Cased-Hole Reservoir Saturation Monitoring in Mixed-Salinity Environments – A New Integrated Approach

Abstract For mature-reservoir description, accurately monitoring oil saturation is a major challenge. Traditional methods for cased-hole saturation monitoring use either pulse-neutron capture or spectroscopic nuclear logging tools. This paper presents a new approach to cased-hole resistivity measurements. This approach has an advantage over nuclear logs because its depth of investigation is more than 10 times that of any nuclear logging measurement. Calculation of saturation from resistivity logs is difficult if formation-water salinity is mixed. In this paper, we overcome this difficulty by combining pseudo formation-water salinity from carbon-oxygen (C-O) logs with bottomhole samples. Using this calibrated formation-water salinity and variable Archie saturation exponents of drainage and imbibition, calculation of reservoir saturation from cased-hole resistivity measurements becomes more accurate. Reservoir saturation is also calculated from C-O logs run at the same time as cased-hole resistivity logs. Wellbore fluid reinvasion is identified from C-O derived saturation across perforation intervals. In unperforated intervals, saturation derived from C-O compares favorably with the original openhole log analysis, well history, and fluid-flow profiles from production logs. Reservoir saturation is finalized by integrating saturations from cased-hole resistivity and C-O measurements. This paper illustrates the importance of integrating all data to obtain accurate saturation calculation of producing reservoirs in mixed-salinity environments.