Effects of moisture, salinity and ethane on the competitive adsorption mechanisms of CH4/CO2 with applications to coalbed reservoirs: A molecular simulation study

Injection of carbon dioxide (CO2) into coal seam gas reservoirs has been considered one of the most popular approaches to enhance methane (CH4) production. However, many geological and engineering factors influence the efficiency of CH4 displacement by CO2. These factors include depth-induced changes in pressure and temperature, accompanied by gas compounds, moisture content, and salinity of the underground water. Thereby, the mechanisms of CH4 adsorption and the competitive adsorption processes between CH4 and CO2 in coalbeds at the microscope level remain to be explored. In this paper, a hybrid of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) method is conducted to probe the adsorption properties of CH4 and competitive adsorption behavior between CH4 and CO2 at 0-5 wt% moisture content, 0-6 mol/L saline of NaCl, and 0-5 wt% C2H6. Gas molecules adsorption processes are undertaken at 308, 338, and 368 K with pressure up to 30 MPa. Effects of Moisture, salinity and C2H6 effects on gas adsorption characteristics are discussed in detail. Based on the simulation results, moisture impedes the CH4 adsorption isotherms, which leads to a 17.1 % reduction in CH4 adsorption amount at 5 wt% moisture content. Moreover, salinity further decreases the CH4 adsorption capacity, which drops 6.1 % at 6 mol/L NaCl within 5 wt% moisture content. In addition, C2H6 is another negative factor in the CH4 adsorption process, which reduces the CH4 adsorption amount up to 36.6 % at 5 wt% ethane content. These mentioned factors influence preferential selectivity SCO2/CH4 shows the competitive adsorption behavior is also analyzed. Moisture content promotes the preferential selectivity SCO2/CH4 owing to CH4's more sensitivity to the presence of H2O molecules. Furthermore, high salinity hinders the CH4 solubility in water, whereas an inverse adsorption characteristic of CO2, thereby, high salinity improves the preferential selectivity SCO2/CH4. Besides, preferential selectivity SCO2/CH4 is also developed with the presence of C2H6. The observations of this study have important implications for a more accurate estimation of gas in unconventional resources and help with the displacement efficiency of CH4 by CO2.