Page 83 - Well Logging and Formation Evaluation

Advanced Log Interpretation Techniques 73

5.2 CARBONATES

Most of the analyses presented in earlier chapters work equally well
with carbonate formations. However, the grain density used for deriving
porosity from the density log needs to be set to a different value. For lime-
stones the most common value used is 2.71g/cc, although a direct mea-
surement on the core is preferred, if available. Dolomite and anhydrite
have higher matrix densities (see Appendix 6). In many fields with good-
quality carbonate reservoirs, evaluation is simpler than with sandstone
because the shaly sand problem does not occur and beds tend to be thicker.
In general, it will be found that Archie’s model works very well.
However, during my career I have also worked on some extremely tight
gas- and oil-bearing carbonate reservoirs where normal techniques did not
give good results. I will therefore discuss these. Typical characteristics of
low-permeability (<1md) carbonate reservoirs are:

• Porosity may still be appreciable but be in the form of isolated pockets
or vugs. This is often seen in chalk reservoirs.
• Entry heights and transition zones may be extremely long (up to 100m).
• Matrix permeability may be extremely low, but the well may flow due
to the presence of natural (or mechanically induced) fractures in the for-
mation. Due to the fact that these fractures may cause significant losses
during drilling, the well may need to produce back a lot of drilling fluid
before formation hydrocarbons start to flow.

In such a field, the development strategy will often be to drill horizontal
wells perpendicular to the natural fracture orientation, thereby optimizing
the productivity of the wells. The STOIIP or GIIP will be stored mainly
in the matrix but flow in the well via the fractures. Hence, if there is not
an extensive fracture system that provides a conduit for the matrix to flow
into, the wells will either water out or die very quickly.
Clearly the ability to detect the presence and orientation of fractures is
extremely important. During drilling it may be useful to monitor the extent
of losses, and depths at which they start to occur, which may provide valu-
able information as to the presence of fractures. Needless to say, the
measures adopted by drilling to cure these losses need to be examined
to ensure that any fracture permeability is not permanently impaired.
Conventional logs that may give indication of fractures are:

• Sonic log (cycle skipping occurrence)
• Microresistivity tool (erratic behavior as the pad crosses a fracture)