WELL LOGGING USING RADIOACTIVE TRACERS

The mean depth of penetration of one or more radioactive tracers injected by a hydraulic fracturing process into a fractured formation disposed about a well bore, as an indication of the extent of radial fractures, is determined by measuring with a radioactivity detector in the well bore at the depth of the formation fracture, the degradation of the gamma ray energy spectra of the radioactive tracer in the fractured formations resulting from the traverse of radiation through the formations and borehole to the detector; obtaining a ratio @@ of the count rates CA and CB in two energy ranges where the range for obtaining CA is sensitive to primary radiation reaching the detector without Compton scattering and the other region for obtaining CB detects radiation degraded through Compton scattering collisions. The ratio @@ is a tracer penetration index (TPI) corresponding to the mean radial distance R between tracer and detector and the extent of radial fracture. For removing potential interference in the determination of R due to the presence of the tracer in the borehole, the method also provides for obtaining a ratio of count rates M/P in two low energy regions of the tracer spectrum - one region "M" sensitive only to Compton scattered radiation and a region "P" sensitive to photoelectric absorption in iron. The ratio M/P is a tracer borehole index (TBI) which indicates the relative presence of tracer in the formation compared to the borehole, and whether the borehole count rate obtained from a selected interval of well bore containing only borehole tracer needs to be removed from the total spectra observed in an interval of interest to obtain a borehole insensitive calculation of tracer penetration index.