Hydrocarbon column: calculation of maximum height

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Evaluating top and fault seal
Author	Grant M. Skerlec
Link	Web page
Store	AAPG Store

Calculation

The maximum column of hydrocarbons that can accumulate under a seal occurs when the buoyancy pressure, P_b, equals the displacement pressure, P_d. If P_b exceeds P_d, the hydrocarbons will leak through the seal. If P_b is less than P_d, it would be possible to seal a greater column of hydrocarbons.

Therefore, for the maximum hydrocarbon column

$P_{\rm {b}}=P_{\rm {d}}$

$(\rho _{\rm {w}}-\rho _{\rm {h}}){\mbox{g}}h={\frac {2\gamma \cos \theta }{R}}$

$h={\frac {\left(\displaystyle {\frac {2\gamma \cos \theta }{R}}\right)}{(\rho _{\rm {w}}-\rho _{\rm {h}}){\mbox{g}}}}$

or, in mixed units,

$h=\left({\frac {P_{\rm {d}}}{\rho _{\rm {w}}-\rho _{\rm {h}}}}\right)0.433$

where:

• ρ_w = density of water, g/cm³
• ρ_h = density of hydrocarbon, g/cm³
• g = acceleration due to gravity, cm/sec²
• h = thickness of the hydrocarbon column, ft or m
• γ is the interfacial tension,
• θ is the contact angle between the wetting fluid and the solid surface, and
• R is the capillary (pore throat) radius (Vavra et al., 1992^{[citation needed]}).

Effect on seal capacity

The height of the hydrocarbon column (h) in the above equations is a theoretical maximum. The actual height is less because of the effect of the reservoir. If the reservoir itself had zero displacement pressure, the height of the hydrocarbon column would attain the theoretical maximum and the oil-water contact would coincide with the free water level. If the reservoir has some displacement pressure greater than zero, then the height of the hydrocarbon column is less than the theoretical maximum, or

${\mbox{h}}=\left({\frac {{\mbox{P}}_{\rm {ds}}-{\mbox{P}}_{\rm {dr}}}{\rho _{\rm {w}}-\rho _{\rm {h}}}}\right)0.433$

where:

• P_ds = displacement pressure of the seal
• P_dr = displacement pressure of the reservoir

See also

• How capillary properties control seal
• Buoyancy pressure
• Top seal displacement pressure

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