Modelling of carbonate rock wettability based on surface charge and calcite dissolution

The lack of consensus on the mechanisms underpinning low salinity waterflooding (LSWF) oil recovery has affected the successful implementation of LSWF in carbonate reservoirs. Many investigators link the improved oil recovery observed from LSWF to wettability alteration. Several models are proposed in the literature to study the wettability alteration mechanism. However, the reliability of some of the models is being contested, as they struggle to accurately predict the oil recovery trend. This study aims to develop a new wettability alteration model that can predict the performance of LSWF oil recovery processes. We propose a new wettability indicator (WI) that integrates three mechanisms (surface charge alteration/ζ-potential, calcite dissolution, and ion exchange) which, are suggested, dictate LSWF wettability alteration. The model was implemented in UTCHEM, which was coupled with PHREEQCRM, and was used to simulate several experimental LSWF oil recoveries.

The results produced from the simulations with the new model are consistent with experimental data. The outcomes from this study show that adsorption of crude oil is still possible even when the oil-brine and the rock-brine interfaces have the same polarities of ζ-potentials. The results further demonstrate that the improved oil recovery observed in LSWF is not always caused by the wettability alteration, but also caused by other factors such as rock quality and physical displacement. The outcome of the study also indicate that early injection of the low salinity brine can significantly improve the oil recovery.

Findings from this study can improve the understanding of the LSWF process. Moreover, the model proposed in this study can be applied in the design and planning of LSWF projects to estimate the performance of the LSWF