Output gas stream composition from methane saturated coal during injection of nitrogen, carbon dioxide, a nitrogen–carbon dioxide mixture and a hydrogen–carbon dioxide mixture

Abstract

Proven global methane reserves can be increased from 177 Tm3 to 210–308 Tm3 (+ 19 to 74%) if 50% of global coalbed methane resources are deemed producible. As primary recovery methods for CBM produce at most 50% of the methane present, there is considerable interest in enhancing recovery by gas injection. It is not clear which composition of the injection gas is most efficient for enhancing methane recovery as the physical and chemical processes associated with gas injection are not fully understood. A previous experimental study has shown that the time for breakthrough increases with increasing sorption capacity of the injected gas when injecting a mixture of gases in a methane saturated coal. The aim of this study is to investigate how the properties of a weakly sorbing gas influence the composition of the produced gas stream when injecting a binary mixture in a CH4 saturated coal. Therefore, the composition of the produced gas is compared for the injection of a mixture of 20 vol.% CO2 in N2 and of 20 vol.% CO2 in H2 in an intact dry CH4 saturated cylindrical coal sample at a pore pressure of 8.0 ± 0.1 MPa and a temperature of 318 ± 1 K. Sorption capacity decreases in the order CO2 ≻ CH4 ≻ N2 ≻ H2. i.e., the methane is displaced by a binary gas mixture of which the main component has a lower sorption capacity and the secondary component has a higher sorption capacity than the methane. The production of H2 when injecting a mixture of CO2 and H2 is earlier than the production of N2 when injecting a mixture of CO2 and N2. This agrees with the observed trend that the breakthrough time of an injected component increases with the components sorption capacity. The production of CO2 does not change if a mixture of CO2 and H2 is injected instead of a mixture of CO2 and N2 mixture. i.e., the properties of the weakly sorbing component have minimal influence on the retention of the strongly sorbing component.