Abstract

Radioactive tracers have been used for years in the petroleum industry to monitor the effectiveness of efforts to fracture formations and to cement casings in place. Normally a single radioactive tracer is used and its distribution is surveyed with a gross gamma ray counting instrument. However, there are many instances where it would be desirable to use two or more tracers in order to obtain more and better monitoring information. It has always been possible to use two tracers with different half-lives, and to make multiple passes over a period time (days or weeks) so that different decay rates at different locations can be used to establish where each tracer was deposited. However, the development of gamma ray spectroscopy devices has made it possible to obtain the desired tracer information in a single trip to the well by observing the specific gamma ray signatures of each different tracer in the frac material or cement.

The Compensated Spectral Natural Gamma Logging System was designed to measure the concentrations of potassium, uranium and thorium in downhole formations, but it is also well suited to monitor radioactive tracers. The CSNG*** tool digitizes 768 channels of gamma ray spectral information spanning the energy range 0–3000 keV, and transmits this data uphole each quater foot to the logging truck computer where it is processed and stored on magnetic tape. This detailed spectral information makes possible to deconvolve the individual gamma rays emitted by specific radioactive traces. Because of the high spectral resolution of the CSNG sonde, the uphole software can be tailored to process virtually any combinaton of multiple tracers. The procedures for background correcting and analyzing the spectral data are described, and examples of Tracerscan*** logs are presented.