Insights into the Impact of Temperature on the Wettability Alteration by Low Salinity in Carbonate Rocks

Abstract

It has been proposed that increased oil recovery in carbonates by modification of ionic composition or altering salinity occurs mainly at a temperature exceeding 70-80 °C. The argument was that elevated temperatures enhance adsorption of the potential determining ions which then modifies wettability to a less-oil-wetting state. According to this rationale, it becomes questionable if diluted brines or brines without these ions can be still applicable. Therefore, the aim of this paper is to investigate if the wettability alteration truly depends on temperature and if so how the trend with temperature can be explained. We followed a combined experimental and theoretical modeling approach. The effect of brine composition and temperature on carbonate wettability was probed by monitoring contact angle change of sessile oil droplets upon switching from high salinity to lower salinity brines. IFT measurements as a function of salinity and temperature along with extensive Ζ-potential measurements as a function of salinity, pH, temperature, and rock type were conducted. Interaction potentials between oil and carbonate surfaces were estimated based on DLVO theory, and its consistency with oil-droplet data was checked to draw conclusions on plausible mechanisms. Three carbonate rocks (two limestones and one dolomite) were used along with two reservoir crude oils, high salinity formation water (FW), seawater (SW), and 25 times diluted seawater (25dSW) as low salinity (LS) brine. It was observed that (i) wettability alteration to a less-oil-wetting state can occur at ambient temperature for specific rock types and brines, and (ii) there is no univocal increase in response to SW and LS brine at elevated temperature. The largest improvement in wettability was observed for dolomite, while, among the limestones, only one rock type showed noticeable wettability improvement at elevated temperature with SW. The difference in behavior between limestones and dolomite indicates that the response to brine composition change depends on rock type and mineralogy of the sample. These observations are consistent with the Ζ-potential trends with salinity at a given temperature. Dolomite generally shows more positive Ζ-potential than limestones. However, even the two limestones react differently to lowering salinity and exhibit different magnitude of Ζ-potential. Moreover, it is observed that, at a specific salinity, an increase in temperature leads to reduction of Ζ-potential magnitude on both rock/brine and oil/brine interfaces toward zero potential. This can affect positively or negatively the degree of wettability alteration (to a less-oil-wetting state) at elevated temperature depending on the sign of oil/brine and rock/brine Ζ-potential in SW/LS. The observed trends are reflected in the DLVO calculations which show consistency with contact angle trends with temperature and salinity. According to the DLVO calculation, the lack of response to SW/LS in some of the systems above can be explained by stronger electrostatic attractive forces under SW/LS than HS. This study concludes that a combined surface-charge change and double-layer expansion is a plausible mechanism for the wettability alteration in carbonate rocks.