Litho-Density Tool Calibration
- D. Ellis (Schlumberger-Doll Research) | C. Flaum (Schlumberger Technical Services) | E. Marienbach (Etudes et Productions Schlumberger) | C. Roulet (Etudes et Productions Schlumberger) | B. Seeman (Schlumherger Well Services)
- Document ID
- Society of Petroleum Engineers
- Society of Petroleum Engineers Journal
- Publication Date
- August 1985
- Document Type
- Journal Paper
- 515 - 520
- 1985. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 5.6.1 Open hole/cased hole log analysis
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A second-generation density logging tool has been developed that uses a gamma-ray source and two NaI scintillator detectors for borehole measurement of electron density, pe, and a quantity Fpe that is related to the lithology of the formation. An active stabilization system controls the gains of the two detectors, which permits selective gamma-ray detection. Spectral analysis is performed 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 of formation density and lithology under a variety of environmental conditions.
In the second-generation density logging tool, as in other logging devices, the principle exploited for the density measurement is that the interaction of medium-energy gamma rays (662 keV) with rock formations is primarily a result of Compton scattering with electrons. Thus, the attenuation of gamma rays can be related to the electron density (pe) in the scattering material, defined by
Z pe = 2< - >Pb,................................(1) A
where less than Z/A greater than is the average value of the ratio of the atomic number to the atomic weight of the scattering formation. For most rocks, less than Z/A greater than is on the order of 0.5, while for hydrogen it is very close to 1.0. Therefore, with a knowledge of the lithology and formation fluid constituents, this measured parameter can be 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 of water-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 on log headings. The theoretical considerations and interpretation of this measurement can be found in Refs. 2 and 3.
Our paper describes, in general, the borehole logging device that 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 a 1.5-Ci radioactive source of (137)Cs and two NaI crystal/photomultiplier assemblies. The two gamma-ray detectors are necessary for mudcake compensation, which is discussed in the section on Environmental Effects. A window made of beryllium allows low-energy gamma rays from the formation to pass through the skid-shielding material 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, by the use of two weak (137)Cs reference sources, one for each detector. These provide references for the two feed-back loops.
Fig. 1 shows the approximate location of the windows used in the energy analysis. Estimates of the density of the formation are obtained from the LS window labeled Ls and the SS window labeled Ss1. The lower-energy edge of these two windows was determined as a compromise between the needs for both high counting rate and a minimization of photoelectric absorption perturbations. The inference of the formation lithology comes from a comparison of the LS window labeled LITH and the Ls window. The long-spacing detector's density estimate is refined further by using the LITH window to compensate for any residual photoelectric absorption in the primary window (Ls). primary window (Ls). SPEJ
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