Advances in Elemental Spectroscopy Logging: A Cased Hole Application Offshore West Africa

Abstract

Rising costs for exploration and developments and more stringent need to secure any additional drop of oil have put operators’ margins under increasing pressure. Coupled with the recent oil price decline, this call for efficiency and diligence to be the main drivers for any formation evaluation and planning for development and production. The reservoirs in Western offshore Africa are so diverse in the settings that two reservoirs hardly show any correlation. The complexity associated with the Rifting of African plate from South American plate has introduced significant geological challenges, adding to even bigger challenges in Petrophysical analysis. The mineralogy is complex; clay characterization is often unsolved. The formation waters are fresh with variable salinity and there is occurrence of thin shale laminations and grain size variations contributing to low resistivity low contrast pay generation. Advanced and fit-to-purpose logging technologies and computational methods are needed for rock quality and potential. Moreover, in some cases the accessibility of the target reservoir is difficult and risky, so that formation evaluation must be performed behind casing.The high definition spectroscopy tool is the latest development in wireline spectroscopy measurements. Its technological advances revolutionize the neutron-induced gamma ray methodology to support robust lithology and saturation interpretation in formations with complex mineralogy and fluid content. The ability to determine both the matrix mineral composition and total organic carbon (TOC) are instrumental to the geoscientist, the petrophysicist, the reservoir engineer, and the completion engineer. In the region, the use of high definition spectroscopy measurement has been pioneered while pursuing better understanding of rock composition and more accurate reservoir models in complex lithology and fresh formation waters with low resistivity contrast. The results are beneficial at the various stages of a field development and provide critical input to the petrophysical reserves estimate.In the example described in this paper, the new technology has proven to be critical to evaluate a complex reservoir system independent of the water salinity and resistivity offshore Gulf of Guinea, even with logging behind casing. A comprehensive set of quality outputs is made available for accurate reservoir quality; the logs data processing is performed within the critical-hours after logging to enable informed decision making.