Abstract
Clay–oil interactions play a critical role in determining the wettability of sandstone oil reservoirs, which, in turn, governs the effectiveness of enhanced oil recovery methods. In this study, we have measured the adhesion between –COOH functional groups and the siloxane and aluminol faces of kaolinite clay minerals by means of chemical force microscopy as a function of pH, salinity (from 0.001 M to 1 M) and cation identity (Na+ vs. Ca2+). Results from measurements on the siloxane face show that Ca2+ displays a reverse low-salinity effect (adhesion decreasing at higher concentrations) at pH 5.5, and a low salinity effect at pH 8. At a constant Ca2+ concentration of 0.001 M, however, an increase in pH leads to larger adhesion. In contrast, a variation in the Na+ concentration showed less effect in varying the adhesion of –COOH groups to the siloxane face. Measurements on the aluminol face showed a reverse low-salinity effect at pH 5.5 in the presence of Ca2+, whereas an increase in pH with constant ion concentration resulted in a decrease in adhesion for both Ca2+ and Na+. Results are explained by looking at the kaolinite’s surface complexation and the protonation state of the functional group, and highlight a more important role of the multicomponent ion exchange mechanism in controlling adhesion than the double layer expansion mechanism.
Highlights
Global crude oil consumption has sharply increased in the last decades; primary and secondary recovery methods may still leave up to 65% of the original oil in place (OOIP) in the reservoir [1,2]
The bulk of experiments shown in this paper were performed over the siloxane face of kaolinite as this will probably be the most exposed face on the pore, at least in terms of the nanoparticles coating quartz grains, which are quite prevalent in real reservoir rocks [8,22]
The effect of the concentration of Ca2+ on the adhesion of –COOH groups on the siloxane face was studied at two different pH values: 5.5 and 8
Summary
Global crude oil consumption has sharply increased in the last decades; primary and secondary recovery methods may still leave up to 65% of the original oil in place (OOIP) in the reservoir [1,2]. Enhanced oil recovery methods include steam and CO2 injection, chemical flooding, pH alteration, inter alia. One method that is currently highlighted is low salinity enhanced oil recovery (LSEOR) owing to its use of a low-cost, environment-friendly substance, its sustainability, and its effectiveness [3,4]. For this reason, a multitude of studies have been conducted to understand the fundamental geochemical processes driving LSEOR; a debate still exists on the exact nature and importance of these processes and whether LSEOR can be applied to any given field [3,4]. On a series of factors or conditions that need to be met for low salinity EOR to be effective, these include: the presence of clay minerals, the presence of a saline connate water (containing multivalent ions), exposure of the rock to acidic and basic oil components, and a significant reduction of salinity in the flooding water [3]
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