The NAFTA Proportionality clause Article 605 is often misunderstood. It comes into effect only if the Federal Government of Canada (or of the US) undertakes to implement measures to limit exports. As long as the governments refrain from intervening in the North American energy market, there is no proportionality requirement. Decisions to export oil or gas from Canada to the US are entirely made by producers and suppliers, subject to regulatory approval by National Energy Board and the Alberta Energy Resources and Conservation Board, primarily for environmental and safety reasons.

The political effect of Article 605 is to severely limit the power of the Federal Government to intervene in the primarily Albertan energy industry, something Albertans wanted to prevent another National Energy Program. My guess is that the FTA negotiations were based on trading US access to Canadian energy for Ontario and Quebec access to US markets for manufactured goods. In effect, Eastern Canada gave up energy security for (supposedly) free access to US markets.

Interestingly, it appears that the Government of Alberta has recently taken steps to remove what may people believe is a requirement that the ERCB (successor to the EUB) hold in reserve 15 years supply of natural gas for Albertans before permitting exports. The 15 year rules appears to never have been written in law, but to be only an interpretation of the clause in the Gas Resources Preservation act (amended 2008) which requires that

Limitations on granting of permit
8 The Board shall not grant a permit unless in its opinion it is in
the public interest of Alberta to do so having regard to
(a) the present and future needs of persons in Alberta,
(b) the established reserves and the trends in growth and
discovery of reserves of gas or propane in Alberta, and
(c) any other matters considered relevant by the Board.
1984 cG-3.1 s5;1986 c17 s3

The EUB interpreted the clause to mean that "core consumers" needs should be met for 15 years before gas could be considered surplus and available for export from Alberta (to the rest of Canada or the US). The term "core consumer" was referenced in the Gas Resources Preservation Act and defined in Gas Utilities Core Market Regulation (AB 44/95):

Classes of consumers
3 Consumers are classified as follows for the purposes of this
Regulation:

(a) Class 1: Industrial Consumers, consisting of consumers who use
or consume gas at a location primarily

(i) as a raw material, or

(ii) as a fuel, whether for space heating, water heating
or otherwise,

in an industrial or manufacturing operation;

(b) Class 2: Alternate Fuel Capability Consumers, consisting of
consumers who have sustainable access to, and facilities that allow them to
use or consume, a source of energy, other than gas, in quantities
sufficient to satisfy the requirements for which gas would otherwise be
consumed by them;

(c) Class 3: Core Consumers, consisting of consumers other than
those within Classes 1 and 2.

According to EUB, core consumer consumption is less than about 18% of total Alberta Consumption (See Energy Utility Board report EUB ST98-2007, calculation of available for export).

Since the regulations that define core consumers have been repealed, it is no longer clear that there is any restriction on export of natural gas from Alberta, as of 2008.