Experimental Analysis of Initial Wettability Effect on the Performance of Hybrid Engineered Water-Assisted Polymer Flooding

Residual oil saturation (Sor) after waterflooding is high in carbonate reservoirs, due to the wettability state, making them a lucrative candidate for novel enhanced oil recovery (EOR) methods such as engineered water/polymer flooding (EWPF). Initial wettability for such reservoirs must be considered during the preliminary screening for an effective EWPF application. This experimental study investigates the carbonate initial wettability criteria to achieve a successful EWPF design. Contact angles are measured at 80 °C to evaluate the temperature effect on wettability shift by engineered water (EW). Coreflood tests are conducted under different initial wettability conditions to analyse EWPF performance in weak, moderate, and strong oil-wet mediums. Capillary numbers and a corresponding reduction in Sor are estimated to develop a screening parameter. EW prepared by 10 times diluted Caspian seawater spiked with optimized active ions has caused a 40-55° reduction in contact angle, making the rock more water-wet. Sor after waterflooding is 25% higher in a strong oil-wet carbonate sample in contrast with a weak oil-wet condition, implying such reservoirs are appropriate candidates for EWPF application. Ion-tuned water flooding has provided 8% of OOIC incremental recovery in strong oil-wet medium while it has given a negligible recovery in the weak oil-wet case. Despite having similar capillary numbers, EWPF has resulted in a 16% more reduction in Sor in the strong oil-wet system compared to the weak oil-wet system. This study presented a preliminary screening criterion for EW-based hybrid methods by considering the initial wettability of a reservoir. The results show that the hybrid EWPF method can reduce Sor more than that predicted by capillary desaturation curves. Hence, it has a higher capillary desaturation tendency and can be successfully applied to carbonate formations that are not water wet.