

[0085] S9 is specifically:

[0086] According to Darcy's law, the flow of CO2 in the circular coal pillar is represented by formula (15):

[0087]

[0088] In the formula: Q-flow of CO2 through the coal sample, m 3 / s;

[0089] k - permeability coefficient of CO2 in coal, 10 -6 m 2 / Pa·s;

[0090] A - cross-sectional area of sample, m 2 ;

[0091] μ - dynamic viscosity of CO2, Pa·s;

[0092] P in - CO2 gas pressure at inlet, MPa;

[0093] P out - CO2 gas pressure at outlet, MPa;

[0094] When the seepage medium is CO2, its seepage velocity in the coal fracture is expressed as formula (16) as follows:

[0095]

[0096] In the formula: μ a - dynamic viscosity of CO2, Pa·s.

[0097] The permeability coefficient of CO2 in coal can be expressed by the slope in the velocity expression, as shown in formula (17):

[0098]

[0099] During the seepage experiment, the average pressure corresponding to the flow rate Q of CO2 through the coal sample is (P in + P out ) / 2, and the relationship between the outlet flow rate Q0 at atmospheric pressure P s and the flow rate Q in the coal sample is obtained from the ideal gas state equation, as shown in formula (18):

[0100]

[0101] In the formula: P s - atmospheric pressure, MPa;

[0102] Q0 - CO2 gas volume flow rate under standard state, m 3 / s.

[0103] By combining formula (15), (16), (17), and (18), formula (19) is obtained as follows:

[0104]

[0105] S10 specifically is: based on the change relation diagram of CO2 permeation coefficient with injection pressure, taking the injection pressure corresponding to the lowest point of CO2 permeation coefficient as the minimum reasonable seepage pressure P 渗 .

[0106] S11 specifically is:

[0107] According to step S7, the reasonable displacement pressure P 置 is determined, and the reasonable displacement pressure interval is determined; according to step S8, the reasonable displacement pressure P 驱 is determined, and the reasonable displacement pressure interval is determined; according to S10, the reasonable seepage pressure P 渗 is determined, and the reasonable seepage pressure interval is determined; the intersection of the pressure intervals formed by the above parameters is obtained by combining the pressure intervals formed by the above parameters, and the intersection is the threshold value of the reasonable injection pressure of carbon dioxide for driving coal seam gas.

[0108] The beneficial effects of the present application are:

[0109] The present application combines similar simulation experiments and the field tests carried out to establish a scientific determination method of the liquid carbon dioxide phase change driving coal seam gas pressure injection pressure. The determination of the reasonable pressure injection pressure of the liquid carbon dioxide phase change driving coal seam gas can effectively avoid accidents caused by too high injection pressure, and the low pressure affects the extraction efficiency of the coal seam, can greatly reduce the cost of the gas control of the working face, effectively alleviate the influence of the gas disaster on the safe and efficient production of the mine, and indirectly the safety and economic benefits are remarkable. At the same time, the research and development of the technology provides a new idea and new technology for the treatment of coal seam gas. The popularization of the technology can effectively improve the utilization rate of gas, reduce the air emission of gas, reduce the greenhouse effect, and create remarkable economic and social benefits for the enterprise. BRIEF DESCRIPTION OF DRAWINGS

[0110] Figure 1 It is the flow chart of the determination method of the reasonable pressure injection pressure of the liquid carbon dioxide phase change driving coal seam gas of the present application;

[0111] Figure 2 It is a schematic diagram of the position of the engineering test area;

[0112] Figure 3 It is a schematic diagram of the drilling arrangement;

[0113] Figure 4 It is the CH4 and CO2 adsorption curve of Huangling weakly caking coal;

[0114] Figure 5 It is the adsorption property curve of Huangling weakly caking coal;

[0115] Figure 6 It is the change rule of CO2 flow with the pressure difference between inlet and outlet;

[0116] Figure 7 the variation law of the CO2 permeability coefficient with the injection pressure;

[0117] Figure 8 a schematic diagram for determining the reasonable injection pressure threshold. DETAILED DESCRIPTION

[0118] The present application will be described in detail below in combination with the drawings and specific embodiments.

[0119] The present application provides a method for determining the reasonable injection pressure of single-hole pressure injection of coal seam CH4 by liquid CO2 phase change displacement, as shown in the following specific steps: Figure 1

[0120] S1, isothermal adsorption experiment is performed on the coal sample of the target coal seam;

[0121] S1 is specifically as follows:

[0122] S1.1, the coal sample of the target coal seam is taken, and a standard coal column with a size of is prepared;

[0123] S1.2, according to the “Coal High Pressure Isothermal Adsorption Test Method GB / T 19560-2008”, single-component CO2 and CH4 gas isothermal adsorption experiment is performed on the coal sample of the target coal seam in the laboratory by using the static capacity method, and the adsorption amount of the coal sample to the single-component CO2 and CH4 gas under different injection pressures can be obtained respectively as

[0124] S2, the virtual saturation vapor pressure of CO2 and CH4 gas is calculated respectively

[0125] The adsorption of coal to gas is established within the critical temperature, and the saturation vapor pressure exceeding the critical temperature has no physical meaning. The virtual saturation vapor pressure of CO2 and CH4 gas is calculated respectively by using the supercritical condition Specifically, as shown in the following formula (1) and formula (2):

[0126]

[0127]

[0128] In the formula: —CO2 virtual saturation vapor pressure;

[0129] —CH4 virtual saturation vapor pressure;

[0130] T—equilibrium temperature, K;

[0131] ​- critical pressure of CO2 gas, MPa (the critical pressure of CO2 is 7.38 MPa);

[0132] - critical pressure of CH4 gas, MPa (the critical pressure of CH4 is 4.62 MPa);

[0133] - critical temperature of CO2, K (the critical temperature of CO2 is 304.4 K);

[0134] - critical temperature of CH4, K (the critical temperature of CH4 is 190.6 K);

[0135] S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure; S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure; S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure; S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure; S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure;

[0136] S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure; S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure; S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure;

[0137]

[0138]

[0139] In the formula: - adsorption potential of CO2 gas, J / mol;

[0140] - adsorption potential of CH4 gas, J / mol;

[0141] - CO2 adsorption equilibrium pressure, MPa;

[0142] - CH4 adsorption equilibrium pressure, MPa;

[0143] - CO2 virtual saturated vapor pressure, MPa;

[0144] - CH4 virtual saturated vapor pressure, MPa;

[0145] - CO2 gas equilibrium pressure at constant temperature, MPa;

[0146] - equilibrium pressure of CH4 gas at constant temperature, MPa

[0147] - absolute temperature of CO2, K

[0148] - absolute temperature of CH4, K

[0149] S4, respectively calculate the CO2, CH4 density

[0150] CO2, CH4 density calculated by formula (5), formula (6):

[0151]

[0152]

[0153] In the formula: - CO2 density, g / cm 3 ;

[0154] - CH4 density, g / cm 3 ;

[0155] - molecular weight of CO2 gas molecules, g / mol

[0156] - molecular weight of CH4 gas molecules, g / mol

[0157] - critical pressure of CO2 gas, MPa

[0158] - critical pressure of CH4 gas, MPa

[0159] - critical temperature of CO2 gas, K

[0160] - critical temperature of CH4 gas, K

[0161] R - gas molar constant, take 8.314 J·mol -1 ·K -1 .

[0162] S5, respectively calculate the CO2, CH4 adsorption space in the coal body

[0163] Under certain temperature and pressure conditions, the place in coal body that can provide adsorption space for CH4 and CO2 is called adsorption space. In the adsorption space, the adsorption potential is greater than the thermal motion of gas molecules. The adsorption space of CO2 and CH4 in coal body is calculated by formula (7) and formula (8)

[0164]

[0165]

[0166] In the formula: Adsorption space of CO2, cm 3 / g;

[0167] Adsorption space of CH4, cm 3 / g;

[0168] Absolute adsorption amount of CO2, mol / g;

[0169] Absolute adsorption amount of CH4, mol / g;

[0170] Molecular weight of CO2 gas molecules, g / mol;

[0171] Molecular weight of CH4 gas molecules, g / mol;

[0172] Density of CO2, g / cm 3 ;

[0173] Density of CH4, g / cm 3 .

[0174] In the isothermal adsorption experiment, the obtained adsorption amount and is the Gibbs adsorption amount, also known as apparent adsorption amount. In the calculation of adsorption space under different temperature and pressure conditions, the absolute adsorption amount is required, that is, the apparent adsorption amount needs to be converted. The absolute adsorption amount is calculated by formula (9) and formula (10):

[0175]

[0176]

[0177] In the formula: Absolute adsorption amount of CO2, mol / g;

[0178] —CH4 absolute adsorption amount, mol / g;

[0179] —CO2 apparent adsorption amount, mol / g;

[0180] —CH4 apparent adsorption amount, mol / g;

[0181] —CO2 density under the test temperature and pressure condition, g / cm 3 ;

[0182] —CH4 density under the test temperature and pressure condition, g / cm 3 ;

[0183] —CO2 density, g / cm 3 ;

[0184] —CH4 density, g / cm 3 .

[0185] S6, establishing the relationship between CO2 gas adsorption potential and adsorption space ; establishing the relationship between CH4 gas adsorption potential and adsorption space ; Here, a cubic polynomial relationship is selected to establish the relationship between the adsorption potential and the adsorption space, as shown in equation (11) and equation (12):

[0186]

[0187]

[0188] In the formula: is a constant.

[0189] The determination of the constant is as follows: the data of adsorption potential ε and adsorption space ω are plotted by using the origin software, after the plotting is completed, the curve fitting of the adsorption potential ε and the adsorption space ω is performed by using the fitting function of the software, and the constant

[0190] of CH4 and the constant of CO2 are obtained.

[0191]

[0192] S7, determining a reasonable displacement pressure P 置 ;

[0193] ​The adsorption characteristic curves of the displacement gas C02 and the displaced gas CH4 are obtained by comparison of step S6, as shown in the figure, the injection pressure interval when the adsorption potential of the displacement gas is higher than that of the displaced gas is obtained, and the pressure interval is the reasonable displacement pressure interval. Figure 5

[0194] S8, determining the reasonable displacement pressure P 驱 ;

[0195] The CO2 displacement coal rock CH4 physical simulation experiment is performed on the target coal sample, the total injection amount of single-component CH4 gas under different injection pressures and the injection of single-component CO2 gas under different pressures are obtained, the total flow rate q of the outlet mixed gas, the total injection amount of CO2 gas CO2 production amount CH4 production amount and displacement time t are obtained; the displacement efficiency η of CO2 displacement CH4 is calculated according to formula (13) and (14) q and displacement replacement ratio μ q , formula (13) and (14) are as follows:

[0196]

[0197]

[0198] In the formula: η q -CO2 displacement CH4 efficiency, %;

[0199] -CH4 production amount, mL;

[0200] -CH4 total injection amount, mL;

[0201] -CH4 concentration at outlet, %;

[0202] q-total flow rate of outlet mixed gas, mL;

[0203] μ q -displacement replacement ratio;

[0204] -CO2 total injection amount, mL;

[0205] -CO2 production amount, mL;

[0206] t-displacement time, min.

[0207] ​The displacement efficiency and displacement replacement ratio can be calculated according to formula (13) and formula (14), and the displacement pressure can be divided into three displacement efficiency intervals, the displacement efficiency < 50% is a low displacement efficiency stage, the displacement efficiency is 50% to 60% is a medium displacement efficiency stage, and the displacement efficiency > 60% is a high displacement efficiency stage. The injection pressure in the medium displacement efficiency stage and the high displacement efficiency stage is selected as a reasonable displacement pressure interval.

[0208] S9, calculating the CO2 seepage coefficient

[0209] The CO2 seepage physical simulation experiment is performed on the target coal sample, the change of the outlet flow is monitored by setting different inlet gas pressures, the outlet flow data of the target coal sample under different inlet and outlet pressure differences are obtained, the influence law of the CO2 permeation coefficient in the coal sample under different inlet and outlet pressure differences is calculated by using formula (15) to formula (18), and the influence law is shown in formula (19), and the specific formula is as follows:

[0210] According to the Darcy law, the flow of CO2 in the circular coal column is represented by formula (15):

[0211]

[0212] In the formula, Q is the flow of CO2 through the coal sample, m 3 / s;

[0213] k is the permeation coefficient of CO2 in the coal body, 10 -6 m 2 / Pa·s;

[0214] A is the cross-sectional area of the sample, m 2 ;

[0215] μ is the dynamic viscosity of CO2, Pa·s;

[0216] P in is the CO2 gas pressure of the inlet end, MPa;

[0217] P out is the CO2 gas pressure of the outlet end, MPa.

[0218] According to formula (15), as shown in Figure 6 , the change law of the CO2 flow with the inlet and outlet pressure difference can be obtained, because the length of the target coal sample is a constant value, when the pressure gradient corresponding to the inlet and outlet pressure difference is low, the CO2 adsorption amount is large, the adsorption expansion effect causes the seepage resistance of CO2 in the coal seam fracture to be large, and the seepage speed is slow, when the pressure gradient is large, the effective stress gradually becomes small, which is beneficial to the seepage of CO2 in the coal seam.

[0219] When the seepage medium is CO2, its seepage velocity in the coal body fissure is expressed as formula (16) as follows:

[0220]

[0221] In the formula, μ a is the dynamic viscosity of CO2, Pa·s.

[0222] The permeability coefficient of CO2 in the coal body can be expressed by the slope in the velocity expression, as shown in formula (17):

[0223]

[0224] During the seepage experiment, the average pressure corresponding to the flow rate Q of CO2 through the coal sample is (P in + P out ) / 2, and the relationship between the outlet flow rate Q0 at atmospheric pressure P s and the flow rate Q in the coal sample can be obtained by the ideal gas state equation, as shown in formula (18):

[0225]

[0226] In the formula, P s is the atmospheric pressure, MPa;

[0227] Q0 is the CO2 gas volume flow rate under standard state, m 3 / s.

[0228] By combining formula (15), (16), (17) and (18), formula (19) is obtained as follows:

[0229]

[0230] According to formula (19), the relationship diagram of the permeability coefficient of CO2 in the target coal sample with the injection pressure can be drawn, as shown in Figure 7 .

[0231] S10, determining a reasonable seepage pressure P 渗 ;

[0232] Based on the relationship diagram of the CO2 permeability coefficient with the injection pressure, as shown in Figure 7As shown in the figure, it can be found that with the increase of injection pressure, the CO2 permeability coefficient first decreases and then increases. When the CO2 permeability coefficient decreases, the CO2 adsorption capacity of coal increases significantly, resulting in coal matrix adsorption expansion, which is not conducive to CO2 seepage; when the CO2 permeability coefficient increases, the CO2 adsorption capacity of coal basically reaches saturation, and with the increase of injection pressure, the CO2 flow and seepage velocity also gradually increase under the action of pressure difference, resulting in the increase of CO2 permeability coefficient, at this time the CO2 seepage effect is obvious. Therefore, the injection pressure corresponding to the lowest point of CO2 permeability coefficient is the minimum reasonable seepage pressure P 渗 .

[0233] S11, determining a reasonable injection pressure P 注 ;

[0234] According to step S7, a reasonable displacement pressure P 置 can be determined; according to step S8, a reasonable displacement pressure P 驱 can be determined; according to S10, a reasonable seepage pressure P 渗 can be determined; combining the pressure interval constructed by the above parameters, the intersection of the pressure interval constituted by the above parameters is obtained, and this intersection is the threshold value of the reasonable injection pressure of carbon dioxide for driving coal seam gas, as shown in Figure 8 .

[0235] Example 1

[0236] Shuanglong Coal Mine of Huangling Shuanglong Coal Development Co., Ltd. is located in the southwest of Huangling mining area. The liquid CO2 phase change driving CH4 engineering test area of coal seam is selected in the south side of the return air crossheading of 112 working face. The mineable coal seam is 2# coal seam, the coal measure stratum is middle-lower Jurassic Yan'an group, the coal seam structure is simple, showing single inclination, the coal seam inclination is 2°-5°, and the average inclination is 3.5°. The coal seam thickness is 3.0-3.2m, and the average thickness is 3.1m. The coal quality of 2# coal seam is low ash, medium-high volatile matter, very low sulfur, medium-low phosphorus, medium calorific value, low metamorphic degree and weak caking coal. The absolute gas emission of the mine is 37.05m 3 / min, the relative gas emission of the mine is 8.72m 3 / t, the maximum absolute gas emission of the coal mining working face is 13.55m 3 / min, and the maximum absolute gas emission of the heading working face is 2.79m 3 / min. The mine employs a comprehensive gas control method combining pre-drainage from boreholes in the coal seam, high-level fracture drainage, and depressurization drainage in the back roadway. This coal seam utilizes closely spaced boreholes for regional gas drainage, with drainage pipelines measuring Ф325mm in diameter and borehole depths ranging from 50 to 120m. The designed borehole spacing is 10m. Analysis of monitoring data on gas concentration and flow rate within the existing drainage boreholes reveals a maximum gas drainage concentration of 9.4% and an initial coal seam temperature of 30℃. The engineering test area and borehole layout are as follows. Figure 2 , Figure 3 As shown. Among them Figure 2 A schematic diagram of the engineering test area. The rectangle with diagonal stripes in the diagram represents the test area for liquid carbon dioxide-driven coalbed methane treatment. Figure 3 The diagram shows the borehole layout. The boreholes drilled in the original comparison area are the original comparison boreholes. Borehole #4 in Test Zone I, Test Zone II, and Test Zone III are injection boreholes, and the remaining boreholes are inspection boreholes. Inspection boreholes with different spacings are arranged along the injection boreholes to verify the extraction effect of coal seam gas driven by liquid carbon dioxide.

[0237] The physical property parameters of Huangling weakly caking coal were analyzed in accordance with the "Industrial Analysis Methods for Coal GB / T 212-2008". The industrial analysis table of the coal sample is detailed in Table 1.

[0238] Table 1 Industrial Analysis Parameters of Huangling Weakly Caustic Coal

[0239]

[0240] S1 was used to conduct isothermal adsorption experiments on coal samples from the target coal seam.

[0241] S1.1 Take a coal sample from the target coal seam and prepare a sample with the following dimensions: Standard coal pillar;

[0242] The adsorption of single-component CO2 and CH4 gases was tested on the target coal sample according to the standard GB / T 19560-2008, "High-Pressure Isothermal Adsorption Test Method for Coal". Coal samples were taken from the No. 2 coal seam of Shuanglong Coal Mine, and the samples were extracted using a core drilling machine and then ground into a fine powder. Cylindrical standard coal samples were selected, and those with neat end faces, no impurities / cracks, and uniform texture were screened and sent to the laboratory after oxygen isolation treatment for isothermal adsorption experiments.

[0243] S1.2 Following the "High-Pressure Isothermal Adsorption Test Method for Coal" (GB / T 19560-2008), single-component isothermal adsorption experiments of CO2 and CH4 gases were conducted on coal samples from the target coal seam using the static volumetric method in the laboratory. The adsorption capacities of the coal samples for single-component CO2 and CH4 gases under different injection pressures were obtained.

[0244] The isothermal adsorption experiment test is carried out on the weakly caking coal in Huangling, and the single-component gas adsorption experiments of CH4 and CO2 are carried out respectively, and the adsorption amounts of CH4 and CO2 of the weakly caking coal in Huangling under different injection pressures are obtained, as shown in the following table 1. Figure 4 According to the adsorption curve, the Langmuir volume and Langmuir pressure of the target coal sample to CH4 and the Langmuir volume and Langmuir pressure of the target coal sample to CO2 can be obtained.

[0245] S2, the virtual saturated vapor pressure of CO2 and CH4 gas is calculated respectively

[0246] The adsorption of coal to gas is established within the critical temperature, and the saturated vapor pressure exceeding the critical temperature has no physical meaning. Here, the virtual saturated vapor pressure of CO2 and CH4 gas is calculated respectively under the supercritical condition Specifically, as shown in formula (1) and formula (2):

[0247] In the formula: —CO2 virtual saturated vapor pressure;

[0248] —CH4 virtual saturated vapor pressure;

[0249] T—equilibrium temperature, K;

[0250] —CO2 gas critical pressure, MPa (the critical pressure of CO2 is 7.38 MPa);

[0251] —CH4 gas critical pressure, MPa (the critical pressure of CH4 is 4.62 MPa);

[0252] —CO2 critical temperature, K (the critical temperature of CO2 is 304.4 K);

[0253] —CH4 critical temperature, K (the critical temperature of CH4 is 190.6 K);

[0254] Under the experimental condition of constant temperature 303.15 K, the equilibrium temperature is 303.15 K. Therefore, the virtual saturated vapor pressure of CO2 is 7.2 MPa; the virtual saturated vapor pressure of CH4 is 11.69 MPa.

[0255] S3, the CO2 gas adsorption potential is established The relationship between the CO2 gas adsorption equilibrium pressure and the CH4 gas adsorption potential is established. The CH4 gas adsorption potential is established. Relationship between the adsorption equilibrium pressure of CH4 gas ;

[0256] Based on the adsorption potential theory, the relationship between the adsorption potential ε of the adsorbed gas and the adsorption equilibrium pressure P J of the adsorbed gas is shown in the following equations (3) and (4):

[0257]

[0258]

[0259]

[0260] In the equations: — the adsorption potential of CO2 gas, J / mol;

[0261] — the adsorption potential of CH4 gas, J / mol;

[0262] — the adsorption equilibrium pressure of CO2, MPa;

[0263] — the adsorption equilibrium pressure of CH4, MPa;

[0264] — the virtual saturation vapor pressure of CO2, MPa;

[0265] — the virtual saturation vapor pressure of CH4, MPa;

[0266] — the equilibrium pressure of CO2 gas at constant temperature, MPa;

[0267] — the equilibrium pressure of CH4 gas at constant temperature, MPa;

[0268] — the absolute temperature of CO2, K;

[0269] — the absolute temperature of CH4, K.

[0270] S4 calculates the densities of CO2 and CH4, respectively

[0271] Adsorbed phase density calculated from equations (5) and (6):

[0272]

[0273]

[0274] In the formula: — CO2 density, g / cm 3 ;

[0275] — CH4 density, g / cm 3 ;

[0276] — Molecular weight of CO2 gas molecules, g / mol

[0277] — Molecular weight of CH4 gas molecules, g / mol

[0278] — Critical pressure of CO2 gas, MPa

[0279] — Critical pressure of CH4 gas, MPa

[0280] — Critical temperature of CO2 gas, K

[0281] — Critical temperature of CH4 gas, K

[0282] R— Gas molar constant, 8.314 J·mol -1 ·K -1 .

[0283] Therefore, according to formula (3), the density of the adsorbed phase CO2 is 10.07 g / cm 3 when CO2 is the adsorbed phase; according to formula (3), the density of the adsorbed phase CH4 is 0.87 g / cm 3 when CH4 is the adsorbed phase.

[0284] S5 respectively calculates the adsorption space of CO2 and CH4 in the coal body

[0285] Under certain temperature and pressure conditions, the place in the coal body that can provide adsorption for CH4 and CO2 is called the adsorption space, and the adsorption potential energy in the adsorption space is greater than the thermal motion of the gas molecules. The adsorption spaces of CO2 and CH4 are calculated according to formula (7) and formula (8)

[0286]

[0287]

[0288] In the formula: — Adsorption space of CO2, cm 3 / g

[0289] Adsorption space of CH4, cm 3 / g;

[0290] Absolute adsorption amount of CO2, mol / g;

[0291] Absolute adsorption amount of CH4, mol / g;

[0292] Molecular weight of CO2 gas molecule, g / mol;

[0293] Molecular weight of CH4 gas molecule, g / mol;

[0294] Density of CO2, g / cm 3 ;

[0295] Density of CH4, g / cm 3 .

[0296] wherein, in the isothermal adsorption experiment, the obtained adsorption amount is the Gibbs adsorption amount, also known as the apparent adsorption amount. In the calculation of the adsorption space under different temperature and pressure conditions, the absolute adsorption amount needs to be used, i.e. the apparent adsorption amount needs to be converted, and the absolute adsorption amount is calculated by formula (9), formula (10):

[0297]

[0298]

[0299] In the formula: Absolute adsorption amount of CO2, mol / g;

[0300] Absolute adsorption amount of CH4, mol / g;

[0301] Apparent adsorption amount of CO2, mol / g;

[0302] Apparent adsorption amount of CH4, mol / g;

[0303] Density of CO2 under the test temperature and pressure conditions, g / cm 3 ;

[0304] Density of CH4 under the test temperature and pressure conditions, g / cm3 ;

[0305] —CO2 density, g / cm 3 ;

[0306] —CH4 density, g / cm 3 .

[0307] S6 Establish the relationship between CO2, CH4 gas adsorption potential and adsorption space ;

[0308] Here we choose a cubic polynomial relationship to establish the relationship between adsorption potential and adsorption space, as shown in equation (11), equation (12):

[0309]

[0310]

[0311] In the formula: is a constant.

[0312] Where the constant is determined by using origin software to plot the adsorption potential ε and adsorption space ω data, and after the plot is finished, the software's built-in fitting function is used to curve fit the adsorption potential ε and adsorption space ω, to obtain the constant of CH4

[0313]

[0314] Based on the relationship between adsorption potential and adsorption space, the relationship between adsorption potential and adsorption space can be plotted, as shown in Figure 5 .

[0315] S7. Determine a reasonable displacement pressure P 置 ;

[0316] By comparing step S6, the displacement gas (CO2) and the displaced gas (CH4) adsorption characteristic curve is obtained, as shown in Figure 5 , the injection pressure interval when the displacement gas adsorption potential is higher than the displaced gas adsorption potential is obtained, then the pressure interval is the reasonable displacement pressure interval.

[0317] Through Figure 5 ​It can be seen from the figure that the CO2 permeability coefficient first decreases and then increases with the increase of injection pressure. It can be seen from the figure that the CO2 permeability coefficient first decreases and then increases with the increase of injection pressure. When the injection pressure is greater than 3.73 MPa, the CO2 permeability coefficient increases, which is beneficial to the displacement of coal seam CH4 by CO2, indicating that there is a reasonable displacement pressure threshold for the displacement of coal seam gas by liquid carbon dioxide injection; In Huangling weakly caking coal, when the injection pressure is in the pressure range of 0.5-1.2 MPa and 3.8-7.0 MPa, the displacement effect of CO2 on CH4 is best, and when the injection pressure is in the pressure range of 3.8-7.0 MPa, the adsorption potential difference is more obvious, so this pressure range is more advantageous.

[0318] S8 determining a reasonable displacement pressure P 驱 ;

[0319] CO2 displacement coal CH4 physical simulation experiment was carried out on the target coal sample, and the total injection amount of single component CH4 gas under different injection pressures and the total injection amount of single component CO2 gas under different pressures were obtained CO2 production CH4 production Displacement time t; displacement efficiency (η q ) and displacement replacement ratio (μ q ) of CO2 displacement CH4 were calculated according to formula (13) and (14), and formula (13) and (14) are as follows:

[0320]

[0321]

[0322] In the formula: η q -CO2 displacement CH4 efficiency, %;

[0323] -CH4 production, mL;

[0324] -CH4 total injection amount, mL;

[0325] -CH4 concentration at outlet, %;

[0326] q-total flow rate of mixed gas at outlet, mL;

[0327] μ q -displacement replacement ratio;

[0328] -CO2 total injection amount, mL;

[0329] - CO2 production, mL;

[0330] t - displacement time, min.

[0331] According to formula (13), formula (14), the displacement efficiency and displacement replacement ratio can be calculated, according to the displacement pressure, which can be divided into three displacement efficiency intervals, displacement efficiency < 50%, which is a low displacement efficiency stage; displacement efficiency is 50% ~ 60%, which is a medium displacement efficiency stage; displacement efficiency > 60%, which is a high displacement efficiency stage. The injection pressure in the medium displacement efficiency stage and the high displacement efficiency stage is selected as the reasonable displacement pressure interval.

[0332] According to formula (13), (14), the displacement efficiency changes with the displacement pressure, as shown in Table 2.

[0333] Table 2 CO2 displacement coal CH4 physical simulation experiment displacement effect quantitative index

[0334]

[0335] According to Table 2, according to the displacement pressure, it can be divided into three displacement efficiency intervals, when the displacement pressure is between 2.25 ~ 2.75 MPa, it is a low displacement efficiency stage, displacement efficiency < 50%; when the displacement pressure is between 2.75 ~ 4.0 MPa, it is a medium displacement efficiency stage, displacement efficiency is 50 ~ 60%; when the displacement pressure is between 4.0 ~ 6.0 MPa, it is a high displacement efficiency stage, displacement efficiency is 60 ~ 75%. Therefore, the reasonable displacement pressure of coal seam CO2 displacement CH4 is greater than 3.75 MPa.

[0336] S9 calculation of CO2 permeation coefficient

[0337] The CO2 permeation physical simulation experiment of the target coal sample is carried out, the change of outlet flow is monitored by setting different inlet pressure, the outlet flow data of the target coal sample under different inlet and outlet pressure difference is obtained; the influence law of CO2 permeation coefficient in the coal sample under different inlet and outlet pressure difference is calculated by formula (15) ~ formula (18) as shown in formula (19), and the specific formula is as follows. According to Darcy law, the flow of CO2 in the circular coal column can be expressed as formula (15):

[0338]

[0339] In the formula: Q - CO2 flow through the coal sample, m 3 / s;

[0340] k - CO2 permeation coefficient in coal body, 10 -6 m 2 / Pa·s;

[0341] A - cross-sectional area of the sample, m2 ;

[0342] The dynamic viscosity of μ-CO2, Pa·s;

[0343] P in —CO2 gas pressure at the inlet, MPa;

[0344] P out —CO2 gas pressure at the outlet, MPa.

[0345] Based on equation (15), the following can be drawn: Figure 6 As shown, the variation of CO2 flow rate with inlet and outlet pressure difference indicates that since the length of the target coal sample is constant, when the pressure gradient corresponding to the inlet and outlet pressure difference is low, the amount of CO2 adsorbed is large. The adsorption expansion effect causes the seepage resistance of CO2 in the coal seam fissures to be large and the seepage velocity to be slow. When the pressure gradient is large, the effective stress gradually decreases, which is conducive to the seepage of CO2 in the coal seam.

[0346] When the seepage medium is CO2, its seepage velocity in the coal seam fractures is expressed by equation (16), as shown below:

[0347]

[0348] Where: μ a —Dynamic viscosity of CO2, Pa·s.

[0349] The permeability coefficient of CO2 in coal can be expressed by the slope in the velocity expression, as shown in equation (17):

[0350]

[0351] During the seepage experiment, the average pressure corresponding to the flow rate Q of CO2 passing through the coal sample is (P). in +P out From the ideal gas law, atmospheric pressure P can be derived as ) / 2. s The relationship between the outlet flow rate Q0 and the flow rate Q in the coal sample is shown in equation (18):

[0352]

[0353] In the formula: P s —Atmospheric pressure, MPa;

[0354] Q0—CO2 gas volume flow rate under standard conditions, m³ 3 / s.

[0355] Combining equations (15), (16), (17), and (18), we obtain equation (19), as shown below:

[0356]

[0357] Based on equation (19), a graph showing the relationship between the permeability coefficient of CO2 in the target coal sample and the injection pressure can be plotted, as shown in Figure (7).

[0358] pass Figure 7 As shown, when the injection pressure exceeds 3.73 MPa, the amount of CO2 adsorbed by the coal sample is basically saturated. With the increase of injection pressure, the permeability coefficient gradually increases under the action of pressure difference, and the seepage velocity also increases. At this time, the seepage effect of CO2 is obvious, which is conducive to CO2 displacing CH4 in the coal seam.

[0359] S10 determines a reasonable seepage pressure P 渗 ;

[0360] Based on the relationship between CO2 permeability and injection pressure, as shown in Figure (7), it can be observed that as the injection pressure increases, the CO2 permeability first decreases and then increases. When the CO2 permeability decreases, the amount of CO2 adsorbed by coal increases significantly, leading to coal matrix adsorption expansion, which is not conducive to CO2 seepage. When the CO2 permeability increases, the amount of CO2 adsorbed by coal is basically saturated. With the increase of injection pressure, under the action of pressure difference, the CO2 flow rate and seepage velocity also gradually increase, leading to an increasing trend in CO2 permeability. At this time, the CO2 seepage effect is obvious. Therefore, the injection pressure corresponding to the lowest point of CO2 permeability is taken as the minimum reasonable seepage pressure P. 渗 .

[0361] Based on the variation of CO2 permeability coefficient with injection pressure, a reasonable injection range for CO2 is derived. When the injection pressure is greater than 3.73 MPa, the CO2 permeability coefficient gradually increases, which is conducive to CO2 seepage in the coal seam.

[0362] S11 Determine the appropriate injection pressure P 注 ;

[0363] Determine the appropriate replacement pressure P based on step S7. 置 A reasonable displacement pressure range can be determined; based on step S8, a reasonable displacement pressure P can be determined. 驱 This allows for the determination of a reasonable displacement pressure range; based on S10, a reasonable seepage pressure P can be determined. 渗 This allows for the determination of a reasonable seepage pressure range. Combining the pressure ranges constructed from the above parameters, the intersection of these pressure ranges is obtained. This intersection represents the threshold for the reasonable injection pressure of carbon dioxide-driven coalbed methane. Figure 8 As shown.

[0364] The reasonable replacement pressure determined in combination with the CO2 and CH4 adsorption characteristics of the weakly caking coal in Shuanglong Mine is 3.8-7.0 MPa; the reasonable displacement pressure determined in the CO2 displacement of CH4 in the coal seam physical simulation experiment is greater than 3.75 MPa; the reasonable seepage pressure determined by analyzing the variation law of the CO2 permeability coefficient is 3.73 MPa; the comprehensive analysis of the above results can determine that the threshold of the reasonable injection pressure of CO2 for displacing CH4 in the coal seam is 5.0-7.0 MPa. The determination of the reasonable pressure injection pressure threshold of liquid CO2 for displacing CH4 in the coal seam by liquid CO2 phase change is shown in Figure 8 .

[0365] Example 2

[0366] The method for determining the reasonable pressure injection pressure of liquid carbon dioxide for displacing coal seam gas by liquid carbon dioxide phase change includes the following specific steps:

[0367] S1, performing isothermal adsorption experiment on the coal sample of the target coal seam;

[0368] S2, calculating the virtual saturation vapor pressure of CO2 and CH4 gas respectively

[0369] S3, establishing the relationship between the CO2 gas adsorption potential and the CO2 gas adsorption equilibrium pressure; and establishing the relationship between the CH4 gas adsorption potential and the CH4 gas adsorption equilibrium pressure;

[0370] S4, calculating the density of CO2 and CH4 respectively

[0371] S5, calculating the adsorption space of CO2 and CH4 in the coal body respectively

[0372] S6, establishing the relationship between the CO2 gas adsorption potential and the adsorption space; and establishing the relationship between the CH4 gas adsorption potential and the adsorption space;

[0373] S7, determining the reasonable replacement pressure P 置 ;

[0374] S8, determining the reasonable displacement pressure P 驱 ;

[0375] S9, calculating the CO2 seepage coefficient

[0376] S10, determining the reasonable seepage pressure P 渗 ; ​​​​​​​​

[0377] S11, determining a reasonable injection pressure P 注 .

[0378] Example 3

[0379] On the basis of Example 2, S1 is specifically as follows:

[0380] S1.1, taking a target coal seam coal sample, and making a standard coal column with a size of ;

[0381] S1.2, referring to "Coal High Pressure Isothermal Adsorption Test Method GB / T 19560-2008", a single-component CO2 and CH4 gas isothermal adsorption experiment is carried out on the target coal seam coal sample in the laboratory by using static capacity method, and the adsorption amount of the coal sample to the single-component CO2 and CH4 gas under different injection pressures can be obtained respectively

Claims

1. A method for determining the appropriate injection pressure of liquid carbon dioxide phase change-driven coal seam gas, characterized in that, The specific steps are as follows: S1. Conduct isothermal adsorption experiments on coal samples from the target coal seam; S2. Calculate the virtual saturated vapor pressure of CO2 and CH4 gases respectively. , ; S3. Establish the adsorption potential of CO2 gas. Equilibrium pressure for CO2 gas adsorption Relationship; Establish the adsorption potential of CH4 gas equilibrium pressure with CH4 gas adsorption Relationship; S4. Calculate the densities of CO2 and CH4 respectively. , ; S5. Calculate the adsorption space of CO2 and CH4 inside the coal body, respectively. , ; S6. Establish the adsorption potential of CO2 gas. With adsorption space The relationship; establishing the adsorption potential of CH4 gas. With adsorption space Relationship; S7. Determine a reasonable displacement pressure. P 置 ; S7 specifically involves: by comparing the adsorption characteristic curves of the displacing gas CO2 and the driven gas CH4 obtained in step S6, the injection pressure range where the adsorption potential of the displacing gas is higher than that of the driven gas is determined, and this pressure range is the reasonable replacement pressure range. S8. Determine a reasonable displacement pressure. P 驱 ; S8 specifically involves conducting a physical simulation experiment on the CO2 displacement of CH4 from the target coal sample to obtain the total injection volume of single-component CH4 gas under different injection pressures and the total outlet flow rate of single-component CO2 gas injected under different pressures. q Total CO2 gas injection volume CO2 production Q CO2 CH4 production Q CH4 and replacement time t The displacement efficiency of CO2 displacing CH4 is calculated based on equations (13) and (14). η q and displacement ratio μ q Equations (13) and (14) are shown below: （13） （14） In the formula: η q —CO2 displacement efficiency of CH4, % Q CH4 —CH4 production, mL; —Total CH4 injection volume, mL; φ CH4 —Export CH4 concentration, % q —Total flow rate of the mixed gas at the outlet, mL; μ q —Displacement ratio; —Total CO2 injection volume, mL; Q CO2 —CO2 production, mL; t —Displacement time, in minutes; The displacement efficiency and displacement ratio are calculated according to Equations (13) and (14). Based on the displacement pressure, the displacement efficiency is divided into three ranges: displacement efficiency < 50%, which is the low displacement efficiency stage; displacement efficiency of 50% to 60%, which is the medium displacement efficiency stage; and displacement efficiency > 60%, which is the high displacement efficiency stage. The injection pressure at the medium displacement efficiency stage and the high displacement efficiency stage is selected as the reasonable displacement pressure range. S9. Calculate the CO2 permeability coefficient. k CO2 ; S10. Determine a reasonable seepage pressure. P 渗 ; S10 specifically refers to: based on the relationship between CO2 permeability coefficient and injection pressure, taking the injection pressure corresponding to the lowest point of CO2 permeability coefficient as the minimum reasonable seepage pressure. P 渗 ; S11. Determine a reasonable injection pressure. P 注 ; S11 specifically refers to: Determine the appropriate replacement pressure based on step S7. P 置 Determine a reasonable replacement pressure range; Determine the appropriate displacement pressure based on step S8. P 驱 Determine a reasonable displacement pressure range; Determine the appropriate seepage pressure based on S10. P 渗 A reasonable seepage pressure range is determined; the intersection of the pressure ranges constructed by the above parameters is obtained, and this intersection is the threshold of the reasonable injection pressure for carbon dioxide-driven coalbed methane.

2. The method for determining the reasonable injection pressure of liquid carbon dioxide phase change-driven coal seam gas as described in claim 1, characterized in that, S1 is as follows: S1.1 Take coal samples from the target coal seam and prepare a standard coal column with a size of φ50×100mm; S1.

2. Referring to the "High-Pressure Isothermal Adsorption Test Method for Coal" (GB / T 19560-2008), a static volumetric method was used in the laboratory to conduct single-component isothermal adsorption experiments on coal samples from the target coal seam using CO2 and CH4 gases. The adsorption capacities of the coal samples for single-component CO2 and CH4 gases under different injection pressures were obtained. , .

3. The method for determining the reasonable injection pressure of liquid carbon dioxide phase change-driven coal seam gas as described in claim 1, characterized in that, S2 is as follows: The virtual saturated vapor pressures of CO2 and CH4 gases were calculated empirically using virtual saturated vapor pressure under supercritical conditions. , Specifically, as shown in formulas (1) and (2): （1） （2） In the formula: —Virtual saturated vapor pressure of CO2; —CH4 virtual saturated vapor pressure; T—Equilibrium temperature, K; —The critical pressure of CO2 gas, in MPa; —The critical pressure of CH4 gas, in MPa; —Critical temperature of CO2, K; —CH4 critical temperature, K.

4. The method for determining the reasonable injection pressure of liquid carbon dioxide phase change-driven coal seam gas as described in claim 3, characterized in that, In S3, the adsorption potential of CO2 and CH4 gases , Equilibrium pressure for adsorption of CO2 and CH4 gases , The relationship is shown in equations (3) and (4) below: （3） （4） In the formula: —Adsorption potential of CO2 gas, J / mol; —Adsorption potential of CH4 gas, J / mol; —CO2 adsorption equilibrium pressure, MPa; —CH4 adsorption equilibrium pressure, MPa; —Virtual saturated vapor pressure of CO2, MPa; —Virtual saturated vapor pressure of CH4, MPa; —Equilibrium pressure of CO2 gas at isothermal temperature, MPa; —Equilibrium pressure of CH4 gas at isothermal temperature, MPa; —Absolute temperature of CO2, K; —CH4 absolute temperature, K.

5. The method for determining the reasonable injection pressure of liquid carbon dioxide phase change-driven coal seam gas as described in claim 3, characterized in that, In S4: CO2 and CH4 densities , Calculated using equations (5) and (6): （5） （6） In the formula: —CO2 density, g / cm³ 3 ; —CH4 density, g / cm³ 3 ; —The molecular weight of CO2 gas molecules, in g / mol; —Molecular weight of CH4 gas molecules, g / mol; —The critical pressure of CO2 gas, in MPa; —The critical pressure of CH4 gas, in MPa; —The critical temperature of CO2 gas, in K; —The critical temperature of CH4 gas, in K; R —Gas molar constant, taken as 8.314 J·mol -1 ·K -1 ; In S5, the adsorption space of CO2 and CH4 inside the coal body is calculated using equations (7) and (8). , : （7） （8） In the formula: — CO2 adsorption space, cm 3 / g; —Adsorption space of CH4, cm 3 / g; —Absolute adsorption capacity of CO2, mol / g; —Absolute adsorption capacity of CH4, mol / g; —The molecular weight of CO2 gas molecules, in g / mol; —Molecular weight of CH4 gas molecules, g / mol; —CO2 density, g / cm³ 3 ; —CH4 density, g / cm³ 3 ; In S6: CO2 gas adsorption potential With adsorption space The relationship between CH4 gas adsorption potential With adsorption space The relationships are shown in equations (11) and (12): （11） （12） In the formula: a CO2 , b CO2 , c CO2 , d CO2 , a CH4 , b CH4 , c CH4 , d CH4 It is a constant.

6. The method for determining the reasonable injection pressure of liquid carbon dioxide phase change-driven coal seam gas as described in claim 1, characterized in that, S9 specifically refers to: According to Darcy's law, the flow rate of CO2 in a circular coal column is expressed as equation (15): （15） In the formula: Q —CO2 flow rate through the coal sample, m 3 / s; k —CO2 permeability coefficient in coal seam, 10 -6 m² / Pa•s; A —Cross-sectional area of ​​the sample, m 2 ; μ —The dynamic viscosity of CO2, Pa ·s; P in —CO2 gas pressure at the inlet, MPa; P out —CO2 gas pressure at the outlet, MPa; L —Length of the coal seam, in meters; When the seepage medium is CO2, its seepage velocity in the coal seam fractures is expressed by equation (16), as shown below: （16） In the formula: μ a —The dynamic viscosity of CO2, Pa ·s; The permeability coefficient of CO2 in coal is expressed by the slope in the velocity expression, as shown in equation (17): （17） During the seepage experiment, the flow rate of CO2 passing through the coal sample Q The corresponding average pressure is ( P in + P out Atmospheric pressure is derived from the ideal gas law by ) / 2. P s Export flow at time Q 0 and the flow rate in the coal sample Q The relationship is shown in equation (18): （18） In the formula: P s —Atmospheric pressure, MPa; Q 0—CO2 gas volumetric flow rate under standard conditions, m 3 / s; Combining equations (15), (16), (17), and (18), we obtain equation (19), as shown below: （19）。

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