Wettability alteration during low-salinity water-flooding in carbonate reservoir cores

Production enhancement by low-salinity waterflood in carbonate formations is a subject of intense speculation. Several mechanisms are attributed to enhanced oil recovery by low-salinity waterflooding in carbonate formations. Review of experimental data in the literature indicates that the main mechanism involves interaction of Na+, Cl^-, Ca²⁺, Mg²⁺, SO₄^2- and crude oil carboxylate ions (R-COO^-) with the rock in the electrical double layer (EDL) near the surface of carbonate pores, leading to wettability alteration. In this study, we performed four seawater floods in heterogeneous low-permeability carbonate cores followed by low-salinity floods. The core permeability is between 0.5 to 1.5 md, and porosity in the range of 18 to 25%. Cores were aged for eight weeks at reservoir pressure and temperature. We also conducted pendant drop oil-brine IFT measurement, and captive oil-droplet contact angle at different brine salinity, with and without the presence of surfactant. The carbonate core flood results show that removing NaCl from seawater or diluting the seawater twice and four times yielded about 8% incremental oil. In one experiment, the change in the effluent ionic concentrations was measured, and it was observed a decrease in Ca²⁺, Mg²⁺, Cl^-, and SO₄^2-. Using pendant drop IFT measurements, oil-brine IFT increased with decreasing salinity both in presence and in absence of 1,000-ppm surfactant. From captive oil-droplet contact-angle measurements, it was observed that cleaned un-aged carbonate core slabs were water-wet, and became more water-wet as salinity decreased (both in presence and in absence of 1000-ppm surfactant). The wettability of crude-aged carbonate core slabs altered from oil-wet to intermediate-wet as salinity decreased. And, the wettability changed from intermediate-wet to water-wet with decreasing salinity in presence of 1,000-ppm surfactant. Moreover, addition of small amount of surfactant alters the wettability of crude-aged or cleaned un-aged carbonate core slabs towards water-wet. The degree of water-wetness achieved by surfactant solution depends on salinity level.