Capillary Pressure and the Transition Zone

Why There Is No Sharp Contact

It is tempting to picture oil sitting cleanly on top of water at a single depth. Reality is gentler. Capillary forces in the small pores hold water up against buoyancy, so above the free-water level (where the capillary pressure is zero) the water saturation falls only gradually with height, through a transition zone, before reaching irreducible water in the cleanest, highest rock.

Capillary Pressure and Height

The link is buoyancy. The capillary pressure at a height $h$ above the free-water level is $P_c = \Delta\rho, g, h$, where $\Delta\rho$ is the oil-water density contrast. A rock will not admit oil until $P_c$ exceeds its entry pressure, which sets how high the oil-water contact sits above the free-water level. The widget plots the resulting saturation-height profile. Adjust the rock quality, the irreducible water, and the oil gravity, and watch the transition zone stretch or sharpen.

What Controls the Transition

Two things set the transition-zone thickness. Rock quality: good rock has a low entry pressure and a narrow pore-size distribution, giving a sharp contact, while poor rock smears it over tens of meters. Density contrast: a light oil far from water density has strong buoyancy and a thin transition, while a heavy oil close to brine density barely lifts the water and the transition runs long. This profile is exactly what populates $S_w$ in the model above the contact, so getting it right matters for the oil volume.