Impact research of CH 4 replacement with CO 2 in hydrous coal under high pressure injection

Purpose. Based on high-pressure gas injection technology that enhances coal seam gas drainage, the effect of CH 4 replacement with CO 2 in aquiferous coal has been studied. Methods. Using the laboratory experimental method and the self-built high-pressure gas injection experimental device, high-pressure CO 2 is injected into coal with different moisture contents to replace CH 4 under different adsorption equilibrium pressures. Findings. With an increase in coal moisture content, the adsorption capacity of coal for CH 4 and CO 2 gradually weakens, the adsorption capacity for CO 2 is always greater than that of CH 4 , and the CH 4 replacement rate and the CO 2 injection ratio gradually decrease. It is concluded that the CH 4 replacement rate and the CO 2 injection ratio are negatively correlated with the water content of coal. With an increase of the pre-adsorption equilibrium CH 4 pressure (0.5, 0.75, 1.0, 1.3 and 2.0 MPa), the CH 4 replacement rate and the CO 2 injection ratio first sharply and then slowly increase. The transition point is 1.3 MPa (pre-adsorption equilibrium pressure of CH 4 ). Originality. Based on the adsorption characteristics of coal seam gas injection, the influence of coal water content and gas injection pressure on CH 4 replacement rate and CO 2 injection ratio is analyzed, and the mechanism is studied. Practical implications. The experimental results have important guiding significance for selecting reasonable gas injection pressure and the source of gas to drive its injection into underground coal seam.


Introduction
Coal contains abundant pore fissure, is a kind of natural adsorbent [1], the performance of different gases adsorption capacity of different [2]- [6] its adsorption properties in addition to the affected by external factors (temperature, pressure), also is determined by its physical and chemical properties, the water content of coal impact on adsorption performance is an important factor, and the effect is more complicated. Zhao-feng, W. et al. [7] believed that the adsorption capacity of coal to gas decreased gradually with the increase of water content, and there was a critical water content affecting the adsorption capacity of coal. When the water content exceeded the critical value, the adsorption capacity would not be affected. Shu-gang, L. et al. [8] analyzed the influence of coal's adsorption capacity on CH4 by its water content by using Langmuir monolayer adsorption theory, and the results showed that water-bearing coal samples still met Langmuir monolayer adsorption theory, and adsorption constants A and B were correlated with the change of water content in coal samples to some extent. Jia-hao, W. et al. studied the displacement effect of water on gas, and measured the displacement desorption amount, pressure relief desorption amount and residual gas content in coal under different moisture content conditions. The results showed that water can promote displacement desorption of adsorbed gas in coal, and the greater the moisture content, the greater the displacement desorption amount.
By analyzing the research contents of scholars, the conclusions of gas adsorption of coal with different moisture content are also different under different experimental conditions, and such conclusions trigger scholars to think that the law of replacing CH4 in coal with high-pressure gas injection in coal with different moisture content should be discussed. Since the successful experiment of gas injection to improve the recovery of coal bed methane (CBM), scholars have carried out many studies on gas injection to promote CH4 drainage in dry coal [9]- [12]. Zhao-feng, W., Hong-min, Y., Li-wei, C. et al. [13]- [18] believed that the mechanism of promoting CH4 in coal drainage by gas injection mainly includes displacement adsorption and desorption, carrying / displacement of gas injection flow, dilution and diffusion of gas injection, and expansion and antireflection.
The weak adsorptive gas N2 can not promote the desorption of CH4 from coal through direct competition of adsorption sites, but can only be achieved by injecting gas to change the partial pressure of gas and destroy the original equilibrium. The strongly adsorbed gas CO2 can compete with the CH4 adsorbed on the surface of coal for desorption of CH4 from coal [19]- [22]. With the deepening of research, scholars have put forward many important conclusions on the CH4 effect of replacing coal by gas injection in dry coal. However, the influence of coal water content on the replacement effect needs further research. The author took anthracite of typical high gas mining area in China as the experimental object to study the difference of replacement CH4 effect of CO2 on coal samples with different moisture content.

The experimental device
The device is mainly composed of high pressure gas supply system, constant temperature adsorption and desorption system, gas component analysis system, vacuum system and data acquisition system, and the safety of the experimental device meets the test requirements. The schematic diagram of the experimental device is shown in Figure 1.

The experimental coal sample
The coal samples used in the experiment were selected from anthracite of Yangquan mine in Shanxi Province. The particle size of coal samples was 60-80 mesh, and the mass was 180 ± 0.01 g. The experimental temperature was kept at a constant temperature of 20℃. The parameters of coal samples were shown in Table 1.

