ABSTRACT

Several years of development and laboratory work have culminated in a pulsed neutron, induced gamma ray spectroscopy logging tool that exploits a high resolution germanium detector. Data have been acquired from a series of laboratory formation models. More importantly, logs have been obtained from a number of wells in active reservoirs. While inelastic and capture process data are both obtained, the high resolution detector's real strength lies in capture spectroscopy. In cased holes, it permits high sensitivity chlorine logs and more reliable logs of such elements as Si, Ca, S, Fe, H, and K, as well as a host of trace elements. The tool has been designed as a practical logging tool. It operates dependably for more than 12 hours at 100C, transmitting digital data over 20,000 feet of logging cable, while withstanding the routine stresses of field handling. When logged very slowly (< 3 ft./min.), it produces two, 4000 channel spectra about every 2 feet. Statistically more accurate results require stationary measurements at selected points, lasting about 15 minutes each. Such results can be used to calibrate the logging passes. The high resolution spectra permit independent determination of the various elemental responses, without resorting to ad hoc assumptions or potentially unstable numerical techniques. It has proven useful in the location of water/oil contacts--even when the water is fresh. Water saturation can also be accurately assessed to aid in residual oil evaluation. Finally, it offers a straightforward analysis of numerous other critical elements. Such information contributes to lithology, mineralogy, and residual oil determination. In short, we have demonstrated that the technology of high resolution, down hole germanium detectors is achievable and can be exploited to extend the utility and range of cased hole elemental analysis for reservoir evaluation and management.

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