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{{publication 
 | image   = exploring-for-oil-and-gas-traps.png
 | width   = 120px
 | series  = Treatise in Petroleum Geology
 | title   = Exploring for Oil and Gas Traps
 | part    = Critical elements of the petroleum system
 | chapter = Formation fluid pressure and its application
 | frompg  = 5-1
 | topg    = 5-64
 | author  = Edward A. Beaumont, Forrest Fiedler
 | link    = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
 | pdf     = 
 | store   = http://store.aapg.org/detail.aspx?id=545
 | isbn    = 0-89181-602-X
}}
==Hydrodynamic influence on trapping==

===Potential of water vs. hydrocarbons===

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

:<math>\rho_{\rm w} \mbox{gH}_{\rm w} - \rho_{\rm h}\mbox{gH}_{\rm h}</math>

where:

* ρ<sub>w</sub> = water density
* ρ<sub>h</sub> = hydrocarbon density
* H<sub>w</sub> = water depth
* H<sub>h</sub> = hydrocarbon column height

Under hydrodynamic conditions, the potential for a hydrocarbon column (Φ<sub>h</sub>) is related to the potential of the water by the equation

:<math>\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}</math>

Dividing through by g (ρ<sub>w</sub> – ρ<sub>h</sub>)/ρ<sub>h</sub> to simplify gives (in a uniformly flat gravity field)

:<math>\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}</math>

Constant values for the left-hand side of the equation are equipotential surfaces for hydrocarbons. Hubbert<ref name=ch05r11>Hubbert, K., 1953, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0037/0008/1950/1954.htm 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.</ref> called this factor U. From the right side, constant values for ρ<sub>w</sub>/(ρ<sub>w</sub> – ρ<sub>h</sub>)H<sub>w</sub> 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

:<math>\mbox{U} = \mbox{V} - \mbox{Z}</math>

Fluid flow is perpendicular to equipotential surfaces.

===Hydrodynamic effect on traps===

[[file:formation-fluid-pressure-and-its-application_fig5-37.png|300px|thumb|{{figure number|1}}Vectors and equipotential lines for a hydrocarbon accumulation in an anticline in a hydrodynamic environment. Modified. Copyright: North, 1985; 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<sub>b</sub>) is interfered with by the hydrodynamic force (P<sub>w</sub>). The resultant interference is the vector known as the confining force (P<sub>cf</sub>). U, an equipotential line, is perpendicular to P<sub>cf</sub> and is tilted because of the effect of P<sub>w</sub>. The diagram in [[:file:formation-fluid-pressure-and-its-application_fig5-37.png|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]]

==References==
{{reflist}}

==External links==
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* [http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]

[[Category:Critical elements of the petroleum system]] 
[[Category:Formation fluid pressure and its application]]