Hydrodynamic influence on trapping

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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

Hydrodynamic influence on trapping

Potential of water vs. hydrocarbons

Fluid pressure equals pgH. Under hydrostatic conditions, the buoyant force equals

where:

  • ρw = water density
  • ρh = hydrocarbon density
  • Hw = water depth
  • Hh = hydrocarbon column height

Under hydrodynamic conditions, the potential for a hydrocarbon column (Φh) is related to the potential of the water by the equation

Dividing through by g (ρw – ρh)/ρh to simplify gives (in a uniformly flat gravity field)

Constant values for the left-hand side of the equation are equipotential surfaces for hydrocarbons. Hubbert[1] called this factor U. From the right side, constant values for ρw/(ρw – ρh)Hw are equipotential surfaces for water. Hubbert called this factor V. The elevation factor (Z) is the difference between the equipotential surfaces for hydrocarbons and water.

Substituting U and V in the above equation gives

Fluid flow is perpendicular to equipotential surfaces.

Hydrodynamic effect on traps

In a hydrostatic environment, the free-water level of a trap is horizontal. In a hydrodynamic environment, the free-water level of a trap is tilted because the buoyant force (Pb) is interfered with by the hydrodynamic force (Pw). The resultant interference is the vector known as the confining force (Pcf). U, an equipotential line, is perpendicular to Pcf and is tilted because of the effect of Pw. The diagram below shows these vectors and the equipotential lines for a hydrocarbon accumulation in an anticline in a hydrodynamic environment.

Figure 5-37 Modified. Copyright: North, 1985; courtesy Allen and Unwin.

See also

References

  1. Hubbert, K., 1953, Entrapment of petroleum under hydrodynamic conditions: AAPG Bulletin, vol. 37, no. 8, p. 1954–2026. The original paper that proposed hydrodynamics as an important trapping mechanism.

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