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.