Geoscience Reference
In-Depth Information
Δ
x
Δ
H
w
Hydrocarbon
Δ
z
Aquifer
flow
Fig. 3.44
Terms defining a tilted oil-water contact (Redrawn from Dahlberg
1995
(Fig. 12.5), Springer-Verlag, New
York, with kind permission from Springer Science and Business Media B.V.)
have for a dynamic modelling of petroleum
accumulations. For simplicity we mainly con-
sider oil-water contacts, but the theory applies
to any hydrocarbon: gas, condensate or oil. The
main principle governing this phenomenon is
potentiometric head. If an aquifer contains
flowing water driven by some pressure gradient
(Fig.
3.44
), then this pressure gradient causes a
slope in the petroleum-water interface of any
accumulation within that aquifer, defined by
Hubbert (
1953
) as:
(
2012
), who include the terms for the effective
permeability in the aquifer, k
aq
, and reservoir,
k
res
, to derive a relationship between the
hydrocarbon-water interface and the hydrody-
namic pressure gradient in terms of steady-state
flow:
: ʔ
ð
ʔ
z
=ʔ
x
Þ
¼
k
res
=
k
aq
ʔˁ
g
ð
H
w
=ʔ
x
Þ ð
3
:
41
Þ
As can be seen from Table
3.6
, the actual
value of the tilted oil-water contact can be quite
small (most documented examples are around
10 m/km), so that uncertainties in detection
become important. There are many situations
which can give an apparent tilt in oil-water con-
tact, including:
Undetected faults (usually the first explana-
tion
ʔ
=ʔ
ˁ
w
=ˁ
w
ˁ
o
ð
Þ: ʔ
ð
H
w
=ʔ
Þ
ð
:
Þ
z
x
x
3
40
where
ˁ
w
ˁ
o
¼
density of water and petroleum
ʔ
z/
ʔ
x
¼
slope
of
the
hydrocarbon-water
interface
to
be
proposed)
or
stratigraphic
ʔ
H
w
/
ʔ
x
¼
potentiometric surface in aquifer
boundaries;
Variations in reservoir properties - systematic
changes in pore throat size across a field can
lead to a variation in the oil-water contact of
5 m or more (Fig.
3.43
);
Misinterpretation of paleo-oil-water contacts
(marked by residual oil stains or tar mats) as
present-day contacts;
Errors in deviation data for well trajectories.
Thus, proof of the presence of a tilted oil-
water contact requires either multiple well data
explained by a common inclined surface
(Fig.
3.45
) or multiple data types explained
The greater the difference in fluid density (i.e.
the lighter the petroleum), the smaller the tilt of
the fluid contact. It is important to differentiate
the free-water level (FWL) from the oil-water
contact (OWC). Where the capillary pressures
are significant (due to small pores), the differ-
ence between FWL and OWC can be significant
(Fig.
3.43
). In Eq. (
3.40
), the
ʔ
z/
ʔ
x term relates
to the FWL (and only approximately to the
OWC). A more comprehensive treatment of this
topic is given by Muggeridge and Mahmode
