Use of Radioactive Isotopes in Gas injection

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Introduction Major advances in the measurement of extremely small amounts of radiation make the use of radioactive isotopes as a tracing material in petroleum reservoirs practical. Isotopes, Inc. has developed equipment capable of extremely low level techniques which, together with a number of low cost radioactive isotopes, make possible many useful applications in reservoir engineering. RADIOISOTOPES AS TRACERS By introducing radioisotopes in the injection wells with the injected gas it is possible to "tag" the injected gas and, by analyzing produced gas, to trace the movement of the injected fluid throughout the reservoir. Directional permeability may be identified early in the life of the project so that steps can be taken to maximize project so that steps can be taken to maximize recovery efficiency. The use of tracers to follow movement of gas, oil, or water is not new. Underground tracer work has utilized many materials; among them certain dyes and chemical compounds, helium, and carbon monoxide. The problems are the same in every application: To get the tracer to behave identically with the material traced To obtain adequate sensitivity To carry out the tracer project at reasonable cost Differences in adsorption, absorption, wetting, precipitation etc., between the tracer and the precipitation etc., between the tracer and the fluid to be traced cast doubt on the results on many of the earlier tests. These tracers also lacked sufficient sensitivity. Such factors discouraged the use of tracing techniques, even though it has been obvious since the beginning of secondary recovery that an ideal tracer would be useful in almost every operation. In principle, radioactive isotopes constitute ideal tracers; first, because the fluid itself may be made radioactive, or the tracer can be made a part of the fluid chemically. For instance, tritiated hydrogen acts the same as hydrogen because it is, in fact, a part of the hydrogen molecule. The only difference is that one of the hydrogen atoms in the H2 molecule has been replaced by a tritium atom. Tritium is an isotope of hydrogen, having two extra neutrons in the nucleus. Tritium has a half life of 12.5 years, and low level counting techniques make possible the detection of these isotopes through seven half lives or approximately 85 years. A number of other low cost radioisotopes are available for use in gas injection projects.