Micro-scale experimental investigation of the effect of pore space deformation on trapping of supercritical CO2

Abstract

In this study, we investigate the impact of pore space deformation on various trapping mechanisms of supercritical CO2 (scCO2) relevant to storage of CO2 in deep saline aquifers. A series of unsteady-state two-phase brine/scCO2 drainage, imbibition, and dissolution tests were performed on a miniature sandpack sample under various stress conditions. High-resolution x-ray microtomography techniques were employed to acquire the pore and fluid occupancy maps at each step of the experiment, which were subsequently utilized to characterize the pore space deformation and changes in the frequencies of various pore-scale fluid displacement events. Our results revealed that, during the scCO2 injection stage, additional scCO2 was stored in the sandpack sample after the sandpack was compressed due to an increase in the pressure gradient even though the pore volume had decreased. On the other hand, the capillary trapping was impaired by the pore space contraction because all the imbibition displacement mechanisms at the pore scale, including the piston-like advance and snap-off, were facilitated as a consequence of an increase in their threshold capillary pressures. The increased frequency of snap-off events caused additional trapping, but more importantly, displaced scCO2 from throats by fragmenting large residual scCO2 clusters into smaller globules. During the dissolution process, the trapped scCO2 clusters and globules shrank in both pore bodies and throats, resulting in movement of the Main Terminal Meniscus (MTM) interfaces and snap-off events, respectively. The water relative permeability during the dissolution process decreased, at a fixed saturation, with the increase in the effective stress since the small pores occupied by the water phase deformed more significantly than the large pores.