Intrinsic Carbon/Oxygen for Borehole Fluid Holdup Characterization and Reservoir Saturation Monitoring

Summary Carbon (C)-oxygen (O) (C-O) logging is essential in the logging program for reservoir management to assess the remaining oil saturation and production optimization. The logging can be performed in wells in either well flowing or shut-in conditions. The current industry practice is that the C/O ratio log is acquired and interpreted for saturation by each service provider using their own proprietary algorithms and workflow. Thus, the C/O ratio from the same well under the same logging conditions will vary if logged by different service providers due to the pulsed-neutron (PN) tool design differences, spectroscopy algorithm differences to compute the C/O ratio from the spectral data, and sometimes interpretation parameter setting differences. Furthermore, the tool-dependent C/O ratio answer product leads to data quality control challenges for the end users, especially for fields where large volumes of data are acquired on a regular basis. This study presents an advanced algorithm for defining and computing the PN intrinsic C/O ratio that uniquely integrates borehole holdup effects in the C/O ratio deliverable answer product, addressing limitations of tool-dependent CO ratio, after enhancing a recent development of intrinsic C/O, which only considers the effects of the formation. In this study, the intrinsic C/O ratio is derived from theoretical atomic concentrations of C and O, accounting for porosity, fluid saturations, and borehole fluid holdup using a unit-cell approach. A comprehensive validation methodology is used, combining theoretical Monte Carlo N-Particle (MCNP) simulations and laboratory experiments. Simulation results demonstrate the robustness of the intrinsic C/O concept, showing consistency across tool configurations while capturing the combined effects of reservoir and borehole conditions. Laboratory testing further validates the approach by correlating measured and intrinsic C/O ratios, highlighting the impact of formation, borehole holdup, and tool eccentricity. This unified intrinsic C/O model advances reservoir saturation monitoring (RSM) by bridging reservoir saturation and borehole holdup into a single coherent framework, overcoming the inconsistencies of traditional methods. The findings enhance the reliability and consistency of PN logging, with potential applications extending to intrinsic elemental yield analyses for improved reservoir description.