Page 66 - Well Logging and Formation Evaluation

56 Well Logging and Formation Evaluation

k hor /k arith

k /k
v h
Figure 3.4.1 Effect on Average Permeability of k v / k h < 1 in horizontal well

related to the flow generated in a production test. Where log-derived
values of k*h are compared with production tests, it is often the case that
the result derived from the arithmetic average will be higher than that seen
in the production test. Reasons for this are:

• Not all the perforated zone contributes to the flow, and the actual h is
smaller than that assumed in the petrophysical calculation.
• Some of the flow is not parallel to the bedding plane.
• Formation damage (called “skin”) has occurred between the openhole
logging and the testing operation. While the test analysis seeks to iden-
tify this as a separate term from k*h, it may still be partly incorporated
into the calculated kh quoted.
• Relative permeability effects, such as gas blocking, may be occurring,
making the lab-calibrated in-situ brine permeability inappropriate.

Differences between the log-derived and test permeabilities are a fact
of life in real reservoirs and do not necessarily invalidate the log-derived