Hydrodynamic influence on trapping

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 ρgH. Under hydrostatic conditions, the buoyant force equals

$\rho _{\rm {w}}{\mbox{gH}}_{\rm {w}}-\rho _{\rm {h}}{\mbox{gH}}_{\rm {h}}$

where:

• ρ_w = water density
• ρ_h = hydrocarbon density
• H_w = water depth
• H_h = hydrocarbon column height

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

$\Phi _{\rm {h}}=\rho _{\rm {h}}{\mbox{gH}}_{\rm {h}}=\rho _{\rm {w}}{\mbox{gH}}_{\rm {w}}-(\rho _{\rm {w}}-\rho _{\rm {h}}){\mbox{gZ}}$

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

$\left({\frac {\rho _{\rm {h}}}{\rho _{\rm {w}}-\rho _{\rm {h}}}}\right){\mbox{H}}_{\rm {h}}=\left({\frac {\rho _{\rm {w}}}{\rho _{\rm {w}}-\rho _{\rm {h}}}}\right){\mbox{H}}_{\rm {w}}-{\mbox{Z}}$

Constant values for the left-hand side of the equation are equipotential surfaces for hydrocarbons. Hubbert called this factor U. From the right side, constant values for ρ_w/(ρ_w – ρ_h)H_w 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

${\mbox{U}}={\mbox{V}}-{\mbox{Z}}$

Fluid flow is perpendicular to equipotential surfaces.

Hydrodynamic effect on traps

Figure 1 Vectors and equipotential lines for a hydrocarbon accumulation in an anticline in a hydrodynamic environment. Modified. Copyright: North; courtesy Allen and Unwin.

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 (P_b) is interfered with by the hydrodynamic force (P_w). The resultant interference is the vector known as the confining force (P_cf). U, an equipotential line, is perpendicular to P_cf and is tilted because of the effect of P_w. The diagram in Figure 1 shows these vectors and the equipotential lines for a hydrocarbon accumulation in an anticline in a hydrodynamic environment.

See also

• Hydrodynamics
• Mapping hydrodynamic traps

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