Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2021 Dec 18;7(1):1395–1403. doi: 10.1021/acsomega.1c06050

Experimental Study on Replacing Coal Seam CH4 with CO2 Gas

Hu Wen †,‡, Jianchi Hao †,‡,*, Li Ma †,‡, Xuezhao Zheng †,‡
PMCID: PMC8757451  PMID: 35036800

Abstract

graphic file with name ao1c06050_0012.jpg

In recent years, many studies have reported the mechanism of CH4 stimulation by gas injection. However, the studies have focused only on monitoring CH4 and CO2 in the tail gas. Thus, it is difficult to distinguish the adsorbed and free gas in the coal and rock and accurately calculate the CO2/CH4 replacement ratio in the displacement process. The low-field NMR technology can effectively overcome the drawbacks of the traditional displacement experiments and distinguish the free and adsorbed gas in the coal and rock. In the present study, the NMR technology analyzed the T2 spectrum for the CH4 desorption amount and CO2/CH4 displacement efficiency in the replacement of methane with gaseous CO2. The results suggested the following: (1) the process of CO2 gas replacing CH4 can be divided into three stages: the initial stage of competitive adsorption, the dominant stage of competitive adsorption, and the weakening stage of competitive adsorption. (2) The cumulative desorption of CH4 gas increases with the increase in replacement time. With the increase in temperature, it first increases and then decreases, and the extreme value is obtained at about 40 °C. Additionally, the greater the CO2 injection pressure is, the greater the cumulative desorption of CH4 is. (3) The cumulative replacement ratio is positively correlated with the replacement time, and with the increase in replacement time, the increment in the cumulative replacement ratio decreases gradually and the upward trend tends to be stable. Overall, the cumulative displacement ratio would increase with an increase in the CO2 injection pressure. With the increase in temperature, the maximum value of the cumulative replacement ratio first increases and then decreases, and the extreme value obtained is about 5.49 at 40 °C.

1. Introduction

China’s high-gas mines account for more than 40% of the total number of mines, and 95% of that are the low permeability coal seams.1,2 Therefore, permeability-modifying technologies, such as hydraulic fracturing and protective layer mining, are extensively used.3−5,30 Among these, inspired by the “gas displacement” technology of the petroleum system, carried out at the end of the 20th century, the experiment of injecting CO2 into the coal seam to improve the CBM recovery (CO2-ECBM) in the San Juan basin, United States, was successful, which explored a new technical way for the rapid gas drainage of low-permeability coal seam.6 CO2 injection into a coal reservoir can realize CO2 geological storage.7 It can also provide a high-pressure potential difference, effectively improving the gas production rate of the production hole, promoting the CH4 desorption in the coal seam, and promoting gas drainage and outburst elimination.8,9 Therefore, the technology of using CO2 gas displacement to replace coal seam CH4 can not only realize mine safety production and protect the atmospheric environment but also help make full use of the underground resources and improve the mine’s economic benefits.

Many researchers have studied the mechanism of CH4 stimulation by gas injection.10−15 Huo et al.16 observed that the increase in CO2 injection pressure increases the CH4 recovery and CO2 storage rates. Sun et al.17 divided the enhanced production process into three stages: initial depressurization, medium-term CO2 adsorption and replacement, and, finally, the CH4 and CO2 coproduction stage. COMSOL was used to simulate the process, and the study concluded that increasing the injection pressure could greatly improve the CO2 storage and CH4 gas production rate.18,19

Extensive research has been performed on the corresponding operating factors, such as the gas injection pressure, mode, and type, to optimize the gas injection displacement technology.4,20,21 Notably, all these studies believed that the experimental process is divided into two stages: seepage diffusion and competitive adsorption.22 Competitive adsorption mainly occurs between the adsorbed gases, and seepage diffusion mainly occurs between the free gases. However, all the above studies monitored CH4 and CO2 in the tail gas.23−25 Further, it is challenging to distinguish the adsorbed and free gas in the coal rock. In the traditional experiment of replacing CH4 with CO2 gas, the free CH4 gas in the coal sample is often considered to be in the adsorbed state, leading to inaccurate calculations, and the CO2/CH4 replacement ratio in the displacement process cannot be calculated. The low-field nuclear magnetic resonance (NMR) technology can effectively overcome the drawbacks of traditional displacement experiments to distinguish the free and adsorbed gas in coal and rock. In addition, although a large number of scholars have applied the NMR technology to the experiment of coal adsorbing methane, no one has used the magnetic resonance imaging technology to carry out the experiment of CO2 gas displacing CH4 gas.26−28 Based on the NMR technology, the present study designed static experiments of the CO2 displacement of CH4 gas to investigate the displacement behavior between the adsorbed gases in this process.

