Geoscience Reference
In-Depth Information
Since the Åre Formation spans such a wide variety
of depositional systems, the challenges in geologi-
cal modelling and reservoir forecasting are not
similar throughout the succession. In heterogeneous
reservoir intervals characterised by facies associa-
tions with large contrast in reservoir properties,
the challenges related to reservoir modelling are
associated commonly with realistic representation
of dimensions, distribution and orientation of
various facies bodies (Jones
et al
., 1995). The Åre
fluvial successions (Åre Zone 1 and 2) comprise a
range of such heterogeneous deposits (see Fig 13).
The seismic data from the Heidrun Field cannot
resolve individual fluvial channels (see Fig. 25)
and a very important model input is provided by
the reconstructed palaeogeographic maps (e.g.
Fig. 7A). Also, for reservoir zones with higher
degree of expected lateral connectivity of sandbod-
ies, the palaeogeographic maps (Fig. 7B/C) in com-
bination with cross-sections (e.g. reservoir zones 4
and 5 in Fig. 18) has been applied as model input
parameters defining dimensions and proximal-
distal trends of reservoir sandbodies.
In zones where the most important variability
in reservoir properties occurs at a much smaller
scale than the modelling grid cell dimension,
upscaling to effective cell values becomes the
main challenge (Kjønsvik
et al
., 1994; Nordahl &
Ringrose, 2008). Examples of such zones in the
Heidrun Field include (but are not restricted to)
the tidal influenced sediments in the Åre Sub-
zone 5.2 and 6.2. Identifying and constraining
these sediments to certain reservoir zones is
another significant and important improvement of
the current stratigraphic model, as compared to
previous versions.
Nevertheless, the sub-zone contains large in place
volumes and represents valuable additional
reserves. However, development of these volumes
is dependent on a robust reservoir characterisation
and a reliable forecast of well production. Several
cross-sections, similar to the one illustrated in
Fig. 18, form important input to the decision pro-
cesses regarding choice of well solutions and
drainage strategies for the Åre 5.1 Sub-zone (e.g.
evaluation of long horizontal zone-dedicated pro-
ducers vs. co-mingled production from several
zones). Essential to these decisions are consistent
definition of bounding surfaces (i.e. top Åre 5.1
Sub-zone and top Åre 4 Zone) as inclusion of any
highly productive intervals in a dedicated hori-
zontal Åre 5.1-well will potentially have a high
impact on well performance. Both revision of the
stratigraphic concepts and consistent implemen-
tation was required in order to exploit this
business opportunity.
CONCLUSIONS
The Åre Formation reservoir characterisation
study has resulted in an updated and more
detailed conceptual understanding of the
Rhaetian to Early Pliensbachian-aged deposi-
tional systems in the Heidrun area. A total of 18
facies associations are defined and form the basis
for a new stratigraphic framework comprising
seven main reservoir zones. The Åre Formation is
transgressive with non-marine coastal plain
deposits at the base, passing into a succession of
lower delta plain and interdistributary bay depos-
its in the middle part, followed by an interval of
mixed wave-influenced and tide-influenced
deposits and, finally, open marine deposits in the
uppermost part. The revised geological charac-
terisation of the Åre Formation permits definition
of a framework with correlateable key surfaces.
Within this framework, the vertical distribution
of facies associations is well-constrained and
packaged into genetically related units. Sub-
sequently, well-to-well correlations can be under-
taken more confidently and the uncertainty in
predictions of spatial facies distribution have
been significantly reduced. Results from this
study have been applied in updated reservoir
modelling strategies and in construction of geo-
logical models of the Heidrun Field. In addition
to a necessary improvement in decision basis for
daily operational activities, these models have
Impact on opportunities for increased
oil recovery
Further development of the Heidrun Field is heav-
ily dependent of various measures for increased
oil recovery. The recognition of Åre 5.1 as a
distinct reservoir zone (see Fig. 18) serves as an
illustrative example of how the current updates of
the sedimentological and stratigraphic models
provide new opportunities for increased oil recov-
ery. Previously, the reservoir sub-zone Åre 5.1, as
defined in this study, had not been recognised as
a distinct reservoir zone and was not implemented
in the reservoir models. The Åre 5.1 Sub-zone is
characterised as a low net to gross reservoir inter-
val with low expected production performance.
Search WWH ::
Custom Search