A Method to Improve Oil Saturation Precision When Monitoring Steam or Water Floods

James L. Hemingway and Thomas A. Zalan

Reservoir monitoring for the purpose of determining oil saturation has become more common due to recent advances in pulsed neutron spectroscopy (PNS) logging techniques. Formations with fresh water or changing salinities cannot be evaluated based on formation sigma, but can be evaluated based on the carbon-to-oxygen ratio (COR).

The accuracy of Schlumberger's PNS log, the RST Reservoir Saturation Tool, is due primarily to the computation of the COR from a full spectral analysis of the inelastic energy spectrum. This requires a great amount of data, which translates to a very slow logging speed. Another way to compute the COR is to use large energy windows that cover the primary energy peaks for carbon and oxygen. This windows technique is more repeatable and therefore more precise, but requires careful correction for biases due to mineralogy and the borehole tubulars. The precision of the windows results can be maintained without sacrificing the accuracy of the spectral stripped results by offsetting the windows results with a filtered difference between the spectral stripped and the windows results. This off et is refered to as alpha.

When monitoring steam or water floods over long periods of time, the precision of the windows COR can be more important than the accuracy of a the full spectral result when looking for small changes in oil saturation on subsequent logging runs. A modified alpha processing technique to evaluate oil saturation under controlled conditions on subsequent logging runs that allows a faster logging speed or greater precision will be presented.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California