Geochemical simulation of wettability alteration and effluent ionic analysis during smart water flooding in carbonate rocks: Insights into the mechanisms and their contributions

Abstract

Production of oil from carbonate reservoirs has always been challenging. The nature of these reservoirs is complex and heterogeneous, and the rock matrix often has low permeability and unfavorable wettability. Hence, waterflooding as the primary method for enhanced oil recovery (EOR) usually results in low recovery in non-water-wet carbonate reservoirs. Alternatively, low salinity/Smart water injection (LSWI) has attracted considerable attention and is widely used for recovery improvement in carbonate rocks. Wettability alteration is believed to be the cause of incremental oil recovery during LSWI in carbonates through two main mechanisms of surface charge change and rock dissolution.In this study, the experimental data of smart water coreflooding of a carbonate rock sample from a real oil field was simulated, focusing on geochemical reactions and dynamic wettability alteration. The contribution of different mechanisms was investigated through careful ionic analysis at elevated temperatures. Oil recoveries from the injection of four brines with different salinities were matched, and the ionic compositions of the effluents were studied individually. Acceptable history match was achieved between the experimental and geochemically simulated concentrations. The trends for Ca2+ and Mg2+ concentrations indicated the presence of Calcite and Dolomite dissolution in all cases. In the case of seawater injection, Ca2+ was underestimated while Mg2+ was overestimated, which shows the dominance of Calcite dissolution in higher salinities. As the salinity was reduced to 5000 ppm, the experimental and simulated Mg2+ concentrations became close, which was translated to higher contribution of Dolomite dissolution. By decreasing the salinity to 2000 ppm, a decreasing trend in the SO42− concentration was observed along with overestimation of Mg2+, which showed the contribution of the surface charge change mechanism in lower salinities. According to the results, the proposed mechanism for wettability alteration was therefore concluded to be mainly mineral dissolution, with a contribution of surface charge change at lower salinities. Moreover, the time effect of LSWI on wettability alteration was also observed.This study provides a simulation approach in which, both practical and fundamental (mechanistic) aspects of LSWI are considered. The findings of this study can help for better understanding of the underlying mechanisms during LSWI and their contributions at reservoir temperatures. Furthermore, by using a systematic set of simulations based on the geochemical ionic analysis, optimal conditions for injection water composition could be provided for maximizing recovery during LSWI operations, without the need for numerous, expensive, and tedious laboratory experiments.