Experimental CO₂ Adsorption in Coal Versus Particle Size: Implications for CO₂ Sequestration

Solano-Acosta, Wilfrido¹, Maria Mastalerz¹, and Arndt Schimmelmann^2
1 Indiana Geological Survey, Bloomington, IN
² Department of Geological Sciences, Indiana University, Bloomington, IN

Gas adsorption experiments are traditionally performed on finely crushed coal samples, < 250 mm. These experiments allow for the estimation of the maximum volumes of gas that a coal can retain at any given pressure/temperature. In this study, an experimental setup originally designed to test uncrushed coal cores was used to investigate the influence of particle size on CO₂ adsorption characteristics in high-volatile bituminous coals from Indiana under conditions that are sub-critical for CO₂. For these coals, standard high-pressure adsorption isotherm data at reservoir temperature, 70 ºF, indicate a maximum adsorption capacity of ~650 scf/ton at 400 psi.

Various coal samples were crushed and sieved to different grain sizes. The particles used in the experiment ranged from very coarse > 32 mm to the standard fraction used elsewhere during adsorption experiments. Our results show a distinct increase in adsorption with a decrease in particle size, probably as a result of the increase in surface area made available for CO₂ and also the faster adsorption kinetics afforded by shorter diffusion pathways into the interior of smaller grains. A relative increase of 20 percent in CO₂ adsorption was observed from the predominantly coarse fraction, 86 % >16 mm to the more finely dominated fraction, 100% <1 mm.

The dependence observed between grain size and adsorbed CO₂ volumes may help predict the adsorption behavior of coal reservoirs under realistic temperature and pressure conditions. The evaluation of CO₂ adsorption characteristics on finely crushed and uncrushed samples is critical for CO₂ sequestration in coal beds.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004