Experimental investigation and analysis on the changing characteristics of the gas flow capacity of coal in CO2-ECBM and N2-ECBM

Compared with many premodification techniques, such as hydraulic fracturing, the improvement in the reservoir permeability lasts throughout the production in the enhanced coalbed methane recovery (ECBM) technique which involves the injection of a gas into the coal reservoir. To determine the changing characteristics of the gas flow capacity of coal in ECBM, N2 core flood and CO2 core flood experiments on CH4 saturated coal were carried out in this study, where the gas composition, coal permeability and total outflow rate were measured. The relationships of the coal permeability in the experiments were presented as: N2-ECBM - k max > k steady > k initial, CO2-ECBM - k max > k initial > k steady, where k initial is the permeability of the CH4 saturated coal. Meanwhile, the trends of both the CH4 composition and the CH4 outflow rate in the two experiments are quite different at the post stage, in which the CH4 composition and the CH4 flow rate in the outlet are stable in the N2 core flood experiments but constantly decrease in the CO2 core flood experiments. Then, this study determined that the pressure hysteresis (reference transient permeability) plays an important role in the foregoing experiment results in both seepage and diffusion. A conceptual model for the distribution of the diffusivity around the seepage aperture in ECBM was proposed, which not only reveals the uneven distribution of the diffusivity in the coal matrix but also provides a new method for further numerical simulations.