| Summary: | “Smart water” refers to the low-salinity brine that can alter wettability and enhance oil recovery. The injection of “smart water” as a low-cost enhanced oil recovery (EOR) approach has drawn increasing attentions in the oil and gas industry. Particularly, the “smart water” EOR has promising applications in oil-wet, naturally fractured carbonate reservoirs where capillary imbibition is extremely important. Successes of “smart water” in carbonate systems have been reported in both laboratory flooding experiments and a field-scale pilot. However, underlying mechanism of the “smart water”-induced wettability alteration in carbonates remains unclear. Therefore, this dissertation systematically investigates the wettability alteration process of carbonate rocks in “smart water”. The first objective of this work is to understand the electrostatic interactions between carbonate rocks and oils. In particular, the surface charge of carbonate minerals in brines has been a focus of literature research because it is generally believed to govern the surface wettability. To model the formation of surface charge, surface complexation models (SCM) are developed based on rock-ion complexations. A SCM was first developed for pure calcite, the primary component of carbonate rocks, in Chapter 3. Divalent ions Ca2+, Mg2+, CO32-, and SO42- are found to bind much more strongly to the calcite than monovalent ions. The equilibrium constants for binding reactions are also found to negatively correlate to the hydrated ion radius for ions of the same charge. Moreover, the weak potential determining ion Na+ is found to significantly contribute to the positive charge of calcite in high-salinity brines (5M NaCl). The synthetic calcite SCM was then extended to work for natural carbonates with surface impurities in Chapter 4. Three carbonate rocks, Iceland spar, Indiana limestone, and “SME” reservoir rock, were investigated. The effects of inorganic impurity silica and organic impurities are examined individually in the model calculation. Both ...
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