Surface and core wetting effects of surfactants in oil-based drilling fluids

Surfactants are used in oil-based drilling fluids to emulsify water and to ensure that cuttings are wetted by oil. The products used are based on drilling conditions and are essentially the same for traditional oil-based and synthetic oil-based fluids. Although much of these surface active materials adsorb on cuttings and filter cake, it is still likely that core samples are exposed to significant concentrations. Additional contamination can occur in subsequent core tests if the oil samples used are themselves contaminated with the drilling fluid.Commercial surfactant packages from several sources have been selected for examination of their effects on the tests normally used to assess the extent of wetting change in crude oil/brine/mineral systems. These products were characterized with respect to their interfacial properties by measurements of interfacial tension as a function of concentration and brine composition. Wetting tests include measurements of contact angles on smooth mineral surfaces and rates of imbibition into porous media. The results show that even very low concentrations of surfactants, much lower than the amounts recommended for use in drilling fluids, can affect the wetting properties of mica surfaces and Berea sandstone cores.