Geoscience Reference
In-Depth Information
Fig. 4.27
Example
reservoir model grid
(Heidrun Field fault
segments, colour coded by
reservoir segment) (Statoil
image archives,
#
Statoil
ASA, reproduced with
permission)
improves the grid quality and flexibility, but
does not solve the whole problem. When
using grids with stair-step faults special atten-
tion must be paid to estimation of fault seal
and fault transmissibility. There is generally
insufficient information in the grid itself for
these calculations, and the calculation of fault
transmissibility must be calculated based on
information from the conceptual geological
model.
• The handling of dipping reverse faults using
stair-step geometry in a corner point grid
requires a higher total number of layers than
required for an un-faulted model.
• Regions with fault spacing smaller than the
simulation grid spacing give problems for
appropriate calculation of fault throw and
zone to zone communication. Gridding
implies that smaller-scale geomodel faults
are merged and a cumulated fault throw is
used in the simulation model. This is not gen-
erally possible with currently available
gridding tools, and an effective fault transmis-
sibility, including non-neighbour connections,
must be calculated based on information from
the geomodel, i.e. using the actual geometry
containing all the merged faults.
• Flow simulation accuracy depends on the grid
quality, and the commonly used numerical
discretisation schemes in commercial
simulators have acceptable accuracy only for
'near' orthogonal grids. Orthogonal grids do
not comply easily with complex fault
structures, and most often compromises are
made
between
honouring
geology
and
keeping “near orthogonal” grids.
Figure
4.28
illustrates how some of these
problems have been addressed in oilfield studies
(Ringrose et al.
2008
). After detailed manual grid
construction including stair-step faults to handle
Y-faults, smaller faults are added directly into
the flow simulation grid. However, some
gridding problems cannot be fully resolved
using the constraints of corner point simulation
grids and optimal, consistent and automated grid
generation based on realistic geomodels is a chal-
lenge. The use of unstructured grids reduces
some of the gridding problems, but robust, reli-
able and cost efficient numerical flow solution
methods for these unstructured grids are not gen-
erally available. Improved and consistent
solutions for construction of structured grids
and associated transmissibilities have been pro-
posed (e.g. Manzocchi et al
2002
; Tchelepi et al.
