Capillary Pressure and Wettability

The Pressure Jump Across the Interface

Capillary pressure is the pressure difference between the non-wetting and wetting phases where they meet inside a pore, $P_c = P_{nw} - P_w$. It climbs as the wetting phase is displaced, because the surviving interfaces must curve through ever smaller pore throats, and a tighter curve holds a larger pressure jump.

Drainage, Imbibition, and Hysteresis

Pushing the non-wetting phase in (drainage) takes a higher capillary pressure than letting the wetting phase soak back (imbibition), so the two follow different paths and trace a hysteresis loop. The gap between them is the energy lost to the difference between the advancing and receding contact angles.

Wettability Sets the Sign

In a water-wet rock, water imbibes on its own wherever Pc is positive, and water must be forced in once the curve crosses zero. Turning the rock oil-wet drops the whole curve, so the range of spontaneous imbibition shrinks. The saturation where Pc crosses zero is the spontaneous-imbibition limit, and it controls how much of a waterflood happens for free.