Page 111 - Geology and Geochemistry of Oil and Gas

WATER DRIVE 83
In studying the water drive systems, the most important variable is the potential
energy (including elastic energy). Energy distribution in the reservoirs determines the
possible directions of fluid (oil, gas, or water) flow. The latter can occur if the
migration avenues are available.
Pressure distribution in the reservoir rocks affects the piezometric surface. The
piezometric surface passes through the points to which the water can rise in different
wells. If the water has uniform density, its movement is possible only if the
piezometric surface is inclined.
8
In 1933, Zhdanov proposed to use a concept of normalized pressure for the water
movement calculations. The World Ocean surface is used as a datum plane. The
datum is considered to be the zero energy surface. All water located either above or
below this surface tends to attain this level. This concept is not exact for at least two
reasons:
The zero energy value is arbitrary.
The World Ocean surface is not horizontal (i.e., geoidal); it has minima and
maxima caused by the gravitational forces of the Earth.
These objections, however, may be neglected for routine hydrodynamic calculations.
All such calculations and construction of piezometric-surface maps utilize only the
differences in liquid column height (at equal density). Any arbitrarily selected surface
may be chosen as a datum plane (this does not affect the height difference used in
calculations). Thus, when calculating normalized pressure, the pressure (in atm, Pa,
2
2
kg/cm , or lb/ft ) measured in the reservoirs is expressed as height of a corresponding
water column with specific gravity equal to one (Fig. 4.5).
Absolute pressure in a water-filled anticline trap (water is not moving) increases
from the crest toward the flanks. The isobars are parallel to the depth contours. The
normalized pressure at all points of isobar is the same. This is not the case if a trap is
filled with oil or gas. At the hydrocarbon–water interface (if there is a contact, and if
the water drive is an infiltration one) pressure will be equal to a hydrostatic one, and
up the section it will be determined by considering the surpressure (see above).
In the case of moving water, the piezometric surface is inclined toward the direction
of water movement due to the pressure drop. The OWC will also be tilted. The
following equation is proposed for determination of the tilt angle a of an OWC surface,
using the tilt angle b of piezometric surface and densities of water, r , and oil, r :
w
o
tan br w
tan a ¼
r r o
w
The slope of the OWC increases with increasing pressure drop and decreasing density
difference. At reservoir conditions, the gas and oil densities are lower than that of water
(specific gr. water41) and the denominator in the above equation is less than 1. Thus,
the inclination angle of the OWC in this case is greater than the one for piezometric
surface ða4bÞ.

8 2 3
p ¼ gh, where p ¼ the pressure, for example, in lb/ft , g ¼ the specific weight of fluid in lb/ft , h ¼ the
height of fluid column in ft.