CHALLENGING THE CURRENT APPLICATION ENVELOPE OF CARBON OXYGEN LOGS

摘要

Neutron capture logs are often used in cased wells to distinguish oil from water in formations where the water is of a known and reasonably high salinity. The carbon-oxygen (C/O) measurement is preferred when the formation water is fresh or the salinity is low. (circa 20,000 ppm in the Niger Delta Basin of West Africa). C/O tools are primarily designed to measure formation parameters behind casing. However, the reality in completion techniques commonly in use, such as, tubing-blast-joint- Internal Gravel Pack (IGP)- casing -formation, the various carbon-oxygen-logging tools in the industry have had to be recharacterized for these conditions. The re-characterization of the tool database for the various completion scenarios has therefore led to a wider scope of application of the tool. However, some issues still persist: - the ability to quantitatively measure residual oil saturation across these completions. This has limited the application of C/O logs to qualitative measurements, such as the determination of fluid contact changes. In this paper, the author focuses on the challenges of carbon-oxygen measurements for quantitative application in reservoir management under the completion conditions mentioned above. Some local examples are presented. The uncertainties in C/O data processing under theses completion scenarios are also demonstrated in the paper. The C/O measurement is found to pose enormous challenges in environments with a combination of one or more of the mechanical components in the completions, such as double/multiple strings, blast joint and IGP etc. However, understanding the challenges in these completions has enabled reliable monitoring of fluid contact changes that would otherwise not have been possible. This is achieved through the integration of PVT data, production data and the field-wide knowledge. The C/O log still has a lot of potential, not fully utilized in reservoir management in the Industry. The challenge therefore, is not only to the Service companies, but the Operators also, to improve the current application envelope for maximum benefit as a total Reservoir management tool. While evaluating C/O logs across complex completions, integrated approach should be adopted, where all available data are used for calibration to ensure satisfactory results. Given the challenges posed to the successful use of C/O tools, further research work on the tool characterization to provide the needed solution for effective reservoir management in the industry should be pursued.