CO2-Induced displacement and diffusive transport of shale geofluids in silica nanopores of varying sizes

Abstract

The need to recover our energy resources in a sustainable manner necessitates the development of technological approaches to reduce water use and enhance the use and storage of alternative processing fluids such as CO2. These societal and technological needs require a fundamental understanding of the pore-scale organization and transport behaviors of confined fluids. Therefore, the aim of this study is to establish a molecular-scale basis for evaluating CO2-induced selective adsorption and diffusive transport behavior of hydrocarbon fluids such as propane and toluene in silica nanopores with sizes in the range of 2–10 nm. The studied fluids are composed of CO2-propane, CO2-propane-toluene, and CO2-toluene. CO2 was found to adsorb preferentially to the silica surfaces by forming hydrogen bonds with the surface hydroxyl (−OH) groups. The selective adsorption of CO2 over propane was significantly higher compared to toluene at the pore sizes in the range of 2–10 nm. Increasing pore size reduces the extent of CO2, propane and toluene adsorbed on the silica surfaces while the corresponding diffusivities increased. Anisotropic diffusivities of the confined molecules were noted. More polar and higher molecular weight constituents of the oil such as toluene diffuse more slowly in confinement. These studies provide detailed scientific insights into the role of CO2 in enhancing hydrocarbon recovery from unconventional and complex porous environments. These molecular-scale investigations suggest that CO2 has a higher tendency to displace lighter aliphatic hydrocarbon molecules such as propane away from the surface as opposed to heavier hydrocarbon molecules such as toluene.