Understanding gas capillary entrapment in sandstone and carbonate aquifer rocks: Impact of gas type and pore structure

Capillary entrapment plays a pivotal role in the long-term and secure storage of CO₂ in subsurface formations. Previous research has indicated that capillary entrapment of gases in porous media is predominantly influenced by the wettability of the rock and its pore structure. However, there is some ambiguity regarding the influence of pore surface hydrophilicity and pore structure on the efficiency of gas capillary entrapment. Certain literature has reported comparable gas entrapment efficiencies for different gas types within the same rock, despite the change in rock wettability due to different fluid pairs. Conversely, other studies suggested that gas capillary entrapment efficiency is contingent upon the type of injected gas. Therefore, this study aims to investigate gas capillary entrapment efficiency using different gases, specifically CO₂ and N₂, in rocks exhibiting varying degrees of hydrophilicity and pore structure, namely sandstone and carbonate rocks. High-pressure, high-temperature core flooding experiments were conducted to examine drainage/imbibition cycles under reservoir conditions. Gas saturation profiles along the core length were derived from X-ray scan results obtained during the experiments, while contact angles were measured to quantify rock sample wettability under experimental conditions. In-depth characterization of the rock samples was achieved through NMR surface relaxometry measurements to ascertain pore size distribution and average pore sizes, along with the collection and analysis of thin sections to obtain two-dimensional images for further insights into pore and pore throat sizes. Our findings suggest compelling evidence that gas type does not significantly influence gas residual trapping efficiency in the tested sandstone and carbonate rocks but does impact sweep efficiency within the rock. Furthermore, residual trapping efficiency increased under conditions favoring snap-off trapping, characterized by a high pore body-throat aspect ratio. Thus, while changes in rock wettability due to different injected gas types had negligible effects on the capillary trapping efficiency of specific samples, pore structure emerged as the predominant factor.