Molecular Simulation on Competitive Adsorptions of CO2, CH4, and N2 in Deep Coal Seams

Abstract

In this paper we have simulated the competitive adsorption of CO2, CH4, and N2 gases in deep coal seams by building a graphite supercell structure and discussed the impact of pressure, pore size, and multicomponent composition on CH4 desorption. The results show that the adsorption capacity of a single component gas changes is in the order of CO2 > CH4> N2. For the CH4/CO2 competitive adsorption, absorbed CO2 can reach saturation at low pressure conditions. CO2 has an adsorptive advantage compared with CH4. It is shown that CO2 can promote the CH4 desorption by the displacement mechanism. For CH4/N2competitive adsorption, the adsorption capacity of N2 is weaker than that of CH4, demonstrating that improvement in coalbed methane (CBM) production by N2 injection is achieved by reducing the partial pressure and creating flow channels. The presence of H2O has a greater impact on the gas with a stronger adsorption capacity in the binary component system. For the CH/CO2/N2 competitive adsorption, the CO2 adsorption is dominant in 1 nm slit pores, while CH4 adsorption is dominant in 2 nm slit pores. This indicates that when the pore diameter increases, the CO2/N2 injection does not promote CH4 desorption. H2O also has a significant impact on the competitive adsorption in the ternary component system. The strong interaction between H2O and CO2 weakens the CO2 adsorption capacity.