ConocoPhillips overcoming viscous oil challenges
Producers have wrestled for three decades with technical problems on how to produce a vast resource of “viscous,” or lower-quality, oil on the North Slope.
Viscous oil is thicker than conventional crude oil and does not flow as easily. It is not heavy oil, although it is sometimes called that.
There is true heavy oil on the Slope too, in the large Ugnu deposit. Viscous oil is somewhat deeper and warmer than Ugnu, and less challenging. The Ugnu heavy oil cannot yet be commercially developed, but ConocoPhillips and BP Exploration Alaska, the two major North Slope producers, are now producing viscous oil.
Problems have impeded past plans to increase that production, but the companies are now working through those challenges.
Using new techniques, ConocoPhillips plans to increase its production of viscous oil from the West Sak project, a part of the Kuparuk field, from about 15,000 barrels per day to 23,000 barrels per day with an expansion project now underway.
The $500 million I-H North East West Sak project, or IH-NEWS, is due to be complete by the end of this year and will add 8,000 barrels per day, at peak production, to the current West Sak viscous oil production of about 15,000 barrels per day, according to Mike Driscoll, ConocoPhillips’ manager for the West Sak expansion.
Potential viscous oil resources on the North Slope are large, with an estimated 5.5 billion to 7.4 billion barrels of oil in place in parts of the Kuparuk River, Milne Point and Prudhoe Bay fields where viscous oil is being developed, according to state geologists. Oil-in-place is the oil physically in the reservoir rock.
Not included in these estimates are substantial viscous oil resources in the western part of the Kuparuk field that are not now targeted for development because they are shallower and cooler, which makes them more difficult to produce than the resource now being tapped, Driscoll said.
While the oil-in-place resource is large, the important number is the estimated recovery, or the amount that can be economically produced from the in-place resource. At West Sak, that is estimated at 15 percent to 20 percent in the parts of West Sak now developed, and which represents the better reservoir areas, Driscoll said.
In comparison, oil recovery is much better in conventional fields, typically 40 percent, and even higher in high-quality reservoirs such as Prudhoe Bay, where recovery may eventually exceed 50 percent.
Viscous oil is essentially conventional oil that has seeped up to shallower levels compared with the large conventional producing fields on the Slope like the Prudhoe Bay and Kuparuk fields, Driscoll said. Because it is shallower, it is cooler and thus thicker and more difficult to flow, he said.
Also, at shallower depths bacterial action has eroded some of the lighter ends of the crude, making it somewhat heavier.
West Sak crude is ranked typically at 18 to 20 degrees API while the conventional North Slope oil is higher, with Prudhoe Bay oil ranked about 29 degrees API and oil from the Alpine field in the high 30s. (API is the American Petroleum Institute measurement for how heavy or light the oil is; lower numbers are heavier and higher numbers are lighter.)
Producing companies began efforts to produce the viscous oil in the 1980s but those were plagued with difficulties. In 2004, ConocoPhillips and BP announced a West Sak expansion aimed at boosting output to 45,000 barrels per day by 2007. The goal was never reached, however. Output was stuck at about 15,000 barrels per day and has remained there since.
“The sand is the number one problem,” at West Sak, Driscoll said.
It breaks loose in the unconsolidated reservoir rock at West Sak and, entering wells, it can damage pumps and other machinery. It is also related to a second problem: how to increase daily production rates of wells while keeping the sand out, he said.
Another serious challenge has been waterflood “breakthroughs,” where water breaks a channel through unconsolidated producing sands. These started happening as producers stepped up waterflood injection in efforts to get well-rates up, and as water broke through the loose reservoir rock it diminished and effectively ended the benefits of injecting water to enhance production.
“These were not good news events,” said Driscoll.
ConocoPhillips addressed this by injecting a polymer into the holes, sealing them. This has been successful. Waterflood breakthroughs, called “Matrix Bypass Events” by ConocoPhillips, have been cut by about a third since the technique was introduced two years ago, Driscoll said.
The company also successfully employed a fine-mesh screen inserted into an open hole section of the well that is now reducing sand coming into producing wells. Previous efforts to keep sand out with screens or gravel “packs” also impeded the oil flow. The new screening technique slows and stops sand inflow but does not impede the oil, Driscoll said.
In the earlier years ARCO Alaska (now ConocoPhillips) was the most aggressive in experimenting with West Sak viscous oil because of the size of the in-place oil resource is larger in the Kuparuk field.
Prior to its merger with Phillips, Conoco, on its own, was able to get production from wells it drilled in Milne Point, a field adjacent to Kuparuk, where the viscous oil is deeper and warmer, which make it easier to flow. In Milne Point and Prudhoe the deposit is called Shrader Bluff but it is the same geologic formation as at West Sak
Conoco’s Milne Point wells typically flowed in the range of 250 barrels per day, which were not economic on the North Slope. ARCO had similar rates with its early wells.
The well-rate problems were overcome to some extent when ARCO and BP, which purchased Milne Point from Conoco in the early 1990s, began drilling multilateral wells with horizontal production well segments. Multi-lateral wells involve several underground production wells, or legs, drilled off one vertical well to the surface.
These allowed the combined flows from several long horizontal production legs to achieve higher per-day rates in the West Sak. Some multilateral wells achieved rates of 3,000 barrels per day, Driscoll said. The wells planned at IH-NEWS will be multilaterals with up to five producing legs.
Driscoll hopes 1H-NEWS is only the first increment, and that nearby undeveloped West Sak reservoir sections that can be added, over time.
There is also future potential in a large area further to the west in Kuparuk, now called “Western West Sak,” that has large in-place resources but is also shallower and cooler, and with even more challenges because of those factors. But it represents a big opportunity for the future as companies work out technical problems.