Salinity-Dependent Adhesion Response Properties of Aluminosilicate (K-Feldspar) Surfaces

Abstract

Flooding sandstone oil reservoirs with low salinity water can lead to a significant increase in oil recovery, a phenomenon called “the low salinity effect”. Although there are many factors that contribute to this response, the surface tension on the pore walls is an important one. Sandstone is composed predominantly of quartz with some clay, but feldspar grains are often also present. While the wettability of quartz and clay surfaces has been thoroughly investigated, little is known about the adhesion properties of feldspar. We explored the interaction of model oil compounds, molecules that terminate with carboxyl, −COO(H), and alkyl, −CH3, with freshly cleaved, museum quality perthitic microcline, KxNa(1–x)AlSi3O8, a K-feldspar. Microcline is a member of the orthoclase family, a type of feldspar that weathers more slowly than the plagioclase series, and thus is more likely to be preserved in well sorted sandstone. Adhesion forces, measured with the chemical force mapping (CFM) mode of atomic force microscopy (AFM), showed a low salinity effect on the fresh feldspar surfaces. Adhesion force, measured with −COO(H)-functionalized tips, was 60% lower in artificial low salinity seawater (LS, ∼1500 ppm total dissolved solids) than in the high salinity solution, artificial seawater (HS, ASW, ∼35 600 ppm). Adhesion with the −CH3 tips was as much as 30% lower in LS than in HS. Density functional theory calculations indicated that the low salinity response resulted from expansion of the electric double layer and that contributions from cation bridging are of less importance. Adventitious carbon, that is, organic material that is inherent on all mineral surfaces exposed to air or water, can enhance nonpolar component adhesion by serving as anchor points.