The mechanism for the alteration of sandstone wettability due to interaction with oil-based mud components such as cationic and anionic surfactants is investigated. Extensive use of such surfactants is made in drilling muds and completion fluids. It is shown through contact angle and capillary pressure experiments, that oil-based mud components can in some cases drastically alter the original wettability conditions of both sandstone and carbonate rocks. The wettability alterations are caused mainly by surfactants in the drilling fluids.

Contact angle measurements were made on quartz that had been equilibrated with the oil-based mud components. Additional measurements were made on quartz in the presence of an aqueous solution of the drilling fluid component.

The changes in the measured contact angles are explained on the basis of disjoining pressure concepts. The van der Waals, electrostatic, and structural components are considered. Zeta-potentials on the silica-surfactant solution and oil-surfactant solution interfaces are measured. Contact angles computed using the van der Waals and electrostatic components only are not in agreement with the experimentally measured angles. It is clearly shown that the structural component needs to be accounted for to calculate a theoretical angle that is comparable with the experimental angle. The structural component was found to vary with the concentration of surfactant presumably due to the formation of monolayer and presumably due to the formation of monolayer and bilayer structures. The results of this study provide a general framework within which wettability alterations caused by surfactants in drilling or completion fluids can be viewed.