Enhanced Coal Bed Methane (ECBM) Recovery by Injecting Different Gas Components

In order to investigate the effects of different injected gas components for enhancing coalbed methane (ECBM), a multifield coupling system is independently developed for the gas injection displacement of coal-containing gas under stress loading. Experimental research has been conducted on the effects of different injection gas components, including flue gas, mixture, N2, CO2, and air for the ECBM experiments. The results show that according to the variation law of each gas volume fraction in the gas mixture at the outlet of the triaxial pressure chamber, ECBM can be divided into four stages. Stage I: the volume fraction of CH4 is maintained at 100%. Stage II: N2 breaks through the coal sample; the gas is a mixture of CH4 and N2 gas. Stage III: O2 breaks through; the gas is a mixture of CH4, N2, and O2 gas. Stage IV: CO2 breaks through; the gas is a mixture of CH4, CO2, O2, and N2 gas with gradually increasing CO2. Additionally, when N2, air, and flue gas are injected into coal samples, the instantaneous flow shows an L-shaped pattern of “rapid decrease-slow decrease-maintain constant”. However, when coal samples are injected with a mixture and CO2, the instantaneous gas flow shows a V-shaped pattern of “rapid decrease-slow decrease-slow increase-maintain constant”. The CH4 recovery rate is finally of the order CO2 >mixture > flue gas > air> N2, with 99.6, 95.5, 93.1, 90.0, and 88.5%, respectively. Therefore, the higher the volume fraction of CO2 in the injected gas component, the more thorough the desorption of CH4, and the longer the time required for ECBM.