For wells in Western Canada, in particular, the challenge is that the oil is so viscous (they're called "tar sands" for a reason) they have to heat (often with

For wells in Western Canada, in particular, the challenge is that the oil is so viscous (they're called "tar sands" for a reason) they have to heat (often with steam) the ground to pump it out. As you'd imagine, this requires a large amount of energy. So stopping production implies letting the ground cool, and restarting implies dumping a lot of (expensive) energy back in before oil will flow.