A continuing effortPrudhoe Bay going beyond 12.4B barrels of oil recovered; gas now dominates ALAN BAILEY Petroleum News
Although the original estimate of 22.4 billion barrels of oil in place for the Prudhoe Bay field on Alaska’s North Slope has remained unaltered since field development first began, the anticipated ultimate oil recovery from the field has climbed over the years, as new development and production techniques have teased more oil out of the field reservoir, Scott Digert, BP’s Prudhoe Bay East Area development manager told the Alaska Geological Society on March 15.
To date the field operators have recovered about 12.4 billion barrels of oil from the field, a figure that represents 55 percent of the oil in place. Ultimate recovery could reach 60 percent, a volume of 14 billion barrels, depending on whether the future oil price is high enough to make economic the increasingly expensive development of the more elusive oil in the reservoir.
“That would take clear seas and a tailwind, and oil prices much above where they are today,” Digert said, commenting that the breakeven price of oil for viable Prudhoe Bay production today is in the high $30s.
Gas dominates Digert also said that, today, Prudhoe Bay is operating more like a gas field than an oil field, with gas production volumes massively exceeding those of oil - most of the gas is injected back into the field reservoir for pressure support and to drive more oil out of the reservoir. In 2014 gas constituted 97 percent of the fluid volume going through the production facilities; 2 percent was water; and just 0.4 percent was oil, Digert said. And nowadays, 50 percent of the oil being produced exists in the gas phase in the reservoir, whereas at field startup almost all of the oil existed underground in liquid form, he said.
Over the years there have been many plans for major gas sales from the field, but now does seem the time to start moving the gas to market, Digert said. While there may be a billion barrels of oil, or perhaps a bit more, to produce from the field, gas sales could amount to more than 3 billion barrels of oil equivalent, he said. Original gas in place in the field was estimated at 40 trillion cubic feet, although only a portion of this could in practice be produced.
At field startup in 1977 the field owners anticipated ultimate recovery of 9.6 billion barrels of oil, with oil production continuing until the late 1980s. At that point major gas sales would blow down the field’s gas cap, with the field’s life coming to an end in the early 2000s, Digert says. But the field operators coasted through those original expectations, with the field now 40 years into its life, longevity well beyond the field’s original 30-year design life, he said.
Ivishak reservoir Most of the oil from the Prudhoe Bay field has come from the field’s massive Ivishak reservoir, which contained a huge gas cap above an equally large oil rim, with an aquifer below the oil. Across much of the field a layer of heavy oil tar seals the oil rim from the underlying aquifer, Digert said. The reservoir thickness between the oil-water contact at the base and the upper edge of the gas cap at the top exceeds 400 feet, he said.
Although the reservoir consist of four distinct zones, each with particular rock characteristics, fluid communication through the reservoir is, in general, good, allowing the vertical migration of fluids and the gravity segregation of fluids of different densities: Oil will tend to flow down through the reservoir, while gas will go to the top and water will sink to the bottom, Digert said.
As a consequence, a primary oil production technique over the years has been gravity drainage, in which the oil has drained down through the reservoir towards production wells, pushed down by gas in the gas cap. After several decades of oil production, the gas has moved most of the way down through the reservoir, so that oil production wells today are mostly producing from near the bottom of the oil column, Digert said. And, with the gas pushing downward in the main central part of the field, a waterflood technique is used around the edges, using injected water to flush oil sideways towards the production wells.
Gas and water injection With gas reaching the production wells and being produced along with the oil, BP injects the gas back into the crest of the reservoir. This gas injection both helps maintain the fluid pressure inside the reservoir and, by flowing back down inside the reservoir, sweeps remnant oil towards the production wells, Digert said. The system works a little like blowing warm air over wet clothing - the lean gas vaporizes the relic oil in the rock pores, enabling the recovery of perhaps 20 percent of the oil left behind from the original oil production, he explained. The injected lean gas can also push out the original gas in place, which contains condensate, a mixture of light hydrocarbons.
The waterflood operations from the edges of the reservoir leave an oil saturation of about 35 percent in place. Consequently, BP alternates water injection with the injection of miscible injectant, a cocktail containing natural gas and natural gas liquids, to flush more oil from the rock pores. That can reduce the oil saturation of the rock pores to perhaps 5 percent, Digert said. But the efficiency of the technique can be relatively low in terms of the volume of oil produced because the water and miscible injectant tend to take different routes through the rock. Consequently, it is necessary to move the production and injection locations around, to improve the contact between the injected fluids and the relict oil.
Also a trademarked technique called BrightWater, involving the injection of polymer particles that expands and blocks already flooded sections of the reservoir, can divert newly injected water into areas where waterflood can be most effectively used.
With oil and some gas being removed from the field reservoir, the reservoir pressure decays over time. Consequently, for the last 10 years BP has been injecting water into the gas cap, to slow down the reservoir pressure decline. The injected water tends to flow down through the reservoir, at the risk of trapping oil or producing water rather than oil from production wells. However, so far the gas cap water injection technique has proved very effective, with more gained than may have been lost, Digert said. The gas cap water injection probably adds 15,000 to 16,000 barrels to the field’s daily production, he said.
Drilling techniques In the effort to extract more oil from the field reservoir, drilling techniques have evolved over the years. Early wells were drilled vertically through the near-surface permafrost, then deviated out at a 40 degree angle before being curved back into a near vertical profile through the field reservoir. Horizontal wells, drilled laterally for 5,000 to 7,000 feet from a steep well bore have since come into use. And in the late 1980s and early 1990s coiled tubing came into use as a cost effective means of drilling lateral wells from existing well bores. Coiled tubing involves the use of a continuous length of flexible, relatively narrow bore steel drill pipe, with a motor driven drill bit at the downhole end.
While there 800 to 850 usable wells in the field at present, there need to be some 650 to 660 producing wells in operation to keep up the full production rates, Digert said.
New plays Development of the Prudhoe Bay field has now reached a point where BP has been designing a series of distinct plays for producing oil in a number of different situations at different locations in the field reservoir. One play, for example, involves drilling a well along the bottom of an area where oil is trapped, with the far end of the well bent upwards into the gas accumulation above the oil. The upward directed end section of the well enables measurement of the oil thickness, while perforations in this section of the well pipe allow gas to enter the well, to help lift oil to the surface from the section of the well in the oil pool.
Another play, in the waterflood area of the field, involves sidetracking a well to sweep the reservoir rock above and below a region where the rock has previously been flooded with water.
BP has also been making moves to develop the Sag River, a relatively thin reservoir unit above the main Ivishak reservoir and originally containing perhaps 1.5 billion barrels of oil in place. Having poorer reservoir quality than the Ivishak, the Sag River has not received much attention until relatively recently. BP is now trying the use of long horizontal wells and hydraulic fracturing techniques, similar to those used for shale oil development in the Lower 48, to produce Sag River oil. At this stage test drilling has successfully demonstrated production, although at today’s oil prices economic success is not certain, Digert said. And the reservoir quality is quite variable from one part of the Sag River to another, he said.
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