Pulsed neutron carbon-oxygen (C/O) logging is a valuable measurement for dynamic reservoir saturation monitoring, especially in mixed salinity and fresh-water environments. Currently, all C/O logs are based on apparent C/O measurements, thus responses of raw data are not comparable between different tools. Tool-specific calibrations are used to convert apparent C/O log to reservoir oil saturation; the objective of C/O logging. In this paper, a new concept of intrinsic C/O logging is introduced so that raw data from different tools can be compared for enhanced log quality control and consistency of raw data and their applications.
For a given system of rock, oil, and water, intrinsic carbon (C) and oxygen (O), thus C/O ratio, are theoretically calculated. For the same system, apparent C/O ratio measured by a generic C/O logging tool is also obtained using existing apparent C/O methodology. A correlation between the two is established, enabling an intrinsic C/O log output.
Eleven laboratory physical tests and 120 Monte Carlo simulated case studies are conducted. Systems investigated include sandstone and limestone with different porosities and fluid saturations. Borehole sizes are 6 and 8 inches, and borehole fluids are water, oil, and their mixtures. Correlations between intrinsic and apparent C/O are established and concept of intrinsic C/O logging is verified.
