The Fluids and Their Contacts

The Three Fluids

A reservoir holds three fluids: gas, oil, and water. They differ in density, gas the lightest, then oil, then the saline formation water, and in the trap they sort themselves by density: gas floats on oil, oil floats on water. The hydrocarbons are buoyant, held up against the water like a beach ball pushed under the surface. Where one fluid gives way to the next is a contact.

Drag the oil gravity and toggle the gas cap, and watch the gradient lines tilt and the contacts move.

Gradients and Contacts

The cleanest way to find the contacts is a pressure-depth plot. Each fluid adds pressure with depth at its own rate, the hydrostatic gradient, set by its density:

in psi per foot, with $\rho$ in g/cc. Water is steepest at about 0.45, oil gentler near 0.35, gas gentlest near 0.10. Plot the measured pressures and the points fall on straight lines, one per fluid; where two lines cross is a contact, the gas-oil contact (GOC) where gas meets oil, and the oil-water contact where oil meets water.

The Free-Water Level and the Transition Zone

The crossing of the oil and water pressure lines is the free-water level (FWL), the depth where the buoyancy pressure is zero. The oil-water contact seen on logs sits a little above it, at the top of a transition zone where capillarity holds the water saturation gradationally between irreducible at the crest and one at the FWL. Finer throats and lighter oil make the transition zone taller, which is why the same field can show a sharp contact in a coarse sand and a smeared one in a fine sand.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Dake, L. P. (1978). Fundamentals of Reservoir Engineering. Elsevier.