Experimental investigation on the dominating fluid-fluid and rock-fluid interactions during low salinity water flooding in water-wet and oil-wet calcites

摘要

Low salinity water flooding (LSWF) has the potential to enhance the oil recovery by affecting the fluid-fluid and rock-fluid interactions. Therefore, a systematic investigation on the effect of initial wetting state (water-wet or oil-wet) of pure calcite is conducted to study the importance of these interactions on the effectiveness of LSWF. In the case of initially water-wet cores, more oil recovery efficiency is observed for more saline water cases. To shed light on the possible involved mechanisms, dynamic IFT, dynamic contact angle (CA), oil/brine and rock/brine surfaces zeta potentials, and effluent pH are measured. It is shown that the short-term effect of IFT reduction and long-term effect of wettability alteration toward neutral-wetness (due to change in the electric charge of the crude-oil/brine and calcite/brine) are the dominating recovery mechanisms. However, the oil recovery behavior of oil-wet cores is in the opposite direction of that of water-wet cores, which demonstrates the wettability dependency of the involved mechanisms. CA measurement shows that diluted sea water (dSW) has more ability to change wettability toward water-wetness. The zeta potential results show that as brine salinity reduces, both crude-oil and rock surfaces become more negatively charged. Therefore, the observed wettability alteration behavior with salinity reduction is attributed to larger repulsion between these surfaces at lower salinity. pH results suggest CO_3~(2-) ions are responsible to make calcite surface more negative with diluting SW brine. The oil recovery behavior is also a strong function of the initial wetting state. SW brine, recovered less oil and experienced an earlier breakthrough in an initially oil-wet core compared to an initially water-wet core. Unlike the previous case, dSW experienced higher ultimate oil recovery and almost the same breakthrough time in an initially oil-wet core compared to an initially water-wet core. The superior performance of the low salinity water in an oil-wet core is a result of the wettability alteration toward more water-wetness which triggers more piston-like advancement.