It seems to me that the only thing Canada will accomplish is increase the overall supply, which will slightly depress prices, and thus increase demand. All the available oil in the market will still be sold at market prices. Venezuelan oil will still be sold. Saudi Arabian oil will still be sold. What mechanism do you think will be stopping these countries from selling their oil?
Demand for oil is finite, and while elastic, it is not infinitely so. Hence the fact that there are close to 3 billion barrels of oil in inventory globally. Expanding our capacity displaces oil sold by other nations. There are questions of logistics, shipping oil to China from Canada is shorter than from Venesuala or Niger for example. Lastly Canada's political stability, adherence to rule of law and reliable infrastructure make it a more favorable partner when it comes to establishing long term supply contracts (A Chinese refinery knows the oil with show up when it's contracted to without fear of a revolution, nationalization, or substantial breakdown in infrastructure).