Micro-scale experimental investigations of multiphase flow in oil-wet carbonates. I. In situ wettability and low-salinity waterflooding

Abstract

In this study, the impacts of temperature, initial water saturation, and aging time on wettability alteration during dynamic aging process were investigated at the pore scale, using a state-of-the-art high-temperature, high-pressure miniature core-flooding system integrated with a high-resolution micro-CT scanner. Furthermore, pore-scale displacement mechanisms impacting the waterflooding oil recovery under varying oil-wet conditions were probed. Subsequently, we investigated the wettability alteration and its corresponding impact on pore-scale displacement mechanisms during low-salinity waterflooding (LSWF). The results showed that the aging-induced wettability alteration reached an equilibrium condition after approximately seventeen days. With higher temperature, lower initial water saturation, and longer aging time, we observed greater in situ contact angles indicative of more oil-wet rock surfaces. Once brine was injected to displace oil, since the majority of pore elements were oil-wet after the aging process, pore-scale piston-like displacements (brine-to-oil drainage) had the largest contribution to the oil recovery. Based on our observations, the waterflooding-based oil recovery decreases as the porous medium becomes more oil-wet. This phenomenon is due to the higher threshold brine pressures of the displacements in more oil-wet conditions, which restricts brine invasion into medium and small size pores. Furthermore, wettability alteration towards neutral-wetness and the consequent reduction in threshold brine pressure required for the fluid to invade the midsize oil-filled pores, result in higher oil recovery by LSWF compared to high-salinity waterflooding (HSWF). The in situ analyses indicate that reservoirs with wide pore size distributions (a large number of midsize pores) could be better candidates for applications of LSWF EOR technique.