2. Experimental Device and Steps

2.1. Experimental Principle

NMR spectroscopy detects the hydrogen nuclei (H1) in methane in coal pores and fissures. The signal amplitude of the NMR spectrum is directly proportional to the methane quality within the detection range. The gas quality of the coal and rock fracture in a single hole corresponds to the ordinate amplitude of the T2 spectrum. The ordinate amplitude integral of the T2 spectrum curve can be used to characterize the gas quantity of the coal and rock hole fracture. The existence of the coal rock porous medium structure makes the NMR transverse relaxation time of pore fissure methane much smaller than that of free-state methane. Different pore diameters in porous media correspond to different transverse relaxation times. Combined with the NMR spectrum characteristic test of free-state gas, micropores, medium–large pores, fractures, and unrestricted free space can be quantitatively divided in the NMR T2 spectrum. Then, the adsorbed and free gas quantity can be obtained by integrating the ordinate of the T2 spectrum.

According to the principle of nuclear magnetic resonance, the transverse relaxation time T2 follows the following formula

2.1. 1

where T2B is the gas relaxation time, given in milliseconds; ρ2 is the transverse surface relaxation strength, given in micrometers per millisecond; S is the pore surface area of the coal measure shale, given in square centimeters; V is the pore volume of coal, given in cubic centimeters; D is the CH4 diffusion coefficient, given in square micrometers per millisecond; G is the magnetic field gradient, given in gauss per centimeter; γ is the gyromagnetic ratio, given in radian per second per tesla; and TE is the echo interval, given in milliseconds.

Since the T2B value is 2000∼3000 ms, which is much greater than the T2 value of gas in the coal sample, and the magnetic field unevenness of the NMR equipment is less than 35 ppm and the echo interval, the last two items in formula 1 can be ignored

2.1. 2

The pore structure of the actual coal seam is extremely complex, and the specific surface area of coal pores has a nonlinear relationship with the pore diameter. For the convenience of analysis, it is assumed that all coal pores and roar channels have a columnar pore structure, which gives

2.1. 3
2.1. 4

where FS is the geometric factor, with Fs = 2 for the columnar pore structure, and is dimensionless; r is the average pore radius, given in micrometers; A is the surface area, given in square micrometers; and C is the conversion coefficient, given in micrometers per millisecond.

According to formula 4, the value of the transverse relaxation time T2 of gas NMR in the coal body corresponds to the pore radius r. The T2 spectrum distribution can be used to evaluate the pore size and pore size distribution. The longer the transverse relaxation time is, the greater the pore fracture radius is. As a typical porous medium material, coal gas mainly exists in the pore structure, and the T2 spectrum amplitude integral represents the total detectable gas signal. Therefore, the T2 spectrum amplitude integral is used to represent the actual gas adsorption capacity of coal samples, and the gas adsorption law in the process of CO2 replacing coal seam CH4 is quantitatively studied in this paper.

During the experiment, the following relationship exists for the CH4 gas

2.1. 5
2.1. 6

where Vdei(CH4) is the desorption amount of CH4, Vad0(CH4) is the initial adsorption capacity of CH4, Vadi(CH4) is the amount of residual CH4 adsorption at time i, Vfreei(CH4) is the free CH4 gas volume at time i, and Vfree0(CH4) is the free CH4 gas volume at the beginning of the experiment.

The partial pressure of free CH4 gas at time i is as follows

2.1. 7

where Pfreei(CH4) is the partial pressure of CH4 gas in the free state at time i, Z is the compression factor of the mixture at the experimental temperature, R is the general gas constant, that is, 8.314 J/(mol·K), T is the temperature in kelvin, and V0 is the volume of the free space.

According to Dalton’s partial pressure law, the partial pressure of CO2 at time i can be obtained as follows

2.1. 8

Therefore, the amount of free-state CO2 at time i is

2.1. 9

Moreover, the amount of adsorbed CO2 is as follows

2.1. 10

2.2. Experimental Scheme and Steps

The conventional methods could destroy the primary pore fracture system of coal or produce some artificial secondary fracture in the sample preparation process, resulting in huge errors. The low-field NMR relaxation time analysis technology is a new detection technology, which can overcome these shortcomings. The analysis principle involves the quantitative characterization of the pore structure and fluid filling attributes of a hydrogen-containing fluid (1H core) in coal seam through the relaxation time and signals’ amplitude.

