AN IMPROVED INSIGHT INTO LOW-SALINITY WATERFLOODING: IN-SITU CHARACTERIZATION OF WETTABILITY ALTERATION AT ELEVATED PRESSURE AND TEMPERATURE CONDITIONS

摘要

Low-salinity waterflooding(LSWF)is known as one of the most effective improved oil recovery techniques that could result in significant additional recovery compared to conventional high-salinity waterflooding(HSWF). Although numerous laboratory studies have confirmed the effectiveness of LSWF under specific conditions,they have mostly failed to present explicit evidences on the pore-level mechanisms that are responsible for the observed improvement in recovery. In this study,we investigated LSWF production mechanisms using X-ray micro-computed tomography(micro-CT)and examined recovery trends at the pore scale. Two core-flooding experiments were performed on miniature reservoir sandstone core samples at elevated pressure and temperature conditions. The preserved core samples(5 mm in diameter)were cut and then saturated to establish reservoir initial saturation conditions. The samples were subsequently waterflooded with low-and high-salinity brines and imaged using a micro-CT scanner. Micro-CT images were then used to obtain fluid saturations and three-dimensional maps of fluid occupancy during each experiment. High-resolution micro-CT images were also analyzed to measure pore-scale contact angles and study in-situ wettability and its impact on oil mobilization during different stages of waterflooding process. The results highlight a significantly improved performance in LSWF compared to that of HSWF. The LSWF test showed more prolonged oil recovery response and gradual recovery at later stages while the HSWF recovery stabilized much earlier. Pore-scale contact angle measurements yielded direct evidence of wettability alteration from weakly oil-wet toward weakly water-wet conditions during LSWF;however,no considerable changes were observed in in-situ contact angles during the HSWF. Investigation of fluid occupancy maps along with in-situ characterization of wettability allowed us to establish a significantly better understanding of pore-scale mechanisms responsible for improved recover by LSWF.