Litho-Density Tool Calibration

A second-generation density logging tool has beendeveloped that uses a gamma-ray source and two NaIscintillator detectors for borehole measurement of electrondensity, pe, and a quantity Fpe that is related to thelithology of the formation. An active stabilization system controls the gains of the two detectors, which permits selective gamma-ray detection. Spectral analysis isperformed in the near detector (two energy windows) and performed in the near detector (two energy windows) and in the detector farther away from the source (three energy windows). This paper describes the results of laboratory measurements undertaken to define the basic tool response. The tool is shown to provide reliable measurements offormation density and lithology under a variety ofenvironmental conditions. Introduction In the second-generation density logging tool, as in otherlogging devices, the principle exploited for the density measurement is that the interaction of medium-energygamma rays (662 keV) with rock formations is primarilya result of Compton scattering with electrons. Thus, theattenuation of gamma rays can be related to the electrondensity (pe) in the scattering material, defined by Zpe = 2<–>Pb,................................(1)A where less than Z/A greater than is the average value ofthe ratio of the atomic number to the atomic weight of thescattering formation. For most rocks, less than Z/A greaterthan is on the order of 0.5, while for hydrogen it is veryclose to 1.0. Therefore, with a knowledge of the lithology and formation fluid constituents, this measured parameter canbe related to the bulk density, pb, of the formation. The traditional transform between measured density values (PLOG)and the electron density is pLOG = 1.0704pe–0.1883........................(2) This ensures that the log-measured density values ofwater-filled calcite agree with the actual bulk density despite the fact that the electron density of water is 11%greater than its bulk density. As the gamma rays emitted from the source are successively scattered, their energy is reduced and they become increasingly subject to photoelectric absorption. This additional attenuation caused by photoelectric absorption is also used to measure the absorption characteristics of the formation, which are determined primarily by its lithology. This measurement is called the primarily by its lithology. This measurement is called the photoelectric factor because it is related to the photoelectric photoelectric factor because it is related to the photoelectric cross section and is referred to as pe in the literature and onlog headings. The theoretical considerations and interpretationof this measurement can be found in Refs. 2 and 3. Our paper describes, in general, the borehole logging devicethat has been designed to meet these goals. The measurements made to define the tool response are presented, as well as the performance of the tool under laboratory and field conditions. performance of the tool under laboratory and field conditions. Description of the Hardware The basic components of the measurement system are a1.5-Ci radioactive source of (137)Cs and two NaIcrystal/photo multiplier assemblies. The two gamma-raydetectors are necessary for mudcake compensation, which is discussed in the section on Environmental Effects. Awindow made of beryllium allows low-energy gamma raysfrom the formation to pass through the skid-shieldingmaterial and pressure housing for use in the lithology measurement. To make the lithology measurement and to improve the response of the density measurement, a spectral analysis of the detected gamma rays is made. Measurements are made in three distinct energy regions at the farther detector(LS) from the source and two at the nearer (SS). To make these spectral measurements, a system of active gain stabilization has been incorporated. This is achieved, bythe use of two weak (137)Cs reference sources, one foreach detector. These provide references for the two feed-backloops. Fig. 1 shows the approximate location of the windows used in the energy analysis. Estimates of the density ofthe formation are obtained from the LS window labeledLs and the SS window labeled Ss1. The lower-energy edgeof these two windows was determined as a compromise between the needs for both high counting rate and aminimization of photoelectric absorption perturbations. The inference of the formation lithology comes from acomparison of the LS window labeled LITH and the Lswindow. The long-spacing detector's density estimate isrefined further by using the LITH window to compensatefor any residual photoelectric absorption in the primary window (Ls). primary window (Ls).SPEJ P. 515