The instrument used in the experiments is mainly composed of a low-field NMR detector, a coal sample container, pneumatic valves, a gas storage tank, a pressure control unit, a temperature sensor, a vacuum pump, and gas cylinders, as shown in Figure 1.

Figure 1.

Figure 1

Open in a new tab

Schematic diagram of the experimental system.

The present study used the long-flame coal of (9–15) 08 working face of the Liuhuanggou coal mine in Xinjiang as the samples for the experiments. Table 1 shows the industrial analysis of the coal sample. Before the experiment, the coal sample was crushed to 80 mesh and dried at 80 °C in the drying oven. After drying, CH4 was replaced by gaseous CO2 at 20–60 °C with an injection pressure of 0–6 MPa. The detailed experimental steps are as follows:

  • (1)

    The pulverized coal was placed into the coal sample tank, and its quality was recorded. Then, valve 1 was closed, valves 2–6 were opened, and all the valves were closed after the vacuum treatment for 8 h.

  • (2)

    According to the ISO 12213:2006 natural gas calculation of the compression factor, the compression factor for helium at different pressures (20–60 °C) was calculated. According to the equation of state PV = nRT, the Vreference was calculated to be 44.87 mL and Vsample was calculated to be 33.39 mL.

  • (3)

    Valves 1–4 were opened, and the CH4 gas was continuously injected into the reference tank and the sample tank at 2.0 MPa pressure. After the pulverized coal adsorption for 12 h, the reference tank and sample bin pressures were recorded.

  • (4)

    Valves 1 and 3 were closed, and valves 2, 4, and 5 were opened. The CH4 gas-filled reference tank was emptied using the vacuum pump for 8 h.

  • (5)

    Valve 5 was closed, and valves 1, 2, and 4 were opened. 2.0 MPa CO2 was filled into the reference tank. Valve 1 was closed when it was stable, and valve 3 was opened to start the replacement of CH4 gas with CO2 gas. The experimental data were continuously recorded for 12 h.

  • (6)

    At the end of the experiment, valves 2–5 were opened to drain.

  • (7)

    Experimental steps 3–6 were repeated, and the CO2 gas was injected at 3, 4, 5, and 6 MPa pressures in turn.

Table 1. Industrial Analysis of Coal Samples.

coal sample moisture/% ash/% volatile matter/%
Long-flame coal 5.58 12.37 42.28
Open in a new tab

3. Results and Discussion

Through the calibration experiment of coal adsorbing methane, the corresponding relationship between the mass of free methane and the signal amplitude can be obtained, as shown in Figure 2.

Figure 2.

Figure 2

Open in a new tab

Fitting curve between the free-state methane mass and the signal amplitude integral.

It can be seen from the figure that the mass of methane is directly proportional to the signal amplitude integral of the NMR T2 spectrum. The greater the mass of CH4 is, the greater the signal amplitude integral is, which conforms to the linear relationship, and the fitting coefficient R2 is determined to be as high as 0.9999. Therefore, when we obtain the NMR T2 spectrum of CH4, we can use the fitting curve between the signal amplitude integral and CH4 mass to calculate the amount of free CH4 at different temperatures and pressures and then calculate the amount of adsorbed CH4 and other parameters.

3.1. T2 Spectrum Analysis

The NMR relaxation time analysis is one of the most important techniques in the industrial application of low-field NMR.29 In the coal reservoir, hydrogen nuclei exist in the coal matrix and reservoir fluid. There is a huge difference in the NMR characteristics of the hydrogen nucleus in these two environments. By selecting appropriate measurement parameters, signals related to pore fluid and independent of coal base block skeleton can be detected. The essence of the coal reservoir system research based on low-field NMR was to analyze the occurrence state of the multiphase fluid in the coal reservoir. It also included establishing the microscopic interaction mechanism between the coal reservoir and fluid using the surface relaxation and free relaxation characteristics of gas in the coal pores. The T2 spectrum of CH4 during the experiment is illustrated in Figures 3–5 below:

Figure 3.

Figure 3

Open in a new tab

T2 diagram test under different CO2 injection pressures.

Figure 5.

Figure 5

Open in a new tab

Relationship between the stage desorption amount of CH4 and CO2 injection pressure.

