Carbon/Oxygen Logging Speed Optimization in the Niger Delta Fields With Improved Pulsed Neutron Technology

Abstract

Abstract Pulsed neutron logging of salinity-independent carbon/oxygen (C/O) mode typically requires slow logging speed (e.g., 2 feet per minute (ft/min)) and multiple passes, and it has always been a concern in the upstream rig operations because of the rig time involved, which translates to higher costs for operators. Reservoir fluid volumes, properties, and contacts change over time in cased wells, such as in the Niger Delta oilfields, produced for over four decades. Time-lapse C/O logging is required in this freshwater environment to determine two-phase saturations and fluid contacts, evaluate fluid dynamics, and identify bypassed oil zones. Due to the higher cost of C/O logging campaigns than pulsed neutron capture (PNC) logging campaigns, it is imperative to consider faster C/O logging speed, hence the advent of an improved pulsed neutron technology capable of logging three times faster than previous tools. The critical properties of the improved pulsed neutron technology are the lanthanum bromide (LaBr3) material chosen for the detector section due to its high density and resolution characteristics, higher pulsed neutron source output, this capable of producing two times the neutrons produced by previous generation tools, and the new digital electronics system capable of processing the increased gamma-ray count rate. The new instrument allows three times faster logging when multiple passes are recommended or improved precision at legacy logging speeds. Indeed, one pass of the new tool at 2 ft/min is equivalent to three passes of the legacy tool at 2 ft/min. Recent papers (Nardiello et al., 2022; Kim et al., 2023) described the comparison demonstrating that the upgraded, faster technology provided the same data precision and saturation interpretation as the slower, legacy technology, achieving considerable time reduction in logging operations. This paper intends to present an example of the Niger Delta field's C/O data logged with the new tool at 6 ft/min and a formation saturation analysis case study. The saturation analysis result identified a watered-out sand due to offset well production (or a formation water-oil contact change).