The behavior and efficiency of methane displaced by CO2 in different coals and experimental conditions

Abstract

CO2 sequestration into coalbed methane reservoirs not only reduces the greenhouse effect by removing CO2 from the atmosphere, but also recovers additional methane from coal which makes coalbed methane (CBM) production more economically attractive. An experimental study on the displacement of methane by CO2 has been carried out using eight bituminous and anthracite coals, to evaluate the displacement efficiency and utilization ratio of CO2. In our experiments, the coal samples were partially saturated with methane, and different amounts of CO2 were then injected. After every completed displacement, the T2 distributions were measured using low-field nuclear magnetic resonance (NMR). The results show that the P1 of the T2 spectra clearly decreased, which indicates that methane was desorbed as a result of CO2 injection. The amount of the adsorbed CO2 and desorbed methane were obtained from a combination of the T2 spectra and the CO2 gas pressure. By comparing the amounts of adsorbed CO2 and desorbed methane, an effective displacement capacity of CO2 has been defined, which varies with coal properties, temperature, and moisture content. The higher rank coals have a lower VL-CO2/VL-CH4 and show therefore a stronger effective displacement capacity. In addition, high temperature, and high moisture content weaken the effective displacement capacity of CO2. The study suggests that the utilization ratio of CO2 is higher in high-rank coals, which makes them more suitable for CO2-enhanced CBM production.