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

•We conducted experimental and numerical studies of enhanced CBM recovery under larger dimension and tighter coal.•We developed a set of displacement device.•We compared Influence of gas species on displacement efficiency.•We researched on the influence of gas injection pressure on displacement efficiency.•We researched on the influence of the stress (permeability) on displacement efficiency.

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 N2 and CO2 through four large parallel specimens (300 × 50 × 50 mm coal briquette). In the first experiment (N2-ECBM), pure N2 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 N2 flooding CH4. Under the same conditions, N2 appears in the outlet in advance than CO2. But the N2 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 N2. Judging from above, CO2 as injectant is superior to N2.

Graphical abstractIn this work, we evaluated how the injection pressure and stress influence displacement coalbed CH4 process and efficiency by CO2 and N2 injection.Figure optionsDownload full-size imageDownload high-quality image (174 K)Download as PowerPoint slide