Wettability Alteration with Brine Composition in High Temperature Carbonate Rocks

Abstract

Abstract Injection of brine with tuned composition has been shown to give improved oil recovery from carbonate rocks. Contact angle studies, spontaneous imbibition and core flood experiments have shown that wettability alteration is responsible for this process. Possible mechanisms include mineral dissolution and ion exchange, which have been investigated by zeta-potential measurements and geochemical modeling of both processes. In this study, the core scale manifestation of these mechanisms is evaluated, and a geochemical model is developed for further insight into reaction pathways. Brines of different compositions were injected into carbonate cores with no oil and the effluent was analyzed for ionic composition. Seawater, sulfate-rich seawater, and dilutions of seawater were tested. Two phase oil displacement core floods were performed for the same brine cases to correlate the oil recovery to the geochemistry. A mechanistic model was developed using our in-house reservoir simulator UTCHEM-IPHREEQC for the wettability alteration process. The single phase core floods with all test brines indicate retention of SO42− within the core, seen by a delay in its effluent concentration reaching the injection concentration. Na+, Ca2+, Mg2+ and Cl− ions mostly behave as tracers in the system. In oil displacement core floods, formation brine recovers 40% OOIP on average and seawater recovers an incremental 7% OOIP. Sulfate-rich seawater and dilutions of seawater increase the recovery to 65-80% OOIP in secondary and tertiary modes, requiring more than 5 PVI. SO42− ion delay is not observed in the two phase core floods. Ca2+ concentrations remain high after 5 PVI of diluted seawater, indicating a slow dissolution process in the low salinity floods. The mechanistic model results were in good agreement with the single phase coreflood experiments and oil recovery experiments. The model showed that the reduction in surface concentration of naphthenic acids was responsible for altering the wettability on the injection of modified brines.