By Nia Williams
CALGARY, Alberta, Aug 15 (Reuters) - Canada's largest pipeline company Enbridge Inc expects maintenance work over the next three months to cut throughput on the western leg of its crude export network by up to 9 percent, a company document seen by Reuters on Friday showed.
Pipeline space lost to maintenance by November will total nearly 190,000 barrels per day of the 2.4 million bpd of operating capacity on the Canadian portion or western leg of the Mainline system, which carries the bulk of Canada's crude exports to the United States.
Traders in Canada's oil capital Calgary keep a close eye on Enbridge pipelines as outages can leave crude bottlenecked in Alberta and Canadian grades trading at steep discounts to U.S. crude.
In its August capacity forecast for September to November, Enbridge said it expects 2.19 million bpd to move through the western leg in November, a drop of 7 percent from 2.36 million bpd moved in June.
The 2.2 million bpd eastern part of the system, which starts in Superior, Wisconsin, and feeds refineries and markets across the Midwest, will be hardest hit in October when 154,000 bpd or 7 percent of operating capacity is expected to go offline.
Enbridge is undertaking a multi-billion dollar Mainline expansion to meet booming oil sands production. However, some traders have warned chokepoints on the system will move upstream as congestion in the Midwest eases.
One Calgary-based trader said he expected crude prices to fall as maintenance ramps up in November, with some barrels getting stranded in Edmonton, Alberta.
He said the light to heavy crude spread could widen as "there is basically zero maintenance on the light lines", particularly in the eastern part of the system.
Front-month light synthetic crude last traded at $4.25 per barrel below U.S. crude futures, while Western Canada Select heavy blend changed hands at $17.75 per barrel below the benchmark.
The Enbridge document showed that on the 2.4 million bpd western part of the Mainline system between Alberta and Superior 83,000 bpd of capacity will be lost to maintenance in September, 120,000 bpd in October and 188,000 bpd in November.
Meanwhile, expansion work on the eastern part of the system means available capacity will rise to 2.2 million bpd in November from 2.1 million bpd in October.
Operating capacity on Line 61 between Superior and Flanagan, Illinois, part of the Mainline, is set to rise to 623,000 bpd by November - although 44,000 bpd will still be curtailed by maintenance.
Capacity on Line 6B between Griffith, Indiana, and Sarnia, Ontario, will reach 401,000 bpd.
As Enbridge moves forward with its Line 9 reversal project to pump Western Canadian crude refineries around Montreal and Quebec City, outages are expected on other lines in the United States and eastern Canada.
"Maintenance activities for the next three months centre around several outages on line 7,9,5,6 in order to accommodate the line 9 reversal project," Enbridge said in the document.
Enbridge spokesman Graham White said the company would not provide detail on specific volumes if outages occurred and would manage the impacts with shippers.
(Reporting by Nia Williams in Calgary; editing by Jessica Resnick-Ault and Marguerita Choy)