Coal sample preparation
The prepared granular coal was put into the air-blast drying oven, and the temperature was set at 105℃. After drying for 24 hours, the coal was put into a drying vessel for cooling, and a certain amount of water was added to stir it fully to make it uniform. Then the sample coal with the required moisture content was prepared by standing for 48 hours.

The experimental steps
Experiment 1: Check the air tightness of the system, vacuum the system, then fill CH4 into each adsorption chamber containing coal samples with different moisture content and balance the adsorption to the set pressure, record the pressure change of gas storage vessel before and after injection, and finally calculate the adsorption amount of gas in the coal. Isothermal adsorption curves of CH4 in coals with different moisture content were drawn based on calculation (the steps for drawing isothermal adsorption curves of CO2 in coals with different moisture content were the same as above). Experiment 2: The air tightness of the system was checked, the system was vacuumized, and then CH4 was filled into the adsorption chamber containing coal samples to balance the adsorption to the target pressure, and quantitative CO2 gas was filled into the coal sample chamber under high pressure. After replacement adsorption equilibrium, data were recorded and static gas samples were collected for gas chromatography analysis. Perform calculations, drawings and analyses based on recorded and analyzed data.

Adsorption law of gas in coal
In order to study the effect of coal with different moisture content on gas adsorption, experiments on CH4 and CO2 adsorption of coal with different moisture content were carried out. Langmuir equation was used to establish adsorption isotherms of pure CH4 and CO2 in coal samples with different moisture content, as shown in Figure 2. The amount of CO2 absorbed by coal with the same moisture content is greater than that of CH4. With the increase of adsorption equilibrium pressure, the amount of CH4 and CO2 absorbed by coal increases, and the amount of adsorbed gas gradually slows down.  That is, the adsorption capacity of gas CH4 is less than CO2, which is consistent with the results of previous studies. The analysis shows that the frequency of gas molecules in the system impacting the coal surface increases with the increase of pressure, which leads to the increase of partial pressure of free gas in the coal body. Because the free state and adsorption state of gas are always in dynamic balance during the adsorption process, the number of adsorbed gas molecules increases. When the pressure is low, some gas molecules are difficult to enter the micropores, and with the increase of pressure, the micropore filling phenomenon will occur in the system. The reason why the growth rate of adsorption capacity gradually slows down with the increase of pressure is that when the pressure is high, there are more adsorption sites in coal body to achieve the dynamic balance between free phase and adsorption phase than in the state of low pressure, while the number of adsorption sites in coal body is fixed, so the growth rate of adsorption capacity slows down with the increase of pressure. Under the same adsorption equilibrium pressure, the adsorption law of coal with different moisture content is shown in Figure 3.   As can be seen from Figure 3, under the same preadsorption equilibrium pressure, the adsorption amount of CH4 and CO2 per unit mass of coal decreases gradually with the increase of coal water content.
Analysis reason, with the increase of water content, water in the coal molecular quantity also will increase, easy water molecules and coal breaking of chemical bonds and the inside of the coal matrix substrate surface hydrophilic functional groups, to a certain extent, reduce the coal surface free energy, make the CH4/CO2coal adsorption system heat release of the balance is less, and, The molecular force between water and coal is stronger than that of CH4/CO2, which can occupy the effective adsorption sites on the coal surface, thus weakening the adsorption capacity of aqueous coal for CH4/CO2.

