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BP says additional potential exists at Northstar, at northwest end Company tells Alaska Oil and Gas Conservation Commission it will minimize flaring associated with startup; if there’s a market, gas used for enhanced oil recovery will be sold at end of field life Kristen Nelson PNA Editor-in-Chief
Both additional oil potential and end-of-field-life gas sales are possibilities at BP Exploration (Alaska) Inc.’s Northstar oil field, scheduled to come online this fall.
BP told the Alaska Oil and Gas Conservation Commission at an Aug. 16 hearing that original oil in place at Northstar is estimated at 247 million barrels, with a 7 billion cubic foot “inferred” gas cap and 480 BCF total gas including solution gas.
The company expects to recover some 176 million barrels of oil and natural gas liquids with enhanced oil recovery — miscible gas injection from five gas injection wells in the central portion of the field and 21 producing wells.
BP said that additional reserve options exist at Northstar, including 7-8 million barrels of oil in the northwest portion of the reservoir beyond the reach of current well technology, which the company believes is limited to 17,500 feet from the production island. However, with experience gained in the initial drilling program and advances in drilling technology, BP said it may be able to tap the northwest portion of the field at the end of the current drilling program.
The company also said that satellite oil accumulations may exist within expected drilling reach from the island, and those targets will be appraised.
Natural gas from both Northstar and Prudhoe Bay will be used as fuel at Northstar and for enhanced oil recovery miscible injection. Prudhoe gas supplements gas available from Northstar, and will be used to commission the gas injection plant and a gas injection well prior to startup of oil production at Northstar, reducing the amount of flared gas traditionally associated with the start up of new production facilities.
At the end of oil production, gas which has been injected to help produce oil could be produced for sale, if a market exists. State and federal leases The Northstar oil pool is an accumulation of hydrocarbons in the Ivishak, Shublik and Sag River formations approximately six miles offshore the Prudhoe Bay unit in the Beaufort Sea in both state and federal waters. Capacity of the stand-alone facilities includes 65,000 barrels per day of oil, 600 million standard cubic feet of gas injection and 30,000 bpd of produced water handling. The island includes slots for 37 wells, and initial development includes 16 production wells, five gas injection wells and one Class I waste disposal wells. BP told the commission it has drilled the disposal well, one gas injection well and two pre-produced gas injection wells. Development drilling will resume following facility startup this fall and will continue into 2003.
Enhanced oil recovery plans involve the initial injection of a large slug of miscible enriched natural gas into the oil column of the Ivishak formation, the main producing formation. Miscible gas injection will last approximately four years, BP said, and will be followed by injection of leaner chase gas through to the end of field life. Late in field life, approximately 16 years after field startup, reservoir pressure will be reduced to maximize recovery of injected gas. BP said gas recovery volumes have not yet been estimated. Reservoir primarily Ivishak BP said that the Shublik formation at Northstar is considered primarily source rock, and not in general a reservoir rock, with most of the Shublik at Northstar tight and non-reservoir quality. The maximum oil column in the Ivishak is estimated to be between 270 and 300 feet thick.
BP evaluated miscible gas injection, water flood, gas cycling and primary depletion as Northstar development methods. Miscible gas injection, the EOR method selected, is estimated to recover 64.5 percent of the original oil in place, compared to 52 percent recovery with water flood, 50 percent with gas cycling and 36.1 percent with primary depletion. BP said miscible gas injection will be implemented concurrent with field startup for maximum benefit.
BP said it also evaluated water alternating gas injection and found no additional recovery would be achieved. However, the company said, if the reservoir turns out to be highly stratified, water alternating gas could be beneficial in high permeability intervals.
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