Influence of Formation Water Salinity/Composition on the Low-Salinity Enhanced Oil Recovery Effect in High-Temperature Sandstone Reservoirs

The mechanism of the low-salinity (LS) enhanced oil recovery (EOR) process in sandstone reservoirs has been debated in the literature for more than a decade. We recently proposed a chemical wettability alteration mechanism for the process, well-founded in experimental observations. Even though the chemical understanding is quite well-described, there are parameters/factors that could influence the main process. Combinations of certain reservoir minerals, temperature, and salinity/composition of the formation water (FW) could have impact on the EOR process by affecting: (1) the initial wetting condition and (2) the wettability alteration process when the high-salinity (HS) FIAT is displaced by LS water. It has been experimentally observed that the LS EOR effect decreases as the reservoir temperature increases. This paper discusses the LS EOR effect related to oil recovery at high temperatures, T-res > 100 degrees C, and high FIAT salinities, similar to 200 000 ppm. In general, the adsorption of active organic polar components onto clay minerals decreases as the temperature and salinity of the FIAT increases. As a result, the rock becomes more water-wet, and the LS EOR potential is decreased. Oil recovery tests at 110 degrees C with HS FW of similar to 200 000 ppm did not show any LS EOR effects, with either seawater (SW) or 50 times diluted seawater (d(50)SW) as LS fluids. However, when the FW salinity was reduced to similar to 23 000 ppm, a LS EOR effect was observed. The combination of a high reservoir temperature and HS FW is most likely not favorable for observing LS EOR effects. The desorption of Ca2+ ions from the clay surface is reduced as a result of both dehydration at high temperature and the common ion effect by dissolution of CaSO4 if the formation contains anhydrite.