Page 57 - Well Logging and Formation Evaluation

Quicklook Log Interpretation 47

bered that they represent only a point measurement. Typically moving the
probe up or down by a few centimeters may result in a completely dif-
ferent measurement of mobility. The lack of a good buildup may be
purely the result of bad luck in the positioning of the probe. Moreover,
the results may not give an accurate idea of the average permeability of
a zone.
In general, pretests should be used to verify that a zone has some per-
meability, but the other methods used (e.g., permeability as derived from
a poroperm relationship) are to determine an average permeability to be
used in dynamic models. A pretest permeability being lower than that
derived from a poroperm relationship may be a result of formation damage
occurring while drilling. This may also be observed when the zone is
tested for production.
Petrophysicists should always try to obtain the actual field print from
the contractor when doing field studies, with a view to assessing perme-
ability and fluid contacts. Reasons for this are as follows:

• Older-generation tools report pressures from a strain gauge, which
measures psi per gauge (psig) rather than the absolute psi (psia) reported
from quartz gauges. If the values are entered incorrectly into a
database, there will be a shift equivalent to atmospheric pressure
(14.7psi).
• When databases are created for fields (e.g., a shared Excel TM spread-
sheet), sometimes not all the field data are entered, such as zones
reported as “tight.” Knowledge of tight zones is crucial if zones are
being considered for recompletion based on log-derived permeability
estimates.
• When zones are reported as being tight or of limited drawdown, it may
be possible in some cases to make an estimate of formation pressure by
extrapolating the buildup pressures.
• The contractors will typically report a measured depth for the pretest,
as well as a true vertical depth (TVD), with reference to the derrick
floor. It is important to check that the pressures used are being refer-
enced properly to the best estimate of TVD relative to the datum
(usually mean sea level). After the pressure tool is run, there will typ-
ically be a gyro survey run once the final casing is set, and this should
be used to convert all measured depths in the well to TVD relative to
the datum.