The T2 relaxation time is associated with the binding force and degree of freedom of the hydrogen proton. Further, the binding degree of the hydrogen proton is closely related to the sample’s internal structure. Specifically, the smaller the pore size is, the more bound the methane in the pore will be, leading to a shorter relaxation time. Thus, it can be considered as the adsorption state. Longer relaxation time could indicate the free state. As shown in Figures 3 and 4, the amount of adsorbed CH4 decreased and the amount of free CH4 gas increased with an increase in replacement time. After opening the connecting valve, the CO2 gas could enter the coal sample tank under the pressure drive, which could have promoted the desorption of CH4 gas in the coal sample through competitive adsorption, resulting in the adsorbed CH4 gas amount decrease and free CH4 gas amount increase in the coal sample.

Figure 4.

Figure 4

Open in a new tab

T2 diagram test at different temperatures.

3.2. Analysis of the Gas Change Law in the Replacement Process

3.2.1. Effect of CO2 Injection Pressure

3.2.1.1. CH4 Desorption by Stages

Low-field NMR (Mesomr12-060 h-i) software was used to analyze the T2 spectrum adsorption peak area and free peak area. The amount of adsorbed CH4 gas and free CH4 gas at different times and under different CO2 injection pressures can be measured. According to eqs 9 and 10, the amount of adsorbed CO2 and free CO2 and the displacement efficiency under various conditions can be calculated. The relationship between the stage desorption amount of CH4 and the displacement time and pressure is shown in Figure 5.

Figure 5 indicates that in the P1 stage (0–120 min), the staged desorption of CH4 gas decreased rapidly, and in the P2 stage (120–480 min), the staged desorption of CH4 gas was increased steadily. At the P3 stage (480–720 min), the staged desorption of CH4 gas gradually decreased. The result could be because at the opening of the connecting valve in the P1 stage, the gas pressure in the coal microcrack was increased when the CO2 gas entered the coal sample tank. The increase in gas pressure hinders the desorption of CH4. Thus, the staged desorption of CH4 decreased rapidly. At the P2 stage, the natural desorption of CH4 was weakened, and the displacement of CO2 gas dominated this stage. Although the staged desorption of CH4 was continuously increasing, it tended to be stable. When it reached the P3 stage, there was little residual adsorbed CH4 in the coal, the replacement of adsorbed CH4 by CO2 gas became difficult, and the competitive adsorption was weakened, resulting in the continuous decline of the stage desorption capacity of CH4 gas. Therefore, the process of replacing CH4 with CO2 can be divided into three stages: the initial stage of competitive adsorption, the dominant stage of competitive adsorption, and the weakening stage of competitive adsorption.

3.2.1.2. CH4 Cumulative Desorption

The relationship between the stage desorption amount of CH4 and the displacement time and pressure is shown in Figure 6.

Figure 6.

Figure 6

Open in a new tab

Relationship between CH4 cumulative desorption and CO2 injection pressure.

Figure 6 shows that the cumulative desorption amount of CH4 gas increases with the replacement time, and the greater the CO2 gas injection pressure is, the greater the cumulative desorption of CH4 gas is. However, at an injection pressure higher than 5 MPa, the influence of CO2 injection pressure on the cumulative desorption of CH4 gas is weakened. This is because when the gas injection pressure of the microcracks in the coal is higher than 5 MPa, the gas pressure causes the micropores in the coal to reach their expansion extreme value, so they cannot accommodate more adsorption sites. If the gas pressure cannot enable its microcracks to continue to expand to accommodate more adsorption sites, the continuous increase in pressure will not have a great impact on the desorption capacity of CH4 gas.

In order to more clearly observe the effect of CO2 gas injection pressure on the cumulative desorption of CH4 gas, the experimental time of 12 h is taken as an example, and the results are given in Table 2.

Table 2. Relationship between the Increase in CH4 Cumulative Desorption and CO2 Gas Injection Pressure.
CO2 injection pressure (MPa) increase in desorption capacity compared with the previous pressure point (%)
3 1.60
4 12.99
5 10.84
6 8.15
Open in a new tab

It can be seen from Table 2 that the increase range of the cumulative desorption amount of CH4 in coal samples first increases and then decreases with the increase in CO2 injection pressure. At 4 MPa, the increase in CH4 gas cumulative desorption reaches the extreme value. When the CO2 injection pressure increases from 3 to 4 MPa, the increase in the CH4 cumulative desorption amount reaches a maximum, which is 12.99%.

3.2.2. Effect of Temperature

3.2.2.1. CH4 Desorption by Stages

Taking the CO2 injection pressure of 4 MPa as an example, the relationship between the stage desorption amount of CH4 and temperature is shown in Figure 7:

Figure 7.

