Fluid Contacts from Pressure
Learning objectives
- Find a fluid contact as the intersection of two pressure gradients
- See that no pressure point need sit exactly at the contact
- Relate the contact's reliability to the gradient contrast
- Recognize a gas-water contact as crisper than an oil-water contact
Two Gradients, One Crossing
One fluid gradient gives a density. Two gradients give a contact. Where the line through the upper fluid and the line through the water meet, the two fluids are at the same pressure, and that depth is the fluid contact. You do not need a pressure point at the contact at all; you extrapolate the two trends until they cross.
The Crossing Angle Is Everything
How well the contact is pinned depends on how sharply the lines cross. Gas at 0.09 and water at 0.45 have very different slopes, so they cross at a steep angle and the contact barely moves even with noisy gauges. Oil at 0.33 and water at 0.45 are close, so they cross at a shallow angle and the same gauge noise smears the contact over tens of feet. The uncertainty scales roughly as the gauge noise divided by the gradient contrast.
Why It Matters
A pressure-derived contact is independent of the logs, so it is a powerful cross-check on a contact picked from resistivity or capillary pressure. When the two disagree, the pressures usually win, because they see the connected fluid system over the whole interval, not just the rock at the wellbore. The contact this gives is the free-water level, the subject of the next section.
References
- Dake, L. P. (1978). Fundamentals of Reservoir Engineering. Elsevier.
- Schlumberger (2006). Fundamentals of Formation Testing. Schlumberger.