Mechanisms of CH4 and CO2 Adsorption and Induced Swelling of Dry and Moist Bituminous Coal

CO2 is usually injected into hydrocarbon reservoirs to enhance production while sequestering CO2 in the subsurface. Adsorption and induced swelling of CH4 and CO2 in dry and moist bituminous coal were simulated at the molecular level. The Langmuir–Freundlich-fitted maximum adsorption capacity and affinity index for CO2 are 1.16 and 3.15 times those for CH4, respectively. Mechanical strain increases linearly with gas pressure. Apparent strain is the sum of mechanical and adsorption strains, which increases significantly at low pressures due to adsorption strain and decreases slightly at high pressures due to large mechanical strain. Gas adsorption strain is a quadratic function of the adsorption amount, with the quadratic coefficient generally increasing with water content except at 0.91 wt % for CO2. The slow increase in the adsorption strain at low pressures is attributed to gas adsorption in rigid small pores. The higher quadratic coefficient of CH4 is attributed to its larger molecular size and its adsorption in larger pores. Water-induced strain increases linearly with the water content, accompanied by the breaking of hydrogen bonds between coal macromolecules and an increase in the number of water–coal hydrogen bonds. The total energy decreases with an increasing water content due to the large reduction in electrostatic energy, as water is a polar molecule. The gas adsorption capacity and interaction energies decrease significantly with increasing water content.