Coalbed methane characterization and modeling: review and outlook

Recently, coalbed methane (CBM) has become one of the major unconventional gas resources in the world. In 2017, the US CBM proved reserves were estimated at 336 Billion Cubic Meter (BCM) while the production was about 27 BCM. Coal formations possess unique characteristics that necessitate accurate characterization and fundamental understanding of the prevailing mechanisms. For instance, gas sorption processes in CBM require a comprehensive characterization integrating sorption factors such as the pressure, temperature, moisture content, ash content, and gas adsorption capacities. In addition, the uncanny gas diffusion behavior in CBM formations poses a challenge for accurate gas diffusion quantification accounting for the unusual varying nature of the gas diffusion coefficient with pressure and temperature. Besides, the stress-sensitivity nature of the CBM permeability still represents an active research area where experimental measurements are needed to develop relationships for the modeling studies. Additionally, CBM reserve estimates are highly dependent on the in-place gas contents. Therefore, one needs to consider not only the adsorbed gas content, but also the free and soluble gas contents – particularly in lower rank CBM reservoirs. Consequently, it is challenging to predict the fluid behavior in CBM formations given the uniqueness and complexity of the governing mechanisms. In this review, we are providing both an overview of the CBM’s fundamental and the recent research studies that revolve around the major aspects of CBM’s characterization and modeling.