Page 86 - Well Logging and Formation Evaluation

76 Well Logging and Formation Evaluation

It is found that changing the value of one parameter can have a drastic
effect on the resulting volume fraction of that mineral.
4. Porosities derived by the program will typically come about from a
combination of the density, neutron, and sonic responses, plus what-
ever assumption is made about the relative saturations in the invaded
zone as opposed to the virgin zone. I am dubious about how correct
such a porosity really is.
5. Unlike with a deterministic approach, where the resulting uncertainty
in porosity and saturation may be directly traced to uncertainty in
the input parameters, statistical programs represent a “black box”
approach, in which there is no clear audit trail between the input and
the output.
6. The programs are very sensitive to log quality and noise on the log
traces. Where even one log is reading wrong, the volume fractions will
be affected and the output may be completely unreliable.

Overall, my impression is that there are some cases in which statistical
models offer real advantages over conventional interpretations. A good
example is in a sandstone reservoir having variable amounts of siderite or
pyrite. The program is also useful in a normal sandstone reservoir where
there are limestone stringers intermittently present. In situations where
the mineralogy is not well known or the logs are of poor quality, I am
extremely dubious about the quantitative correctness of the output. Even
when neither of these situations arises, it is still my experience that it
is necessary to make dozens of runs, investigating the effects of minor
changes to the input parameters, before a solution can be produced in
which one can have any confidence.
All the main logging contractors are able to offer software for statistical
analyses, which can be run either by the contractor or in-house by the oil
company. The values for typical parameters for various minerals are usually
built into the software as default values, so will not be repeated here.

5.4 NUCLEAR MAGNETIC RESONANCE LOGGING

When first introduced in the late 1980s, NMR logging attracted a lot of
interest because it was a whole new type of measurement and offered the
possibility of direct measurement of porosity and the differentiation of
fluid types and the relative contributions arising from clay-bound water
from free water. In this respect it offered to solve one of the problems
occasionally confronting petrophysicists, namely, low-resistivity pay.