Abstract
Abstract In SmW-EOR it is generally believed that precipitation of brines must be avoided since it can have a negative impact on the SmW sweep efficiency. But substitution of Mg2+ by Ca2+ on calcite surfaces (a well-accepted phenomenon) can change the brine combination and enhance the possibility of fine formation at speciation. Considering this phenomenon we analyze the possibility of fines formation and its influence in SmW-EOR. To calculate the brine speciation and the amount of precipitate formed at different pressure and temperature conditions, we use the Extended UNIQUAC model for 61 SmW-EOR experiments reported in literature. Both the amount of available soluble SO42- (aq) in the solution and the amount of CaSO4 precipitation has been calculated and correlated to the corresponding oil recovery. Brine speciation after considering the substitution of Ca2+ by Mg2+ on chalk mineral surfaces showed possible precipitation of CaSO4 in most of the brine solutions. The amount of CaSO4(s) precipitate formed, increased with temperature and consequently the dissolved soluble SO42-(aq) decreased. Increased concentration of Ca2+,Mg2+ or SO42- ions compared to Sea Water-(SW) also enhanced the amount of precipitate formed. Increasing NaCl concentrations increased the solubility of SO42- and Ca2+ in SmW and decreased the amount of precipitate formed. The amount of precipitate formed in the pore space (after the substitution of Mg2+ on calcite surface) consistently correlated to the increased oil recovery observed in various experiments. While in several cases additional oil recovery did not correlate to the amount of soluble SO42- present in the SmW at equilibrium. These results together suggest that the substitution reaction on the mineral surface followed by solid fines formation is the fundamental reason behind the observed EOR. These results indicated that the observed wettability alteration in chalks may not be the primary cause but an additional effect of SmW-EOR. This study shows how Extended UNIQUAC model can be used as a rigorous mathematical tool for understanding of the SmW-EOR in chalk. It also quantitatively explains a series of previous experiments reported in the literature and shows the importance of fine formation in SmW-EOR, both through correlation between modeling and experiments.
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