Comparison of Numerical Simulators for Greenhouse Gas Storage in Coalbeds, Part I: Pure Carbon Dioxide Injection

The injection of carbon dioxide (CO2) in deep, unmineable coalbeds is a very attractive option for geologic CO2 storage: the CO2 is stored and at the same time the recovery of coalbed methane (CBM) is enhanced. The revenue of methane (CH4) production can offset the expenditures of the storage operation. Coalbeds form complex gas reservoirs characterized by their dual porosity: they contain both primary and secondary porosity systems. The primary porosity system contains the vast majority of the gas-in-place volume while the secondary porosity system provides the conduit for mass transfer to production wells. Primary porosity gas storage is dominated by adsorption. Mass transfer for each gas molecular species is dominated by diffusion that is driven by the concentration gradient. Flow through the secondary porosity system is dominated by Darcy flow that relates flow rate to permeability and pressure gradient. A full understanding of all of the process mechanisms is essential to performing a numerical simulation of this CO2 storage process in which CO2 is injected into the coalbed to replace the adsorbed CH4. Existing CBM numerical simulators which are developed for the primary CBM recovery process, have many important features such as: (1) a dual porosity system; (2) Darcy flows of gas and water (i.e., multiphase flow) in the natural fracture system; (3) pure gas diffusion and adsorption in the primary porosity system; and (4) coal shrinkage due to gas desorption; taken into consideration. However, process mechanisms become more complex with CO2 injection. Additional features such as: (1) coal swelling due to CO2 sorption on coal; (2) mixed gas adsorption; (3) mixed gas diffusion; and (4) nonisothermal effect for gas injection; have to be considered. This paper describes the first part of a comparison study between numerical simulators for CO2 storage in coalbeds with pure CO2 injection. The second part of the comparison study will be for CO2 storage in coalbed with flue gas injection. Proposed problem sets are presented along with preliminary simulation results obtained from various numerical