الملخص الإنجليزي
Modification of potential determining ions (PDIs') concentration in the brine can result in alteration of the surface wetting properties towards more water-wet state, which is the desired wetting state in fractured reservoirs. However, few researches discussed the interactions between PDIs and their mechanisms on wettability alteration. The aim of this study is to investigate the effect of combinations of PDIs at different molar ratios on the wettability of oil-wet calcite surfaces as well as the interactions between the ions. Brines concentration of two combinations (Mg2+, s042) and (Calt, s04) were considered. Contact angle and zeta potential were measured after treating the oil-wet calcite surfaces with a combination of PDIs. Results revealed that both groups are able to reduce the contact angle significantly and expel carboxylate group from the surface.
Besides, combinations of PDIs were more effective in wettability alteration compared to
those ions alone. It was observed that equal concentrations of magnesium and sulfate provide more water-wet state. However, for combinations of calcium and sulfate, the inost water-wet state was achieved when lower concentrations of calcium and higher concentrations of sulfate were utilized. Zeta potential measurements indicated that the performance of each PDI for changing surface wettability is critically affected by the concentration of the other ions present in the same solution. It has been shown that at low concentration of magnesium ions, the adsorption of sulfate ions on the rock surface and the interaction of magnesium ions with the carboxylate group is the main mechanism of wettability alteration by magnesium and sulfate ions. However, at high concentration of magnesium ions, no adsorption of sulfate ions was detected which means that sulfate plays as a catalyst of wettability alteration by (Mg2+, s04) and the formation of the complex Mg-carboxylate group is the dominant mechanism. Moreover, the effect of modified low salinity brine in the absence and presence of cationic surfactant on the oil recovery during spontaneous imbibition experiments was investigated in the current study. Results showed that, a notable oil recovery can be achieved by adjusting the ionic composition of brine without using of costly surfactant solutions. However, the co-presence of the modified brine and cationic surfactant can result in higher rates as well as oil recovery during the spontaneous imbibition process.
