Dual‐spaced Neutron Logging for Porosity

Abstract

Two‐group neutron diffusion theory has been applied to two concentric cylindrical regions representing borehole and formation to show that the ratio of thermal neutron flux at two appropriately large distances along the axis is virtually identical to the epithermal flux ratio, and is a measure of a single epithermal parameter of the formation, viz., the slowing down length. The slowing down length is most strongly dependent on hydrogen content. It is shown that variations in salinity of the liquid in the formation or the borehole and uncertainties in borehole size have relatively little effect on the flux ratio. Experiments in a borehole model showed that the thermal flux ratio does not depend on eccentricity of location of the logging tool in the borehole. Fits of theory to experiment allowed determination of slowing down lengths for Pu‐Be neutrons in salt water filled sandstone at two porosities for which experimental values have not previously been available. These investigations suggest that an improved porosity log can be obtained by use of 70‐ and 90‐cm source‐detector spacings with a neutron source of [Formula: see text] neutrons/sec or greater. Proportional counters containing [Formula: see text] are recommended as detectors. Such a system employing flux ratios is expected to be insensitive to casing and cement in cased holes.

Investigation of Neutron-Porosity Log Uncertainties: Ocean Drilling Program Hole 642E

“…This deficiency limits the scope of any theoretical investigation, and the spirit of the present effort is to seek only qualitative answers to the neutron-log problems. Thus the relatively simple, homogeneous, one-dimensional, diffusion approximation to the transport equation developed by Allen et al (1967) for neutron logs will be followed. A discussion of the approximations pertaining to the diffusion model and to the constitutive coefficients in the diffusion equation may be found in Henry (1975) and in Glasstone and Sesonske (1967).…”

“…An important concept was recognized by Allen et al (1967). Specifically, "... essentially all dependence of a thermal neutron measurement upon thermal neutron parameters can be eliminated simply by making measurements at two sufficiently distant points... and taking their ratio."…”

“…The effect of various rock constituents on L x has been investigated by Kreft (1974). Allen et al (1967) generated data on the parameters L x and L 2 for fresh and salt water (100,000 ppm NaCl) saturated sands. A fit to these data for L x is…”

“…In general, any cross section of a saturated material is where 35 CU is the cross section of sea water and 5.2 CU is the cross section of the reference sandstone (calculated from the data of Allen et al, 1967). Note one capture unit (CU) equals lO^cm-1 .…”

“…Furthermore, neutron logs can be influenced by the presence of elements that are strong absorbers of neutrons. This influence can be minimized if two neutron detectors with long source-to-detector spacings are utilized in the logging tool (Allen et al, 1967). A good logging strategy for use in scientific drilling programs that penetrate unknown formations is to run both gamma density and neutron-porosity logging devices.…”