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

Coalbed methane (CH4) is an important energy source. With increasing climate change concerns, coal beds are also considered potential sinks for underground carbon dioxide (CO2) storage.(1,2) Roughly twice the amount of CO2 can be adsorbed than CH4 in most bituminous coals.(1) Another advantage of CO2 injection into coalbeds is the additional production of CH4. This process is known as CO2-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. ECBM field operations can be better managed by an improved understanding of laboratory measurements and coalbed reservoir simulations.(6) Both CH4 and CO2 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 heterogencity(11) and complex response to CO2 adsorption.(12) In this study, molecular simulations are used to identify energetically favorable adsorption regions, thereby testing the molecular exchange of CH4 by CO2.(13,14) Our goal is to discuss the adsorption distribution at a molecular scale.