Geoscience Reference
In-Depth Information
zone. There is now an increasing awareness that
even if a fault core is sealing, the fractures in the
fault damage zone may be open to flow
up or
along
the fault zone.
The Douglas Field in the East Irish Sea
provides an example of this, for which a simple
modelling workflow was designed (Bentley and
Elliott
2008
). The interpretation of open damage
zones was prompted by the anomalous water-cut
behaviour of some wells and the inability to
match history using conventional simulation
modelling of the reservoir matrix. The anoma-
lous behaviour took three forms:
1. Water breakthrough not matched in wells
drilled close to or through major faults.
2. Gas breakthrough not matched in wells post
gas injection into a flank well in the field.
3. Flowing bottom-hole pressures not matched
in most wells.
In order to address these observations, three
activities were initiated (a) re-visit the core store;
(b) understand fault-related processes using out-
crop analogues, and (c) re-design the reservoir
modelling approach using the new insights.
The revised geological concept which
emerged from these studies was one of zones of
damage around seismic-scale faults containing
both permeability-reducing and permeability-
enhancing elements. The sealing elements were
the small shear fractures, observed as deforma-
tion bands in quartz-rich layers or as small dis-
crete faults in more mud-rich intervals, and the
master fault slip surfaces themselves. Evidence
of fracturing in the core material is sparse, as the
core was taken in vertical appraisal wells. Frac-
turing was nevertheless observed in core in the
form of deformation bands and small faulted
intervals. One fault plane in particular was well
preserved in core, was not cemented and,
crucially, was observed to be hydrocarbon
stained (Fig.
6.50
). Some fractures were clearly
permeable.
The structural concept which emerged from
the review is summarised in Fig.
6.51
. Although
major faults tend to seal to lateral cross-flow,
either through juxtaposition, the formation of a
sealing fault gouge or the generation of deforma-
tion bands, the damage zones around the faults
Fig. 6.50
Open fractures associated with deformation
bands in Ormskirk Sandstone core from the Douglas Field
include open fractures, either joints or small
faults. The joints will tend to be stratigraphically
sensitive and bed-limited to the quartz-rich
intervals, but the slip surfaces will be through-
going. The major faults therefore exhibit a
tendency to seal laterally but also a tendency
for vertical flow along open damage zones.
In order to model the effects of this concept,
involving both conductive fractures and sealing
faults, a novel approach to modelling the fault
damage zones was implemented, using artificial
wells ('pipes') to create flow conduits along
suspected fracture zones.
The pipes were assigned with open flow
completions in each simulation grid block along
the fracture corridor to allow cross-flow within the
formation. Altering the radii of the pipes provided
a method of history matching the rapid onset of
water production (Fig.
6.52
). The approach readily
allowed a successful history match, successfully
replicating the observed water break through
patterns, the gas-oil ratio changes and the recorded
flowing bottom-hole pressures.
