Gamma Ray Spectroscopy Tool: Environmental Effects

Abstract

Summary Elemental analysis of inelastic and capture gamma ray spectra from specially constructed laboratory formations is presented. The analysis provides formation oil saturation, water salinity, porosity, and lithology provides formation oil saturation, water salinity, porosity, and lithology after accounting for elements in casing, cement, and borehole fluid. These data permit quantitative formation analysis for many borehole geometries and elemental contents. Introduction The Gamma-Ray Spectroscopy Tool (GST TM) is a down hole NaI(Tl) based nuclear gamma ray spectrometer system, which detects gamma rays produced from reactions induced by 14-MeV neutrons irradiating the formation. Elemental data are obtained in two modes of operation: inelastic and capture-tau. The inelastic mode is based on a pulsed 100- sec timing cycle during which three spectra are accumulated:a gross inelastic gamma ray spectrum obtained during the neutron burst,a background spectrum obtained immediately after the burst, which is scaled and subtracted from the gross inelastic spectrum to obtain a net inelastic spectrum, anda 50- sec capture gamma ray spectrum obtained after the background spectrum. The net inelastic spectrum and the 50- sec capture spectrum are analyzed by a weighted-least-squares (WLS) fitting procedure to obtain information on the atomic concentration of carbon (C), oxygen (O), hydrogen (H), silicon (Si), calcium (Ca), chlorine (Cl), iron (Fe), and sulfur (S) in the formation. The capture-tau mode acquires an optimized capture spectrum (that provides information on H, Si, Ca, Fe, Cl, and S concentrations) and also measures the formation thermal neutron decay time, . The basic design and interpretation of both the inelastic mode and the capture-tau mode 3 of the GST tool have been presented previously. The use of inelastic neutron scattering for carbon and oxygen spectroscopy logging to measure oil saturation has been investigated for many years. More recently, the use of other elemental yields to improve the interpretation of oil saturation and to provide valuable information on formation salinity, porosity, and lithology has been demonstrated. While these investigations have emphasized the value of data on elemental concentrations for defining formation properties, they also have clarified further the advantages of having prior knowledge of tool responses under a wide variety of borehole conditions. The ultimate goal of the GST tool is to provide a measurement of elemental concentrations in the formation. These concentrations then can be used to provide detailed descriptions of formation fluid types and minerals. However, the measurement also is sensitive to the elemental concentrations in the region surrounding the tool, where borehole fluid, casing, and cement contain elements found in the formation. Since a gamma ray from an element in the cement looks the same as the gamma ray from that same element in the formation, the determination of the formation elemental concentration can become ambiguous. However, if the cement thickness is known, then the yield that results from an element in the cement can be obtained from laboratory measurements of cement in combination with formations that are free of the particular element. Similar techniques can be used to investigate the contribution to the spectral yields from other changes in the borehole environment. The potential number of formation and borehole conditions would lead to an unmanageably large set of measurements to determine all GST tool environmental effects. We have, therefore, obtained sufficient data from laboratory measurements to provide the expected response of the tool, in both modes of operation, to the most commonly encountered field conditions. The measured environmental effects data also provide the critical results for defining and testing interpretation models. The parameters that have been varied, with indications of the range of variation are shown in Table 1. Both open- and cased-hole responses have been obtained. For all cased-hole measurements, the casing is cemented. It should be noted that all laboratory measurements were made with a freshwater Type H portland cement. If, for example, other cements had been used, the results portland cement. If, for example, other cements had been used, the results would be altered by the relative elemental concentrations to those in Type H portland cement. Data have been obtained with a laboratory sonde and a GST prototype in specially constructed laboratory formations. The inelastic mode data were obtained with no external boron-loaded fluid excluder while the capture-tau mode data were obtained with a 5 1/2-in. [14-cm] OD boron-loaded fluid excluder. This corresponds to normal field operation. Although the absolute value of GST elemental responses may vary slightly with changes in tool design, the overall systematics of the responses reflect the actual physical changes in formation and borehole elemental concentrations and physical changes in formation and borehole elemental concentrations and will be invariant to small changes in final tool design. JPT p. 1527