Page 62 - MSc_thesis_R A Kil

6 Reservoir modelling

The results from the fieldwork campaign, consisting of the facies description, the inferred conceptual
geological model and the reservoir properties, will be used to generate a model of the eastern part of
Favignana. The geological modelling will be performed as a (potential) reservoir model, and will therefore
not aim for accuracy to a metre or centimetre scale. As all upscaled geological models, it is an attempt
to capture key architectural and sedimentological characteristics of these deposits. Therefore, a model
will never be as accurate as the reality, but it will try to be as useful and correct as possible. To quote a
well known statistician:

’Essentially, all models are wrong, but some are useful.’ (Box & Draper, 1987)
In order to build a model, the present boundary surfaces have to be gridded. Before this step, data for
those surfaces was available in a vector-like grid with high resolution. Gridding introduces the concept
of averaging certain properties and geometries over a 3D volume. In order to limit the time needed for
calculation to acceptable dimensions, a grid size of 25 by 25 metres was chosen. Compared to reservoir
models used in dynamic simulations, this is a rather high resolution. However, compared to the amount
of details observed in the field, this grid could be considered a gross oversimplification. Because this
study focusses on the reservoir implications, this trade-off was made. The layers are defined between
the sea level and the surface of eastern Favignana, with a varying thickness. Some lower areas like Cala
Rossa and Cala Azzurra require more detail than others, so a proportional approach is chosen where the
layering builds up from the bottom.

6.1 Facies model

The facies model is the starting point of a reservoir model. The flow behaviour of a reservoir is strongly
affected by petrophysical properties of the rock it consists of. These parameters can be tested in a
laboratory. However, the results of these tests only represent a very small piece of reservoir rock. The
aim of a facies model from a reservoir engineering point of view is to visualise the subsurface in order to
populate the reservoir model with petrophysical properties. For example, a rock sample originating from
a certain facies influences parts of the reservoir that are modelled as the same facies.

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