Salt precipitation in carbonates during supercritical CO2 injection: A pore-scale experimental investigation of the effects of wettability and heterogeneity

Abstract

Salt precipitation in near-wellbore regions of subsurface reservoirs is a significant phenomenon that may occur during carbon dioxide (CO2) injection for carbon sequestration and enhanced oil recovery. It induces pore plugging, which causes reductions in porosity and permeability and hence well injectivity. Previous research in this area focused predominantly on salt precipitation in fully brine-saturated media, such as those found in deep saline aquifers. However, studies on salt precipitation in hydrocarbon reservoirs where brine and oil coexist are rare, particularly at the pore scale. In this study, we elucidate the occurrence and dynamics of salt precipitation at the pore-scale during supercritical CO2 injection into depleted carbonate and sandstone oil reservoirs at elevated temperature and pressure conditions. Specifically, we examine the influence of wettability and morphological heterogeneity on salt precipitation evolution and dynamics. Investigations on the influence of wettability reveal that the time required for the initiation of precipitation is shorter for an intermediate-wet carbonate compared to that for a weakly oil-wet carbonate. Relatively higher amounts of salt deposits are formed in the more hydrophilic (intermediate-wet) carbonate, leading to an intensification of porosity reduction in this rock. In addition, the presence of oil within the pore space does not hinder the precipitation process but suppresses the reverse flow of solutes toward the evaporation front, thereby creating localized precipitation at the front. The level of heterogeneity affects the precipitation initiation and its duration as well as the amount of precipitates formed, with lower salt saturations encountered in the less heterogeneous sandstone compared to the more heterogeneous carbonate. Heterogeneity also influences the salt cluster size distribution within pore elements such that, for the same wettability state, medium-sized clusters dominate in sandstone, while large clusters are the main contributors to the total salt volume in the more heterogeneous carbonate. This study forms a fundamental basis for the prediction of salt precipitation during three-phase flow processes induced by supercritical CO2 injection into porous media containing high-salinity brine and oil.