MONITORING CO2 ADSORPTION INTO THE COAL: APPLICATION OF SANS AND USANS TECHNIQUES

Injection of CO₂ in unmineable deep coal seams is one of the options of geologic CO₂ sequestration. Whereas it has been demonstrated that organic matter has high gas adsorption capacity, the mechanisms and the consequences of this adsorption in subsurface conditions are poorly understood. Small-angle scattering techniques can provide unique, pore-size-specific insight into the density of adsorbed CO₂. This study reports the results of the first small-angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) studies on coal, using the Seelyville Coal from the Illinois Basin as an example. Experimental conditions employed in this work were chosen to simulate a range of coal subsurface conditions including those at 518 feet depth (P,T) = (1-50 bar, 16ºC), and the coal was saturated with subcritical CO₂. Experimental results illustrate that coal microstructure is unaffected by pressurised subcritical CO₂, and these findings suggest that depths of burial do not constitute a stability barrier to storage of CO₂. The physical density of CO₂, fluid phase adsorbed in the porous coal matrix exceeds by a factor of 3-4 the density of the bulk fluid at the same thermodynamic conditions. The applied methodology can be extended to studies of the sorption kinetics and capability of other naturally occurring porous materials of interest for carbon geological storage (saline aquifers, porous rocks, basalts, etc.) as well as investigations of supercritical fluid mixtures (e.g. CO₂ and methane) in various coals.