1/ NAFTA can be abrogated with 6 months notice

The overall effects of such a step would collapse the Canadian economy. NAFTA's energy provisions cover just 12 out of a total of over 2000 pages.
So while it is possible in theory, it's not in practice. The signees were well aware of this. That's probably why they chose to leave it in.

Another effect of NAFTA is the facilitation for US investors to purchase and hold Canadian assets. Hence, abrogation would mean endless litigation with potentially devastating financial penalties.

The only feasible step would seem to be a separate renegotiation of the energy part of NAFTA. But the first demands the US would bring to the table, if it were to agree to negotiations, would be a long list of extremely expensive ones.

2/ The NEB's mandate superseeds NAFTA

From  
Resource Imperialism
Energy Policy under NAFTA
by David Lapp

 The U.S.-Canada Free Trade Agreement provides insights into what can be expected to result from NAFTA. The evisceration of Canada's National Energy Board (NEB) due to the U.S.-Canada trade deal encapsulates NAFTA's broad reach. After examining a number of natural gas projects being developed by several U.S. companies, the NEB determined the projects would provide no net benefit to Canada and rejected the companies' applications for export licenses. The companies appealed the ruling, arguing that the NEB's cost-benefit analysis was, in effect, a price test that violated the free- market intent of the Free Trade Agreement. In the end, the NEB conceded that its cost/benefit analysis might violate the Free Trade Agreement, and, rather than risking potential arbitration, it backed down.

Dillon explains in an upcoming book entitled The Political Economy of North American Free Trade that before the deregulatory measures of the free trade agreement were implemented, "the NEB applied a 'surplus test' requiring that minimum supplies of nonrenewable hydrocarbons be available for Canadians before sanctioning exports. However, under [the trade agreement] the NEB has been reduced to a monitoring agency with limited ability to restrict exports."