Pulsed Neutron Capture Logging - Oil and Gas Industries

The pulsed neutron capture tool is used to determine water saturation in formations having high water salinity. The range of applicability for determining water saturation is generally a minimum of about 15% porosity, with a formation water equivalent sodium chloride salinity of at least 50,000 ppm. New tools and repeat runs can reduce the statistical error inherent in a nuclear tool, correspondingly reducing these limits. For purposes of comparison with earlier logging to detect changes in saturation, these limits may be reduced even further, since such comparisons are not intensely quantitative. Be sure to consult with the wire-line service company prior to running any pulsed neutron capture logs if conditions are at or below these limits. Some of the more recently developed tools (e.g., Schlumberger’s Reservoir Saturation Tool, or RST) can be run through tubing. Schlumberger’s RST also has carbon-oxygen (C/O) measurement capabilities, and so can be used in environments of low or unknown water salinity.

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Pulsed Neutron Capture Hardware

Pulsed neutron capture logging tools are typically small-diameter through-tubing tools, 1 11/16 in. (4.29 cm) diameter or less. These tools include an electronically activated neutron generator, which periodically emits bursts of 14 MEV neutrons, at rates ranging from 800 μs to 5000 μs between bursts. The burst rate varies with the service company and tool model. All modern tools have a near (short-spaced) and a far (long-spaced) detector that count gamma rays associated with neutron interactions with the formation. Figure 1 shows a schematic of this tool The detectors are typically sodium iodide crystal scintillation detectors and do not discriminate with regard to gamma ray energies. As a result, the tools also measure a background count rate to distinguish natural from induced gamma rays. At present, the main industry versions of the pulsed neutron capture log are the following:

Figure 1 - Logging tool pulse.

• Schlumberger Wireline & Testing: thermal decay time log (TDT-K, TDT-M, TDT-P); as noted above, Schlumberger has incorporated pulsed neutron capture and carbon-oxygen measurements into the reservoir saturation tool (RST).

• Halliburton Energy Services: thermal multigate decay time tool (TMD-L).

• Western Atlas: reservoir monitoring system (RMS).

Other types of pulsed-neutron capture tools are available, and may be obtained from smaller
independent service companies.

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