Calpine Corp., San Jose, Calif., agreed Thursday to buy Canadian energy producer Encal Energy Ltd. for $868.8 million in stock, giving the US independent power producer a strategic natural gas transportation link between western Canada and the US. The Encal acquisition will more than double Calpine's proved and probable natural gas reserves to 1.7 tcfe of gas, net of royalties.Senior Calpine executives have said the company's goal is to own 25% of its gas-fired electric generation needs.