Response of the Carbon/Oxygen Measurement for an Inelastic Gamma Ray Spectroscopy Tool

Abstract

Summary In the inelastic mode of operation, a gamma ray spectroscopy tool (GST) performs a measurement of the carbon and oxygen in the formation and borehole on the basis of a spectral analysis of gamma radiation induced by fast neutrons. The carbon-to-oxygen ratio contains valuable information concerning the location of oil-and/or gas-bearing rock. Hundreds of laboratory formation measurements have been performed to determine the partitioning between formation and borehole response of this tool. An interpretation model has been developed for the carbon-to-oxygen ratio that determines the borehole effects as a function of the following parameters: borehole size, borehole fluid composition, borehole fluid displacer size, casing size, casing weight, and formation porosity.