Optimization of Pulsed Neutron Logging for Real-Time Water Management Through Oxygen Activation and Water Flow Logs
Authors: Yusuf N. , Cavalleri C. , Tolioe W. A. , How L. H. , Daud S. H. , Johare D. , Hendrawati M. S. , Halim A. G. A. 
Published: February 2024

Abstract
Pulsed neutron logging (PNL) is often part of production enhancement strategies to evaluate remaining potential and water flow, guiding intervention plans and de-risking water shutoff opportunities. The qualitative assessment of the oxygen activation (OA) curves response can accurately pinpoint zones potentially affected by water flow and can be used on real-time to optimize the selection of representative depths for water flow logging (WFL) station survey.
Both OA and WFL operate under the assumption that oxygen nuclei represent the borehole water phase. The OA continuous measurement of the oxygen activation is made during a window when the burst is off and only the activation background from the formation and the borehole is recorded. Depending on the toolstring configuration, the OA curves are sensitive to up or down flow of water, while the signal amplitude varies with velocity and water holdup. Regions where the OA curve amplitude and separations are more pronounced indicate possible crossflow or movement where the water velocity is faster than the tool.
The OA curves are recorded during standard depth logging passes performed for petrophysical parameters and saturation monitoring, without the need of separate passes; these curves help in highlighting changes in the borehole conditions, detecting unexpected flow, and indicating where to do WFL stations. In some logging programs where WFL stations were not planned, the real-time findings from the OA curves have enabled the extension of the log data acquisition to further investigate potential water movement with WFL mode, confirming the presence of fluid movement or recirculation at the wellbore. A good qualitative match between the OA curves behavior and the WFL stations’ result is observed in most cases.
As indicated by the physics of the measurement, the pulsed neutron tool can only be configured to detect one flow direction per descent; reconfiguration for inverted positioning of transmitter and detectors is required for sensitivity to flow in the opposite direction. However, in a recent example, the intelligent combination of the pulsed neutron and gamma ray sensors made it possible to detect flow in both up and down directions in one single pass.
The examples in this paper demonstrate the usage of the OA continuous logs and WFL stations where pulsed neutron logging (PNL) have been run in a few wells in Malaysia. These logs have been recorded in wells with variation/complex completions ranging from single completion with gravel pack assembly to openhole completions with screens. As demonstrated in these case studies, real-time evaluation of the pulsed neutron data and logging program optimization and updates are instrumental to enable a comprehensive interpretation, from petrophysical analysis to water management, facilitating fast, informed decision making.
The novel OA curves from advanced PNL technology, complemented by other pulsed neutron borehole indicators and temperature profile, have been able to help indicate potential water movement in the borehole affecting the reservoir performance. This information successfully assisted the interpretation of remaining hydrocarbon potential and current fluid dynamics at the wellbore.