Prudhoe bay field is one of the largest oilfields in the US, located in Alaska. The reservoir is of the Permo-Triassic period, which covers 213,000 acres and contains over 25,000 million barrels of oil as well as 46,500 billion cubic feet of gas (Baker et al. 2). The field has been exploited since the late 1970s, although its potential has been known for some time, unable of being exploited due to the remote location of the field. Its peak production was at 1.5 million barrels a day in 1988, having substantially dwindled since, to 281,000 barrels a day today (Baker et al. 3). The field is operated by British Petroleum (BP) and has been implementing enhanced recovery methods for over 30 years.
Characteristics of the Prudhoe Bay Reservoir
The reservoir is dated to have appeared during the Permo-Triassic period. It is formed from a thick sandstone of deltaic form, which is about 500 feet thick (Simon and Petersen 1). Average porosity ranges between 15 and 30% BV (Simon and Petersen 2). Permeability range varies between 50-3,000 MD. The reservoir features an oil layer overlaid by gas, with the original gas-oil ratio being at 730 SCF/STB (Simon and Petersen 3). Underneath the oil column, there is a 50-feet thick tar layer, which is found above the oil-water contact line. The reservoir’s geometry presents itself as a structural stratigraphic trap dipping to the south and south-west directions with a 1-2° inclination (Simon and Petersen 3). The main area, as well as the western wing, has oil-to-water contact. Initial water saturation is at 35%, oil gravity is 28° API, while its viscosity is at 0.8 CP (Simon and Petersen 3). Natural depletion mechanisms included gravity drainage due to the large gas cap, as well as a potential solution-gas drive. There was a presence of a water drive as well, but its effects were very weak and negligible.
The reservoir utilized various methods of enhanced oil recovery, such as waterflooding, gas-cap cycling, and enriched-hydrocarbon miscible methods (Thrasher et al. 1). Waterflooding was part of the original development plan and required infill-drilling, as well as water, which was obtained from the sea. It was used in areas that were not covered by the gas cap. The gas-cap cycling project started shortly after and was continued up until the early 2000s, managing a capacity output of 8,0 BCF/D (Thrasher et al. 5). Finally, the enriched-hydrocarbon miscible solution included the use of a water-alternating gas injection, which was used in various parts of the field. The efforts managed to significantly increase recovery while reaching displacement efficiency grow from 68% to 76% after nearly three decades of draining (Thrasher et al. 6).
Economic Side of EOR
Oil recovery methods increase the cost of oil by 0.5-8.0 USD per ton of the gas used (James 271). At the same time, the method allows for increased extraction, allowing to provide more product to the buyer and benefit from increasing oil prices. Therefore, the method is viable only when oil prices are high to warrant excess production. In the future, however, it is estimated that such methods would become a requirement since oil and coal resources in the US and around the world are finite (Babadagli 106930). The project requires sources of materials used for EOR practices to be delivered or taken from the natural environment, which may increase or reduce the costs of the project (Babadagli 106930). Gas-based EOR is typically more expensive than water-based solutions, due to water being readily available in some areas, like Prudhoe Bay (Babadagli 106930).
Works Cited
Baker, Richard, et al. “The myths of waterfloods, EOR floods and how to optimize real injection schemes.” SPE Improved Oil Recovery Conference. Society of Petroleum Engineers, 2016.
Babadagli, Tayfun. “Philosophy of EOR.” Journal of Petroleum Science and Engineering, vol. 188, 2020, p. 106930.
James, Alexander. “The long-run vanity of Prudhoe Bay.” Resources Policy, vol. 50, 2016, pp. 270-275.
Ning, S. X., et al. “Optimizing the Utilization of Miscible Injectant at the Greater Prudhoe Bay Fields.” SPE Western Regional Meeting. Society of Petroleum Engineers, 2016.
Simon, Andrew D., and Eric J. Petersen. “Reservoir Management of the Prudhoe Bay Field.” SPE Annual Technical Conference and Exhibition. Society of Petroleum Engineers, 1997.
Thrasher, David, et al. “Waterflood Sweep Improvement at Prudhoe Bay, Alaska.” SPE Improved Oil Recovery Conference. Society of Petroleum Engineers, 2016.