The Free-Water Level and the Transition Zone

Part 11, Chapter 11: Capillary Pressure and Saturation-Height

Learning objectives

Convert capillary pressure to height above the free-water level
Distinguish the FWL (Pc=0) from the OWC (entry-pressure height above)
Read the transition zone from full water to irreducible
Explain how oil density and rock quality set its height

Pressure Becomes Depth

Capillary pressure turns into depth the moment you divide by the buoyancy. The height a fluid stands above the free-water level is

h = \frac{P_c}{0.433\,(\rho_w - \rho_{hc})},

so the rock's capillary-pressure curve, replotted against height, is the saturation profile of the reservoir.

The FWL Is Not the OWC

Two surfaces fall out, and confusing them is a classic blunder. The free-water level is where $P_c = 0$ , the datum of the whole system. The oil-water contact is where the logs first see oil, and it sits above the free-water level by the height of the entry pressure, because oil cannot enter the rock until $P_c$ beats $P_e$ . In a tight rock that offset can be tens of feet, so a contact picked off the logs is not the free-water level the pressure data define.

What Sets the Zone

Between the contact and the top lies the transition zone, where saturation grades from full water up to irreducible. Its height is set by buoyancy: a light oil, with a large density contrast, lifts water fast and makes a short transition, while a heavy oil makes a tall, smeared one that can swallow much of the column in producible-but-wet rock. Tighter rock, with a higher entry pressure, pushes the contact further above the free-water level. This profile, saturation against height, is the model the next sections build and calibrate.

References

Worthington, P. F. (2002). Application of saturation-height functions in integrated reservoir description. Geological Society Special Publication, 215.
Tiab, D. and Donaldson, E. C. (2015). Petrophysics, 4th ed. Gulf Professional Publishing.