Pulsed neutron generator (PNG) tools have been used historically in cased hole environments to provide formation sigma and carbon/oxygen (C/O) ratios to characterize reservoir depletion, gas/oil/water contacts, and waterflood sweep efficiency. However, one of the main challenges has always been planning the right acquisition time for the first sigma measurement. The implementation of a ruggedized PNG source in the multifunction logging-while-drilling (LWD) formation evaluation platform, has allowed reservoir saturation using capture sigma measurements in open hole environments.

Comparisons of LWD sigma with wireline cased hole sigma has helped cross-validate the reliability and shortcomings of both measurements. The results of this specific analysis demonstrate that the sigma-while-drilling measurement can be an alternative as a baseline for monitor logs. On the other hand, and going beyond the historical application, new methodologies to apply sigma-while-drilling in for-mation evaluation have been developed. Through the integration of LWD resistivity and sigma measurements, which are collocated in the multifunction LWD platform, both formation water salinity and water saturation can be simultaneously determined. New developments in Multi Depth of Investigation (MDOI) sigma help us ensure that a non-invaded or true formation sigma value is determined. With this in mind, this presentation is aiming to demonstrate a unique application for sigma-while-drilling to determine saturation under unknown water formation values.

The implementation of sigma-while-drilling technology has opened new frontiers that go beyond the common application as a baseline for monitoring logs. With the integration of collocated, invasion-free measurements, it is possible to determine water saturation in unknown formation water salinity that could be key information in enhanced oil recovery (EOR) projects or exploratory wells.