The CD Carbonate in X-Field, East Java Basin has multiple reservoir targets with varying flow units, predominantly because of secondary porosity development from karst processes. Each of these reservoirs are relatively thin and the continuity of the karst within the reservoir is random. The amount of producible hydrocarbon will rest on the length of the drain section along the reservoir and the amount of karst intersected by the trajectory. Thus, maintaining the trajectory inside the carbonate reservoir while targeting the karst position is important for productivity. To maintain the production in the field, a new platform was built to the southeast of the first platform, aiming for a new development area. Five horizontal wells were planned as part of the second development phase. The first horizontal well is the most challenging one because of the high uncertainty of the structural dips along the lateral length triggered by the low seismic resolution and the limited nearby offset wells for control points. Moreover, the offset wells show inconsistent log properties that complicate the correlation to land into the targeted karst level within the reservoir. Initially, a pilot hole was planned to reduce the landing uncertainties; however, it was being challenged for cost efficiency. Therefore, a well placement strategy was proposed instead, including landing and geosteering using a new logging-while-drilling (LWD) combination of advanced high-definition reservoir mapping technology, high-resolution laterolog borehole imaging technology, and a multifunction LWD tool. In this paper we demonstrate the complete preparation of the well placement project, strategy, and evaluation using this new LWD combination for better interpretation of the reservoir. The deeper reading and higher resolution of the new reservoir mapping technology have permitted continuous mapping of the target reservoir, which typically has 35- to 50-ft thickness, to reduce the structural uncertainties from seismic. For the first time, it successfully revealed the karst network within the reservoir with greater details, identified by a blue-vein color spectrum of the resistivity inversion caused by seawater invasion or clay-filled karst. This high-definition karst mapping has helped to land the well precisely at the target karst sweet spot, improving the understanding of the karst characters along the trajectory, and providing higher confidence in the real-time geosteering decision. The high-resolution borehole image revealed the carbonate rock texture and karst/vugs appearance on a smaller scale, which was used to analyze the secondary porosity distribution and contribution along the trajectory using a quantitative image-based porosity analysis method. By integrating the high-definition reservoir mapping inversion interpretation and porosity analysis from a high-resolution borehole image, we were able to bridge the gap from seismic to reservoir scale, and finally to link the karst scale down to the vug pore sizes, for a better geological understanding and an improved geosteering strategy in the field.
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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