Insights into the Mechanism of Wettability Alteration by Low-Salinity Flooding (LSF) in Carbonates

Abstract

The low-salinity effect (LSE) in carbonate rock has been less explored in comparison to sandstone rock. Laboratory experiments have shown that brine composition and (somewhat reduced) salinity can have a positive impact on oil recovery in carbonates. However, the mechanism leading to improved oil recovery in carbonate rock is not well understood. Several studies showed that a positive low-salinity flooding (LSF) effect might be associated with dissolution of rock; however, because of equilibration, dissolution may not contribute at reservoir scale, which would make LSF for carbonate rock less attractive for field applications. This raises now the question whether calcite dissolution is the primary mechanism of the LSF effect. In this paper, we aim to first demonstrate the positive response of carbonate rock to low salinity and then to gain insight into the underlying mechanism(s) specific to carbonate rock. We followed a similar methodology as in sandstone rock [Kinetics of low-salinity-flooding effect Mahani H. Berg S. Ilic D. Bartels W.-B. Joekar-Niasar V. Mahani H. Berg S. Ilic D. Bartels W.-B. Joekar-Niasar V. SPE J.201520820, DOI: 10.1021/ef5023847] using a model system comprised of carbonate surfaces obtained from crushed carbonate rocks. Wettability alteration upon exposure to low-salinity brine was examined by continuous monitoring of the contact angle. Furthermore, the effective surface charge at oil–water and water–rock interfaces was quantified via ζ-potential measurements. Mineral dissolution was addressed both experimentally and with geochemical modeling using PHREEQC. Two carbonate rocks with different mineralogy were investigated: limestone and Silurian dolomite. Four types of brines were used: high-salinity formation water (FW), seawater (SW), 25× diluted seawater (25dSW), and 25× diluted seawater equilibrated with calcite (25dSWEQ). It was observed that, by switching from FW to SW, 25dSW, and 25dSWEQ, the limestone surface became less oil-wet. The results with SW and 25dSWEQ suggest that the LSE occurs even in the absence of mineral dissolution, because no dissolution is expected in SW and none in 25dSWEQ. The wettability alteration to a less oil-wetting state by low salinity is consistent with the ζ-potential data of limestone, indicating that, at lower salinities, the charges at the limestone–brine interface become more negative, indicative of a weaker electrostatic adhesion between the oil–brine and rock–brine interfaces, thus recession of the three-phase contact line. In comparison to limestone, a smaller contact angle reduction was observed with dolomite. This is again consistent with the ζ-potential of dolomite, generally showing more positive charges at higher salinities and less decrease at lower salinities. This implies that oil detachment from the dolomite surface requires a larger reduction of adhesion forces at the contact line than limestone. Our study concludes that surface charge change is likely to be the primary mechanism, which means that there is a positive LSE in carb...

Enhanced oil recovery from carbonate reservoirs by spontaneous imbibition of low salinity water

“…Currently, increasing oil production through enhanced oil recovery (EOR) techniques is one of the important subjects that many researchers are studying in carbonate and sandstone reservoirs (Austad et al 2010;Lager et al 2008a;Mahani et al 2015;Shehata and Nasr El-Din 2015;Tang and Morrow 1999). More than 60% of world's oil is trapped in carbonate reservoirs.…”

“…It is costly to provide formation water for the experiments while sea water is cheaper and more appropriate to conduct injection and imbibition tests. Mahani et al (2015) focused on mechanisms of wettability alteration by low salinity flooding in carbonate rocks through measuring contact angles and f-potentials. They suggested a combination of mechanisms including surface charge change and mineral dissolution, the former being the most effective and the latter having a positive but insignificant effect.…”

Growth and Dissolution of Calcite in the Presence of Adsorbed Stearic Acid

“…12,14,15 In calcareous oil reservoirs, fatty acids also play an important role and are often used to model the heavy organics interaction with calcite's surface during oil recovery. 16,17 Aqueous solutions with various salt contents are usually injected into the reservoir to further enhance recovery [18][19][20] , inducing a complex interplay between ions, organics and calcite.…”

Efficiency of enhanced oil recovery by injection of low-salinity water in barium-containing carbonate reservoirs

“…, Mg 2? , and SO 2À 4 ) and rock dissolution (Mahani et al 2015;Hiorth et al 2010;Al-Shalabi et al 2014). When the low-salinity water is injected into carbonate reservoirs, the negatively charged SO 2À 4 ions in the injection water (IW) absorb onto the rock surface and decrease the positively charged surface potential of the rock.…”