Technical Paper: Neutron-Gamma Density (NGD): Principles, Field Test results and Log Quality Control of a Radioisotope-Free Bulk Density Measurement

Society: SPWLA
Paper Number:
Presentation Date: 2012


Bulk density is among the most critical formation parameters for geoscientists. Traditional acquisition technology employs a radioisotope (137Cs) as a source of gamma rays. With a half-life of 30.17 years, this radioisotopic source continually emits 662-keV gamma rays, posing health, security, and environmental risks, which need to be minimized through rigorous and expensive transportation, storage, handling, and deployment procedures. The well logging industry has been seeking an alternative to the radioisotope-based technology for years; some of these alternatives have focused on finding a way to "make do" without a density measurement. However, the industry-standard technique of comparing neutron and density measurements favors the development of a cleaner and safer way to acquire a true formation bulk density.

Pulsed-neutron generators (PNG) have been successfully used by the well logging industry to acquire a variety of measurements. Electronically controlled pulses of high-energy neutrons can be emitted in precisely timed bursts. The inelastic collision of high-energy neutrons with the nuclei of formation atoms can put them into excited states, which decay through the emission of gamma rays. The characterization of the transport of these gamma rays in the formation can be used to determine the bulk density, in a manner similar to the traditional gamma-gamma density (GGD) measurement. While the principles behind this method are well understood, the practical development of an industry-grade measurement has taken years of careful development and testing.

This paper reviews the principles behind the neutron-gamma density (NGD) measurement and explains its measurement specifications, applications, and quality-control indicators. The new measurement was tested and benchmarked using data from a multifunction logging-while-drilling tool that incorporates the PNG and detectors required for the NGD measurement and a GGD density section. Comparing the results from both density measurements allows a rigorous benchmarking and performance evaluation of the NGD measurement versus the GGD in a diverse range of environments. This is illustrated through field test results detailing the accuracy and precision of the new measurement.

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