Nowadays many exploration and production companies have high interest in producing deep, high pressure gas reservoirs that contain sour gas. Carbonates in East Java, Indonesia faced challenges related to workover and completion operations that are prone to kick-loss cycles based on the drilling experiences.The presence of hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ) in the formation made the completion operation dangerous and difficult to be carried out in the field where is adjacent to a populated area, thereby increasing the requirement and need to prevent the release of sour gases during completion operations.

The PEP structure in Matindok block in Central Sulawesi has been proven to produce gas reserves in the Minahaki formation based on the first exploration well PEP-001 that was drilled in 2018. The PEP structure has a Miocene carbonate build-up play, and the target reservoir is the M member pinnacle carbonate reef. One of the main challenges in this area is low resolution of seismic data, leading to a high depth uncertainty of top M position. The PEP-001 well was planned to set the 9 ⅝-in casing point above top M. Offset wells did not show any clear markers in the thick shale above M formation, that could have been used for log correlation. In previously drilled offset wells correlation was done conventionally by taking cutting samples and relying on drilling parameters break. However, when PEP-001 was drilled, no apparent drilling break was observed. By the time cuttings reached surface, the bit had drilled into 20-m thickness of M formation. Since casing covered most part of the upper carbonate formation, open hole logging and well testing data were not acquired to delineate target M formation optimally. The second exploration well PEP-002 was planned with an objective to set 9 ⅝-in casing ~5 m above top M to acquire a full interval coverage of coring, open hole wireline logging, and well testing program. This information was critical for optimal reservoir delineation to allow for accurate reserve calculation and future structure development. Conventional correlation (well-seismic pairing and correlation) has proven insufficient for casing point placement and was a lesson learned from PEP-001. The presence of limestone stringers observed in offset wells within proximity of top M presented an additional challenge. The stringers could have been mis-interpreted as the main carbonate body, if interpretation was solely based on cutting samples. Based on these challenges, a technology with the capability to map and detect lithology changes ahead of the bit in real time was required. Real-Time EM Look Ahead technology uses deep directional electromagnetic (EM) technique to detect formation feature ahead of the bit. A feasibility study was done to simulate the tool response and define the placement of transmitter and receivers (spacings) in the BHA, as well as frequency selection based on the resistivity properties from offset wells. Based on the simulation, this technology was expected to detect top M formation as early as 10 m ahead of the bit. While drilling, top M could be resolved at 6 m ahead of the bit with an uncertainty of <1 m, therefore making this the fit-for-purpose technology to place the 9 ⅝-in casing point ~5 m above the M formation. As a result, 9 ⅝-in casing was successfully placed 5 m above top M. High resistivity contrast, that is expected to be top M, was mapped continuously from 10 m ahead of the bit. The decision was to set the casing point at 1706 m MD, 4-5 m from the estimated top M. Coring results and open hole logging in the subsequent 8 ½-in hole section confirmed that top M was at 1711 m MD, 5 m from the casing point, which was precisely as estimated from Real-Time EM Look Ahead Technology. This technology also helped detect thin limestone stringer and differentiate it from the target carbonate formation. It prevented 9 ⅝-in casing to be set ~118 m shallower where the first limestone stringer was observed. This has avoided extra operating days in case of drilling problems due to long exposed shale in the subsequent 8 ½-in hole section. The potential cost savings were estimated at USD 1.35 million for an additional 6-in hole section. Following this success, the same technology will be used in the next planned exploration wells.

Exploration in the basement could uncover the potential oil and gas resources in the South Sumatera area. However, drilling in the basement always brings different challenges compared to shale or sandstone formation. These challenges include low penetration rate due to hard lithology, deteriorating drilling bit and tools due to abrasive environment and high vibration, and unpredicted loss circulation zone that can increase mud consumption. In long interval sections, multiple bit runs are inevitable, causing higher open hole exposure time that can increase the risk of well integrity and well control. All challenges will have a significant effect on increasing drilling cost. Pertamina EP planned to run 4 - 6 bits based on the last two wells drilled through basement formation. Comprehensive steps were taken to minimize the drilling risks and optimize the drilling operation In terms of drilling assembly, including bit and Bottom Hole Assembly (BHA), enhanced design was selected based on the lesson learned from previous campaigns. The bit equipped with durability proven conical diamond element with other features to compromise basement formation. BHA was designed by considering motor selection, stabilizer placement, and collar combination. To optimize drill string stability, several BHA options, combined with a detailed bit model, were analyzed in advanced Finite Element Analysis (FEA) drilling dynamic simulation. The process generated the most stable bit-BHA configuration, ROP-bit durability prediction and drilling parameter roadmap. Extensive drilling strategy planning was taken in place to set the interval, ROP target, and parameter management. To have a step change from the previous basement drilling campaign, Pertamina EP set the target to drill 880-m basement intervals in 8 ½-in sections by maximum two bit runs with minimum on-bottom ROP of 5 m/h. By implementing all engineering preparation, basement formation in PLD-001 exploration well was successfully drilled in one run to section TD from 2,690 m to 3,573 m. The engineered bit-BHA combination drilled metamorphic basement rock with 10 – 15 klbs of WOB, 40 – 60 SRPM, and 550 – 595 gpm of flow rate. Close supervision was performed to monitor drilling dynamics, with no significant shock and vibration was observed. In overall, the bit generated 6.2 m/hr of on bottom ROP, 24% higher than the initial target. After drilling, the bit was pulled out of hole with considerably good condition despite heavy drilling in the 880 m of basement.