Page 390 - Hydrocarbon Exploration and Production Second Edition

Risk Analysis 377

strengthened through an effective communication plan. Those with little influence
may become valuable if they are brought together through organisation.
The tools that exist to win over stakeholders include face-to-face meetings,
workshops, visits, presentations, briefing papers and written information, or a
mixture of these, and the medium needs to be tailored to the target audience. The
position of stakeholders on the plot may change over time, and so keeping this plot
updated for movements and any new entrants is important.

15.3.5. More complex problems: specific subsurface considerations

The spider diagrams and tornado plots are useful in displaying the impact of input
assumptions when the problems are relatively simple and there is a linear relationship
between inputs and outputs. In more complex cases (e.g. dependency of reserves on
permeability distribution) there are several ways of analysing the risk on the subsurface
side. Below we suggest three alternative approaches to subsurface uncertainty handling;
the most appropriate one will depend on the amount of data and the complexity of
the problem. In each case, we are trying to capture the range of uncertainty in the
subsurface descriptions, which will involve all subsurface disciplines. These can be
termed multiple realisations – in other words possible alternatives (Figure 15.13).
The three approaches suggested to creating the multiple realisations are

Build a limited number of cases, anchored around a preferred ‘Best Guess’ model
Use statistical methods to build a ‘Multiple Stochastic’ model, constrained by
imposed boundary conditions
Build ‘Multiple-Deterministic’ models which are designed manually, based on
discrete alternative concepts, and can be assigned a relative probability.
In each case, we are trying to build a range of possible reservoir descriptions, all
of which honour the limited amount of raw data available (Figure 15.14). A useful
initial step is to take a team-based approach to identify the key inputs which affect

Multiple Realisations

Geophysical
Interpretation
Geological
Modelling
Petrophysical
Interpretation

Reservoir
Engineering,
Simulation &
Forecasting

Figure 15.13 Multiple realisations of the subsurface description.