Methane/CO2 Sorption Modeling for Coalbed Methane Production and CO2 Sequestration

Abstract

A thorough study of the sorption behavior of coals to methane and carbon dioxide (CO 2 ) is critical for carbon sequestration in coal seams and enhanced coalbed methane recovery. This paper discusses the results of an ad/de -sorption study of methane and CO 2, in single gas environment, on a set of coal samples taken from the San Juan and Illinois Basins. The results indicate that, under similar temperature and pressure conditions, coals exhibit higher affinity to CO 2 as compared to methane and that the preferential sorption ratio varies between 2:1 and 4:1. Furthermore, the experimental data were modeled using Langmuir, BET, and Dubinin−Polanyi equations. The accuracy of the models in quantifying coal−gas sorption was compared using an error analysis technique. The Dubinin−Radushkevich equation failed to model the coal−gas sorption behavior satisfactorily. For methane, Langmuir, BET, and Dubinin-Astakhov (D-A) equations all performed satisfactorily within comparable accuracy. However, for CO 2, the performance of the D-A equation was found to be significantly better than the other two. Overall, the D-A equation fitted the experimental sorption data the best, followed by the Langmuir and BET equations. Since the D-A equation is capable of deriving isotherms for any temperature using a single isotherm, thus providing added flexibility to model the temperature variation due to injection/depletion, this is the recommended model to use.