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

Abstract

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.