Sorption behavior of coal for enhanced gas recovery and carbon sequestration

The concept of displacement of sorbed methane and enhancement of coalbed methane recovery by injecting CO2 into coal reservoirs, while simultaneously sequestering the CO2, has been studied over the last few years. Preferential adsorption of CO2 over methane is considered to be responsible for the ability of the former to displace the adsorbed methane. The primary objective of this study was to investigate the ability of CO2 to displace and enhance the recovery of sorbed methane, when injected in coals after partial production. A secondary objective was to study the sorption of methane and CO2 on coals to investigate the degree to which coal exhibits preferential sorption for one over the other.; During the first experimental phase, single component adsorption experiments were carried out using pure methane and CO2 on coals from the San Juan and Illinois Basins, and the relative (CO2:CH4) sorption values were calculated. As a part of this study, applicability of different adsorption models, such as Langmuir, Dubinin-Polanyi (D-P), and BET equations were tested for adsorption of methane and CO2 on coals. In the second phase, CO2 injection experiments were carried out after partial desorption of methane to study its effect on desorption of additional methane. Finally, numerical simulation was undertaken to estimate the incremental methane recovery by injection of CO2 as compared to production by the primary recovery method.; The results showed that CO2 adsorbed preferentially (approximately 3 times more) over methane. CO2 injection experiments showed that, in most cases, injected CO2 displaced all of the sorbed methane, while the total pressure was maintained at high levels. However, this effect was not universal. In some cases, injection of CO2 was effective only until a certain point, after which it did not result in recovery of additional methane. In other cases, the injected CO2 led to re-adsorption of methane, thus delaying the methane recovery. The results also suggested that competitive sorption, as in the case of CO2 injection in coals, is very complex and the mechanism may be significantly different from that in pure gas adsorption. The D-A equation best fit the adsorption data for both methane and CO2 for the pressure range tested. The Langmuir equation also gave a good fit, while the BET equation did not fit well. The simulation study showed that it is possible to obtain significant improvement in methane recovery by injection of CO2.