(84c) Confined Fluid Phase Behavior of Carbon Dioxide in Nanoporous Media

Geological formations possessing nanometer-sized pores, such as shale reservoirs, are promising candidates for the sequestration of vast quantities of carbon dioxide. To improve the current understanding of the storage mechanisms in such geologic systems, further research on the fundamental physics of storage and phase behavior of CO₂ in the confinement of nanopores under varying conditions is needed. To this end, systemic experimental investigations utilizing model adsorbents of varying pore diameters at different temperatures can be employed. In this study, we use a novel gravimetric apparatus to study the phase behavior and capillary condensation of CO₂ in a silica-based nanoporous medium (MCM-41). Four different pore sizes (i.e., 6, 8, 10, and 12 nm) are used, and the adsorption and desorption isotherms are generated at temperatures ranging from -23.1 to 20 °C. The onset and end pressures of the capillary condensation and evaporation processes as well as the capillary condensation mean pressure, are reported for each pore size. The experimentally measured bulk condensation pressures are successfully compared against their counterparts provided by NIST. The results reveal that the capillary adsorption of CO₂ increases as the temperature decreases for the same pore size. Similarly, capillary adsorption increases as the pore size is reduced for a fixed temperature, confirming the predominance of the confinement effect in smaller pore sizes. The data generated under this study indicate that CO₂ can be stored as a liquid phase at, due to confinement, lower pressures in nanoporous materials than in bulk. The findings from this work can be used to develop and validate thermodynamic models capable of predicting the phase behavior of CO₂ in nanoconfinement.