Methane and carbon dioxide adsorption–diffusion experiments on coal: upscaling and modeling

Abstract

Numerical modelling of the processes of CO 2 storage in coal and enhanced coalbed methane (ECBM) production requires information on the kinetics of adsorption and desorption processes. In order to address this issue, the sorption kinetics of CO 2 and CH 4 were studied on a high volatile bituminous Pennsylvanian (Upper Carboniferous) coal (VR r=0.68%) from the Upper Silesian Basin of Poland in the dry and moisture-equilibrated states. The experiments were conducted on six different grain size fractions, ranging from <0.063 to ∼3 mm at temperatures of 45 and 32 °C, using a volumetric experimental setup. CO 2 sorption was consistently faster than CH 4 sorption under all experimental conditions. For moist coals, sorption rates of both gases were reduced by a factor of more than 2 with respect to dry coals and the sorption rate was found to be positively correlated with temperature. Generally, adsorption rates decreased with increasing grain size for all experimental conditions. Based on the experimental results, simple bidisperse modelling approaches are proposed for the sorption kinetics of CO 2 and CH 4 that may be readily implemented into reservoir simulators. These approaches consider the combination of two first-order reactions and provide, in contrast to the unipore model, a perfect fit of the experimental pressure decay curves. The results of this modeling approach show that the experimental data can be interpreted in terms of a fast and a slow sorption process. Half-life sorption times as well as the percentage of sorption capacity attributed to each of the two individual steps have been calculated. Further, it was shown that an upscaling of the experimental and modelling results for CO 2 and CH 4 can be achieved by performing experiments on different grain size fractions under the same experimental conditions. In addition to the sorption kinetics, sorption isotherms of the samples with different grain size fractions have been related to the variations in ash and maceral composition of the different grain size fractions.