Abstract

Throughout the history of logging, resistivity had been one of the most basic of surveys and central to rock-fluid saturation computations. Capability of measuring resistivity through casing has opened up new possibilities in formation evaluation and reservoir surveillance. Acoustic and nuclear measurements were possible behind casing, with the pulsed neutron techniques as the main stay for reservoir monitoring. Resistivity measurements score over the nuclear methods in most situations due to deeper depth of investigation, less limitations and fewer assumptions about the borehole environment. Besides resistivity is a high contrast measurement that affords simple and direct interpretation. Cased-hole resistivity finds its utility in two circumstances; as a first resistivity measurement in old and new wells to assess the formation in the present conditions, or as a time-lapse survey where different resistivity measurements (open or cased) are analyzed to describe the temporal behavior of a dynamic reservoir. Combining salinity-based resistivity with the pulsed neutron measurements adds confidence to the analysis. Case histories of four of the initial six wells logged in the Bombay offshore fields of India are presented. The objective of the surveys varied from back to back open and cased-hole resistivity surveys to verify tool validity, reservoir monitoring in old wells, supplementing casedhole data with resistivity measurements in an old well to contingency logging in new wells where open-hole data was missed in sections. Raw curves and the results of cased-hole analysis are juxtaposed with other open and cased-hole surveys as well as well history for matter-of-fact assessment.