Geoscience Reference
In-Depth Information
Fig. 6.22
Anisotropic
relative oil permeability for
an example shallow marine
facies (wavy-bedded
facies, Rannoch Formation,
Ringrose and Corbett
1994
)
(Redrawn from Ringrose
and Corbett
1994
, The
Geological Society,
London, Special
Publications, No. 78
#
1
Wavy-bedded facies
Horizontal
Vertical
Geological Society
of London [1994])
S
wor
S
wi
0
1
Water Saturation
shallow marine reservoirs have such good overall
recovery factors. Thin shales, however, have a
negative impact.
The small-scale (microscopic) effects of cap-
illary forces in causing strong directional anisot-
ropy on two-phase immiscible flow processes
(Fig.
6.22
) are surprising to many, although the
effect has been clearly documented using
modelling (Corbett et al.
1992
; Ringrose et al.
1993
), laboratory analysis (Huang et al.
1995
)
and full-field history matching (Rustad et al.
2008
).
Reluctance to acknowledge the importance of
small-scale heterogeneities may also be due to
the fact that the effect operates at a scale much
smaller than most models can resolve, and so
must be incorporated implicitly - using upscaled
relative permeability functions. Here, the fluid
system itself plays a determining role - the geo-
logical factors discussed above are for water
displacing oil, an immiscible flow process. Gas
displacing oil - generally a miscible flow process
- tends to be most influenced by the large-scale
permeability architecture. Strong gas over-ride
should be expected, driven by both gravity and
viscous forces working in concert, leading to a
gas thief zone at the top of any progradational
shallow marine unit, in contrast to the waterflood
shown in Fig.
6.20
.
The same parasequence model was used by
Carruthers (
1998
) to simulate oil migration into a
detailed model of a rock formation. Oil was
introduced at the base and allowed to invade the
rock model, using a capillary-dominated
invasion-percolation technique (Carruthers and
Ringrose
1998
). The result (Fig.
6.23
) illustrates
how a capillary-dominated drainage flow process
picks out critical flow pathways, filling individ-
ual sand layers as local accumulations. If oil
migration is allowed to continue out of the
model then little more than a few percent of the
rock volume is contacted by oil. However, impo-
sition of a structural closure on the model would
result in the unit back-filling to create an oil
reservoir.
In summary, shallow marine reservoir
systems generally provide us with a 'dream
ticket' for oil recovery. This is due partly to
geology - shallow marine systems are generally
laterally-continuous, sand-rich and well-sorted -
but also due to the positive interaction between
flow processes and geology. Two-phase flow
effects at the lamina-scale and the coarsening
up profile both have a positive effect on lateral
water injection strategies; the geology assists the
reservoir engineer. However, the small scale
factors are important and need to be included in
any modelling exercise.
