Commercial CBM production is almost exclusively through reservoir pressure depletion (primary recovery). Enhanced coalbed methane recovery (ECBM), involving injection of pure N2/CO2 or a mixture of the two, such as power plant flue gas, is a technology that has the potential to recover a larger fraction of methane in place without excessively reducing the reservoir pressure. CO2-ECBM has an added benefit that a potentially large volume of greenhouse gas can be stored in deep coal seams globally. The use of numerical models is essential in the development of the ECBM and CO2 storage technology. The quasi-steady state Fickian diffusion equation, together with the Langmuir equation, has been almost exclusively used in numerical simulation of primary coalbed methane production. Assuming that the individual gas component in a multicomponent system diffuses independently, the quasi-steady state Fickian diffusion equation can be readily extended to model mixed-gas diffusion. In the recent years, attempts have been made to use this approach, in conjunction with the extended Langmuir equation, to model ECBM/CO2 storage field pilots as well as laboratory flooding tests. However, this modelling approach for mixed-gas diffusion lacks theoretical rigour as the interaction between different gas components is not accounted for, though the practical significance of doing so in coalbeds is not well understood at present.
A reservoir simulation sensitivity study on the Yubari CO2 storage pilot project, Japan, is presented. The sensitivity study with CO2 and CH4 sorption times indicates that the sorption time for CO2 has a much pronounced impact on the back-produced gas composition. The study has also highlighted the limitation of the extended Langmuir equation for ternary system predictions. Although inconclusive, the sensitivity results suggest that the N2 component sorption in the coalbed reservoir is over-estimated by at least 20% by the model.