Page 84 - Well Logging and Formation Evaluation

74 Well Logging and Formation Evaluation

• GR (spikes occurring where fractures have become cemented up with
radioactive minerals)
• Caliper (borehole will tend to become elliptical, with the major axis
perpendicular to the tectonic stress direction)

In order to properly characterize the types and orientation of fractures,
it is necessary to use imaging tools. Resistivity tools are preferred because
it is easier to differentiate open fractures filled with fluid from cemented
fractures. However, ultrasonic-based tools can also be used and are the
only option in oil-based mud (OBM).
Note that naturally occurring fractures will tend to be oriented in the
direction of maximum horizontal stress in the field. Particularly in areas
close to major fault systems, the difference between the stresses in differ-
ent directions may be very large and conducive to fracturing. Cores may
also prove invaluable in characterizing any fractures that may be present,
although care has to be taken to exclude drilling-induced fractures. It is also
necessary that an orientation tool be run with the coring assembly. Once
the fracture system has been analyzed, it is useful to derive a fracture
density curve that may be included with the other logs. Such a curve may
be correlated with the horizontal permeability k h derived during a well test
and used to predict the producibility of future well penetrations.
Carbonate reservoirs, unlike clastic reservoirs, may well be amenable
to HCl acidization treatments either with or without mechanical fractur-
ing (“fraccing”) of the reservoir. However, such treatments can affect only
the region around the wellbore and will not compensate for poor perme-
ability and lack of fractures over the wider extent of the field. Pressure
testing/sampling of tight carbonate reservoirs using a conventional probe
is nearly always unsuccessful. In order to have any hope of success, one
would need to use a packer-type of tool, and even then the success rate is
typically low.
In chalk reservoirs, compaction may well be an issue during field life.
While this has the advantage of providing an additional pressure support
mechanism, extensive studies will be required during the design phase of
any installations, particularly offshore.

5.3 MULTIMINERAL/STATISTICAL MODELS

As stated in Chapter 2, my preferred method of calculating porosity is
the density log using the appropriate matrix and fluid densities. This
approach can go badly wrong if heavier minerals are also present in the