Experiences With Casedhole Resistivity Logging For Reservoir Monitoring

Over the past decade, service companies overcame significant difficulties in low signal measurement in cased-hole resistivity logging development. Consequently, resistivity logging through steel casing has progressed from theoretical concepts to some field applications. Through-casing resistivity measurement was developed to differentiate the formations containing hydrocarbon from those with water in a cased-hole environment. The technology can be applied to reservoir monitoring, bypassed reserve evaluation, or hostile drilling environment where open-hole logs cannot be recorded. We have conducted a series of field trials of through-casing resistivity tools from two service companies. We have run the service in both a sandstone formation, where its resistivity is in the range of one ohm-m to a few tens of ohm-m, and in carbonate, where formation resistivity reaches hundreds of ohm-m or more. We applied the tools to newly drilled wells with "perfect" casing as well as to old wells with casings of years or tens of years of age. We observed anomalies possibly related to casing joints, casing-end, perforations, or cement. It is also noted that logging speed (measurement duration per station) contributes to the log quality, especially in high resistivity formation. Despite some of the issues observed, the arrival of cased-hole resistivity technology is an exciting development in the field of petrophysics and reservoir development. Cased-hole resistivity logs generally correlate very well with open-hole resistivity logs. In the zones where cased-hole and open-hole logs differ, the differences often can be attributed to the saturation changes which occurred between log runs. Cased-hole resistivity has advantages over other cased-hole saturation evaluation methods in that it is not limited by low porosity and it has a much deeper depth of investigation. On the other hand, the evaluation of water saturation from the cased-hole resistivity log requires good knowledge of formation water salinity. In fields under enhanced oil recovery, care must be taken in deriving accurate water saturations because of the mixing of connate and injection water.