Evaluating the Effect of Wellbore Dynamics and Concentric Casings on Pulsed Neutron Technology PNL – A Case Study from Mature Carbonate Reservoir

Abstract Determining an accurate saturation profile in old oil wells completed with multi-casing and producing at high water cut through a gas lift system involves many challenges. These challenges include the effects of concentric casing environment on saturation logs and the reinvasion phenomenon expected in artificially lifted high water cut wells. As the number of wells completed with concentric casings grows due to ongoing corrosion, Sirte Oil Company (SOC) decided to leverage an ongoing workover operation to evaluate the effects of reinvasion and concentric casing on pulsed neutron logs (PNL). Well UU-1, an old oil well in the Lehib Field, is completed with a 7 in casing inside 12.25 in open hole and produces at a high water cut, suffering from casing leaks. SOC took advantage of a workover operation to set a second 5.5 in casing and evaluate the reinvasion phenomenon and the effect of concentric casings on saturation logs. The process involved running four PNL surveys in sigma and Carbon Oxygen (CO) modes: the first survey in flowing condition after flow stabilization using the gas lift system, the second survey in shut-in condition immediately after shutting down the gas lift system, the third survey four hours after shut-in, and the fourth survey twelve hours after shut-in to evaluate the possibility of reinvasion. A final fifth survey was performed after running the second 5.5 in casing to compare the saturation profile with the previous log acquired in the 7 in casing. The main observation was that the PNL did not suggest an ongoing reinvasion process up to twelve hours after shutting down the gas lift compressor, despite the pay zone being covered with segregated water for twelve hours. All four surveys were overlaying and identical, clearly indicating the remaining hydrocarbons in place. Furthermore, the fifth survey in concentric casing suggested an excellent sigma contrast between oil and water, with slight variation in sigma compared to the single casing log due to the multi-pipe effect. This small difference could be overcome with comprehensive knowledge about hole size, casing properties, and annulus material. Following this evaluation, many wells were logged in concentric casings with excellent results, helping SOC achieve significant production gains. This paper details the processing workflows developed to navigate the multi-pipe effects, ensuring accurate saturation profiles for reservoir modeling and reserve estimation. Utilizing a major workover operation to evaluate the most problematic phenomena affecting the PNL has increased SOC's confidence in performing PNL for reservoir monitoring in concentric pipe conditions.