A New Technique for Carbon-Oxygen Logging Through Gravel Packs and Other Special Completions

Abstract

Abstract Productive horizons in the Niger Delta basin of West Africa comprise clastic tertiary sequences that formed coastal barrier bars, beaches, tidal distributor channels and lagoonal deposits. Such reservoirs are often made up of very high permeability unconsolidated sands that require gravel-packed completions for efficient production. Water influx from edge- or bottom-aquifers is a dominant drive mechanism, and in this basin water salinity is often less than 30 kppm (i.e., predominantly fresh). It is no surprise that in this low-salinity environment carbon-oxygen (C/O) logging has become particularly popular for reservoir monitoring. In the past, operators were often forced to exercise the high-cost option of pulling the completion string in order to run C/O logs. This was necessary because no logging tool was rated for saturation evaluation inside gravel-packs or other special completion elements such as blast joints. In this paper a new characterization for a slim C/O logging tool is presented that now extends its applicability to these special completion types. The technique is based on laboratory measurements made with the tool inside typical Niger Delta gravel-pack and formation lithology environments. These measurements were used to benchmark a Monte Carlo nuclear model from which a database was constructed. The questions of gravel-pack quality and of the pack's fluid content have been addressed in this work, as has the impact of uncertainties associated with these variables upon the final saturation answer. The new characterization database has been used to evaluate C/O logs run in several wells in the Niger Delta. Three examples where the tool logged inside an internal gravel-pack (IGP) completion are presented in this paper. The examples demonstrate a significant improvement over a qualitative approach and provide for a more robust and reliable saturation computation.