Neutron-Induced Gamma Ray Spectroscopy for Reservoir Analysis

The weighted least-squares (WLS) approach to spectral analysis has enabled more information to be extracted from the downhole recorded induced gamma ray spectra than was previously possible. GST (gamma ray spectroscopy tool), with its optimized inelastic and capture spectral modes, permits analysis of most and often all significant elements present in the formation and provides the possibility of evaluating hydrocarbons, salinity, lithology, porosity, and shaliness. Data have been obtained in a wide range of conditions in open and cased holes with the GST tool both in its present and experimental versions. This paper presents field examples to demonstrate the versatility and potential of the technique, not only as an effective oil-finder independent of water salinity conditions but as a valuable input to a more complete interpretation of well logs. Introduction Nuclear well logging has been long established as a means of evaluating reservoir porosity and hydrocarbons in open hole and behind casing. The count rates of neutrons or gamma rays returning to one or more detectors are measured and related to the formation rock characteristics according to the physics of the neutron inter-actions that have occurred. For example, high-energy neutrons interact with the surrounding formation nuclei and can induce gamma ray emission. Most conventional neutron/gamma spectroscopy techniques for well logging that have been developed to date are based on integral counts in rather broad energy windows. In this paper, we discuss an alternate technique that allows an accurate and detailed formation evaluation. Gamma rays emitted from the formation nuclei are limited to specific and well-defined energies governed by the laws of quantum mechanics. Each element (isotope) has a characteristic spectrum of gamma rays that can be emitted from a given neutron interaction. Therefore, an element may be identified by its gamma my spectral shape or signature whose emission intensity is related to the elemental concentration. The GST tool measures the relative yields of gamma rays resulting from the interactions of neutrons with different elements present in the formation. The measurements are based on a WLS shape analysis of the observed gamma ray spectral distribution. This is a recently introduced approach to induced nuclear logging. Neutron induced gamma rays are analyzed downhole in terms of intensity in each of more than 200 discrete, narrow energy increments. From this detailed measurement of formation spectral response to neutron bombardment, eight constituent elements can be identified and their proportions estimated. These elements, C, 0, C1, H. Si, Ca, Fe. and S, are significant in formation mineralogical and fluid analysis. A considerable amount of new information is thus made available in the form of a continuous or quasicontinuous well log for a more comprehensive evaluation of the formation. Because of its immediate commercial interest, emphasis in a previous publications was placed on the application of the carbon and oxygen measurements in estimating hydrocarbon saturation. This approach has the advantage of being unaffected by the presence of salts (particularly NACl) in the pore fluid, and has had encouraging success in the monitoring of reservoirs where salinities were either unknown, variable, or too low for conventional neutron logging. SPEJ P. 553^