By David Anna
May 14, 2005
Alaska's North Slope is home to the largest oilfield ever found in North America, the legendary Prudhoe Bay. Also linked to the Prudhoe Bay infrastructure is Kuparuk River oilfield, the Nation's second largest oilfield by production. A number of smaller oilfields that still would be considered giants by Lower 48 standards have been developed nearby.
But perhaps the biggest potential source of oil on the North Slope is the less-heralded heavy oil formations overlying the main producing zones at Prudhoe and Kuparuk. As much as 36 billion barrels of original-oil-in-place lie within the Ugnu, West Sak, and Schrader Bluff formations. That surpasses the original-oil-in-place of Prudhoe and Kuparuk combined.
The largest undeveloped accumulations in North America, these reservoirs lie tantalizingly in reach of existing infrastructure. Their recovery, however, has proven a daunting challenge. At a depth of 3,0003,500 feet, these formations' proximity to the subsurface permafrost renders the already low-gravity crudes extremely viscous. Viscous crude, which can be as thick as molasses, has trouble flowing to the wellbore for production. Low formation strength presents an additional hurdle to high recovery and productivity rates. North Slope producers have struggled for years to devise an economic plan to recover the heavy oils. This plan has become more important for America's energy security because of production declines in other North Slope fields.
North Slope operators thus far have focused on the less-viscous crudes in the West Sak and Schrader Bluff heavy-oil formations, where viscosities range from ~30 centipoise to ~3,000 centipoise. Combined original-oil-in-place volumes for these two formations total about 1020 billion barrels. Even assuming modest recovery rates, producible reserves could total several billion barrels of oil. Because there are even higher viscosities seen in the Ugnu formation, that resource's exploitation must await future technological advances.
North Slope operators have had some success producing the less-viscous crudes in the West Sak and Schrader Bluff formations by injecting slugs of water alternating with gas ( WAG ) into the reservoirs; the gas acts as a solvent to reduce oil viscosity, while the water front helps sweep the reservoir, pushing the crude to producing wells.
There are several gas streams available on the North Slope that contain natural gas liquids and carbon dioxide. Natural gas liquids have been used for years as part of a miscible gas enhanced oil recovery effort to bolster recovery at Prudhoe Bay. CO2 flooding is the fastest-growing form of enhanced oil recovery in the United States and creates the opportunity for environmentally safe disposal of the greenhouse gas.
DOE-funded research by the University of Houston has developed tools for modeling the optimum WAG flood design. The goal of the research-managed for DOE's Office of Fossil Energy by the National Energy Technology Laboratory-was to focus on modeling tools that would determine the best solvent, injection schedule, and well architecture for a WAG process in North Slope shallow-sand viscous oil reservoirs.
In the project, which took place from September 2001 through September 2004, the researchers were tasked with developing a compositional model, devising a new relative permeability model, creating a reservoir simulator, designing the ideal well architecture, and optimizing the WAG flood.
In their final project report, researchers also found a potential for greatly increased production rates for deploying WAG floods in horizontal wells vs. vertical wells, depending on well design and other factors. The research also showed that well productivity for these viscous oil reservoirs can be doubled via electromagnetic heating.
Such models will serve as useful tools in fully commercializing a vast oil resource needed to help stem the decline of production on Alaska's North Slope. And that could make the difference in keeping America's most prolific oil province on line.
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