Page 148 - Well Logging and Formation Evaluation

138 Well Logging and Formation Evaluation

(HIIP), etc. Such models may be either deterministic (i.e., using fixed
distributions of reservoir properties) or probabilistic (i.e., using prob-
ability distributions for reservoir properties).
6. Create an upscaled subsurface model in digital format that may be
exported to the reservoir simulator and used for dynamic simulation
and reserve estimation.
7. Provide the geological background for any proposed well stimulation
or secondary recovery projects.

Poor communication between the petrophysicist and the geologist can
sometimes lead to some expensive mistakes, particularly with respect to
proposing future wells and assessment of reserves. Here are some of the
major pitfalls I have come across:

• Where the reservoir zonation is quite coarse (i.e., one zone covers a
large depth interval), there may be considerable variation of the reser-
voir properties over the zone. One petrophysical average may be quite
inappropriate. For instance, say a zone consisted of 3m of good-quality
sand of 1 darcy permeability overlying a 100-m interval having 10-md
sand. The zone, seen as an average, might appear unproducible when
actually the good sand, taken in isolation, might be commercial. It could
also happen that the whole zone is interpreted as commercial when in
practice only a small part will ever contribute to production.
• Where the reservoir zonation is too fine, there is a danger of incorrect
correlation between wells. This often leads to an incorrect assessment
of fluid contacts or to some serious errors in estimation of reserves.
• Great care needs to be taken in the allocation of permeabilities in the
production geologist’s model. Bear in mind that this may be done in a
number of ways. The petrophysicist may apply a poroperm equation to
his porosity log and supply the production geologist with a curve to use
within his software. In this case the petrophysicist needs to ensure that
the permeabilities in nonreservoir units are set to an agreed-upon value.
In particular, very high permeabilities arising from spurious porosity
values need to be edited out.
• The petrophysicist may supply the production geologist with a poro-
perm equation to apply himself. In this situation there is an even greater
danger of incorrect values entering the model. If the geologist is resam-
pling the data to a coarser depth interval before applying the equation,
the resulting permeabilities are very likely to be incorrect due to the
nonlinearity of the poroperm equation.