Environmental corrections and tool

Environmental corrections and tool selection
The neutron log needs corrections for borehole size, formation temperature, formation pressure, mud
salinity and barite content in mud. The borehole size is automatically adjusted based on the caliper log
readings. The correction charts are shown on page 178 of reference 2. The corrections are additive.
There are a number of tools available from the service companies. The absorption of thermal neutrons
causes a less precise estimate of apparent porosity, so it would be preferable to rely exclusively on the
epithermal neutron count. Improvements in the neutron source have led to the development of neutron
logs which determine porosity using only epithermal counts.
In air or gas filled wells, the thermal porosities can not be calculated. It is possible to improve gas
detection using four detectors, two for epithermal porosity detection and two for thermal porosity
detection. Schlumberger refers to this as CNL-G.
Newer LWD tools have additional complications, as discussed in Reference 2, V-268. LWD tools are
evolving rapidly. I will try to update this area later.
Interpretation
If the lithology is known, the neutron log can be used to calculate porosity. Generally, the neutron and
density logs are run together. The Pe (photoelectric log) may also be run to further improve the lithology
description.
The effects of components of the matrix on neutron porosity and bulk density are proportional to their
fraction in the matrix. For example a rock matrix
with a 50% limestone and 50% sandstone, with
20% porosity will be half way between the
limestone and sandstone lines, as shown on the
crossplot on the left. The crossplot is correct for
readings in the invaded zone using fresh water
muds.
The M/N lithology plot is used to identify a
complex mixture of sandstone, dolomite and
limestone using all three porosity logs. The
rational for the M/N plot is that the porosity should
be the same in all three porosity logs for the same
composition of lithology.
Modern log analyses today allows the inclusion of
numerous minerals, including shale and various
clays into a lithology model. The computer
program then must calculate the percentage of
each component, based on all relevant logs, including acoustic, spectral gamma ray, and NMR, if
available. The Pe log is particularly useful in complex lithology determinations, as the readings are
much more sensitive to lithology than porosity.
All log analyses is solution to an inverse problem. With log analyses software, the solution to the forward
problem (assume the reservoir formation and borehole properties and generate what the theoretical log
readings) can also be obtained and compared to the readings from the actual raw logs.
Gas Effects
Gas has a very marked effect on both density and neutron logs. If it is assumed that the formation fluid is
water and the invasion zone is shallow, then gas will result in a lower bulk density (note on the cross plot,
this results in a point higher on the y-axis), and a lower apparent neutron porosity.
In limestone and dolomites, the lithology of a gas zone may show up as a
sandstone (a move up and to the left). The correction for gas is then down and to
the right. The gas contact can sometimes be easily identifiable in carbonates from
the raw logs, as they are frequently plotted on a limestone scale (∅