ABC-1 exploration well was drilled through a carbonate build-up structure of Ngimbang Formation in offshore East Java, Indonesia. Standard triple-combo open hole logs were acquired by means of logging while drilling, while more advance wireline loggings were planned subsequently. Unfortunately, there were total losses during drilling which had to be managed by pressurized mud cap drilling (PMCD) which prevent from cuttings recovery for the rest of the interval. Multiple trips were also required to drill the well safely resulting in rugose and enlarged borehole. These conditions did not allow open-hole wireline log to access the target located at the lower interval of the well.

It was inevitable to complete the well despite not enough data had been acquired to perform a comprehensive formation evaluation. In order to obtain remaining required data, it was decided to complement the compromised open-hole data with an advance pulsed neutron log (PNL) device, which offered several unique measurements to tackle the harsh conditions. Some of these key measurements are: (1) a self-compensation algorithm which provided robust sigma (SIGM) and cased-hole porosity measurement (TPHI), which was used to further validate neutron from LWD. (2) A combination of both capture and inelastic high definition elemental spectra measurement were utilized to obtain accurate mineralogy fraction. (3) carbon-oxygen ratio (COR) high precision measurement to calculate oil saturation. Lastly (4), fast neutron capture cross-section (FNXS) measurement was also acquired to give insight on possible gas occurrence even in tight zones.

The advance PNL, acquired over 3 passes, showed consistent reading of sigma, TPHI, FNXS and elemental spectroscopy measurement. However, there were some discrepancies in between COR passes, which eventually has shed some light on what happened in this well. The first pass did not really show any potential oil along the carbonate body. Then, the second pass started to reveal potential oil around the top part of the carbonate, where resistivity is low with no distinctive neutron-density crossover. The third pass revealed an even more oil volume along the top carbonate. There is a possibility that the increase of oil reading might be due to the changing environment during logging, allowing some invasion to dissipate along the carbonate tops. This implies that there might be yet another oil zone below the revealed oil interval, should the invasion fluid start to dissipate.

Subsequent well test showed significant oil production over the interval identified from the PNL interpretation, which put ABC-1 as one of the most successful Indonesian exploration well in 2021. This case study shows the success of utilizing advance pulsed neutron log to perform comprehensive formation evaluation under challenging condition, which can be used as reference for tackling similar drilling challenges in the future.

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