Radioactive Tracer Techniques

Abstract

Abstract This paper summarizes the principal applications of radioactive tracer materials in the oil field, and describes the different tools and techniques used in conducting various sub-surface surveys. Radioactive tracer surveys have been used to locate thief zones during drilling, and are unaffected by lost circulation agents present in the mud. During long string casing cementing operations, they have been used to locate the top of the cement, and the effectiveness of the cement job has been determined by channel location surveys through perforations. The success of remedial cement squeeze jobs also has been evaluated by this method. Radioactive tracers have been used to control two-pump selective acidizing treatments, as well as determining which zones have been stimulated, during conventional acidizing operations. Considerable experimental work has been conducted in connection with fracturing operations to locate and determine the orientation of any fractures so formed. Other multi-tracer experiments have been conducted in to determine the extent of co-mingling the various fracturing materials undergo in the formation, and in what order they return to the wellbore following the treatment. Other applications for radioactive tracers include the performance of relative permeability surveys, injectional capacity surveys and water location surveys; the detection of casing or tubing leaks, or leaks around casing seats or packers; evaluation of shaped charge perforating jobs, and subsequent acidizing treatments; and the identification of produced brine following break-through in secondary recovery projects. Introduction The oil industry has employed gamma ray detectors to log formation characteristics since the early 1940's. With the development and exploitation of atomic energy following World War II, the availability of various radioactive isotopes made possible their utilization as tracing agents. They have since proven a valuable aid to the oil operator in solving his drilling, completion and remedial problems. Radioactive tracers, as used in oilfield applications, may be defined as chemical materials that emit gamma rays capable of being detected by suitable electronic equipment. A number of artificially produced radioactive materials have been used for a number of years. The choice of which type tracer to use for any given application depends on a number of factors: availability, cost, solubility, radioactive intensity and halflife. The half-life of a radioactive compound is the time required for the gamma ray emission to decrease by half. Materials in use may have half-lives ranging from a few minutes to several thousand years. One of the radioactive isotopes that is popular for oilfield work at the present time is Iodine-131. It emits gamma rays that can penetrate steel tubing, casing and several inches of cement, and is readily located by means of a subsurface Geiger counter. This material is chemically versatile, in that it can be modified to be water-, acid-, or oil-soluble. Thus, it can be readily incorporated into any of these carrying media, without "hot spots" that might complicate the interpretation of the surveys. Once mixed in, the isotope does not settle out of solution, neither does it "plate out" on the formation.