Significance of Time-Lapsed Hydrocarbon Saturations From Pulsed-Neutron-Capture (PNC) Logs and the Impact of Borehole-Environment Parameters on PNC-Derived Saturations

Abstract

Summary Reservoir surveillance with pulsed-neutron-capture (PNC) logs has shown its importance historically in providing reservoir hydrocarbon saturations behind the casing. However, there is little in the literature to show the importance of PNC logs for contact tracking and accessing bypassed/moved hydrocarbons. In addition, no one has ever attempted to characterize the various borehole environments and subsequently understanding their impact on the PNC-derived hydrocarbon saturations. This paper is divided into two parts. The first part shows the importance of time-lapse PNC logs in understanding reservoir-fluid movement and chasing migrated hydrocarbons behind the casing. A case study from the UK Central North Sea is presented to highlight the importance of the PNC sigma data. The second part of the paper shows the computation of borehole-environment parameters from simple to complex completions including multiple casing/tubing strings, varying hole sizes, and annuli fill. For the first time, the work presented in this paper shows the importance of having correct environmental parameters and the impact of using incorrect parameters on the PNC-derived hydrocarbon saturations. Not making such detailed checks may result in misinterpretation of the PNC data, leading to incorrect conclusions about fluid dynamics, and risky decisions in adding perforations and water-shut off (WSO) opportunities.