Application of Pulsed-Neutron Spectroscopy Logging as Crucial Data Input To Modify Reservoir Characterization Understanding: Case Study in Carbonate Reservoir, Indonesia
Authors: Kusuma D.P. 
Published: March 2023

Bukit Tua field is located in the north of Madura Island & its main reservoir are Kujung 1 & Kujung-2 which are consist of isolated patch reef with carbonate stringer. Pulsed-Neutron Spectroscopy Logging (PNL) was planned since this field enter next development stage, the formation oil/gas saturation & fluid contacts movement is crucial for development planning and behind casing opportunities. PNL can accomplish the objective since its building on the three types of nuclear reactions between neutrons and formation, four kinds of pulsed neutron logging modes are commonly utilized: sigma mode which based on the thermal neutron capture & primarily measure the time decay of the capturing process; C/O which based on the fast neutron inelastic scattering reactions; Ratio-Based Gas Saturation and 3-Phase Saturations. Sigma processing result can be specifically calculated from Sigma equation, but the C/O and Ratio-Based Gas Saturation and 3-Phase Saturation data processing need to apply the Monte Carlo models to eliminate the impact of the downhole environment to the log data. This paper introduces a novel pulsed-neutron log design that was successfully practiced in Bukit Tua, where it took the advantages from Sigma log modes to efficiently overcome the challenges of defining or to monitor hydrocarbon sweep and Gas-water contact movement. PNL was run in the Kujung-1 interval with activated sigma mode. This interval is known as a thick build-up platform carbonate with 95 feet TVT of gas column. Previous understanding was that the Kujung-1 reservoir was connected and there was no fault or facies compartmentation. There are 16 wells that penetrate this interval, consisting of 6 exploration wells, 6 non-producing wells and 4 producing wells. X1 well has produced since 2017, followed by X6 well in 2018. X11 well was drilled in 2020 and showed the GWC higher by 15 feet from the initial GWC. But surprisingly, two other wells (X8 & W2) drilled in 2020 and 2021 and penetrating the Kujung-1 interval showed original gas water contact but with pressure data below original pressure. To answer this anomaly, the surveillance PCINO team proposed to run PNL in two wells that are located in the X11, X6 & X1 area. X4 and X1 well were selected to represent a producing and non-producing well. PNL was run successfully in X4 and X1 wells, with three pass data that showed good repeatability. X4 is an oil well, and SGS data acquisition was run to confirm fluid annulus that will affect PNL data interpretation. X4 results show that the current GWC has risen by 17 feet, close to data from X11 which showed GWC higher by 15 feet. Meanwhile, X1 as producing well shows that the current GWC has risen by 23 feet, which is higher than the other data and possibly affected by water coning. Based on this PNL reservoir data, the previous understanding that Kujung-1 intervals are defined as connecting reservoir should be revised and thus will have implications for OGIP and reserve calculation.