Geochemical Logging With Spectrometry Tools

Abstract

Summary A geochemical logging tool (GLT SM) string, measuring natural, activation, and prompt neutron-capture gamma rays, produces logs of the most abundant and a few trace inorganic element concentrations. Direct measurements of Al concentrations are provided. A geochemically based closure model is used to derive Si, Ca, Fe, S, Gd, and Ti concentrations. The only significant spectroscopically undetermined element, Mg, is inferred by comparing measured with derived photoelectric factor. Analysis algorithms, demonstrations of accuracy and precision, and applications of geochemically derived formation properties are discussed. Introduction Elemental Analysis by Spectroscopy. This paper describes a nuclear geochemical tool string designed to determine a sufficient number of elemental concentrations through logging measurements to permit a satisfactory geochemical description of the formation. The tool string combines measurements of natural radioactivity with the natural gamma ray tool (NGT SM), delayed activation (for aluminum) with a new tool called the aluminum activation clay tool (AACT TM) and tau-gated thermal neutron-capture spectroscopy with the gamma ray spectrometer tool (GST TM). The components of the tool string are described in the next section. Three new features of nuclear spectroscopic logging measurements are introduced in this paper. First, an algorithm is presented to determine Al concentrations, in weight percent, from delayed-activation count-rate measurements. The count rates are corrected for the complex effects of the subsurface environment on the neutron and gamma ray physics. Natural activity measurements of Th, U, and K are directly calibrated in weight percent. Second, a new method is presented to calculate elemental concentrations from pulsed-neutron-capture measurements. Thus, all elemental concentrations are provided in weight percent. In most gamma ray spectroscopy measurements, the fraction of the detected spectrum that can be attributed to a particular element is linearly proportional to the concentration of that element in the volume of the measurement. However, determining the proportionality constant can be essentially impossible under neutron-capture logging conditions when pulsed neutron generators with variable and uncalibrated neutron yields are used. Thus, we have taken a new approach that renormalizes the relative yields from each clement measured through thermal neutron-capture reactions in a self-consistent manner to obtain the elemental concentrations. The key to this approach is focusing only on elements that are contained in the rock and not present in the fluids. The renormalization procedure is based on the geochemical fact that in all core analyses, the rock elemental oxides, measured in weight percent, sum to 100 %. The elements measured by capture, activation, and natural spectroscopy compose, with few exceptions, most of the significant elements seen as oxides, or carbonates, in the formations. Therefore, one can use the absolute elemental concentrations measured by natural K activity and delayed All activity, induced with a calibrated neutron source, to renormalize the prompt capture elemental contributions from the formation rock in a closure relationship. Third, the most significant element not measured spectroscopically, magnesium, can be determined when it is present in a significant concentration by comparing the measured photoelectric cross section for the formation with the theoretical cross section calculated with the assumption that no Mg is present. After the means for determining the elemental concentrations are described, examples are provided to demonstrate the viability of the elemental concentrations determined by neutron-induced gamma ray spectroscopy through comparisons of logging results with laboratory analysis of cores from the logged wells. Examples of elemental concentration repeatability are provided. Finally, the last part of this paper discusses several applications in which elemental concentrations can be used to enhance the formation description. Spectroscopic measurements of gamma rays were originally introduced to separate the contribution of particular elements to the total activity seen in a detector. Natural activity can be separated into the Th, U, and K components. Other logging measurements use a neutron source to initiate reactions that produced gamma rays. However, there are many difficulties associated with relating the fraction of a gamma ray spectrum to the absolute concentration of an element in the formation, and most of the original work concentrated on the use of relative spectral contributions, such as carbon/oxygen, to indicate gross formation properties like oil saturation. Recent geochemical research has shown that when a sufficient number of elemental concentrations is determined, a detailed mineralogy of the formation can be estimated. From this mineralogical description, many formation properties can be better characterized and other properties can be derived that could not otherwise be obtained except, perhaps, by detailed analyses of core samples. These properties include better porosity determination from logs, sandstone classification, cation exchange capacity, grain-size distribution, and permeability. Tool String Description Nuclear geochemical logging involves three separate modes of gamma ray spectroscopy to make a comprehensive elemental analysis of the formation. Fig. 1 shows the recommended configuration of this new openhole tool string. The first measurement is performed by the NGS tool, which passes by the formation before any neutron source can induce radioactivity. The concentrations of K, Th, and U in the formation are derived from the gamma ray spectrum recorded from these naturally radioactive elements and their daughter products. The second and newest measurement in the nuclear geochemical logging toot is performed by the AACT. The AACT is a modified NGS tool. The modification consists of three more windows added to the spectrometer to remove potentially interfering Mn activation. The AACT, the NGS tool above it, and the 252Cf neutron source [carried in the compensated neutron tool with epithermal measurements (CNT-G)] between them allow a measurement of activation gamma rays to derive formation aluminum concentration. The GST at the bottom of the tool string, in Fig. 1, measures the spectrum of capture gamma rays from elements in the formation. The GST uses a pulsed 14-MeV neutron generator to induce the capture reactions. The spectrum from the GST, in conjunction with elemental concentrations from the NGS tool and AACT, allows derivation of the concentration of elements in the formation rock, such as Si, Ca, Fe, S, Ti, K, and Gd. The GST is also sensitive to H and Cl, but these elements are not used in determining the rock geochemistry. Secondary measurements are made by the auxiliary measurement sonde (AMS TM) for determining borehole salinity and mud temperature, the CNT-G for apparent neutron slowing length of the formation, Ls, and as a carrier for the 252Cf source, and the formation neutron capture cross section, sigma form, obtained from the GST. These measurements are used in the environmental correction of the derived Al concentration. Data from the tools in the string are sent by telemetry to the surface by the telemetry communication cartridge or cable communication cartridge (cable communication electronics), also shown in Fig. 1.