CH4 replacement rate of coal with different moisture content
High pressure injection is when coal sample chamber CH4 adsorption equilibrium pressure reaches a constant, using the equivalent replacement of piston pump will be higher pressure gas chamber into the coal samples, its purpose is to after the replacement source gas injection, reduce the free volume fraction of CH4 in the gas phase after injection pressure is reduced, resulting in CH4 in coal adsorption quantity is reduced, the free volume increase, CH4 is displaced from coal.
In order to study the difference of CH4 replacement effect of CO2 in coals with different moisture content under high pressure injection, the CH4 replacement rate was selected to measure the CH4 replacement effect of injection source gas in coals with different moisture content. CH4 replacement rate refers to the ratio of the change in the amount of CH4 absorbed by coal before and after gas injection to the amount of CH4 absorbed by coal before gas injection, as shown in Formula 1: where: Rr,CH4the replacement rate of CH4, %; Q1,CH4, Q2,CH4the adsorption capacity of CH4 in coal before and after the experiment, cm 3 /g.
After the high-pressure injection test of coals with different moisture content, the replacement amount and replacement rate of CH4 are shown in Figure 4. As can be seen from Figure 4, both the replacement amount and replacement rate of CH4 increase rapidly before the equilibrium pressure of CH4 pre-adsorption is 1.3 MPa, and slowly after that. This indicates that the pursuit of high gas injection pressure should not be pursued in the downhole gas injection to replace CH4, wasting resources and increasing costs.
After coal with different moisture content is injected under different high pressure, the replacement amount and replacement rate of CH4 are shown in Figure 5. That under the same equilibrium pressure of CH4 pre-adsorption, the displacement amount of CH4 decreases with the increase of coal water content. Within the range of 0.75 to 3% moisture content in coal, CH4 desorption rate decreases in a "downhill" pattern.
Based on the analysis, in the process of high pressure injection, injection of CO2 cavity after total volume, total pressure, reduce the free volume fraction of CH4 in CH4 in the gas phase after the injection of CO2 partial pressure is reduced, resulting in CH4 in coal adsorption quantity is reduced, the free volume to increase, and free of CO2 adsorption on coal, to establish a new equilibrium. In addition, AS an adsorbent gas, CO2 is bound to have competitive adsorption with CH4, and the coal sample holes are bound to absorb a small amount of CO2, occupying part of the adsorption space and inhibiting part of CH4 adsorption. The adsorption state of CH4 tends to be transformed into a free state for various reasons, which explains why the injection of highpressure CO2 can make CH4 desorbed out.     The replacement rate of CH4 in coal with high moisture content is low, because water can inhibit the adsorption and desorption of CH4 in coal. For example, when the adsorption equilibrium pressure of CH4 is 1.3 MPa, the adsorption amount of CH4 (21.45 cm 3 /g) of coal with 0.75% moisture content is 1.11 times that of coal with 1.5% moisture content (19.38 cm 3 /g). It is 1.22 times that of 3% coal (17.53 cm 3 /g). The water molecular weight of coal with high moisture content is large, and water molecules can occupy effective adsorption sites on the coal surface, which weakens the adsorption capacity of CH4. Finally, with the increase of water content, CH4 replacement rate weakens.

CO2 injection ratio of coal with different moisture content
In order to study the replacement efficiency of CO2 injection on CH4 adsorption in coal, it is expressed by the replacement and injection ratio of CO2 to CH4 in coal, referred to as CO2 injection ratio, which refers to the ratio of the replacement amount of CH4 and the injected amount of CO2 after balanced high-pressure injection. On the other hand, it represents the efficiency that CO2 per unit volume can replace CH4 in coal. See where: Rr,CH2injection ratio, %; QCO2 is the injected amount of CO2 before high-pressure gas injection, cm 3 /g. Figure 6 and 7 show CO2 injection after high-pressure injection experiment for coals with different moisture content.

Figure 6. Variation of CO2 injection ratio in coals with different water content with equilibrium pressure of CH4 pre-adsorption
In the coal with the same moisture content under high pressure injection, CO2 injection ratio increases sharply with the increase of CH4 preadsorption equilibrium pressure and then increases slowly. Under the same CH4 preadsorption equilibrium pressure condition, CO2 injection ratio decreases with the increase of coal moisture content.
A comparative analysis of the replacement amount of CH4, replacement rate of CH4 and CO2 injection ratio after high pressure injection equilibrium showed that: 1. Increasing CO2 injection pressure can make the replacement amount and replacement of CH4 increase rapidly at first and then slowly, resulting in the similar phenomenon of CO2 injection ratio as above. The moisture content，% Pre-adsorption equilibrium pressure 0.5MPa Pre-adsorption equilibrium pressure 0.75MPa Pre-adsorption equilibrium pressure 1.0MPa Pre-adsorption equilibrium pressure 1.3MPa Pre-adsorption equilibrium pressure 2.0MPa

Figure 7. Variation of CO2 injection ratio with coal water content at different CH4 pre-adsorption equilibrium pressures
Therefore, for the engineering technology of underground CO2 injection to replace coal seam CH4, blindly increasing the gas injection pressure during high-pressure injection will lead to a decline in CO2 replacement efficiency.
2. With the increase of coal water content, the replacement amount of CH4 and CO2 injection ratio decrease, and the replacement efficiency decreases. In other words, under the same gas injection condition, the displacement amount of coal with high moisture content is small and the displacement efficiency is low.

Conclusions
In coal with the same moisture content, the adsorption capacity of CH4 and CO2 in coal increases with the increase of injection pressure; With the increase of coal water content, the amount of CH4 and CO2 absorbed by coal weakens, and the amount of CO2 absorbed by coal is greater than that of CH4.
In the process of high pressure injection, both CH4 replacement rate and CO2 injection ratio are negatively correlated with the water content of coal. With the increase of CH4 pre-adsorption equilibrium pressure, the CH4 replacement rate and CO2 injection ratio show a trend of sharp increase at first and then slow increase, and the transition point is 1.3 MPa (CH4 pre-adsorption equilibrium pressure).
The replacement amount of CH4, replacement rate of CH4 and CO2 injection ratio are mainly affected by CO2 injection volume, coal water content, coal adsorption to gas, "adsorption competition" between gases and "partial pressure" caused by high pressure injection.