Abstract
Abstract The latest pulsed neutron spectroscopy tool for logging behind casing provides for enhanced formation evaluation due to improved self-compensated sigma and porosity measurements and a new measurement, the fast neutron cross section (FNXS), which is independent of the formation hydrogen index (HI). The FNXS is highly sensitive to gas filled porosity and can be used to differentiate between gas-filled and low-porosity zones, which was previously not possible. The FNXS measures the ability of the formation to interact with fast neutrons which is highly dependent on atomic density and not dominated by any particular isotopes, in contrast to traditional sigma and porosity measurements. This paper examines and compares the results of gas identification and lithology identification using pulsed neutron spectroscopy in an open-hole and a cased-hole environment. Most pulsed neutron tools are run after casing; this study provides a unique example to compare the effect of casing on the spectroscopy by comparing the cased hole measurements to measurements taken in the open hole, before the casing was run.
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