Low tension and wettability alteration EOR approaches for a high temperature high salinity carbonate reservoir

Carbonate reservoirs account for more than half of the world’s oil reserves. Many carbonate reservoirs have low oil recovery due to their heterogeneity and oil-wetness. This research work focuses on developing chemical enhanced oil recovery methods for a reservoir with high temperature (100°C), high salinity (35000 ppm) and light oil (40° API). The work tested deployment of both the ultra-low interfacial tension method and the wettability alteration method to increase the oil recovery. The ultra-low interfacial tension formulation was found to incorporate three surfactants at 0.5 wt% and one co-solvent at 0.75 wt%. It also used a polymer at 5000 ppm to control the mobility and increase the sweep efficiency. The solution was found by conducting phase behavior experiments on a range of surfactants and was tested in two sand packs made of the reservoir rock. The first flood (143mD) employed a salinity gradient and led to an oil recovery of 99.9%. The second flood (202mD) was conducted at constant salinity and led to an oil recovery of 98.1%. A single surfactant solution was found for the wettability alteration method. This process does not use any polymer. The solution was arrived at by conducting contact angle and spontaneous imbibition experiments. During the spontaneous imbibition experiment, the surfactant recovered 33% of total oil in place compared to only 10% recovered by the control sample (sea water). The surfactant was tested in a secondary core flood (0.17 mD) and led to an oil recovery of 54.3%. Additionally, it was tested in a tertiary core flood (2.87mD) and managed to increase the oil recovery from 51.9% during water flood to 69.2% after surfactant flood. Both methods have been successful in demonstrating an increase in oil recovery during a flood. The ultra-low interfacial tension approach could not be tested in reservoir core samples due to their low permeability and lack of polymer transport. While the wettability alteration approach has demonstrated a significantly less recovery than the ultra-low interfacial tension approach, it is expected to be much more cost effective due to the less amount of surfactant used and no use of polymersPetroleum and Geosystems Engineerin