Terang/Sirasun Field Development: Geohazards Engineering

Abstract

The Terang, Batur, East and West Sirasun gas reservoirs are located 120 km north of Bali on the Kangean shelf of the North Bali Basin, in water depths ranging from 100 to 300 metres. The reservoirs collectively contain around 1 TCF of recoverable gas. It is proposed to jointly develop the reservoirs by means of a subsea development and associated floating production unit (FPU), with gas being exported to the nearby East Java Gas Pipeline for onward transmission to Java. The North Bali basin lies above a major subduction zone, and is consequently an area of extensive seismic and volcanic activity. The proposed Terang/Sirasun development location is subject to numerous geohazards, including fault rupture, shallow gas, slope instability, liquefaction, and strong earthquake shaking. These geohazards pose risks to safety, the environment and the field facilities, both during construction and during production operations. Consequently, a key element of the early engineering definition phase of the project, prior to undertaking FEED, has been the analysis and quantification of the geohazard risks. A geohazards engineering programme was undertaken during 2000–2001. An integrated multidisciplinary team was assembled, covering the geophysics, geology, drilling, completions, subsea and facilities disciplines. An extensive data acquisition programme was undertaken, comprising some 5,600km of 2D seismic lines, swathe bathymetry, sidescan sonar, sub-bottom profiling and geotechnical sampling. The team used state-of-the-art visualisation technology to map the hazards and plan wellbore trajectories and facility locations to minimise risk. Limit state analyses of the wells and seabed facilities were performed to determine capacity to respond to seismically triggered events. Finally, extensive probabilisitc analysis utilising decision (or logic) trees was undertaken to quantify the safety and economic consequences of geohazard events in terms of environmental impact, asset damage and production interruption. This paper describes the geohazards engineering programme and presents lessons learned. The programme received a commendation in BP's internal Helios Awards scheme during 2001. Development Description Location. The Terang, Batur, East and West Sirasun gas reservoirs are located 120km north of Bali on the Kangean shelf of the North Bali Basin, in water depths ranging from 100 to 300 metres. The area is a relatively benign environment in metocean terms, with a 100-year return significant wave height of 2.5 metres. The climate is monsoonal. Reservoirs. The Terang, East Sirasun, West Sirasun and Batur reservoirs collectively contain around 1TCF of recoverable gas. The reservoirs are extremely shallow (less than 600 metres subsea), and cover a large area (27 km by 5 km). The reservoir fluids are over 98% methane, with no recorded H2S and around 0.1% CO2. Produced water rates vary from 0.3 to 1,000 bbls/mmscf. Reservoir temperatures are moderate, less than 60°C, and reservoir pressures are low (1,100 psi). The reservoir rocks are carbonate sands composed of foraminifera (globigerinids) which pose a potential compaction hazard during production. Location. The Terang, Batur, East and West Sirasun gas reservoirs are located 120km north of Bali on the Kangean shelf of the North Bali Basin, in water depths ranging from 100 to 300 metres. The area is a relatively benign environment in metocean terms, with a 100-year return significant wave height of 2.5 metres. The climate is monsoonal. Reservoirs. The Terang, East Sirasun, West Sirasun and Batur reservoirs collectively contain around 1TCF of recoverable gas. The reservoirs are extremely shallow (less than 600 metres subsea), and cover a large area (27 km by 5 km). The reservoir fluids are over 98% methane, with no recorded H2S and around 0.1% CO2. Produced water rates vary from 0.3 to 1,000 bbls/mmscf. Reservoir temperatures are moderate, less than 60°C, and reservoir pressures are low (1,100 psi). The reservoir rocks are carbonate sands composed of foraminifera (globigerinids) which pose a potential compaction hazard during production.