Chapter 15. pdf

The Chlorine Effect.
Some types of neutron tool measure the thermal neutrons and gamma rays
produced during the capture of neutrons. There are only two elements that are found in reservoirs that
contribute significantly to neutron absorption; hydrogen and chlorine. The presence of hydrogen in the
fluids is what we want to measure, so this is not a problem.
However, if the drilling mud, mud filtrate or formation fluids contain a significant amount of dissolved
chloride ions, as is often the case, the tool will measure a lower flux of neutrons and hence
overestimate the porosity. This is called the 
chlorine effect
, and is present in wells drilled or logged in
the presence of drilling muds containing dissolved chlorine, or in formations where the formation
waters are particularly salty.
The Shale Effect.
Shale contain clays that have a significant amount of surface absorbed (bound)
water. Hence shales can contain a significant proportion of hydrogens despite being low porosity. The
apparent porosity read from the neutron tool in shale formations is therefore always significantly
higher than it really is. This is called the 
shale effect
or the 
bound-water effect
, and will be discussed
further later in this chapter.
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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 156