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BP applies for pool rules for viscous Schrader Bluff Polaris accumulation Company tells AOGCC western Prudhoe satellite will be developed with water flood, EOR test deferred; initial wells proving up productivity, but hydrate formation causing problems in keeping wells operating; Ugnu sand included Kristen Nelson PNA Editor-in-Chief
Development of viscous oil is more of a challenge than development of conventional North Slope oil, and BP discussed some of the challenges — low temperatures, artificial lift, hydrate formation — at the Alaska Oil and Gas Conservation Commission Dec. 9.
Prudhoe Bay field operator BP Exploration (Alaska) Inc. is developing the Polaris viscous oil accumulation and the commission held a hearing on the company’s September application for pool rules and an area injection order.
Polaris overlies the Sadlerochit reservoir in the northwestern area of the Prudhoe Bay field in the vicinity of pads S, M and W. It also overlies the Aurora pool Kuparuk River formation in the vicinity of S pad. BP said that initial production is planned from the N and O sands in the Schrader Bluff formation, but asked to have the adjacent sand above Schrader Bluff — the lower Ugnu M sand — included in the pool rules.
Two wells tested long term BP described Polaris development as a challenge: the production is there, the company said, but hydrate formation has made it hard to keep wells operating.
Two wells, the S-200 and W-200, have been tested long term, BP said, both producing since late 1999 and both “have successfully sustained rates.” Both wells use gas lift for artificial lift, “which has caused hydrate problems to occur,” the company said. Those problems “have been somewhat resolved by frequent hot oil tubing washes or by methanol injection.”
BP said stable production has also been established in the W-201 and S-213 wells with continuous methanol injection.
The S-200 well has produced some 200,000 barrels of oil, and averaging 400-500 barrels per day after 18 months, but was down 30-50 percent of the time because hydrates formed in the tubing. The well has been shut in since October 2001 after it developed mechanical problems with the liner.
The W-200 well, on the other hand, has produced some 585,000 barrels of oil, is producing 600 bopd after 27 months, experiences minimal downtime and has not experienced significant hydrate formation.
BP was unable to establish stable production in two other wells, the S-201 and S-216, due to hydrate formation and those wells have been converted to jet pumps as an alternative lift method after lift gas created hydrate problems that were not remedied with hot oil tubing washing and methanol injection.
“Jet pumps have resolved the hydrate problems,” BP said.
Vertical wells planned BP said horizontal multi-lateral wells have been successful at other Schrader Bluff pools, but the initial development plan at Polaris calls for primarily vertical wells, although some horizontal wells may be used.
The W-201 was drilled as a horizontal well to provide productivity information. BP said production from the well has been “substantially less than expected,” but the likely source of the problem has been identified, and horizontal well potential is being evaluated at W pad where a tri-lateral well, W-203, has been drilled and is on production.
Three-phase development BP said Polaris development is planned in three phases: phase I will develop and establish water flood operations in key areas at S pad and W pad, and includes a tri-lateral horizontal producer. Phase II will complete development of additional fault blocks in the core area of the field, while phase III would involve developing areas that require improved understanding or refinements in drilling techniques.
BP said production results from Polaris “confirm initial evaluations of reservoir deliverability” while hydrate formation is still causing problems in keeping wells operating, and “has been more of a problem in recent wells than indicated from initial production. Keeping the wells on line with a combination of low rates, cool production temperatures, presence of water, and lift gas composition and temperature, have proven both challenging and costly,” BP said. The company expects to improve well operations through alternative artificial lift methods and enhanced production rates from better fracture stimulations and use of horizontal wells.
Recovery only 15-30 percent Estimates of original oil in place at Polaris are 350 million to 750 million barrels of oil and 84-250 billion cubic feet of gas. BP said model results indicate that primary recovery would be approximately 5-10 percent of original oil in place, the low rate the result of a low gas-oil-ratio (the accumulation is under saturated), low initial reservoir pressure and viscous oil.
BP will develop Polaris with water flood, which is expected to improve total recovery to 15-30 percent of original oil in place, with production rates expected to peak at 12,000-15,000 barrels of oil per day from water injection, with water volumes expected to peak at 20,000-25,000 barrels a day.
In its September application for pool rules BP said enhanced oil recovery using miscible injectant could yield an additional 6 percent recovery at Polaris, and requested that the commission allow an EOR pilot on three wells for a two-year period. In an October letter, however, BP said it was withdrawing the miscible injectant EOR request. If the Polaris owners decide to institute MI-based EOR in the future, BP said in its October letter, it would apply for an amendment to the rules as appropriate.
Fourteen sands The Schrader Bluff and Ugnu formations at Polaris are subdivided into 14 sand units in the O and N intervals of the Schrader Bluff and the M interval of the lower Ugnu, BP said.
The Polaris pool was first penetrated in 1969 commercial production at Polaris was confirmed in late 1999 and 2000 with the W-200 and S-200 wells; several additional wells have since been drilled.
BP said that all current Polaris production is from the N and O sands at S pad and from the OB sands at W pad. There are no Polaris producing wells on M pad. N sands at W pad have not been a target, BP said, because the oil has a 14 API gravity. M sands (the lower Ugnu) at both S and W pads contain 12-14 API gravity oil, and BP said they are not considered economic targets because of “production complications related to heavy oil and unconsolidated sands.”
But, BP said, the lower Ugnu M sands “may be a future development target at Polaris.”
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