Page 18 - Well Logging and Formation Evaluation

8 Well Logging and Formation Evaluation

• Thermal decay tool (TDT): This neutron tool works on the same prin-
ciple as the neutron porosity tool, that is, measuring gamma ray counts
when thermal neutrons are captured by the formation. However, instead
of measuring the HI, they are specifically designed to measure the
neutron capture cross-section, which principally depends on the amount
of chlorine present as formation brine. Therefore, if the formation water
salinity is accurately known, together with the porosity, S w may be
determined. The tool is particularly useful when run in time-lapse mode
to monitor changes in saturation, since many unknowns arising from
the borehole and formation properties may be eliminated.
• Gamma ray spectroscopy tool (GST): This tool works on the same prin-
cipal as the density tool, except that by measuring the contributions
arising in various energy windows of the gamma rays arriving at the
detectors, the relative proportions of various elements may be deter-
mined. In particular, by measuring the relative amounts of carbon and
oxygen a (salinity independent), measurement of S w may be made.
• Production logging: This tool, which operates using a spinner, does not
measure any properties of the formation but is capable of determining
the flow contributions from various intervals in the formation.
• Cement bond log: This tool is run to evaluate the quality of the cement
bond between the casing and the formation. It may also be run in a cir-
cumferential mode, where the quality around the borehole is imaged.
The quality of the cement bond may affect the quality of other pro-
duction logging tools, such as TDT or GST.
• Casing collar locator (CCL): This tool is run in order to identify the
positions of casing collars and perforated intervals in a well. It produces
a trace that gives a “pip” where changes occur in the thickness of the
steel.

1.2.4 Pipe-Conveyed Logging

Where the borehole deviation or dogleg severity is such that it is not
possible to run tools using conventional wireline techniques, tools are typ-
ically run on drillpipe. In essence, this is no different from conventional
logging. However, there are a number of important considerations.
Because of the need to provide electrical contact with the toolstring, the
normal procedure is to run the toolstring in the hole to a certain depth
before pumping down a special connector (called a wet-connect) to
connect the cable to the tools. Then a side-entry sub (SES) is installed in
the drillpipe, which allows the cable to pass from the inside of the pipe