Initialization and Equilibration

Setting the Model at Rest

Before the first timestep the model must be in hydrostatic balance. Equilibration sets each phase's pressure to climb with depth along its own density gradient; the water and oil gradients cross at the free water level, where the capillary pressure is zero.

Contacts and the Transition Zone

The oil-water contact sits a capillary-entry height above the free water level. Between the contact and the top, the saturation grades from all water up to connate water through the transition zone.

What Sets the Zone Thickness

A larger density contrast or better rock compresses the transition into a thin contact; weak contrast in poor rock smears it over a tall interval. Getting this initial state right is what keeps the very first volumes and pressures physical.