Schlumberger

CNL Compensated Neutron Log

Thermal and epithermal neutron measurement for porosity analysis

The CNL compensated neutron log tool makes thermal and epithermal neutron measurements to calculate porosity and identify lithology and the presence of gas.

The chemical source of the CNL tool bombards the formation with fast neutrons. The fast neutrons are slowed and then captured, primarily by hydrogen atoms in the formation. The slowed neutrons deflected back to the tool are counted by the two detectors. The neutron count rates measured at the two detectors are used to compute a ratio related to formation porosity that is much less affected by environmental factors than that of single-detector tools.

Because the CNL tool responds primarily to the hydrogen content of the formation, in clean formations with pores filled with water or oil, the CNL measurement reflects the amount of liquid-filled porosity.

Combinability and versatility

CNL tools are combinable and are usually run simultaneously with other services. CNL measurements are used for quick identification of gas zones by the crossover with the porosity curve from another tool. The combination of CNL and other porosity logs results in more accurate porosity values and lithology identification, including the evaluation of shale content.

The CNL tool can be run in open or cased liquid-filled boreholes. In cased holes, it can be also used for depth correlation.

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CNL Correlation for Open and Cased Hole Logs

CNL measurements are used to calculate porosity, detect gas, determine lithology and correlate between openhole and cased hole logs. The caliper curve in track 1 was recorded simultaneously by a Litho-Density tool.
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