The Trans Mountain pipeline may have been slowly leaking for an extended period of time before Kinder Morgan Canada crews found crude oil on the ground.
The small spill now estimated at about five barrels of oil 40 kilometres southwest of Merritt was discovered Wednesday morning and prompted the company to shut down the pipeline.
Hugh Harden, Kinder Morgan Canada's vice-president of operations, said Thursday crews are expected to install a permanent repair sleeve allowing the line to reopen by Friday evening.
He said it's difficult to say how long oil had been escaping.
"I would say it was a matter of days or weeks rather than months or years," Harden said. "It was a very, very, very slow leak. It's almost classified as a weep."
At five barrels of oil, the spill would technically be below the National Energy Board's 1.5 cubic metre (approximately 10 barrel) threshold for reporting spills that don't reach water courses.
Harden said the leak was found because Kinder Morgan crews arrived to check that section of the pipeline after an in-pipe inspection device flagged it as having a potential defect.
They noticed an oily patch the size of a wagon wheel and called the control centre, which shut down the pipeline.
Otherwise, workers would have excavated the pipe to inspect its exterior for leaks or problems.
"The fact that the tool found it is actually a good thing," Harden said. "It tells us our inspection process is valid and we're finding these very small defects."
Environmental groups who oppose Kinder Morgan's plan to build a twin pipeline tripling capacity to 890,000 barrels per day held up the incident as proof spills are inevitable.
"By a stroke of luck, the spill did not enter any waterways, saturate a farmer's field, or worse yet, a school yard," said Chilliwack resident Jordan Wilson of the Pipe Up Network.
"Kinder Morgan says that there has been no impact to any water course and no threat to the public, but what about the next spill?" asked Gabriel George of the Tsleil-Waututh First Nation. "They are proving that they can't eliminate the threat posed by their pipeline."