Quantification of wettability characteristics for carbonates using different salinities

Low salinity water flooding has received strong interest recently. It has been reported that injecting low salinity water can improve oil recovery in carbonate reservoirs through wettability alteration, whose mechanism however remains unclear, especially regarding the contributions from adsorbed polar organic components. This work investigates systematically the key parameters that govern the complex interactions of rock/brine/crude oil in carbonate reservoirs. The effects of crude oil composition and salinity on surface wettability alteration are studied by using three types of dead oil from Norwegian and North Sea with various acid numbers and two types of carbonate rocks (outcrop-calcite and subsurface-dolomite). Ion-substitution-adsorption is evaluated using the infrared spectrometer (IFR) and energy-dispersive X-ray spectroscopy (EDX). Oil/brine interactions by means of the interfacial tension (IFT) and contact angle as a function of oil composition, rock type, and salinity are experimentally investigated. The IFR and EDX data indicate a compositional change when the carbonate minerals are in touch with saline solutions, supporting the proposed rock-brine interaction mechanism. The salinity effect is more salient at the liquid/rock interface than the liquid/liquid interface, and the response to the brine composition is highly dominated by the oil composition with respect to its content of polar organic components. There is a reduction of contact angle as the seawater water is diluted twice while its influence on the IFT is small, and the effect is more pronounced for dolomite rock, showing different results for different crude oils used. This suggests that the wettability alteration of carbonate rock is an intricate phenomenon that is influenced not only by the salinity but also by the chemical composition of crude oil and rock mineralogy. A combination of three possible mechanisms, including multiple ion exchange, EDL expansion (electrokinetics repulsive), and salting-out, is discussed to illustrate the mechanism of wettability alteration during low salinity injection.