Normal hydrostatic pressure gradients

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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

The hydrostatic pressure gradient is the rate of change in formation fluid pressure with depth. Fluid density is the controlling factor in the normal hydrostatic gradient. In the U.S. Rocky Mountains, a formation water gradient of 0.45 psi/ft is common. In the U.S. Gulf Coast, a gradient of 0.465 psi/ft is common.

Factors controlling fluid density

Fluid density changes with depth as a result of changes in the following factors:

  • Temperature
  • Pressure
  • Fluid composition (including dissolved gases and solids)
  • Fluid phase—gaseous or liquid

These factors must be taken into account when estimating fluid pressure at depth. The small amount of dissolved gas contributes little to the density and can be ignored in the exploration state.

Factors controlling oil density

Oil density varies greatly because of the large variety of oil compositions and quantity of dissolved gases. Also, oil composition is inherently much more variable than formation water composition.

Factors controlling gas density

Gas density is strongly affected by pressure, temperature, and composition. In the reservoir, gas may be in the liquid phase; if so, we should treat it as a very light oil.

Predicting gas phase can be complicated. Consult an experienced reservoir engineer when making this prediction.

Ranges of fluid density and gradient variation

Oil-field liquids and gases occur in a wide range of compositions. The table below shows typical density ranges and gradients for gas, oil, and water. However, because exceptions occur, have some idea of the type of fluid(s) expected in the area being studied and use appropriate values.

Fluid Normal density range (g/cm3) Gradient range (psi/ft)
Gas (gaseous) 0.007-0.30 0.003-0.130
Gas (liquid) 0.200-0.40 0.090-0.174
Oil 0.400-1.12 0.174-0.486
Water 1.000-1.15 0.433-0.500

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