Pressure Gradients and Fluid Density
Learning objectives
- Plot formation pressures against depth and read the slope
- Convert a gradient to a fluid density, gradient = 0.4335 times rho
- Tell gas, oil, and water apart by their gradients
- See how gauge noise over a short interval blurs the density estimate
A Line Whose Slope Is a Density
Stack several pretest pressures down a well and plot them against depth, and the points in a single fluid fall on a straight line. The slope of that line is the fluid density, because a static column obeys
with the gradient in psi/ft and density in g/cc. Reading the slope tells you what fluid you are standing in.
Gas, Oil, Water
The three fluids separate cleanly. Gas is light, near 0.08 to 0.10 psi/ft, so its line is steep, almost vertical, gaining little pressure with depth. Brine is dense, near 0.43 to 0.45 psi/ft, a much shallower line. Oil sits between at roughly 0.30 to 0.35. A glance at the slope is a fluid identification.
The Catch: a Short Interval
The density comes from a slope, and a slope needs length. Over a thin column the pressure change is small, gauge noise of even a psi or two swings the fitted gradient, and the lighter the fluid the worse it is. A confident density needs pressure points spread over tens of feet, which is exactly what makes a fluid contact, where two long gradients meet, so powerful.
References
- Dake, L. P. (1978). Fundamentals of Reservoir Engineering. Elsevier.
- Schlumberger (2006). Fundamentals of Formation Testing. Schlumberger.