ABSTRACT

A simple model of inelastic neutron scattering has been applied to the case of 14 MeV neutron bombardment of limestone and sandstone. The model assumes a mono-energetic, isotropic 14 MeV neutron source with neutron absorption characterised by a total neutron absorption coefficient. Inelastic neutron scattering is described by a cross-section and a function describing the angular distribution of emitted gamma-rays. Gamma-ray absorption is described by a total absorption coefficient. Collimation can be specified so that realistic borehole geometries can be modeled. Using this model, the gamma-ray response from various oil and water saturated formations is calculated and the carbon/oxygen ratios predicted. As a confirmation of the model predictions, an oil saturated sand target was irradiated with 14 MeV neutrons and gamma-ray spectra acquired using a high resolution gamma-ray detector. The detector efficiency was measured in geometry identical to that of the sand target and so gamma-ray yields were accurately determined. Agreement between prediction and experiment is such that confidence may be placed in the performance of the model. In particular, the response of the Schlumberger Gamma Spectroscopy Tool (GST) has been modeled giving good agreement between predicted and published response curves. The response of the Dresser-Atlas Carbon/Oxygen Log is also examined. On the basis of the above considerations two features of the carbon/oxygen log can be elucidated. First, the depth of investigation can be better characterised and consequently the extent to which the uninvaded zone is interrogated by the tool evaluated. Second, the response of the tool to components of the formation is determined and also the usefulness of a silicon/calcium ratio as a lithology indicator.

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