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Abstract: Multidiscipline Studies of the Depletion Behavior of the F23 Gas Field, Offshore Sarawak, East Malaysia

H. B. Heijna, S. Leong Mooi Sin, S. Tiang Ting Ing, A. Van Vliet, K. Wong, Wan Manan W. Hassan

The F23 gas field is located 178 km north-northwest of Bintulu in the central Luconia province, offshore Sarawak. The accumulation consists of a late Miocene layered platform-type carbonate buildup of 22 km2 and a maximum gas thickness of over 1000 ft. Eleven development wells and one observation well were completed in 1983, with initial gas delivery to Malaysia LNG in October 1983.

Annual TDT surveys are conducted in the observation well to monitor the movement of the GWC in the central area of the field. As of April 1992, a cumulative rise of 25 ft was observed. This contrasts with material balance calculations predicted rise of some 60 ft based on Sgr of 50% inferred from core measurements. Among the potential explanations were irregular bottom-water encroachment, preferential flank water advance, or larger GIIP. As all had potentially important consequences to depletion strategy and ultimate reserves, multidiscipline studies were initiated. Time-lapse seismic data were used to better determine the existing fluid levels across the field and reservoir and simulation studies were used to match observation well data and predict future reservoir behavior.

Concurrent reservoir and simulation studies suggested that the material balance and observation well data could be reconciled by (1) a reduced aquifer, and/or increased GIIP, and/or a lower residual gas saturation, or (2) preferential flank water influx due to internal low-permeability layers. As (1) would not lead to early water breakthrough, the simulation effort was aimed to investigate (2), the possibility and implications of early edge water breakthrough via the flank zone.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994