Radioactivity Logs for Gas Location

Gas Technology Symposium, 17–18 April, Shreveport, Louisiana Abstract A brief and basic discussion of the theory of radioactivity logging along with types of instrumentation now in use explain how gas bearing zones are detectable in cased hole or open hole by use of radioactivity logs. Numerous examples of the successful use of single and multispaced neutron logs are presented. Also included are examples of the detection of gas, distillate, oil-, and water-bearing sands by proper interpretation of radioactivity logs. A new technique for determining the L.P.G.-Brine interface in an L.P.G. storage cavity is discussed. A new technique, developed in Venezuela for determining the point of gas entry into a bore hole is discussed. A great deal has been written concerning the theory of radioactivity logging and to go into this subject deeply would be repetitious, however, to enter into a discussion of applications of this type of log it is necessary to briefly cover the fundamentals of theory and instrumentation. The gamma ray log is a measure of the natural radiation of formations. All earth formations contain radioactive elements, some more than others. Figure 1is a table of natural radio-elements showing the progression down the scale with the accompanying emission of radioactivity at the various stages. There are three families of elements which are radioactive. These are the Uranium series, the Actino-Uranium series and the Thorium series. This chart clearly shows that an atom of uranium, in progressing through the various stages, will be at one stage ionium, then radium, then radon, etc., and end the progression as non-radioactive lead. At almost every stage of this progression some form of radiation is given off. The radiation will be an alpha particle, a beta particle or a gamma ray. Although each moves with very high velocity, a thin sheet of bond paper is sufficient barrier to stop an alpha particle. The beta particle has more energy than the alpha, however, several sheets of cardboard will successfully shield one from the radiation of betas. Gamma rays have the ability to penetrate 12 inches of steel or 8 inches of lead. From this it is easily seen that only gamma rays may be detected with the instruments now in use because the instrument wall is sufficiently thick to shield out the alpha and beta particles. Furthermore the gamma ray has such great penetrating qualities that we may case a well with concentric pump liners and still obtain a formation gamma ray log. The neutron is very little more affected by casing so we may also obtain a neutron in the well cased with pump liners.