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^[1] 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;^[2] 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.

References

↑ 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.
↑ North, F. K., 1985, Petroleum Geology: London, Allen & Unwin, 246 p.

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Hydrodynamic influence on trapping

[ch05r11-1] 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.

[North1985-2] North, F. K., 1985, Petroleum Geology: London, Allen & Unwin, 246 p.

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Hydrodynamic influence on trapping

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