OIL QUALITY PREDICTION IN THE WEST SAK VISCOUS OIL FIELD, ALASKAN NORTH SLOPE – INTEGRATION WITH DRILLING AND DEVELOPMENT PLANNING

The West Sak super giant viscous oil resource in the Kuparuk River Unit (KRU) comprises 7-9 billion barrels OOIP of 12° to 22° API gravity crude. The initial development of West Sak in the KRU commenced in 1997 utilizing vertical wells with frac and pack completions. Typical well rates were 150-250 BOPD. Current well design has evolved to extended reach dual and tri-laterals with individual horizontal slotted liner completions in excess of 6000 feet and first year average rates over 1500 BOPD.

Biodegradation is the primary control on West Sak regional oil quality trends in the KRU. Since the West Sak occurs at depths between 2500 and 4000 feet subsea, reservoir temperatures are relatively cool due to proximity to the base of permafrost around 1500 feet subsea. Oil samples acquired in the 1980's showed that API gravities regionally improve with depth to the east. This, in combination with warmer reservoir temperature and more favorable oil viscosities, was crucial in selecting the eastern KRU for the initial waterflood development. More detailed oil sampling from development wells at drillsites 1D and 1C showed that oil quality varied inversely to the regional trend within the development area. This is believed due to the local influences of greater biodegradation near the eastern oil-water contact, secondary light-end charge, and compositional segregation. Better oil gravity up dip locally helps to moderate the viscosity increase caused by cooler reservoir temperature nearer the permafrost in drillsite 1E, and has allowed development planning in the 1J area using fewer wells at greater spacing. This model is now being applied to predict oil quality and evaluate height above oil-water contacts in adjoining undeveloped reservoir blocks. Application of this relationship is also helping to focus delineation and development planning in shallower, colder parts of the West Sak accumulation.