A Geochemical Model for Investigation of Wettability Alteration during Brine-Dependent Flooding in Carbonate Reservoirs

Abstract: The impact of brine salinity and compositions on improving brine-dependent recovery processes has been an active research area over the past two decades. Various studies have demonstrated an improvement in oil recovery, attributed predominantly to the ability of the brine to alter rock’s wettability towards water-wetness. The proposed hypothesis is that the wettability of carbonate rocks is altered due to desorption of oil carboxylic groups from rock surfaces by the adsorbed sulfate, while the divalent cations are co-adsorbed.

We developed a reactive transport model to test this hypothesis and considered wettability alteration through geochemical interactions among brine, oil and rock surface. In this model, we used various reaction pathways to account for the competition between oil acid-groups and active ionson the rock surface. The equations developed from various reactions are coupled with multiphase flow equations to control flow functions that ultimately determine the oil recovery. The model we developed was used to investigate the effects of ionic variations during carbonate coreflooding experiments. Thereafter, we extended DLVO (Derjaguin, Landau, Verwey and Overbeek) theory of surface forces to explain the molecular interactions between rock−brine−oil system by generating interfacial disjoining pressure and interaction energy.

The model was tested by matching and predicting results from recently published flooding experimental studies related to ionically-modified brines conducted under single-phase and two-phase flow conditions. In these experiments, sulfate concentration in seawater was halved and quadrupled, and compared to injections of formation water and seawater. We found remarkable agreement with the produced ion histories reported from the single-phase tests, although there were delays in the produced sulfate concentration because of retention within the core. For two-phase experiments, the model yielded excellent replication of the produced ion histories and oil recoveries obtained during injection of various brines. Results show that increasing sulfate while maintaining the concentration level of other ions improved oil recovery up to 10% OOIP. However, when sulfate was reduced, neither additional oil was recovered nor sulfate ion production was delayed. Application of DVLO theory shows that disjoining pressure, which dictates the water film thickness, is extremely sensitive to sulfate content of the brine. Brines with more sulfate content exhibit higher disjoining pressure and energy barrier compared to brines with fewer sulfates. This suggests that increasing sulfate in the injected brine is essential to alter rock wettability.

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