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 S_CO2/CH4 shows the competitive adsorption behavior is also analyzed. Moisture content promotes the preferential selectivity S_CO2/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 S_CO2/CH4. Besides, preferential selectivity S_CO2/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.

将二氧化碳 (CO ₂ ) 注入煤层气藏已被认为是提高甲烷 (CH ₄ ) 产量的最流行的方法之一。然而，许多地质和工程因素影响CO ₂置换CH ₄的效率。这些因素包括由深度引起的压力和温度变化，以及地下水的气体化合物、水分含量和盐度。因此，CH ₄吸附的机理和CH ₄和CO ₂之间的竞争吸附过程在显微镜水平的煤层中仍有待探索。在本文中，进行了分子动力学 (MD) 和正则蒙特卡罗 (GCMC) 方法的混合，以探讨 CH _{4 的}吸附特性以及 CH ₄和 CO ₂在 0-5 wt% 水分含量下的竞争吸附行为， 0–6 mol/L NaCl 盐水和 0–5 wt% C ₂ H ₆。气体分子吸附过程在 308、338 和 368 K 下进行，压力高达 30 MPa。详细讨论了水分、盐度和 C ₂ H ₆对气体吸附特性的影响。根据模拟结果，水分阻碍 CH ₄吸附等温线，这导致 CH ₄吸附量在 5 wt% 水分含量下减少 17.1% 。此外，盐度进一步降低了 CH ₄吸附能力，在 6 mol/L NaCl 和 5 wt% 水分含量下，CH ₄吸附能力下降了 6.1%。此外，C ₂ H ₆是CH ₄吸附过程中的另一个负面因素，当乙烷含量为5 wt% 时，CH ₄吸附量降低至36.6 %。这些提到的因素影响优先选择性 S _CO2/CH4显示竞争吸附行为也被分析。由于 CH _4，水分含量促进了优先选择性 S _CO2/CH4对 H ₂ O 分子的存在更敏感。此外，高盐度阻碍了 CH ₄在水中的溶解度，而 CO ₂的逆吸附特性，因此，高盐度提高了优先选择性 S _CO2/CH4。此外，在C ₂ H ₆的存在下，S _{CO2/CH4 的}选择性也得到了提高。本研究的观察结果对于更准确地估算非常规资源中的天然气具有重要意义，并有助于提高CO ₂对 CH ₄的置换效率。