Figure 7

Open in a new tab

Relationship between the stage desorption amount of CH4 and temperature.

Figure 7 shows that the relationship between the stage desorption amount of CH4 and the replacement time is consistent with the above conclusions. That is, the process of replacing CH4 with CO2 gas can be divided into three stages: the initial stage of competitive adsorption, the dominant stage of competitive adsorption, and the weakening stage of competitive adsorption. At the same time, it can be seen that under the influence of temperature, the stage desorption amount of CH4 reaches the maximum at 40 °C. The order of the desorption amount of CH4 at different temperatures from large to small is 40, 50, 60, 30, and 20 °C. This is because the desorption of methane in coal is an endothermic reaction. Desorption essentially means that the adsorption speed of adsorbate molecules on the adsorbent surface is less than its leaving speed. The formation of the interaction between the adsorbate and adsorbent needs to release energy, and the fracture of the weak bond between them also needs to absorb energy from the surrounding. Therefore, the increase in temperature is beneficial to the desorption. However, when the temperature exceeds 40 °C, the activity of free CH4 gas increases and the partial pressure of CH4 gas increases, which hinders the desorption of CH4.

3.2.2.2. CH4 Cumulative Desorption

The relationship between the CH4 cumulative desorption and temperature is shown in Figure 8:

Figure 8.

Figure 8

Open in a new tab

Relationship between CH4 cumulative desorption and temperature.

The cumulative desorption of CH4 increases with the increase in replacement time. Under the influence of temperature, the cumulative desorption of CH4 gas reaches the maximum at 50 °C.

In order to more clearly see the effect of temperature on the cumulative desorption of CH4, the case with the CO2 injection pressure of 4 MPa and the experimental time of 600 min is taken as an example, and the results are given in Table 3.

Table 3. Effect of Temperature on the Cumulative Desorption of CH4.
temperature (°C) increase of CH4 cumulative desorption (relative to 20 °C) (%)
30 52.67
40 39.26
50 54.54
60 48.31
Open in a new tab

Compared with that at 20 °C, the cumulative desorption of CH4 measured at 50 °C increased the most, about 54.54%. This is because the desorption of CH4 is an endothermic process. When the temperature increases to 40–50 °C, the increase in temperature provides energy for the desorption of CH4 and promotes the desorption of CH4. When the temperature is higher than 50 °C, due to the increase in the amount of free CH4 and the coal having not been discharged in time, the increase in temperature will reduce the density of CH4 in the coal and increase the gas partial pressure, which hinders the desorption of CH4.

3.3. Replacement Efficiency Analysis

The stage displacement ratio and cumulative displacement ratio were introduced to characterize the relationship between CO2 adsorption and CH4 desorption. According to Avogadro’s law, any gas with the same volume at a certain temperature and pressure contains the same number of molecules. It also suggests that the gas volume ratio must be equal to the number of molecules at the same temperature and pressure. The relationship between different gases at the same temperature and pressure can be obtained as Inline graphic.

Therefore, the stage replacement ratio can be defined as the ratio of CO2 staged adsorption to CH4 staged desorption, as given in eq 11.

3.3. 11

Here, S is the stage replacement ratio, A1 is the staged adsorption capacity of CO2 in mole, and D1 is the staged desorption of CH4 in mole.

The cumulative displacement ratio is defined as the ratio of cumulative CO2 adsorption to cumulative CH4 desorption, as shown in eq 12.

3.3. 12

C is the cumulative replacement ratio, A2 is the cumulative adsorption capacity of CO2 in mole, and D2 is the cumulative desorption of CH4 in mole.

Figure 9 shows that the stage replacement ratio of CO2/CH4 increased with an increase in the replacement time. The stage replacement ratio shows a rapid growth trend before 120 min, a relatively stable state between 120 and 480 min, and a rapid upward trend after 480 min. This may be because in the initial S1 stage (before 120 min), CO2 gas quickly occupies the remaining adsorption site after entering the coal. Moreover, because part of CH4 gas is replaced by competitive adsorption, this stage includes not only the natural adsorption of CO2 gas but also the competitive adsorption of CO2 and CH4, resulting in the fact that the stage adsorption amount of CO2 is much greater than the stage desorption amount of CH4. Therefore, the stage replacement ratio increases rapidly. With further progress of the experiment, competitive adsorption dominates in the S2 stage (120–480 min), resulting in the steady growth of the stage replacement ratio in this stage. With the increase in experimental time, the reduction of the residual adsorbed CH4 in coal makes CH4 desorption more challenging. However, the competitive adsorption of CO2 and CH4 increases the adsorbed CO2 amount in coal. Thus, the stage replacement ratio increases rapidly in the S3 stage.

