Fig. 2.14 Critical order of magnitude permeability contrasts for a range of fluid and productionmechanisms - 'Flora's Rule'
Fig. 2.15 What type of reservoir model? A choice based on heterogeneity and fluid type
The decision as to which candidate elements to
include in a model is therefore not primarily
a geological one. Geological and petrophysical
analyses are required to define the degree of
permeability variation and to determine the
spatial architecture, but it is the fluid type and the
selected displacement process which determine the
level of geological detail needed in the reservoir
model and hence the selection of 'model elements'.
Alabert 1990 ) and was generally received as a
welcome answer to some tricky questions such
as how to handle uncertainty and how to represent
geological heterogeneities in 3D reservoir models.
However, the promise of geostatistics (and
'knowledge-based systems') to solve reservoir
forecasting problems sometimes led to disap-
pointment. Probabilistic attempts to predict
desirable outcomes, such as the presence of a
sand body, yield na¨ve results if applied blindly
(Fig. 2.16 ).
This potential for disappointment is unfortu-
nate as the available geostatistical library of tools
is excellent for applying quantitative statistical
algorithms rigorously and routinely, and is essen-
tial for filling the inter-well volume in a 3D
Determinism and Probability
The use of geostatistics in reservoir modelling
became widely fashionable in the early 1990s
(e.g. Haldorsen and Damsleth 1990 ; Journel and