Compensated Neutron Log (CNL*)

Description

The compensated neutron log tool (CNL) contained a 16-curie Am/Be radioactive source that bombarded the formation with fast neutrons and two detectors at different spacings from the source. The neutrons were slowed down through elastic collision with the formation nucleii and then captured, primarily by hydrogen atoms in the formation. The slowed neutrons deflected back to the tool were counted by detectors. The tool responded primarily to the hydrogen content of the formation because hydrogen has a mass close to that of the neutrons and thus is most efficient in the slowing process. The measurements (ratios of far to near detector counts) were transformed to porosity units on the basis of laboratory calibrations. The 16-curie source produced neutrons at four times the rate of a standard nuclear source, greatly reducing statistical variation. The use of longer source-to-detector spacings increased the depth of the investigation.

The CNL was used during the Deep Sea Drilling Project.

Applications

Porosity. In reservoir engineering its importance is quite evident; in the study of the volcanic rocks that make up the upper oceanic crust, a good in-situ porosity measurement is most important to the correct understanding of the crustal structure: first, because it samples both the small-scale (microcrack, vesicle) porosity seen in the cores and the large-scale fractures not sampled by drilling; and second, because other properties such as density, seismic velocity, and permeability, depend strictly on porosity variations and on the geometry of the pore space. In the presence of clays or hydrous alteration minerals a correction is required to account for the presence of bound water.

Lithology. Because the hydrogen measured by the tool is present not only as free water but as bound water in clay minerals, the porosity curve, often combined with the density log, can be used to detect shaly intervals or minerals such as gypsum, which has a high hydrogen index due to its water crystallization. Conversely, the neutron curve can be used to identify anhydrite and salt layers (which are both characterized by low neutron readings and by high and low bulk density readings, respectively).

Environmental Effects

The tool was designed to minimize the effect of hole size. When run in combination with the FDC (formation Density Compensated Tool), the caliper measurement was used to apply an automatic correction for hole diameter. Other factors that might affect the porosity reading were temperature, salinity, lack of eccentralization (standoff) and type of drilling fluid.

Log Presentation

The CNL was recorded in linear porosity units (or %) for a particular matrix lithology (limestone, sandstone, dolomite, usually limestone). When a CNL was run in combination with another porosity log, both curves were recorded on the same porosity scale. This overlay-type presentation permitted visual qualitative interpretation of porosity and lithology or the presence of gas.

Major Outputs

Tool Specifocations

Deployment Notes

The CNL was usually run in combination with the FDC or LDT tools.

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