Molecular Exchange of CH4 and CO2 in Coal: Enhanced Coalbed Methane on a Nanoscale†

Coalbed methane (CH 4 ) is an important energy source. With increasing climate change concerns, coalbeds are also considered potential sinks for underground carbon dioxide (CO 2 ) storage. 1,2 Roughly twice the amount of CO 2 can be adsorbed than CH 4 in most bituminous coals. 1 Another advantage of CO 2 injection into coalbeds is the additional production of C H 4. This process is known as CO 2 -enhanced coalbed methane (ECBM) and is very attractive considering the increasing energy demand. Several ECBM pilot programs have been implemented in several parts of the world; 3,4 however, its full potential has yet to be exploited. 5 ECBM field operations can be better managed by an improved understanding of laboratory measurements and coalbed reservoir simulations. 6 Both CH 4 and CO 2 are predominantly stored in the micropores of the coal matrix, 7 which appear as "isolated" locations. 8 No consensus exists on measuring the total surface area, 9 the heat of adsorption, 1 or the theoretical description of adsorption, 10 partially because coal is difficult to characterize as a result of its heterogeneity 11 and complex response to CO 2 adsorption. 12 In this study, molecular simulations are used to identify energetically favorable adsorption regions, thereby testing the molecular exchange of CH 4 by CO 2 . 13,14 Our goal is to discuss the adsorption distribution at a molecular scale.