An Experimental Investigation into the Impact of Sulfate Ions in Smart Water to Improve Oil Recovery in Carbonate Reservoirs

Carbonate reservoirs are considered to be in the range of mixed to oil wetting state. Their present wetting state seems unfavorable to recover more oil via conventional waterflooding especially as the water cut increases with huge residual oil saturation. In recent years, studies have shown that tuning the injected brine chemistry can help decrease the oil-wetness, resulting in additional oil recovery and reduction in residual oil saturation. Thus, rock wettability is being dictated by the surface chemistry associated with the water-film stability between crude oil and the rock surface. This study presents series of experiments conducted on Middle Eastern carbonate core plugs at high pressure and high temperature in order to determine the impact of sulfate ions in smart brine on oil recovery. Each experiment was initially conducted using the formation brine, and subsequently, various versions of seawater with varying sulfate concentrations were used. Coreflooding experiments showed an additional recovery of about 7-9 % oil originally in core when sulfate concentration was increased while none when decreased. Decrease in rock oil-wetness was observed to be responsible for the improved recovery as demonstrated by the contact angle analysis. Similar trend was observed with the zeta potential analysis which further proved that the magnitude of surface charge alteration contributes immensely to wettability modification toward more water-wetness.