Wettability Estimation by Surface Complexation Simulations

Abstract Accurate wettability estimation is essential in optimizing oil production, because it controls the fluid phase distribution and flow properties in the reservoir. The ionic composition of the brine, the oil chemistry and the mineralogy of the reservoir rock are believed to have weighty effect on the wettability. In this study, the objective was to estimate the wettability by Surface Complexation Modelling (SCM). The simulation results were confirmed by the findings from their corresponding wettability estimation using the flotation technique. Quartz, kaolinite and calcite minerals were selected for this study because they dominated the compositions of the studied reservoir rock. Both the SCM and the flotation test results elucidate the role of the reservoir rock mineralogy, the composition of the Formation Water (FW) and the oil chemistry on wettability estimation. The simulation results show that quartz is strongly water-wet while calcite is also strongly oil-wet which is consistent with the flotation test results. The kaolinite on the other hand was less water-wet as compared to quartz but more water-wet as compared to calcite. The SCM results show that oil adhesion is due to bridging by divalent ions when the mineral surface and oil-brine interface have similar charge. The oil adhesion was observed to increase with increase in divalent ion concentration. For positively charged mineral surface such as calcite, direct adsorption of carboxylic acid was the dominating mechanism for oil adhesion. Nevertheless, divalent ions bridging mechanism also occurred for the electrostatic pair linkages in calcite. From the simulation results, it can be concluded that the surface charge of the mineral has an overriding effect on oil adhesion compared to the oil-brine interface. To add to the above, the carboxylic acid has a huge influence on the wetting properties of the minerals than the basic counterpart. This is mainly due to pH of the studied systems. SCM provides a cost-effective technique of estimating the wettability of minerals at reservoir conditions. Finally, the SCM approach of characterizing the wettability can be used to screen possible injection water compositions to assess their potential to alter the wettability of the reservoir rock surface to more water-wet. Thus, compelling the adsorbed oil to be released, mobilized and produced with the injected water.