Geoscience Reference
In-Depth Information
outcome - a range of water-cut breakthrough
times - is illustrated in Fig.
5.13
.
The Gannet B study offered some additional
insights into mature field scenario modelling:
• Although the truism is offered that multiple
models can match production data (there is no
uniqueness to history matches), the converse
is not necessarily true - everything cannot
always be matched;
• The above is more likely to be true in smaller
fields, where physical field limitations con-
strain possible scenarios;
Time to water break through
(year 0 = study time)
Well 1
Well 2
•
In the specific case of Gannet B, the principal
matching tool was 4D seismic data (not well
production data), and it was the seismic which
was the matching target
for the multiple
model scenarios;
• A base case selection from the quantified range
in water breakthrough times would have been
highly misleading; “between 9 months and 4
years” was the answer to the question based on
the available data. Making a median guess
would have simply hidden the risk.
1
2
3
4
year
Static
Dynamic
Faulted case
Aquifer
Sandy case
5.7
Scenario Modelling - Benefits
Muddy case
The scenario-based approach as defined here
offers specific advantages over base case
modelling and multiple probabilistic modelling:
Determinism
: the dominance of the underlying
conceptual reservoir model, which is deter-
ministically applied via the model design.
Although the models may use any required
level of geostatistical simulation to re-create
the desired reservoir concept, the
geostatistical algorithms are not used to select
the cases to be run, nor to quantify the uncer-
tainty ranges in the model outcomes.
Lack of anchoring
: the approach is not built on
the selection of a base case, or best guess.
Qualitatively, the natural tendency to under-
estimate uncertainties is less prone to occur if
a best guess is not required - the focus lies
instead on an exploration of the range.
Dependence:
direct dependence between para-
meters is maintained through the modelling
process;
High volume case
Fig. 5.13
Application of deterministic scenarios to a
brownfield case: forecasting water breakthrough
(Redrawn from Bentley and Smith 2008, The Geological
Society, London, Special Publications 309
#
Geological
Society of London [2008])
Gannet B Field in the Central North Sea (Bentley
and Hartung
2001
; Kloosterman et al.
2003
).
The issue to model in the Gannet B case was
the risk and timing of potential water break-
through in one of the field's two gas producers,
and placing value on the possible contingent
activities post-breakthrough. As with the cases
above, the study started with a listing and quali-
tative ranking of principal uncertainties in a
cross-discipline forum. Unlike the previous
cases, it proved not to be possible to match all
static reservoir models with history. The lowest
volume realisation would not match. The model
a
contrast between two model
