Page 113 - Geology and Geochemistry of Oil and Gas

WATER DRIVE 85
If in the above equation the density is changing linearly, and z o is at the sea level,
then:
gðr þ r Þ
o
p ¼ p þ z
norm
2
If the water density does not remain constant both horizontally and vertically,
neither of the above techniques is applicable. In such a case, a vector of the filtration
force for limited bed volume is calculated.
As Gattenberger (1984) stated, ‘‘the possibility of obtaining objective information
on the subsurface water movements based on the analysis of static levels (Hubbert’s
potentials) or normalized pressure is available only if the changes in the static levels,
potentials, or normalized pressure drops are greater than their combined calculated
error.’’
In order to better define the role of hydrochemical and geochemical studies in the
field surveys (and production), the following formation water classification is
presented below:
1. Lower edge water. The lower edge water resides within the same reservoir as the
accumulation outside the OWC/GWC. In case of hydrodynamic drive, such
water is the main source of the potential energy pushing oil (and, to some extent,
gas) toward the well. This is clearly observed at depths with pressures close to the
critical one for water (2–2.5 km). Approximately at that depth a distinct water–oil
(gas) separation disappears to be replaced by a transition zone (see above). All
previously developed concepts and practical recommendations as to the
‘‘contour’’ behavior at these conditions must be reconsidered.
2. Upper edge water. The upper edge water occurs within the same reservoir as the oil
(gas) accumulation, up-dip of it in a monocline or the flank of a structure. It is rare,
which may be simply due to the lack of knowledge: it was believed that such an
occurrence is impossible. As Confucius remarked, ‘‘no sense in trying to catch a
black cat in a dark room if the cat is not there.’’ The first mention of such a water
goes back to the early 20th century when it was encountered (Gubkin, 1915) in the
so-called ‘‘inlet-like’’ oil accumulations in the Maykop Formation (Krasnodar
Region, Russia). At that it did not receive a due attention (‘‘a black cat’’). The
existence of such water may be explained by the change in porosity, and
the capillary forces. This explanation, however, was not valid in the case of San
Juan Field in Alberta, Canada. There, a gas accumulation in a 3,000-ft-high
syncline (surplus pressure of over 30MPa), is trapped by the upper edge water
with a simultaneous increase in the porosity up-dip from 14% to 25% and increase
in the average pore diameter. The pressure increases from the syncline flanks
to its trough. Two possible explanations were proposed: (a) change in the
wettability of rock properties from hydrophilic to hydrophobic and (b) the volume
of currently forming gas exceeds the gas loss due to migration through the upper
edge water.
3. Bottom water. The bottom water occurs within the same reservoir as the oil (gas)
underneath the entire accumulation or its near-edge portions. Reservations
expressed above (No. 1) are applicable.