Control Instrumentation for Wellheads and Mud-Kill Systems

Summary

This paper describes the instrumentation and control systems used on the wellheads and mud-kill systems at the Mobil Oil Arun natural gas field, situated onshore in the province of Aceh, North Sumatra, Indonesia (Fig. 1). The reservoir is a carbonate reef containing an estimated 15 Tcf (0.42 x 1012 m3) gas at approximately 7,000 psig (48 263 kPa) and 360 deg. F (182 deg. C). The wellstream from the field is separated into natural gas, condensate liquid, and water. The gas and condensate are then shipped through separate pipelines to the Arun liquefied natural gas (LNG) facility at Lho' Seumawe, about 40 mi (25 km) away on the northeastern coast of Sumatra. After liquefaction the LNG is shipped by tanker to Japan. The field was discovered in 1971 and became operational in 1977. The LNG delivery contract complied with the Japanese calls for regular delivery; thus continuity of supply to the LNG plant was of paramount importance for meeting transportation and supply paramount importance for meeting transportation and supply schedules. The overall design of the Arun field facility had to be of the highest possible standard to maintain a reliable but safe production facility in relatively difficult operating conditions. The difficult wellhead conditions demand that the control systems be designed for the ultimate in reliability to maintain continuous production safely. Should a well develop a serious problem, the well mud-kill system must be immediately operable and reliable in case a kill operation is required. The consequence of not being able to kill a well because of failure of the kill system could be disastrous. Two actual blowouts in the Arun field have provided valuable experience in evaluating both equipment and systems in terms of design, reliability, and application for this type of field service. High-pressure protection of the flowlines, and the gas-gathering system in particular, required a highly reliable and safe system, yet one that would not inadvertently trip or be difficult to maintain. This paper concentrates on the design and installation of the control systems associated with the wellhead and mud-kill systems and highlights the problems encountered during the past five years.

Description of Field Facilities

The Arun natural gas field production facility is designed around a cluster concept (Fig. 2). There are four clusters along the length of the field plus a common facilities area called Point A. Clusters 1, 2, and 3 are fully operational and the fourth is under construction. Each cluster is an entirely independent, self-contained production area except for electrical power, which is generated at Point A. Point A is also the location of the administration offices, Point A is also the location of the administration offices, maintenance workshops, warehouse, and the tie-in to the LNG plant gas and condensate pipelines. The clusters are each designed for up to 16 production wells, although only one-half of these are required during the present initial stage of development because of the high reservoir pressure. There are two trains per cluster and each has independent facilities for pressure reduction, cooling, liquid separation, and condensate pumping (Fig. 3). pumping (Fig. 3). JPT

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