What is a measurement of the ability of a fluid to coat a rock or mineral surface?

• A. Porosity
• B. Permeability
• C. Viscosity
• D. Wettability

Answer: (D)

Wettability describes the ability of a fluid to coat a rock or mineral surface. It reflects how readily the liquid spreads across and adheres to the solid, which is tied to the balance of interfacial tensions between the solid, the fluid, and any other immiscible phase. This is often expressed through the contact angle: a small angle means the liquid wets and coats the surface well, while a large angle means poor wetting. In rocks, wettability determines whether water or oil more strongly coats the mineral surfaces, influencing how fluids distribute in pores, capillary pressures, and recovery behavior.

Porosity is about how much void space exists in the rock; permeability is about how easily fluids can move through connected pore networks; viscosity is the fluid’s internal resistance to flow. None of these directly measure how well a fluid coats a surface, which is captured by wettability.