The Keystone XL Pipeline could bring more than 800,000 barrels of Canadian oil to the area’s coastal refineries.
If and when the the pipeline is completed, those barrels coming from the northern oil sands would run into competition for refinery access with oil from Latin American countries. 
Mexico relies on coastal U.S. refineries since its own facilities cannot process the heavy crude the country produces. The Mexican state-run oil company Pemex holds a financial stake in the Shell Deer Park refinery. Industry experts and analysts debate the effects of Keystone XL on this tight-knit relationship.
It may be more expensive for Canada to move oil through the new pipeline than it is for Latin American countries to transport oil to U.S. coastal refineries like the one in Deer Park. Shell and others have even sold off their oil sands holdings. Oil demand and pricing can change the balance, tipping it in favor of Canada. The third major player in the equation comes in the form of U.S. shale, which serves as competition to the Canadian oil and further obscures the potential effects of the planned pipeline.
Source: HoustonPublicMedia.org, “Considering the Economics of the Keystone XL Pipeline,” by Travis Bubenik, April 3, 2017.