This paper discusses the planning, results, problems, and lessons learned during the first use of the Extended-Range Electromagnetic (EM) MWD system for underbalanced drilling in an offshore well. The use of EM MWD in offshore applications has historically been limited due primarily to the difficulty of seabed deployment of the receiving antenna. In underbalanced drilling, however, EM is the MWD system of choice because mud-pulse telemetry has great difficulty operating in two-phase flow conditions commonly used. If underbalanced drilling is to become more frequently used offshore, then one limitation that must be overcome is the difficulty for MWD systems to provide real-time annular pressure, directional, and formation evaluation information. Repsol-YPF-Maxus successfully used underbalanced drilling on Krisna Well D12 in offshore Indonesia to prevent massive lost circulation and stuck pipe in the reservoir. Although the total vertical depth of the well was relatively shallow (approximately 4,200 ft subsea), the very low resistivity of the formations above the reservoir precluded use of standard EM MWD services. The Extended-Range EM MWD system was used to extend the depth capability. Background SPE/IADC 67689 described the overall planning and drilling of Well D12 in Repsol-YPF-Maxus' Krisna Field in offshore Indonesia, using underbalanced drilling. 1 The primary goal of underbalanced drilling on Well D12 was to eliminate the significant drilling problems encountered in previous wells using conventional overbalanced drilling methods. Low pore pressure (2. 98 ppg) in the reservoir, the Upper Rata Baja, caused costly and time-consuming drilling problems in these earlier wells. The most significant drilling problems were massive lost circulation (in excess of 50,000 bbls of fluid in one well) and stuck pipe. Reducing formation damage and identifying fractures while drilling were secondary goals of the project. Both secondary goals relied on the ability to maintain an underbalanced condition. The Reading and Bates Falcon jackup rig Roger W. Mowell was positioned in 77 feet of water over Well D12. Seven-in. casing was set at 5,029 ft measured depth (MD) at an 80 deg. angle of hole inclination. After drilling out the casing shoe, 1,159 feet of formation were drilled underbalanced using 6-1/8 in. bits and 4-3/4 in. EM MWD tools with hole inclination ranging from approximately 73 to 83 degrees. After experiencing some minor startup problems using the underbalanced drilling system, all the operational and technical problems were solved. One major change was swapping fluid systems from a KCl mud gasified with nitrogen to a diesel mist. By the end of the well, the underbalanced drilling system was working well. All objectives of underbalanced drilling were achieved and the well was drilled without any major drilling problems. Planning of Krisna Well D12 This paper describes the use of the Extended-Range EM MWD system on Well D12. The main reason for using EM MWD was to provide real-time data for annular pressure and directional information.

Due to the previous history of high mud losses and stuck pipe, Repsol - YPF - MAXUS Southeast Sumatra B. V. decided to implement underbalanced drilling technology in the development drilling campaign in the Krisna Field offshore Indonesia. Although Underbalanced drilling is not a new technology, it has been used infrequently in drilling from an offshore structure. Underbalanced drilling from a jack-up rig causes some unique planning and implementation issues. Planning and implementation are made more complex by the space limitations and logistics. These were compounded by the lack of infrastructure in Indonesia for underbalanced applications. The Krisna D12 production interval was successfully drilled underbalanced from 10 June 2000 through 20 June 2000 from the Reading and Bates Falcon jack-up Roger W. Mowell. A total of 1159 feet of formation was drilled Underbalanced without any significant drilling related problems. Although the project encountered problems during the initial start up, modifications to the system and changing the fluid system to a diesel mist eliminated the majority of the issues. By the end of the well, the Underbalanced system was working well. This paper presents the pre-engineering, project management and field implementation results for the drilling of the Krisna D12. Introduction The Krisna field is located 60 miles offshore Java Indonesia, a part of the South East Sumatra (SES) production sharing contract, and is operated by Repsol-YPF-MAXUS Southeast Sumatra B. V. The main production comes from the Upper Bata Raja formation, which has a BHP of 650 psi (2.98 ppge). Massive lost circulation (losses in excess of 50,000 bbls of fluid) occurred while attempting to drill the first horizontal well in the field. The primary goal for drilling the production interval Underbalanced was to eliminate the loss of fluids to the formation. Secondary goals of the project were to demonstrate the ability to maintain an Underbalanced condition, reduce formation damage and increase productivity of the well, and evaluate the formation and identify fractures real time while drilling the formation. Planning and Engineering Underbalanced drilling is a multi-disciplined operation that involves drilling engineers, production systems engineers, drilling operations personnel, geologist and reservoir engineers.The rig contractor and the service sector heavily supported the Krisna UBD project. The project integrated the different disciplines into one team to come up with an engineered solution specific to the Krisna wells. Rig interface coordination was a critical step. The drilling operation was to be tied in to the Krisna D production platform; therefore it was deemed critical that proper interfaces were set up between the production and the drilling departments.Shutting down the production platform for any reason would have significant economic consequences. In view of this arrangement, on-site engineering and interdepartmental coordination was vital.

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