oilwet

Oilwet, usually written as oil-wet, refers to a condition in which a solid surface or porous medium is preferentially wetted by oil rather than by water. In geology and petroleum engineering, oil-wet surfaces indicate that oil is the wetting phase in the rock’s pore space, whereas water-wet surfaces have water as the wetting phase. Mixed or intermediate wettability, where neither fluid completely dominates, is also common.

Wettability strongly influences fluid flow in reservoirs. It shapes capillary pressure, relative permeability, and residual oil saturation. Oil-wet rocks typically trap more oil and respond less favorably to waterflooding, reducing sweep efficiency and overall oil recovery compared with water-wet rocks. The degree of oil-wetting can vary within a reservoir and may evolve with time or exposure to crude oil, brine, and injected recovery fluids.

Measurement and characterization are performed through contact angle tests on rock surfaces, spontaneous imbibition experiments, and analyses of relative permeability curves. A contact angle greater than about 90 degrees is generally considered oil-wet; angles below 90 degrees indicate water-wet behavior, with intermediate values signaling mixed wettability.

Wettability can be altered by chemical and physical treatments. In carbonate reservoirs, aging oil films can be removed or reversed by acidizing or surface-active agents to promote more water-wet conditions. Surfactants, polymers, low-salinity water flooding, and nanoparticles are also studied as methods to modify wettability and improve oil recovery.

Oilwet is most discussed in the context of reservoir engineering and enhanced oil recovery, where understanding and managing wettability helps predict production performance and select appropriate recovery strategies.