Influence of H2O on Adsorbed CH4 on Coal Displaced by CO2 Injection: Implication for CO2 Sequestration in Coal Seam with Enhanced CH4 Recovery (CO2-ECBM)

CO2-ECBM is capable of realizing CO2 sequestration and coalbed methane (CH4) production simultaneously. Practical coal seam contains H2O, thus significantly influencing adsorption, desorption, diffusion, and flow capability of CO2 and CH4. Accordingly, the impacts of H2O on adsorbed CH4 on coal displaced by CO2 were investigated to gain a further understanding on CO2-ECBM. The results derived from this study indicate that the occurrence of H2O with coal positively relates to oxygenic functional groups and mesopores of coal. Particularly, the oxygenic functional group of the coal matrix acts as a primary adsorption site for H2O. The mesopore of coal is the main space for H2O occurrence. Furthermore, the impact of H2O on CO2 adsorption on coal depends on dissolution capability and competitive adsorption of H2O. With respect to coal with a low H2O content, the dissolution of CO2 in H2O is dominant at high CO2 adsorption equilibrium pressure, thus leading to increasing CO2 adsorption capability of coal, while the opposite trend is applicable for coal with a high H2O content. With regard to the displacement process, injecting CO2 promotes desorption of adsorbed CH4 from dry and moist-equilibrated coals. The absolute adsorption amount of CO2 at the equilibrium state for displacement is lower than that in single-component adsorption for both dry and moisture-equilibrated coals at equilibrium pressures below 3 MPa, whereas the adverse trend exists for a high equilibrium pressure range of 3–4 MPa. Moreover, the elevated CO2 injection pressure favors both CO2 adsorption and CH4 desorption on dry and moisture-equilibrated coals. The presence of H2O decreases the CO2 adsorption amount and CH4 desorption amount of the coals. Therefore, practical implementation of CO2-ECBM should focus on the H2O dependence of CO2 sequestration and CH4 recovery.