Accurate monitoring of oil saturation is a key challenge in developing a mature reservoir description. For managing mature fields, the time-lapse cased hole saturation log is one of the most crucial datasets. However, the inability to acquire the data in unconventional completion environments—for example, completions with external gravel packs (EGPs) or with uncemented horizontal wire-wrapped screens (WWSs)—has been a major challenge to surveillance activities, particularly in the mature fields of south Oman.

Traditional nuclear logs such as neutron capture sigma logs or neutron inelastic carbon-oxygen ratio (C/O) logs are affected by the tools' shallow depths of investigation. Gas segregation in horizontal drains and borehole fluid reinvasion is likely to change near-wellbore saturation. Mechanical artificial lift equipment further complicate the data acquisition effort and make specialized sensor conveyance techniques necessary. This surveillance paradigm has recently changed with the introduction of cased hole resistivity measurements. The ability to measure formation resistivity directly through casing adds a new dimension to water saturation measurement deeper into the formation, and it is unaffected by the borehole inhomogeneity.

This paper presents results and logging data acquired for the first time in a variety of difficult completion configurations, sizes, and types: large-OD EGPs, WWSs, casing with expandable zonal inflow profilers with dual-wellhead completion, and long horizontal in-fill wells. The cased hole resistivity data provided information that increased oil gains by locating un-swept oil zones, and even more importantly, provided much-needed calibration points for reservoir dynamic models.

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