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The carbon/oxygen (C/O) method of pulsed neutron logging has been widely used since the mid-1970s for saturation monitoring in cased hole wells. C/O logging measures the ratio of gamma rays created by inelastic neutron scattering by carbon and oxygen to determine formation water and oil saturation. A key benefit of C/O logging is that it provides a salinity-independent measurement. However, very slow logging speeds are required to achieve the necessary statistical accuracy. This paper introduces a faster salinity-independent water saturation method based on oxygen activation (OA) logging, including the physical principles, modeling and interpretation of laboratory test pit data. Oxygen activation can be induced when fast neutrons with an energy greater than 10 MeV interact with isotope O-16, resulting in the creation of radioactive N-16 and subsequent generation of gamma rays with a predominant energy of 6.13 MeV. These detected activation gamma rays are directly indicative of the amount of oxygen in the vicinity of the neutron source. Modeling of the oxygen activation response accounts for formation lithology, porosity, borehole fluid, wellbore geometry and logging speed. Formation water saturation is determined by comparing the measured oxygen activation gamma rays to the modeled activation response envelope. A custom-designed test pit was fabricated to evaluate OA water saturation feasibility. Comparison of C/O and OA measurements obtained over a range of logging speeds, and with different wellbore fluids, has confirmed that OA measurements can be successfully used to determine formation water saturation. In addition, the OA water saturation measurement is feasible at much faster logging speeds than C/O logging and can significantly reduce operation time. Indeed, the maximum practical logging speed in the test facility was 16 ft/min, and the upper speed limit of OA logging remains to be determined. This will best be evaluated through field testing in actual well conditions. OA logging is a new alternative to C/O logging for through-casing salinity-independent water and oil saturation measurement. With the ability to log at much faster speeds, OA logging can provide significantly improved performance considering the operational limitations of C/O logging. This paper presents the laboratory experiments, modeling and interpretation that confirm the viability of the OA water saturation measurement. In continuation of this work, the OA method is being applied to acquire and interpret field data from different wells globally. Various logging scenarios will be evaluated, including mixed lithology formations, flowing wells, and openhole completions. This will be presented in a separate publication. |