Figure 9.

Figure 9

Open in a new tab

Staged replacement ratio of CO2/CH4.

At the same time, it can be seen that the stage replacement ratio of the coal sample increases with the increase in CO2 injection pressure, but this law is not very obvious. The reason may be that the stage adsorption amount of CO2 and stage desorption amount of CH4 are not only related to the injection pressure of CO2 gas but also related to the parameters such as micropore expansion, permeability, and the diffusion rate of coal.

The stage replacement ratio of coal samples first increased and then decreased with the increase in temperature, and it reached the peak at 40 °C. The reason is that the formation of the interaction between the adsorbate and adsorbent needs to release energy, and the fracture of the weak bond between them also needs to absorb energy from the surrounding. The adsorption of CO2 by coal is an exothermic reaction, while the desorption of CH4 gas is an endothermic reaction. The higher the temperature is, the more conducive it is to the desorption of CH4 gas but not to the adsorption of CO2 gas. Therefore, after 40 °C, with the increase in temperature, the stage adsorption amount of CO2 decreases and the stage desorption amount of CH4 increases, resulting in the decrease of the stage replacement ratio.

Figure 10 shows that the cumulative replacement ratio is positively correlated with the replacement time, and with the increase in replacement time, the cumulative replacement ratio increases rapidly at first and then tends to become stable gradually. This is because when the replacement time is less than 60–120 min, after opening the connecting valve, CO2 gas will first quickly occupy the remaining adsorption position in the coal sample, and then replace CH4 in the coal due to competitive adsorption. From the previous analysis of CH4 desorption capacity, the CH4 desorption capacity is less at this stage, while the CO2 adsorption capacity increases rapidly. It can be seen that the increment of CO2 adsorption capacity is much greater than that of CH4 desorption capacity. Therefore, in the P1 stage, the cumulative replacement ratio of the three coal samples increases rapidly. In the 60–600 min period, it can be seen from the above analysis that in this stage, CO2 gas has filled the remaining adsorption sites. At this time, the competitive adsorption of CO2 gas and CH4 gas occupies the dominant position, so the cumulative replacement ratio in this stage shows a slow upward trend. When the experimental time exceeds 600 min, the adsorbed CO2 has reached saturation, there is little residual adsorbed CH4 in the coal sample, and the CH4 gas that can be replaced is also decreasing. Therefore, the cumulative replacement ratio of CO2/CH4 tends to be stable in the later stage of the experiment. In addition, there is no obvious change law between the cumulative replacement ratio and the increase in CO2 injection pressure.

Figure 10.

Figure 10

Open in a new tab

Cumulative replacement ratio of CO2/CH4.

Table 4 shows that at 2 and 6 MPa CO2 injection pressures, the maximum cumulative displacement ratios were about 3.61 and 4.25, respectively. Overall, the cumulative displacement ratio increases with an increase in the CO2 injection pressure. With the increase in temperature, the maximum value of the cumulative replacement ratio first increases and then decreases, and the extreme value is about 5.49 at 40 °C.

Table 4. Maximum Cumulative Displacement Ratio.

injection pressure 2 MPa 3 MPa 4 MPa 5 MPa 6 MPa
maximum ratio 3.61 3.45 4.32 4.42 4.25
temperature 20 °C 30 °C 40 °C 50 °C 60 °C
maximum ratio 4.82 4.99 5.49 4.83 4.53
Open in a new tab

4. Conclusions

Based on nuclear the NMR technology, the stage desorption amount, cumulative desorption amount, stage replacement ratio, and cumulative replacement ratio of CH4 under different CO2 injection pressures and temperatures are deeply analyzed. The effects of CO2 injection pressure and experimental temperature were studied. The main conclusions are as follows:

  • (1)

    Through the study of the change law of CH4 gas in the replacement process, it is concluded that the stage desorption amount of CH4 increases with the increase in CO2 injection pressure. Under the influence of temperature, the stage desorption amount of CH4 first increases and then decreases, reaching the maximum at 40 °C. According to the variation law of the stage desorption amount of CH4 with replacement time, the process of CO2 gas replacing CH4 can be divided into three stages: the initial stage of competitive adsorption, the dominant stage of competitive adsorption, and the weakening stage of competitive adsorption.

