Waterflooding of Preferential Oil-Wet Carbonates: Oil Recovery Related to Reservoir Temperature and Brine Composition

Abstract

Abstract Much effort has been focused on wettability modifying methods to improve the water-wetting nature of carbonates in order to enhance the oil recovery by spontaneous imbibition of water. The use of expensive surface active additives like cationic surfactants of the type R-N(CH3)3+ have been suggested, as well as steam injection. In this study, we will discuss possible wetting modifications of carbonates in relation to potential determining ions present in the injected fluid. Artificial seawater is used as the base or reference imbibing fluid. Outcroup chalk cores of high porosity and low permeability and reservoir limestone cores were saturated with oils with high acid number at residual water saturation to render the cores preferential oil-wet. Spontaneous imbibition tests were performed at different temperatures ranging from 70-130 °C using modified seawater with various concentrations of sulfate, which is regarded as a potential determining ion towards carbonates. Major observations were: (1) Spontaneous imbibition of seawater took place only at elevated temperatures, (2) For the chalk samples, the oil recovery increased beyond the recovery at completely water-wet conditions when the concentration of sulfate was increased 3 times relative to seawater at 130 °C, (3) Reservoir limestone cores also responded with increased oil recovery at 120 °C as the sulfate concentration increased, (4) at lower temperatures, increased spontaneous imbibition was obtained when adding cationic surfactant to the imbibing fluid, (5) The activity of sulfate as a potential determining ion, and thus a wettability modifier, appeared to increase as the temperature is increased. The results show that sulfate is a very efficient wettability modifying agent towards carbonates at elevated temperatures.