Hydrocarbon pressure gradient: plotting

We can estimate the downdip free-water level from a valid fluid pressure measured within a reservoir.

Plotting gradient

The list below outlines the procedure for plotting a hydrocarbon pressure gradient on a hydrostatic pressure plot when a measured pressure is available from the reservoir.

1. Plot measured fluid pressure on a hydrostatic pressure–depth plot.
2. Determine the hydrocarbon pressure gradient from one of two ways:
   • Measured hydrocarbon density
   • Estimates of hydrocarbon density
3. Determine the buoyancy pressure gradient: static water pressure gradient minus hydrocarbon pressure gradient.
4. Determine a pressure above or below the measured depth point. The table below lists the steps for determining this number.
   1. Pick a depth above or below the measured point.
   2. Multiply the difference in depth by the buoyancy pressure gradient.
   3. Add the number from step 2 to the measured pressure if the depth is deeper; subtract if shallower.
      • Example: Measured pressure at depth::7607 ft is pressure::3530 psi and buoyancy pressure gradient is 0.076 psi/ft. What is the hydrocarbon pressure at depth::7507 ft?
      • Solution:
        • $7607{\text{ ft}}-7507{\text{ ft}}=100{\text{ ft}}$
        • $100{\text{ ft}}\times 0.076{\text{ psi/ft}}=7.6{\text{ psi}}$
        • ${\text{Hydrocarbon pressure at 7507 ft }}=3530{\text{ psi}}-7.6{\text{ psi}}=3522.4{\text{ psi}}$
5. Plot the pressure number from step 4 on the pressure–depth plot and draw a line between this point and the measured pressure point. This line is the hydrocarbon pressure gradient.

See also

• Static hydrocarbon pressure gradients
• Estimating static oil pressure gradients
• Estimating static gas pressure gradients
• Free water level determination using pressure

External links

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