Dynamic characterization of multiphase methane during CO2-ECBM: An NMR relaxation method

Injecting CO2 into coal reservoirs is essential both for enhancing coalbed methane recovery (CO2-ECBM) and CO2 geological sequestration. However, due to the difficulty in simultaneously monitoring multiphase methane (adsorbed and free phases) by conventional testing methods, the real-time variations in different phases of methane throughout the whole process of CO2-ECBM still remains to be clarified. In this study, with the introduction of self-designed nuclear magnetic resonance (NMR) measurement, three subbituminous and anthracite coals collected from deep-well drilling were used to investigate the dynamic characteristics of multiphase methane in CO2-ECBM. The results show that the adsorbed methane desorption efficiency could be additionally improved by similar to 14%-26% with the injection of CO2 after conventional reservoir depressurization. In the process of CO2-ECBM, three different CO2-CH4 displacement rates emerged: the rate rapidly decreased during the initial CO2 injection time period and then began to slowly decrease until reaching CO2-CH4 competitive-adsorption equilibrium. Comparison of the NMR transverse relaxation (T-2) characteristics of CO2(-)ECBM after natural and in-situ methane adsorption suggests that a low concentration ratio of CH4/CO2 could significantly improve the methane recovery. The promising results from the CO2-CH4 displacement experiments demonstrate the great significance of the CO2-ECBM method in enhancing methane recovery and CO2 geological sequestration for coals.