Page 149 - Well Logging and Formation Evaluation

Production Geology Issues 139

• The petrophysicist may supply the geologist with a constant average
permeability to use for the horizon in question, or supply averages for
each well. There is a danger here that either the average does not take
into account known geological variation over the structure or that con-
touring of well averages leads to an incorrect interpretation of the areal
variation.
• The above arguments with respect to permeabilities may also apply to
water saturations. The petrophysicist may supply the production geol-
ogist with saturations in the form of either logs, averages, or a satura-
tion/height function. It is essential that a clear audit trail for the
saturations in the model be supplied.
• Probably the greatest source of error in the petrophysics/production
geology interface lies in the realm of net-to-gross values, and this has
led to some huge mistakes in the past. Probably the safest approach,
where the petrophysicist is supplying the geologist with evaluated logs
for inclusion within a static model, is for porosity to be set to zero in
all nonreservoir units, and net/gross to be set to unity throughout.
However, even this can cause problems where upscaling is occurring.
It is really essential for the petrophysicist to sit with the geologist and
see just how net/gross is being handled within the software used to
generate the static model and how this is passed on to the dynamic
model.
• Picking of coring points or well TD (total depth) is often done by the
production geologist on the basis of his correlation. Failure to incorpo-
rate all of the petrophysical information available may often result in
bad decision making.
• Sometimes the petrophysical interpretation itself depends on the geo-
logical interpretation. For instance, if gas/oil differentiation is not pos-
sible from the logs alone, use might be made of the known production
history from neighboring wells, which will depend on the correlation.
If this correlation is wrong, the fluid allocation will be wrong. It may
also happen that the petrophysicist makes an interpretation of the fluids
that leads to an unresolved inconsistency between wells. In this case
the production geologist may be forced to introduce a fault in the struc-
ture, which may or may not exist in reality.
• The logging program may incorporate items that have a positive value
of information (VOI) only in the event of the well being a success.
Therefore, the petrophysicist, using good communication with the pro-
duction geologist, may be able to save money on the well through pro-
vision of early information on the well’s results. Poor communication