Hydrocarbon pressure gradient: plotting

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Critical elements of the petroleum system
Chapter	Formation fluid pressure and its application
Author	Edward A. Beaumont, Forrest Fiedler
Link	Web page
Store	AAPG Store

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

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