The CD Carbonate of Ngimbang Formation in East Java basin was deposited in early Oligocene and divided into 5 reservoir targets for development. These reservoir targets were subdivided into different flow units, predominantly due to secondary porosity resulting from karst processes. Identifying these karst zones therefore became important for formation evaluation. Classic high resolution borehole images, combined with sonic, density and neutron logs, and NMR are the most commonly used tools to identify secondary porosity. However, due to the horizontal well design, conventional wireline logging was no longer possible thus Logging While Drilling (LWD) has become the primary data acquisition method. The LWD tool selection criteria have to meet both the geosteering and formation evaluation objectives by considering all drilling challenges. Even though all these measurements are currently available with LWD technology, the complexity of the long horizontal well trajectories and associated drilling risks have been assessed as being too high to run all the LWD services. Therefore, a different technique is required to identify karst zones and to determine saturation along the deeply-invaded karst intervals. A statistic-based correlation technique using principal component variance between advanced measurements from the multifunction LWD tool such as sigma, neutron capture spectroscopy, and standard measurements such as gamma ray, neutron porosity, density, caliper, and resistivity invasion profile is used to show an indication of karst- or matrix-dominated porosity. The spectroscopy provides accurate lithology volumes and information on grain properties. A simultaneous sigma–resistivity inversion technique is used to determine Archie's parameters in zones where the water salininty is known. The applicability and limitations of the technique are discussed for this particular reservoir environment. The combination of all these techniques along with information from state-of-the-art reservoir mapping technology has provided more complete characterization of the reservoir along the horizontal section.
Carbonate environments are complex by nature and the characterization, based on their petrophysical properties, has always been challenging due to the pore heterogeneity. In this paper, we present the integration of factor analysis applied to while-drilling Nuclear Magnetic Resonance (NMR) data, full-suite data from a multifunction logging-while-drilling (LWD) tool, and modeling of the NMR T2 transverse relaxation time to improve the fluid typing interpretation in complex carbonate reservoirs. The interpretation results are essential for perforation and completion decisions in a high-angle development well. The carbonate reservoirs in this case study are within the Kujung formation in the East Java Basin. Kujung I is a massive carbonate reservoir with abundant secondary porosity, while Kujung II and III consist of interbedded thin carbonate reservoirs and shale layers. High uncertainty in identifying the fluid type existed in the Kujung II and III formations due to the presence of multiple fluids in the reservoir, the effect of low water salinity, as well as pore heterogeneity and diagenesis. Due to the high-angle well profile, LWD tool conveyance became the primary method for data acquisition. NMR while drilling and multifunction LWD tools were run on the same drilling bottomhole assembly (BHA) to provide complete formation evaluation and fluid identification. The NMR factor analysis technique was used to decompose the T2 distribution into its porofluid constituents. Thorough T2 peaks modeling was performed to interpret the fluid signatures from the factor analysis results. Borehole images, caliper, triple-combo, density-magnetic resonance gas corrected porosity (DMRP), as well as time-lapse data were evaluated to identify the presence of secondary porosity and narrow down the T2 fluid signatures interpretation. Each of the porofluid signatures were identified and validated in the Kujung I formation with its proven gas and thick water zone. These signatures were then used as references to interpret the fluid types in the Kujung II and III formations. Gas was identified by a low-amplitude peak in the shorter T2 range between 400 ms to 1 s. Oil or synthetic oil-based mud (SOBM) filtrate was indicated by a high-amplitude peak in the longer T2 range (>1.5 s). The water signatures are very much dependent on the underlying pore sizes. Larger pore sizes will generate longer T2 values, which could fall into the same T2 range as hydrocarbon. For that reason, it is important to combine the NMR porofluid signatures interpretation with other LWD data to restrict the fluid type possibilities. This integrated methodology has successfully improved the fluid type interpretation in the Kujung II and III thin carbonate reservoir targets and was confirmed by the actual production results from the same well. This case study presents excellent integration of LWD NMR with other LWD data to reduce fluid type uncertainties in complex carbonate reservoirs, which were unresolved by conventional interpretation methods. Based on this success, a similar integrated NMR factor analysis method can be applied to future development wells in the same 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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