1. n. []

The angle of intersection of the interface between two fluids at a solid surface. The angle is measured from the solid surface through the aqueous phase, or in an oil and gas test through the oil phase. The contact angle displays hysteresis based on direction of motion of the interface. Surface roughness affects the equilibrium contact angle, so measurements are typically made on smooth, flat surfaces. A contact-angle test uses carefully captured and preserved samples of reservoir oil to determine the wetting preference. A droplet of the crude oil is suspended between two parallel plates of quartz or calcite inside a simulated formation water bath at reservoir temperature and sometimes at reservoir pressure. By periodically displacing one of the plates sideways, a contact angle is determined at the side of the droplet where water is forcing the oil from the solid. A small angle indicates water-wetting preference, while a large angle indicates oil-wetting. Angles near 90 degrees are intermediate-wetting. Different minerals display different wetting preferences, although most are more likely to be water-wet.

See related terms: imbibition,  oil-wet,  water-wet,  wettability