Carbon-oxygen logging is used primarily to estimate oil saturation in cased-hole conditions when the formation water is fresh or unknown. The drawbacks of current techniques are: 1) slow logging speed, 2) large tool diameter, and 3) excessive sensitivity to borehole fluid composition. A new, slim, neutron-induced gamma ray spectroscopy logging system has been developed to overcome some of these limitations. The new logging service is called the Reservoir Saturation (RST*) Tool. Initial field tests are being carried out in the Middle East.

The RST tool uses multiple detectors to separate the signal contributions from the borehole and the formation. Therefore, even when the borehole fluid composition is unknown, oil saturation can be determined in addition to the borehole oil fraction. This presents the possibility of logging flowing wells, which ensures that reinvasion and crossflow will not affect the results, and eliminates the costs of well preparation.

A reduction in sonde diameter, without sacrificing logging speed, has been made possible through the use of specially developed technologies in the areas of scintillation materials; photomultiplier tubes; pulsed neutron generators; pulse acquisition circuits; and high-density, digital, signal-processing electronics. The new tool fits through production tubing and operates at temperatures to 150 C. Since no Dewar flasks are used, there is no limit on how long the tool can stay in the well.

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