Page 112 - Geology and Geochemistry of Oil and Gas

84 WATER

Fig. 4.5. The concept of normalized pressure. H A is the height of the liquid column at point A (pressure
head at point A), H B the height of the liquid column at point B (pressure head at point B), h A ¼ the
normalized height of liquid column at point A (normalized pressure head at point A), h B ¼ the normalized
height of liquid column at point B (normalized pressure head at point B). ðH A þ z A Þ
ðH B þ z B Þ ¼ h A h B , where z A is the potential head at point A, z B the potential head at point B. p A
ðpressure at point AÞ ¼ g f H B ; p B ðpressure at point BÞ ¼ g f H B , where g f is the specific weight of fluid. If
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specific weight is in lb/ft and H is in ft, then pressure is in lb/ft .
The above analysis enables one to formulate the main condition for the
preservation of an accumulation within a hydrodynamic trap. The inclination angle
of the OWC must be lower than the dip angle of the trapping flank. If these two
angles are equal (and even more so, if their magnitudes are reversed), the
accumulation can be squeezed-out of the trap down-dip. The speed of squeezing is
determined by the speed of water flow, which is rather slow in a porous medium
(Darcy law). All this is true when the water density remains constant.
If the subsurface water density changes only vertically and remains constant along
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any horizontal plane, the A. I. Silin-Beckchurin or M. Hubbert equation can be
used to calculate the normalized pressure (p norm ):
z
Z
p ¼ p þ grðzÞ dz
norm
z 0
where rðzÞ ¼ the water density changing in the vertical direction, z o ¼ the elevation
of a comparison (reference) surface (horizontal datum plane) with equal normalized
pressure, and g ¼ the gravitational acceleration.

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F ¼ gðz z o Þ þ ðp p o Þ=r. If the datum plane is taken at sea level [z o ¼ 0 and p o ¼ 0 (atmospheric
pressure)], the equation becomes F ¼ gz þ p=r.