Sourceless Neutron-Gamma Density SNGD: A Radioisotope-Free Bulk Density Measurement: Physics Principles, Environmental Effects, and Applications

Abstract

For many years there has been a need to find an alternative to the radioisotope-based gamma-gamma density (GGD) measurement. The traditional GGD measurement uses the scattering of 662-keV gamma rays from a 137 Cs radioisotopic source to determine formation density. A statistically precise measurement requires a 40-GBq or higher source strength and such a logging source, with a 30.17-year half-life, may pose health, security, and environmental risks.Pulsed-neutron generators have been used in the industry for several decades in wireline tools and more recently in logging-while-drilling tools. These generators produce 14-MeV neutrons, many of which interact with the nuclei in the formation through inelastic collisions. These inelastic interactions are typically followed by the emission of a variety of highenergy gamma rays. Similar to the case of the GGD measurement, the transport and attenuation of these gamma rays is a strong function of the formation density. However, the gamma-ray source is now distributed over a volume within the formation, where gamma rays have been induced by neutron interactions and the source can no longer be considered to be a point as in the case of a radioisotopic source. In addition, the extent of the induced source region depends on the transport of the fast neutrons from the source to the point of gamma-ray production.Even though the physics is more complex, it is possible to measure the formation density if the fast neutron transport is taken into account when deriving the density answer. This paper reviews the physics underlying the sourceless neutron-gamma density (SNGD) measurement, explains the various facets of the algorithm used for its computation and details the different environmental effects that may influence the measurement.The successful application of the method is shown in several log examples.

Exploring for oil with nuclear physics

“…It accounts for unmeasured elements as not all elements provide a capture signature that can be measured [5].…”

Benchmarking LWD Sourceless Neutron Gamma Density Measurements in Southeast Asia

“…Environmental corrections are applied for borehole size, mud weight, borehole salinity and formation sigma. Details of the physics and corrections can be found in Evans et al (2012) and Reichel et al (2012). The SNGD measurement is characterized for standard 8 ½-in.…”

Bulk Density Response and Experimental Study of Pulsed Neutron-Gamma Density Logging

“…This physical process mainly includes neutron transport, inelastic gamma ray production, and gamma ray transport (Streeter et al, 1996;Odom et al, 1998). The accuracy of the neutron-gamma density logging in bulk density measurement is lower than that of conventional gamma density logging techniques because of the complex physical mechanism (Evans et al, 2012). The induced gamma ray source is produced by the collision of fast neutrons with the formation.…”

“…Jacobson et al (2004) pointed out that neutron capture gamma rays could be used for correction in density calculation. Schlumberger developed the first commercial logging-while-drilling (LWD) pulsed neutron-gamma density measurement tool with thermal and epithermal neutron detectors (Yu et al, 2011;Evans et al, 2012;and Reichel et al, 2012). For all the methods previously mentioned, it is assumed that the inelastic gamma rays are affected by the hydrogen index and gamma ray transport, and fast neutrons, epithermal neutrons, thermal neutrons, and capture gamma rays can be used to correct the hydrogen index.…”