“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 the silica surface binding reactions and the organic acids surface coverage (%) are included in the extended model. The SCM successfully fits all 63 zeta potential data of synthetic calcite and three natural carbonates in various mixed-electrolyte brines with varying ionic strengths and CO2 partial pressures. The organic impurities are found to play the dominant role in making the natural carbonates more negatively charged than synthetic calcite. Humic acids extracted from a humus sample were used to treat the synthetic calcite sample in an experiment. The treated calcite had a significantly more negative zeta potential, demonstrating the effect of organic impurities on the carbonate surface charge.
The second objective of this work is to experimentally evaluate the carbonate wettability alteration in “smart water”. Spontaneous imbibition was chosen over contact angle to characterize wettability alteration due to the poor reproducibility of contact angle measurements. The effects of brine chemistry, especially Mg2+, SO42-, and salinity, were investigated in a model oil system in Chapter 5. Both the reduction in Na+ and addition of SO42- are found to contribute to wettability alteration. Mg2+ is found to be unfavorable for wettability alteration. Ca2+ is believed to facilitate SO42- with wettability alteration. Rock/brine and oil/brine zeta potentials are measured, and the electrostatic component of disjoining pressure is calculated to understand the role of electrostatics in this process. The surface concentration of charged species on the Indiana limestone surface is also analyzed based on the SCM developed in Chapter 4. The reduction of the Na+ surface complexation (>CaOH…Na+0.25) in low salinity brines is believed to be a critical mechanism responsible for wettability alteration based on the SCM calculations.
In Chapter 6, the effect of oil physicochemical properties on carbonate wettability alteration was also investigated by spontaneous imbibition measurements. The results were also used to evaluate two possible wettability alteration mechanisms: rock/oil electrostatic repulsion and microdispersion formation. Seven oils were fully characterized and used in spontaneous imbibition measurements in low-salinity water. For the first time, the effectiveness of low-salinity water is found to positively correlate with the oil interfacial tension in low-salinity water. Oils with higher interfacial activity are found to respond more positively to low-salinity water. Moreover, cryogenic transmission electron microscopy (Cryo-TEM) images suggest that microdispersion is essentially macroemulsion, and its formation is an effective indicator – but not the root cause – of wettability alteration. Rock/oil electrostatic repulsion based on zeta potentials is found to be an insufficient condition for wettability alteration in carbonate minerals.
Finally, low-salinity water coreflooding was performed using a crude oil that responded positively to low-salinity water in spontaneous imbibition. 41% of original oil in-place (OOIP) was recovered after the initial high-salinity water flooding. An additional 12% (of OOIP) is observed after the injection of low-salinity water. The reduction of NaCl concentration is confirmed to effectively improve oil recovery in the forced displacement experiment.