The pipeline resumed operations on June 29, and the leak was discovered on July 15.
Cleanup at the site continues. But the company still has no clear idea exactly when the leak began or what caused it, said Ron Bailey, vice-president of Nexen's Canada operations.
Bailey said the company doesn't know how long it will take to clean up five million litres of bitumen, sand and water that leaked near the Long Lake oilsands facility.
Alberta's Environment Minister Shannon Phillips and Energy Minister Marg McCuaig-Boyd plan to visit the site Friday.
The spill covers about 16,000 square metres, an area approximately the size of two CFL football fields. It is mostly contained within the pipeline corridor.
The area can only be reached by a winter access road, so the company had to build a road into the site. Vacuuming of the oil started last Friday. The spill has been contained by berms and other abatement equipment.