  • (2)

    Variation law of the cumulative desorption of CH4 gas: the cumulative desorption of CH4 gas increases with the increase in replacement time. With the increase in temperature, it first increases and then decreases, and the extreme value is obtained at about 40 °C. Additionally, the greater the CO2 injection pressure is, the greater the cumulative desorption of CH4 gas is. However, when the CO2 injection pressure is higher than 4 MPa, the effect of CO2 injection pressure on the cumulative desorption of CH4 gas is weakened.

  • (3)

    The cumulative replacement ratio is positively correlated with the replacement time, and with the increase in replacement time, the increment in the cumulative replacement ratio decreases gradually, and the upward trend tends to be stable. Overall, the cumulative displacement ratio increases with an increase in the CO2 injection pressure. With the increase in temperature, the maximum value of the cumulative replacement ratio first increases and then decreases, and the extreme value is about 5.49 at 40 °C.

Acknowledgments

This work was supported by the National Natural Science Foundation of China [grant number: 51974240].

The authors declare no competing financial interest.

References

  1. Zhang J. Research on key technology of cooperative mining in deep coal seam with high gas and low permeability. Coal Sci. Technol. 2020, 48, 66–74. 10.13199/j.cnki.cst.2020.09.007. [DOI] [Google Scholar]
  2. Wen H.; Li Z. B.; Wang Z. P. Experiment on permeability enhancement process and fracture propagation characteristics of liquid CO2 injection fracturing in coal seam. J. China Coal Soc. 2016, 41, 2793–2799. 10.13225/j.cnki.jccs.2016.0124. [DOI] [Google Scholar]
  3. Ma G.; Zhang F.; Liu X. Experimental study on hydraulic fracture propagation in fractured reservoir. J. Min. Saf. Eng. 2017, 34, 993–999. 10.13545/j.cnki.jmse.2017.05.025. [DOI] [Google Scholar]
  4. Zhang L.; Chen S.; Zhang C. The characterization of bituminous coal microstructure and permeability by liquid nitrogen fracturing based on μCT technology. Fuel 2020, 262, 116635. 10.1016/j.fuel.2019.116635. [DOI] [Google Scholar]
  5. Ren W.; Zhou H.; Xue D. Strong disturbance dynamic behavior and permeability characteristics of coal and rock under upper protective seam mining. J. China Coal Soc. 2019, 44, 1473–1481. 10.13225/j.cnki.jccs.2019.6011. [DOI] [Google Scholar]
  6. Yu H.; Fan W.; Sun M. Characteristics and predictions for adsorption isotherms of methane/carbon dioxide binary gas on coals. Coal Convers. 2005, 28, 43–47. [Google Scholar]
  7. Yu H.; Jiang Q. Z.; Song Z. Z. The economic and CO2 reduction benefits of a coal-to-olefins plant using a CO2-ECBM process and fuel substitution. RSC Adv. 2017, 7, 49975–49984. 10.1039/c7ra07994j. [DOI] [Google Scholar]
  8. Fujioka M.; Yamaguchi S.; Nako M. CO2-ECBM field tests in the Ishikari Coal Basin of Japan. Int. J. Coal Geol. 2010, 82, 287–298. 10.1016/j.coal.2010.01.004. [DOI] [Google Scholar]
  9. Edo P.; Ismail M. S.; Syahrir R. Identification of coal seams suitability for carbon dioxide sequestration with enhanced coalbed methane recovery :acase study in South Sumatera Basin ,Indonesia. Clean Technol. Environ. Policy 2018, 20, 581–587. 10.1007/s10098-017-1383-4. [DOI] [Google Scholar]
  10. Hao D.; Ye Z.; Fang S. Application status and Prospect of coal seam gas displacement technology by gas injection in China. China Min. Mag. 2016, 25, 77–81. [Google Scholar]
  11. Liang W.; Zhang B.; He W. Experimental study on supercritical CO2 displacement of CH4 from different rank coal. J. China Coal Soc. 2020, 45, 197–203. 10.13225/j.cnki.jccs.YG19.1498. [DOI] [Google Scholar]
  12. Wang L.Experimental Study on Gas Injection Displacement of Ch4 in Deep Coal Seam and Characteristic Trace after Displacement; China University of Mining and Technology, 2013; pp 33–35. [Google Scholar]
