Laboratory Investigation on the Permeability of Coal During Primary and Enhanced Coalbed Methane Production

Coalbed Methane (CBM) has grown into all important natural gas resource in North America in recent decades. The economical development of CBM heavily relies on coal permeability. The coal permeability is provided through a network of coal cleats and natural fractures. It is greatly affected by the confining stresses and the coal matrix response during gas adsorption and desorption in primary and enhanced recovery processes. This paper is focused on a laboratory investigation of coal permeability variability with different operating parameters. Core flood experiments in coal have been conducted with methane production and methane displacement by CO2. The permeability to different gases (CH4, CO2) under equilibrium conditions was measured at different confining stresses. The coal permeability to helium, which was reported in previous work, provides a comparison baseline. The permeability of a coal core to gases shows a strong dependence on net confining pressure and exhibits strong hysteresis. The CH4 permeability is relatively smaller than the corresponding He permeability, demonstrating a swelling effect of coal by CH4. The adverse effect of CO2 on coal permeability is also shown. The paper focuses on the experimental results obtained to date.