Comparison of enhanced coalbed methane recovery by pure N2 and CO2 injection: Experimental observations and numerical simulation

Enhanced coal bed methane recovery (ECBM) displacement simulation experiments are a direct way to research on the gas displacement process. We carried out two sets of ECBM experiments by injecting pure N-2 and CO2 through four large parallel specimens (300 x 50 x 50 mm coal briquette). In the first experiment (N-2-ECBM), pure N-2 is injected at different pressures to a coal briquette saturated with CH4, in the second experiment (CO2-ECBM), pure CO2 is injected at the same injection pressures and various permeability to the same coal briquette outgassed and resaturated with CH4. The gas composition and outflow rate were measured with time. In order to master the displacement process, we employed a one-dimensional, two-phase (gas and solid) model. They are in accordance with our experimental results. Coal seam permeability significantly affects the effect of CO2 or N-2 flooding CH4. Under the same conditions, N-2 appears in the outlet in advance than CO2. But the N-2 displacement time (CH4 is completely displaced) is longer than CO2, namely CH4 gas in the coal is completely displaced by CO2 injection in advance. And flow in the output of CO2-ECBM is larger than that of N-2. judging from above, CO2 as injectant is superior to N-2. (C) 2015 Elsevier B.V. All rights reserved.