  13. Hao J.; Wen H.; Ma L. Theoretical Derivation of a Prediction Model for CO2 Adsorption by Coal. ACS Omega 2021, 6, 13275–13283. 10.1021/acsomega.1c00957. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Han J.; Liang W.; Zhang J. Simulation experimental study on carbon dioxide disposal in coal seam. Coal Sci. Technol. 2014, 39, 531–536. 10.13225/j.cnki.jccs.2013.1656. [DOI] [Google Scholar]
  15. Yang H. M.Study on Mechanism and Law of Underground Gas Injection Displacing Coalbed Methane; Henan University of Technology, 2010. [Google Scholar]
  16. Huo P.; Zhang D.; Yang Z. CO2 geological sequestration: Displacement behavior of shale gas methane by carbon dioxide injection. Int. J. Greenhouse Gas Control 2017, 66, 48–59. 10.1016/j.ijggc.2017.09.001. [DOI] [Google Scholar]
  17. Sun H.; Yao J.; Gao S. Numerical study of CO2 enhanced natural gas recovery and sequestration in shale gas reservoirs. Int. J. Greenhouse Gas Control 2013, 19, 406–419. 10.1016/j.ijggc.2013.09.011. [DOI] [Google Scholar]
  18. Li X.; Elsworth D. Geomechanics of CO2 enhanced shale gas recovery. J. Nat. Gas Sci. Eng. 2015, 26, 160–1619. 10.1016/j.jngse.2014.08.010. [DOI] [Google Scholar]
  19. Du X.; Gu M.; Duan S.; Xian X. The Influences ofCO2 Injection Pressure onCO2 Dispersion and the Mechanism ofCO2 −CH4 Displacement in Shale. J. Energy Resour. Technol. 2018, 140, 012907–012916. 10.1115/1.4037687. [DOI] [Google Scholar]
  20. Li Y. X.Mechanism and Experimental Research on Coalbed Methane Displacement by Continuous and Intermittent Air Injection; Taiyuan University of Technology, 2017. [Google Scholar]
  21. Shi Q.; Chen J. B.; Huang H. Experimental study on improving coalbed methane recovery by gas injection. Saf. Coal Mines 2018, 49, 10–13. 10.13347/j.cnki.mkaq.2018.05.003. [DOI] [Google Scholar]
  22. Sang S. Research review on technical effectiveness of CO2 geological storage and enhanced coalbed methane recovery. Coal Geol. Explor. 2018, 46, 1–9. 10.3969/j.issn.1001-1986.2018.05.001. [DOI] [Google Scholar]
  23. Zheng S.; Yao Y.; Elsworth D.; Liu D.; Cai Y. Dynamic fluid interactions during CO2-ECBM and CO2 sequestration in coal seams. Part 2: CO2-H2O wettability. Fuel 2020, 279, 118560. 10.1016/j.fuel.2020.118560. [DOI] [Google Scholar]
  24. Zheng S.; Yao Y.; Elsworth D.; Liu D.; Cai Y. Dynamic fluid interactions during CO2-ECBM and CO2 sequestration in coal seams. Part 1: CO2–CH4 Interactions. Energy Fuels 2020, 34, 8274–8282. 10.1021/acs.energyfuels.0c01371. [DOI] [Google Scholar]
  25. Sun X.; Yao Y.; Liu D. The behavior and efficiency of methane displaced by CO2 in different coals and experimental conditions. J. Nat. Gas Sci. Eng. 2021, 93, 104032. 10.1016/J.JNGSE.2021.104032. [DOI] [Google Scholar]
  26. Yao Y.; Liu D.; Xie S. Quantitative characterization of methane adsorption on coal using a low-field NMR relaxation method. Int. J. Coal Geol. 2014, 131, 32–40. 10.1016/j.coal.2014.06.001. [DOI] [Google Scholar]
  27. Yao Y.; Liu J.; Liu D.; et al. A new application of NMR in characterization of multiphase methane and adsorption capacity of shale. Int. J. Coal Geol. 2019, 201, 76–85. 10.1016/j.coal.2018.11.018. [DOI] [Google Scholar]
  28. Zheng S.; Yao Y.; Liu D.; Cai Y.; Liu Y. Quantitative characterization of multiphase methane in coals using the NMR relaxation method. J. Pet. Sci. Eng. 2021, 198, 108148. 10.1016/J.PETROL.2020.108148. [DOI] [Google Scholar]
  29. Coates G. R.; Xiao L.; Prammer M. G.. NMR Logging Principles and Applications; Gulf Professional Publishing: Houston, 1999. [Google Scholar]
  30. Hao J.; Ren L.; Wen H.; Zhang D. Experimental Study of Gangue Layer Weakening with Deep-Hole Presplitting Blasting. Shock Vib. 2021, 2021, 4796500. 10.1155/2021/4796500. [DOI] [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES