Five wells in the Big Horn Basin of Wyoming were perforated in the Phosphoria formation and subsequently treated with acid. Each of the acids was tagged with a radioactive isotope. Spectral gamma ray logs were then run to determine the effectiveness of the treatments. After processing and analyzing the data, two facts became evident: 1) Several pitfalls need to be avoided in selecting the isotope and designing how the acid is tagged. For example, the correct isotope must be selected with the correct specific gravity to allow the isotope to stay suspended in the treatment fluid and the borehole fluid must be displaced with non-radioactive fluid. 2) Looking at total gamma ray counts alone to determine isotope distribution is inconclusive and sometimes misleading because this does not distinguish between tracer material in the borehole from tracer material in the formation.

When gamma rays pass through any median such as formation, cement, and casing, Compton downscattering occurs.(1) By applying a weighted least squares (WLS) algorithm to the gamma spectroscopy data, the compton downscattering was taken into account to determine both borehole and formation components.

The geology of the formation, treatment design, isotope design and isotope selection are discussed as they apply to the log data. The logging procedure, processing of the log data, and log examples are presented in more detail as this is the main thrust of this paper.

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