EOR in chalk: Optimized oil recovery with modified seawater

Abstract

The effect of using seawater or modified seawater as a wettability modifier in chalk has been previously investigated at higher temperatures. This work has shown that “smart water” improves the oil recovery due to wettability alteration at temperatures above 90°C. In this work, the effect of modifying the salinity and the ionic composition of seawater on oil recovery from chalk is studied at lower temperatures, 90 and 70°C.respectively. The chalk cores with 10 % initial water saturation were saturated ,flooded and aged with the synthetic crude oil (with AN= 0.5mgKOH/g oil). Then, the cores were imbibed with different imbibing fluids. The brine VB0S, with no ability to change the wetting condition of the core was used as the reference brine. Seawater was modified by reducing the concentrations of the non-active ions, Na  and Cl  .Also, the concentration of sulfate which has a catalytic effect on the wettability alteration was increased. The chromatographic wettability test was used to determine the water-wet surface area of the chalk cores after the spontaneous imbibition tests. The experimental work showed improved oil recovery when modified seawater was used both at 90 and 70°C. The surface active ions (SO2 4 - and Ca 2 ) had easier access to the chalk surface by reducing the concentrations of the non-active ions in seawater. Increasing the sulfate concentration, improved both imbibition rate and ultimate recovery. For a slightly water-wet system, “smart water” was able to improve the oil recovery at temperatures down to 70°C. The wettability alteration process was confirmed by the chromatographic wettability tests. The tests indicated that the water-wet surface area in the cores increased with increasing “smartness” of seawater.