Abstract
Several laboratory studies observed wettability alteration of carbonate rocks towards a more water-wet state through modifying the composition of injected water.The change in carbonate surface charge was reported as the underlying reason behind this wettability alteration and the corresponding incremental oil recovery by engineered water injection(EWI).Although the reported results of carbonate rock sensitivity to injected water ionic strength,pH,salinity,and concentration of potential determining ions(Ca~(2+),Mg~(2+),and S0_4~(2-)),there is still an ambiguity on the way carbonates interact with injected water/ions.This is due to the different miner-alogical compositions and electrokinetic properties of carbonates.Therefore,this paper provides more insight into the role of major rock-forming carbonate minerals(calcite and dolomite)in engineered water injection at different water compositions by utilizing surface complexation modeling(SCM).The study utilizes a geochemical simulator to highlight the effect of dolomite surface complexation reactions.After determining the total rock surface charge and surface potential,the disjoining pressure was calculated to estimate the water film thickness and consequently,infer about wettability alteration.The results showed that a successful EWI operation creates three regimes:displacement regime,surface reaction regime,and quasi-equilibrium regime.These regimes can be characterized by the proposed surface complexation technique to calculate/optimize the injection pore volumes needed for each regime,determine the location of EWI effect in the reservoir,and predict the time ranges when EWI will reach to the surface in the form of additional oil recovery.Also,it was found that the surface reaction regime is the most important one where main surface reactions and oil recovery occur.Furthermore,the study utilizes SCM for both calcite and dolomite minerals for the first time highlighting their importance in geochemical reactions of EWI when the rock has dolomite in its composition.Existing SCM considers oil-brine and rock-brine interfaces where only calcite mineral is captured in the rock-brine interface.The latter is based on assuming that the rock composition is pure calcite,which is not necessary the case.The findings of this study confirm that seawater is capable of inducing wettability alteration in carbonates under certain favorable conditions due to adsorption/desorption of the potentially determining ions.
NSTL