A Relative Distance Indicator from Gamma Ray Spectroscopy Measurements with Radioactive Tracers

Abstract

Abstract The effectiveness of downhole operations is increasingly being evaluated using gamma ray spectroscopy tools inconjunction with one or more radioactive tracers. The materials placed in the well are selectively tagged with tracer isotopes. The spectroscopy measurements are typically used to distinguish the vertical distributions of two or more tracers which have distinct gamma ray energy signatures. In addition, it is possible to use the characteristic shape of the Compton scattered portion of the gamma ray spectrum to determine qualitatively how far each tracer is located from the tool, to within the depth of investigation of the measurement. A series of calibration experiments was performed to establish the response of three gamma ray spectroscopy tools to six common tracer isotopes. Two artificial formations were constructed to simulate a typical cased hole environment. In order to investigate radial distribution effects, one test formation was subdivided so the formation region consisted of three annuli. The principal purpose was to determine tool sensitivities to different isotopes so that a weighted-least-squares algorithm could be implemented to process multi-tracer logs. The shapes of the tracer spectra were also evaluated for Compton scattering and photoelectric absorption effects. photoelectric absorption effects. The results of this study were combined with extensive field experience to show that, in addition to measuring the relative distribution of the tracer isotopes as a function of depth, it is also possible to make a qualitative determination of the average radial distance from the spectroscopy tool for each tracer. A log example illustrates the utility of this analysis. Introduction The use of radioactive isotopes as tracers is widespread throughout the oil industry. Such applications are particularly important to productive wells in field operations. It is almost always necessary to case and cement a well before it comes on-line so that the zones of interest can be isolated and so that a seal can be maintained between adjacent geological formations and the surface. In addition the stimulation of hydrocarbon-bearing formations via hydraulic fracturing is a routine operation for many wells. It is commonplace in each case to add one or more radioactive tracers to the constituents of the slurries pumped downhole. Subsequently, a gamma ray tool which pumped downhole. Subsequently, a gamma ray tool which detects the gamma rays emitted by each tracer is run in the well. The resulting log shows the locations of the tracers, which are presumably where the accompanying materials were placed in the well. Many times the desired well completion or stimulation procedures can require that several different materials be procedures can require that several different materials be placed in a single operation. In the past it has usually been placed in a single operation. In the past it has usually been necessary to limit the complexity of such operations because only one tracer could be monitored. This is because gross gamma ray counting tools are sensitive only to the overall presence of gamma rays and not to the individual gamma ray energy signatures characteristic of different tracers. However, gamma ray spectroscopy tools make it possible to efficiently and accurately monitor multiple radioactive tracers, and thus multiple materials, in downhole operations. In particular, the Tracer Scan technique uses the Compensated Spectral Natural Gamma Logging System to measure the energies of gamma rays emitted by radioactive tracers located in a well. Tracer-Scanlogs have been obtained in a variety of situations and the utilization of this technique is continuing to expand. P. 357