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Gamma Ray-Density Logging

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Gamma Ray-Density Logging SOCIETY OF PETROLEUM ENGINEERS OF AIME PAPER Fidelity Union Bldg. NUMBER 1253-G Dallas, Tex. THIS IS A PREPRINT --- SUBJECT TO CORRECTION Gamma Ray-Density Logging By Downloaded from http://onepetro.org/SPEPBOGR/proceedings-pdf/59PBOR/All-59PBOR/SPE-1253-G/2087146/spe-1253-g.pdf by guest on 27 September 2021 Albin J. Zak and Joe Ed Smith, Junior Members AIME Core Laboratories Inc., Dallas, Tex. ABSTRACT In the practical logging tool, a beam of gamma rays is emitted into the formation. A This paper presents a review of current small fraction of these photons find their way methods of interpreting and applying gamma ray­ back to the detector. Normally, the detector density logs in evaluating fundamental reservoir used is a Geiger-Muller counter or scintillation data. The basic theories and physical principles counter. Studies of other detecting devices, underlying the techniques of density logging with such as cloud chambers and ionization chambers, gamma rays are discussed along with some of the have shown them to be unsuitable for this purpose. problems associated with the instrumentation of The most common gamma ~ay source for this tool is such a tool. An effort has been made to evaluate radioactive cobalt (CobO ). The source is Shielded the instrument calibration techniques, and discus­ from the detector with lead such that the majority sion and figures are presented which describe the of the gamma rays counted by the detector are effect of borehole conditions, pressure, tempera­ those back-scattered from the formation. ture, etc. The relationship of natural density, grain density, and interstitial fluid density to Initial calibration of the instrument is made porosity is presented. Field data illustrate how in special testholes in which the density of the density logs can be correlated with porosity and calibrating material is carefully controlled. grain density data. With the aid of adequate core These experiments are made in various hole sizes analysis data, a reasonably accurate estimate of in each of several different mud densities. From formation porosity can be obtained from density this, an adequate prediction has been established log interpretation. The paper also offers a dis­ as to the effect of borehole diameter and borehole cussion of the limitations of the density log as a fluid density on the log response. To date, there porosity tool. The current applications of the are no valid correction factors proposed for the density log to geologic work are cited, and the effects due to poor borehole geometry. The rules prospective uses and future developments are and limitations of logging speed, time-constant, briefly explored. bed thickness, etc., that apply to conventional radioactivity logs also apply to density logging. SUMMARY The effects of temperature and pressure are those effects that are imposed on the response of the The principle of gamma ray absorption as a gamma ray counter, and the effects on the densi­ function of density was adapted to the logging of ties of the interstitial fluids. petroleum formations through the utilization of a phenomenon known as "back-scattering". This tech­ The basic interpretation of the density log nique uses a tool which contains both a gamma ray is fairly simple; the possible applications are source and detector. The detector measures the numerous. The most prominent denSity log applica­ intensity of the gamma rays emitted from the tions are: (1) lithological eorrelations, (2) source that have been back-scattered to the de­ i determination of density grad:"ents for gravity tector by the formation. After proper instrument , meter surveys, (3) determination of borehole fluid calibration, the log response can be used to denSity for gradient surveys, (4) location of compute the natural density of the formation. casing shoe and cement top, (5) location of casing Since there is a definite relationship between leaks, (6) aid in the interpretation and evalua­ natural density and porosity, it is possible to tion of other logs, and (7) method of estimating predict formation porosity from density logs. It formation porosity. Application 7 is probably is necessary to correlate carefully density log the most important and most widely used. data with core analysis data before the direct correlation of density log response with porosity Maximum use of the density log nas not been can be made. fully realized. Although there are some severe limitations in its use, there are many cases where References and illust!13,tion~~t,~~?f p_~_~~_.