Salinity-dependent contact angle alteration in oil/brine/silicate systems: The effect of temperature

To understand the success of low salinity water flooding in improving oil recovery, it is important to identify the molecular scale mechanisms that control the wettability and thus the adhesion between oil and rock. Previous experiments have attributed the wettability alteration in core flood experiments either to the expansion of the electric double layer or to multicomponent ion exchange reactions or a combination of both. Here, we explore changes of the contact angle of brine droplets on mica in ambient oil (n-decane plus added fatty acid) at variable temperature as a function of the concentration of mono- and divalent cations. For 20 °C and 40 °C, we find that the contact angle decreases by up to 30° with decreasing divalent cation concentration but remains constant upon decreasing the total salinity by removing only monovalent cations, i.e. upon double layer expansion at constant divalent cation concentration. At 60 °C, we find a remarkable increase of the water contact angle of artificial sea water to values of approximately 120°. This value decreases upon dilution to values in the range of 10-40 °C, where the lowest values are again only obtained upon removing the divalent cations. These findings corroborate the conclusion of earlier measurements at room temperature that divalent cations play an essential role in controlling the wettability of carboxylic acid groups to mineral surface, presumably in an ion bridging type mechanism. We also demonstrate that the contact angle reduction occurs very quickly upon flushing a sessile droplet of artificial sea water with divalent cation-free or simply diluted brine, suggesting fast equilibration as required for a successful tertiary water flooding process. Our experiments also demonstrate that, despite the simplicity of the present system, the origins of wettability alteration are rather complex and that synergistic effects can lead to dramatic variations such as the unexpectedly high contact angle at 60 °C.