The decision throws into doubt whether Enbridge will be able to lock customers on the Mainline into contracts for as long as 20 years, a dramatic shift from the current system in which space on the system is allocated on a monthly basis. While Enbridge has said the change would give shippers certainty, producers including Suncor Energy Inc. and MEG Energy Corp. have said the revamp represented an abuse of market power at a time of constrained pipeline capacity out of western Canada.

“The commission has concerns regarding the fairness of Enbridge’s open season process and the perception of abuse of Enbridge’s market power,” the CER said in the letter. “Shippers have few alternative transportation options at this time.”

Enbridge said it will evaluate the decision and the next steps it will take toward implementing long-term contracts on the Mainline.

“Enbridge remains committed to moving ahead with contract carriage on the Mainline and has strong support for our offering,” spokeswoman Tracie Kenyon said in an emailed statement.

Best Compromise

The company previously had said the proposal was the result of 18 months of discussions with a wide variety of shippers and that it represents the best compromise of all of their competing views. Enbridge argued that regulators needed to let the open season continue so they could have commercial data to factor into their decision on whether to approve the tolling change. The plan also calls for 10% of the Mainline’s space to be kept open for spot shipments.

The debate takes on heightened importance because of the lack of options the oil-sands industry has for getting its crude to market. Major projects like TC Energy Corp.’s Keystone XL, Enbridge’s Line 3 expansion and the Trans Mountain expansion have faced years of delays. That has weighed on Canadian heavy crude prices and last year prompted Alberta’s government to introduce production limits.

Some producers have said they want more clarity on those major pipeline projects before committing to decades of shipments on the Mainline. The change also could mean that a relatively few large refiners in the U.S. will gain control of a large portion of the Mainline, according to Mike Walls, an analyst at Genscape Inc.

But opposition to the plan wasn’t unanimous. Oil-sands producer Cenovus Energy Inc. and Exxon Mobil Corp.’s Imperial Oil Ltd. came out in support of letting the open season run its course. Among buyers of Canadian crude, Saudi Arabia’s Motiva Enterprises LLC, which operates the largest U.S. refinery, and Houston-based LyondellBasell Industries NV also expressed support for the process.