Impact of injection pressure on CO2-enhanced coalbed methane recovery considering mass transfer between coal fracture and matrix

- Results of coal core flooding tests under different injection pressure are presented.
- The influence of the mass transfer rate on the production efficiency is investigated.
- The mass transfer rate and the dispersion coefficient are affected by the injection pressure.

Although the production mechanism of CO2-enhanced coalbed methane (ECBM) recovery has been previously reported and corresponding models have been proposed, investigations on the effect of the injection pressure on the production efficiency considering the mass transfer between the coal fracture and matrix are lacking. In this study, laboratory core flooding experiments were conducted on the coal sample from South Sichuan Basin, China. Experimental results indicate that earlier breakthroughs and a higher initial CH4 flow rate and cumulative CH4 flow rate occurred under a higher injection pressure in the CO2-ECBM recovery. However, as CO2 was continuously injected, the CH4 flow rate and cumulative CH4 flow rate under a lower injection pressure exceeded those at the higher injection pressure. Furthermore, a dual porosity model was used to perform numerical simulations. The simulation results under four injection pressures matched the experimental results well. The simulation results indicate that the dispersion in fractures and the mass transfer rate between matrices and fractures significantly affect the CH4 flow rate. The injection pressure leads to changes in the experimental observations by affecting the dispersion and the mass transfer rate.