De-Risking Shutoff Opportunities in Complex Dynamic Multiphase Environment with Advanced Pulsed Neutron Measurements: From Petrophysics to Production Logging Applications
Authors: Cavalleri C. , Rahman R. Mohd Abd , Khong C. Kin , Ng H. 
Published: February 2024

Abstract
Accurate characterization of current hydrocarbon volumes and distribution is essential for production optimization. The use of advanced logging technology de-risked decision making related to zonal isolation and justification for a sidetrack in a complex scenario in a long horizontal producing drain completed with uncemented pre-perforated liner from a sandstone reservoir.
In a shallow water offshore well, a combination of advanced pulsed neutron (PNL) and multispinner and holdup sensors in production logging tools (PLT) were recorded for borehole diagnostics and evaluation of fluids behavior for water contribution and zonal distribution of fluids production along a horizontal section. The integrated evaluation of neutron porosity, sigma, fast-neutron-cross-section, elemental concentrations, carbon/oxygen ratios, and total organic carbon (TOC) from PNL quantified the hydrocarbon volumes. Moreover, PNL provided critical input to multiphase production profile otherwise compromised due to unforeseen downhole challenges. PNL three-phase holdup measurements were able to detect hydrocarbon and water in the borehole for fluid stratification. This was complemented by the different phases detected using the electrical and optical probes of the multispinner array PLT tool. Simultaneously, stationary water flow logs (WFL) assisted by novel continuous oxygen activation curves measured upward water movement otherwise overlooked.
The presence of unstable slug flow during flowing condition and fluid redistribution during shut-in required the use of new fit-for-purpose processing and interpretation techniques. A tailored approach was developed for oil holdup input to carbon/oxygen analysis and TOC correction. This was validated by novel application of inelastic gas ratio. The continuous change in fluid contact level in the borehole was used to strengthen the analysis of fluid behavior.
The comparison between fluid contact levels detected at different times highlighted the time required for the well to stabilize, adding to the global understanding of fluid behavior around the wellbore. Moreover, the PNL WFL survey, assisted by prompt interpretation of continuous oxygen activation at different spacings, effectively measured slow fluid redistribution velocity during shut-in. Despite the complication related to solids clogging the bottommost spinner, the PLT was able to detect the velocity profile inside the slotted liner; the water velocity below the liner was measured using WFL. The log responses and data analysis assisted the decision-making process. Based on the interpretation results and contextualization, integrated with understanding of the reservoir, it was decided to sidetrack the existing well to an up-dip location to maximize the oil recovery, rather than investing in water shutoff operations.
High-quality data acquisition coupled with novel evaluation approaches enabled solving hydrocarbon profile and zonal contribution in an environment with variable holdup over time. The interpretation methods were developed to compensate for wellbore dynamics and the lessons learned in this complex scenario can be applied to other similar cases where comprehensive evaluation is required.