Volume 54

Pore-Scale Heterogeneity and Salinity Impacts on CO2 Storage in Deep Saline Aquifers: A Microfluidic and Computational Investigation Rajat Dehury, Jitendra S. Sangwai

Abstract

Achieving net-zero emissions and curbing the long-term increase in global warming requires large-scale carbon dioxide (CO2) sequestration through various techniques. Among the plethora of options for large-scale CO2 storage, geological storage stands out as the most promising, given its expansive storage capacity and minimal environmental impact. Deep saline aquifers, in particular, have emerged as frontrunners due to their vast storage potential and broad global availability. In saline aquifers, various trapping mechanisms, including structural and stratigraphic trapping, residual or capillary trapping, solubility trapping, and mineral trapping, work synergistically to store CO2. The displacement dynamics of CO2-brine two-phase flow in pore-scale dictates the structural and residual trapping of CO2 in subsurface formations. In this regard, a highly heterogeneous porous microchannel was used to investigate the impact of salinity and porous heterogeneity on CO2-brine displacement dynamics. A computational fluid dynamics (CFD) simulation based on the volume-of-fluid (VOF) method was used to study the immiscible two-phase flow dynamics under deep reservoir extreme temperature and pressure conditions. Microfluidic experiments, conducted under a high-resolution digital microscope with different brine salinities, provide pore-scale flow visualization and quantification of trapped CO2. Meanwhile, CFD simulations on the same porous media consider thermophysical property changes for CO2 and brine under high-pressure and high-temperature conditions, elucidating their impact on CO2 trapping or storage. The observed unstable finger-like displacement pattern in the microchannel, attributed to very low capillary number (Ca) and viscosity ratio (M) values, results in CO2 channeling with limited water displacement, thus restricting CO2 saturation at around 50 %. Along with viscous fingering, small pore-throats pronounce the snap-off effects at the micro-scale, which increase the residual trapping of CO2 in the porous media. An increase in brine salinity increases viscosity, interfacial tension, and contact angle, leading to a more vertical sweep of water and increasing the CO2 storage capacity of a saline aquifer. Under reservoir conditions, the comparatively higher density of CO2 and increase in contact angle resulting from the adsorption of CO2 increases CO2 trapping in porous channels. This combined approach of microfluidic experiments and CFD simulations provided valuable visualization and insights into CO2-brine dynamics at the pore-scale, which would contribute significantly towards efficient, enhanced, and secure storage of CO2.

Keywords capillary number, CO2 geo-sequestration, micromodel, porous media, saline aquifers, snap-off effect