The Free-Water Level
Learning objectives
- Define the free-water level as the depth where capillary pressure is zero
- Place the log oil-water contact above the FWL by the capillary entry height
- Compute the entry height h = Pce / (0.433 (rho_w - rho_o))
- See that one FWL gives different OWCs in different rock
One Datum for the Whole System
The contact the pressures gave is the free-water level, the depth where the capillary pressure is zero and below which the rock is fully wet. It is a property of the connected fluid system, so a single reservoir in pressure communication has one FWL, no matter how the rock varies. That is what makes it the datum the whole field is built on.
The OWC Is Not the FWL
The oil-water contact you pick on a log, the depth where oil first appears and water saturation leaves 100 percent, is not the FWL. Oil cannot enter the rock until buoyancy overcomes the capillary entry pressure, so the OWC stands above the FWL by the entry height
One FWL, Many OWCs
Because the entry height depends on the rock, the same free-water level produces a different log OWC in different rock. A clean sand with a low entry pressure has its OWC almost at the FWL; a tight sand with a high entry pressure carries its OWC tens of feet higher. Tighten the rock and watch the OWC climb while the FWL stays put. This is why mapping OWCs from logs alone can look chaotic, and why the pressure FWL is the anchor.
References
- Dake, L. P. (1978). Fundamentals of Reservoir Engineering. Elsevier.
- Tiab, D. and Donaldson, E. C. (2015). Petrophysics, 4th ed. Gulf Professional.