Wettability alteration of carbonate surfaces in the presence of modified salinity brines
Keywords: Energy Resources Engineering.

Low salinity water injection (LSWI), also called ''smart waterflooding" injects modified salinity brine with controlled ionic composition to achieve increased oil recovery compared to conventional waterflooding. Evidence from laboratory experiments and field trials suggest that LSWI leads to an increase in oil recovery ranging from 5% to 20% of the original oil in place in carbonate rocks. Although many mechanisms have been proposed to explain the low salinity effects, conflicting results were reported and little agreement exists. The underlying mechanisms dictating the low salinity effects in carbonate reservoirs remain an open question. Motivated by the current lack of understanding in the fundamental mechanisms at work, this dissertation applies multiple experimental and modeling methodologies to investigate important low salinity mechanisms for carbonate porous media. This work first examined the influence of different ions on the short-range non-DLVO (Derjaguin, Landau, Verwey, and Overbeek) forces at the calcite/brine interface. An amplitude modulated Atomic Force Microscope (AFM) operating in contact mode was used to acquire Force-Distance Spectroscopy (FDS) movements at the calcite surface immersed in various electrolyte solutions containing NaCl, Na2SO4, MgCl2, MgSO4, and synthetic formation water. Experimental results reveal that, in single-component solutions, a greater concentration of Na+ ions decreases the decay length of short-range repulsion while a greater concentration of Mg2+ ions increases decay length. These results imply that Na+ ions reduce the affinity of calcite surfaces for water whereas Mg2+ ions make calcite more water-wet. Importantly, the relationship between the behavior of non-DLVO forces at small separations and concentrations of ions is not monotonic in multiple-component brines. The fitted parameters for short-range repulsive forces are useful to more accurately construct the total disjoining pressure curve and calculate contact angle of calcite/brine/oil interfaces when combined with measurement, or theory, of other DLVO forces. Second, we applied the extended-DLVO theory to explain the fundamental difference between two types of crude oil that show different responses to LSWI. C oil and H oil are crude oil from carbonate reservoirs located in Central Asia and the Middle East, respectively. Based on the laboratory core-flooding and imbibition tests, the C oil showed little response when the saline connate water was switched to diluted connate water and other brines with lower salinity. The H oil, however, achieved an additional oil recovery of more than 5% when diluted seawater and Mg-rich brine was injected into the core samples. We use the measured and modeled zeta potential data, parameters of the hydration forces, and the extended-DLVO framework to calculate the total disjoining pressure and contact angles under different scenarios. In the C oil system, diluted brine solutions cause decreases in the zeta potentials of calcite/brine and oil/brine interfaces, but this does not lead to less attractive electrostatic forces because of the great difference in the magnitude of the two zeta potentials. For the calcite/seawater/H oil system, however, diluted seawater and Mg-rich brine cause the difference in the magnitude of zeta potentials of the two interfaces to decrease. This leads to less attractive electrostatic forces for the two interfaces that have zeta potentials with opposite polarity. Importantly, this study provides insight about why low salinity effects were not observed in some carbonate systems. Third, a pore network modeling approach was used to evaluate low salinity effects. A thin-film model solved by the level-set method was adopted to characterize the movement of an oil droplet in a water-filled tube given two different wetting conditions. A repulsive and an attractive disjoining pressure curve were input into the thin-film model, respectively, to represent a water-wet condition and an oil-wet condition. Results from the thin-film model reveal that the oil phase conductance in the repulsive disjoining pressure case is 1.4 times of that in the attractive disjoining pressure case. In addition, we upscaled the results from the thin-film model to the pore-network level using an open source pore network modeling tool. The upscaled lubrication effects on relative permeabilities predicted from the pore network model depends on the geometry of the network. Sensitivity analysis shows that networks with longer throat length, greater throat diameter, and smaller difference in pore size and throat size are more susceptible to the lubrication effects