Modeling of low salinity waterflooding in carbonates: Effect of rock wettability and organic acid distribution

Abstract

Carbonate reservoirs hold a substantial amount of our existing oil reserves. However, waterflood recovery from these reservoirs is often low due to their oil-wet nature. Recent studies have shown that oil recovery from oil-wet carbonate reservoirs can be increased by wettability alteration using low salinity brines having carefully modified ionic compositions. In this study, we developed a multiphase, multicomponent, finite-difference reservoir simulator and incorporated an improved mechanistic model to model low salinity waterflood in carbonates. The model considers key geochemical reactions relevant to low salinity waterflooding as reported by many authors recently. However, our modeling approach differs in two ways. Firstly, our model assumes that geochemical reactions of injected brines with the rock take place only on water-wet surfaces of these rocks and at the three-phase contact line; compared to other existing models that consider reactions to occur on the entire rock surface for oil-wet rocks. Secondly, previous models consider organic acids as a single species. In reality, oil is likely to have a mixture of different organic acids of different polarity, chain length, etc. In our model, we consider the oil to have a mixture of different organic acids and studied its impact on wettability alteration and oil recovery. Using this model, single phase and oil recovery experiments reported in the literature were modeled. The effect of rock wettability on geochemical reactions of low salinity brines with carbonate rocks was studied. Similarly, the effect of organic acid distribution on wettability alteration and oil recovery was investigated. Our simulation results showed that the extent of reaction of low salinity brines with carbonate rocks decreased as the rock became more oil-wet due to a decrease in reactive surface area. For example, less delay in effluent sulfate ions was observed in case of oil-wet rocks. The water-wet regions grew as a result of wettability alteration at the three-phase contact line. The initial wettability, extent of wettability alteration, and oil recovery profiles were dependent on the organic acid distribution. Furthermore, the inclusion of only key geochemical reactions (dolomitization, sulfate adsorption, and anhydrite dissolution/precipitation) were found to be reasonable for modeling various experiments reported in the literature. The proposed model will help in improving our understanding of mechanisms responsible for wettability alteration.