The Effect of Salinity and pH on Wettability Alteration of Kaolinite Films Studied by Contact Angle Measurement

Abstract

Summary Low-salinity enhanced oil recovery (LSEOR) has shown great effectiveness in improving the oil recovery ratio and therefore extending the life of mature oil reservoirs. This effectiveness is related to the ability of low salinity injection water to change the wettability state of the reservoir to more water-wet. However, the complexity of the chemical relationship between brine, oil and rock is such that no single mechanism has been identified for being responsible for this change in wettability. This said, the role of clay minerals (which are widely present in the pore space of clastic reservoirs) in their interaction to oil molecules has been regarded as critical. In this work, we aim to study the effect of formation water (varying composition), model oil (decanoic acid/dodecane) aging and low salinity treatment (with varying degrees of concentration and cation identity) on clay mineral wettability, as studied by contact angle measurements. To this end, we have developed a method to prepare smooth, oriented, kaolinite films (with the siloxane face exposed to the solution/oil) of several square cm and a submicron roughness. Contact angles of model oil droplets over the clay films were measured using the sessile drop method. Results show that formation water containing Na+ does not result in large contact angle (wettability) variation, but the presence of Ca2+ does. Increasing pH from 6.5 to 8 leads to an overall increase in the measured contact angle, i.e. to decreased oil sorption (or increasing water-wetness) by the kaolinite film. This effect can be explained by the protonation state of the polar molecules (decanoic acid) as well as the surface complexation (which would lead to the formation of cation bridges in the case of Ca2+). Treatment (washing) of oil-aged films by brines of different concentrations (and either CaCl2 or NaCl composition) showed no clear trends in the modification of contact angle but, in general, increasing CaCl2 concentrations leads to lower contact angles, i.e. less oil desorption (oil wet surface). On the other hand most treatment with NaCl led to higher contact angles, or increased waterwetness of the kaolinite films.