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ConocoPhillips explains West Sak plans Tells AOGCC that expanded oil pool and use of new EOR technique will enable increase in viscous oil recovery in Kuparuk River unit Alan Bailey Petroleum News
The many billions of barrels of thick, viscous oil known to exist at relatively shallow levels above the major oil fields of Alaska’s central North Slope present an enticing yet challenging opportunity for oil companies seeking new avenues for oil development. Syrupy in consistency and thus difficult to entice into production wells, this oil resource is relatively expensive to produce. And because of production difficulties, the proportion of the oil in place that can actually be extracted tends to be rather low compared with a more conventional oil resource.
Pool expansion As part of continuing efforts to exploit North Slope viscous oil ConocoPhillips has applied to the Alaska Oil and Gas Conservation Commission to expand the definition of the West Sak viscous oil pool that the company has been developing for a number of years in the Kuparuk River unit. And in an April 23 hearing experts from the company explained to the commission the reasoning behind the expansion application.
Both ConocoPhillips and BP have been developing North Slope viscous oil resources for several years, with BP’s developments targeting what the company refers to as the Schrader Bluff formation, a rock system upper Cretaceous in age and contiguous with what ConocoPhillips refers to as West Sak. Eni has also been developing relatively viscous oil from Schrader Bluff rock units in its offshore Nikaitchuq field.
ConocoPhillips wants to expand its West Sak pool definition upwards by about 200 vertical feet, to include some as-yet untapped reservoir sand units. The company also wants some minor adjustments to the areal extent of the pool. And, in parallel, the company is seeking approval of the use of a new oil recovery technique called viscosity reducing water alternating gas, or VRWAG, to encourage more viscous oil to flow into production wells.
Water alternating gas, or WAG, is a technique commonly used in the North Slope oil fields to enhance oil recovery by alternately flushing oil from a reservoir with water and injecting natural gas into the reservoir. The VRWAG variant of this technique involves mixing natural gas liquids components such as butane and propane with the injected gas. Scott Redman, ConocoPhillips reservoir engineer for the Greater Kuparuk unit, explained to the AOGCC commissioners that the more liquid components of the gas cocktail tend to mix with the oil in the reservoir, causing the oil to swell and hence become less viscous. And the less viscous the oil, the more readily the oil will flow towards a production well.
Schrader Bluff sands Michael Werner, principal geologist for ConocoPhillips heavy oil development, said that the existing West Sak oil reservoir is equivalent to what are referred to as the Schrader Bluff O sands. The requested vertical expansion would encompass what are known as the Schrader Bluff N sands, a sequence of mudstones and sands above the Schrader Bluff O. The expansion, motivated by successful development activity in adjacent units and by the results of appraisal drilling in the Kuparuk River unit, would bring the pool definition at Kuparuk into line with those adjacent units, the Milne Point, Prudhoe Bay and Nikaitchuq units, Werner explained.
The N sands consist of four distinct intervals, with one interval, the Nb, being particularly sandy and, hence, effective in holding oil. Other intervals tend to contain relatively impervious mudstones that can confine the oil in the more sandy parts of the section. The complete rock sequence, from bottom to top, represents a transition from the nearshore area of an ancient sea through a river delta system to a system characterized by ancient river channels.
The thickness of the N-sands varies greatly across the entire Kuparuk River unit but in the more eastern part of the unit, where the sands hold hydrocarbons, the unit is about 180 feet thick, Werner said. The Nb interval in this region typically contains from five to 30 feet of sand strata, he said.
According to paperwork submitted with ConocoPhillips’ application to AOGCC there are an estimated 320 million barrels of oil in place in the Nb sands.
Segmentation Two sets of geologic faults crisscross the West Sak and Schrader Bluff strata, typically displacing the strata by 20 to 30 feet, but sometimes with throws as much as 150 feet. And, because of the relatively thin nature of the strata, faults with vertical displacements of as little as 20 feet can trap oil, Werner said. Different segments of the reservoir can have different oil qualities and different oil-water contacts, he said.
This segmentation of the reservoir led to a brief discussion of what is actually meant by an oil pool, with Werner commenting that ConocoPhillips views the West Sak pool as a single “charge event.”
Development history Marc Jensen, a drill-site engineer for ConocoPhillips’ West Sak assets, recounted the history of West Sak development, showing how that history has led to ConocoPhillips’ latest development proposals.
Development of the West Sak core area began in 1983 with a pilot project involving the use of nine vertical production wells, with seven additional vertical wells used to inject water into the reservoir, Jensen said. The pilot, which continued to 1986, demonstrated the viability of this waterflood technique for viscous oil recovery and led to the first phase of commercial development between 1997 and 1998, he said.
A second phase of development, from 2000 to 2004, involved the use of horizontal production wells, but with the injector wells still being drilled vertically. And, with loose sand from the oil reservoir proving problematic, a mesh system in the well completions was used to exclude sand from the production wells.
The current production phase began in 2004. Both the production wells and the injection wells are now horizontal, being drilled as dual and trilateral wells from vertical well bores and with sand production being handled at the surface, rather than downhole.
Currently ConocoPhillips has 54 production wells and 55 injection wells in the West Sak, using waterflood to produce around 15,000 barrels per day of oil, Jensen said.
EOR testing In 2003 ConocoPhillips conducted a small-scale test in the use of gas for enhanced oil recovery from West Sak. And in 2006 BP sought approval to use enriched gas for viscous oil recovery from its neighboring Orion oil pool, Jensen said.
In 2009 ConocoPhillips began a large-scale pilot program, to test the use of VRWAG techniques in the West Sak pool. That pilot ran until 2013 and, having proved successful, triggered ConocoPhillips’ current application for approval of the full-scale use of VRWAG, Jensen said.
Redman said that results from the pilot and from accompanying laboratory tests indicated that VRWAG could increase viscous oil recovery by 5.2 percent of original oil in place compared with waterflood by itself. Potential oil recovery of West Sak viscous oil through the use of waterflood is estimated at 15 percent of original oil in place.
NGL availability However, there is apparently an issue of how much natural gas liquid may be available for VRWAG use. ConocoPhillips is in the process of converting a pipeline that currently carries natural gas liquids from the Prudhoe Bay field to the Kuparuk field for enhanced oil recovery. After conversion that pipeline will ship natural gas for use as fuel for the Kuparuk facilities. Once shipment of Prudhoe Bay natural gas liquids ceases, VRWAG oil recovery in West Sak will rely on just natural gas liquids produced from the Kuparuk field.
Redman told the commissioners that the scale of the increased oil recovery from VRWAG depends on the proportion of natural gas liquids used in the injected gas, and that the improved recovery rates would be somewhere above at least half the optimum level, depending on the availability of the natural gas liquids.
Commissioner Dan Seamount questioned a potential conflict in gas availability between gas use for enhanced oil recovery and proposals to export Prudhoe Bay gas by pipeline to a major liquefied natural gas plant. Developing the estimated 26 trillion barrels of viscous oil under the North Slope would likely require all of the gas in the Prudhoe Bay field, he suggested.
“Just talking about this I can just see … tar and feathers and getting marched down to the state,” he quipped.
Redman commented that gas would be continuously cycled in and out of the oil reservoir in the VRWAG process, with the possibility of ultimately recovering much of the gas.
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