1. Field of the Invention
The present invention relates in general to nuclear well logging, and pertains in particular to an improved method for analyzing neutron-induced gamma ray energy spectra detected in a borehole in order to provide more accurate information about the elemental composition and other parameters of an earth formation surrounding the borehole.
2. Description of the Prior Art
Knowledge of the elemental composition of an earth formation surrounding a borehole (well bore) is useful in identifying possible oil and gas producing regions and in evaluating the degree of hydrocarbon saturation of such regions. For example, an analysis of the amounts of carbon and oxygen in a formation provides useful information concerning the presence of oil in the formation. Similarly, information as to the relative abundance of calcium and silicon in the formation permits the lithology of the formation to be determined. Still other formation parameters, such as the salinity of the formation fluid, the formation porosity, etc., can be ascertained from knowledge of the abundance of other formation constituents, e.g., chlorine, hydrogen, and the like.
An important and basic method for performing an accurate constituent (elemental) analysis of a formation is disclosed in U.S. Pat. No. 3,521,064, issued on July 21, 1970, to Moran et al., which is commonly owned by the assignee of this application. In accordance with the Moran et al. technique, a detected gamma ray energy spectrum for a formation of unknown composition is compared with a composite energy spectrum made up of weighted standard gamma ray energy spectra of the constituents (elements) postulated to comprise the formation. As used herein, the terms "spectrum" and "spectra" will mean "energy spectrum" and "energy spectra," respectively. The word "energy" may or may not appear before these terms, but in either case, no difference in meaning is intended. Each standard spectrum represents the detector response over a wide energy range to the excitation of a single constituent or element. The weight coefficients (yields) for the standard spectra that provide the best fit of the composite spectrum to the unknown spectrum, as determined, for example, by the method of least squares, are functions of the relative abundance of the constituents of the formation. By appropriate selection of the elemental standard spectra, the relative abundance of each constituent of interest, such as carbon, oxygen, calcium, silicon, etc., may be obtained, from which the desired information regarding oil content, lithology, porosity, etc., may be derived.
Typically, the elemental standard spectra, or elemental standards, are determined only for those elements commonly found outside the tool, either in the borehole or in the formation, whose neutron-induced gamma ray production cross sections are large enough to produce at least a few percent of the total detected signal, i.e., H, Si, Ca, Cl, Fe, and S for thermal neutron reactions (capture reactions) and C, O, Si, Ca, Cl, Fe, and S for high energy neutron reactions (primarily inelastic scattering reactions, but also neutron-proton and neutron-alpha reactions). Elemental standards are usually determined from a set of laboratory measurements made by the tool in test formations, each containing a high concentration of one of the above elements. These measurements are made under known conditions of temperature, pressure, and detector resolution. Each measurement is carefully stripped of the secondary elemental responses using conventional stripping techniques to leave only the response due to the predominant element.
It has been found in accordance with the present invention, however, that the elemental standard spectra determined in this way include an ever-present tool contribution, or "tool background," spectrum that results from neutron interactions with the components of the tool, and most significantly with the detector itself. Moreover, when gamma rays are measured at a location in a borehole in order to obtain a gamma ray energy spectrum for a formation of unknown composition, the measured gamma ray energy spectrum also includes a tool contribution spectrum.
If the tool contribution were always a constant fraction of the total response, the errors caused by the presence of the tool contribution spectrum in the standards and the unknown spectrum could easily be corrected. As has further been determined, however, the tool contribution is not constant, but rather is highly environmentally dependent. The tool contribution for high energy neutron reactions, for instance, has been found to increase significantly at low formation porosities and when casing is present. Casing has the same effect as a low-porosity formation, that is, its presence results in fewer slowing down events and, therefore, a greater high energy neutron population in the borehole. The tool contribution for thermal neutron reactions, on the other hand, has been found to be especially dependent upon borehole salinity and formation salinity. Furthermore, as implied previously, the tool contribution varies with the energy of the neutrons producing the reactions detected. Consequently, if the tool contribution is ignored, the analysis of the measured spectra may not be sufficiently accurate to provide reliable information concerning the constituent composition of the formation investigated.
Accordingly, a need exists for a method for investigating the composition (namely, the identity and the relative abundance of the constituent elements) of an earth formation traversed by a borehole that is free from errors caused by contributions of the logging tool to either the elemental standard spectra or the measured formation gamma ray spectra.