___ -,- ______________________________________________---' 2 GAMMA RAY-DENSITY LOGGING 1253-G the density log is an effective tool, and serves recall that atoms with nuclei of the same Z but as a valuable aid in f'ormation evaluation. different A are forms of the same element and are called isotopes. INTRODUCTION Beta rays (~) are believed to be physically Gamma ray-density logging has progressed far equivalent to electrons if the beta radiation since its original experimental stage. Numerous has a negative charge. If the beta rays have a major oil companies and several service companies positive charge, they are equivalent to positive­ have made significant contributions to its ad­ charged electrons and are called positrons. When vancement. Although the tool was originally con­ talking about positrons, the term negatron is ceived as an al~iliary exploration device, con­ usually used instead of electron; negatrons and tinued research and development has created new electrons are exactly the same. Beta radiations, Downloaded from http://onepetro.org/SPEPBOGR/proceedings-pdf/59PBOR/All-59PBOR/SPE-1253-G/2087146/spe-1253-g.pdf by guest on 27 September 2021 and improved equipment and techniques. This positrons, and negatrons all have the same rela­ paper is an analysis of the theories, modern tive charge, either plus or minus one, and they techniques and interpretation procedures applic­ all have the same relative mass of one. (As able to formation evaluation. shown in Table 1, a proton (p+) has a relative charge of plus one, and a relative mass of ap­ Often, the reservoir geologist, petroleum proximately 1,836; a neutron (n) has a relative engineer, and log analyst are not familiar with charge of zero, and a relative mass of approxi­ the basic theory and physical principles that mately 1,840.) Unless otherwise stated, the underlie a particular logging tool. This results charge of beta radiation is always assumed to be in the interpreter being unable, or limited in negative and equivalent to an electron. Hence, his ability, to evaluate logs. An attempt is when a nucleus decays by beta emission, Z will made herein to "bridge the gap" between theory increase by one unit and A will remain constant. and application. It should be pointed out that the exact value of mass will decrease very slightly; this is a The purpose of this paper is to inform the necessary conclusion in order to explain the re­ user of the basis of gamma ray-density logging; lease of energy. to describe the equipment used; present the ac­ cepted principles of interpretation; illustrate Alpha particles (0<) are equivalent to the uses and applications; and cite the known accelerated helium atoms that have been stripped advantages and limitations. of their electrons. Thus, the alpha particle will have a nuclear charge of plus two and an THEORY AND PHYSICAL PRINCIPLES atomic mass of four. Therefore, if a nucleus decays by alpha emiSSion, Z will decrease by two Basic Nuclear Concepts and A will decrease by four. Note that in either beta or alpha decay, isotopes of the mother ele­ In order to fully understand the nuclear ment are not formed; the residual that is formed processes that occur when a gamma ray-density log is always a different element. However, the is run, it will be necessary to briefly review series disintegration of an element tr~ough both some basic radiochemical theory and physical con­ alpha and beta decay will often produce isotopes cepts. The discussion here will not begin with I of the original element. the most elementary principles; for introductory material on the subject of atomic PhYSicS the In atomic physics, it is conventional to reader may refer to the works of Bttter,l3 express energy in terms of elec~ron-volts. An Fermi,25 Friedlander and Kennedy,2b Lapp and electron-volt is equal to 1.6 x 10-12 ergs; it is Andrews,31 and Semat.36 From the beginning, the further defined as the kinetic energy required by inquirer must realize that gamma ray-density an electron to move through a potential difference logging is different from conventional gamma ray­ of one volt. Since the electron-volt is a rather neutron logging. The gamma ray-density log is small unit, thousands of elect:ron-volts (Kev) , often referred to as the gamma-gamma log, which I and millions of electron-volts (Mev) have been means that the source emits gamma rays and the adopted as standard units. Occasionally, units detector counts gamma rays. This paper will of electron-volts (ev), and billions of electron­ present only that material which is related and volts (Bev) are used. Alpha particles will nor­ necessary to the understanding of gamma ray­ mally exhibit energies in the range of several density logging. Mev; beta particles have no normal energy range -­ some are as low as one ev, whereas some are as The definitions of nuclear terms presented high as ten Mev. Gamma rays have energies in the by Faul,24 will be sufficient for this paper; the order of one Mev for naturally occurring decay. atomic number Z, or nuclear charge as it is some­ It is interesting to note that cosmic rays have times called, is the integral number of protons energies in the order of several Bev, and are in the nucleus of an atom.
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