Exxon Mobil Corp.’s Canadian subsidiary is considering plans for what would be the deepest offshore well ever drilled in the Arctic, increasing some environmentalists’ concerns about how the company would respond to a blowout. Imperial Oil Ltd. submitted a project description last September to Canadian regulators for a proposed exploratory well so deep that it would likely take two to four years to complete. The well could extend about 6 miles beneath the floor of the Beaufort Sea, according to a study commissioned by the Pew Charitable Trusts, an environmental watchdog. Imperial’s filing didn’t specify a well depth, but the unpublished Pew study, prepared by a third-party engineering consultant, calculated it as roughly 34,000 feet. “These wells may be drilled over 6 miles deep, and will need complex well casing, cementing and drilling plans to address the technical challenges of drilling deep high pressure wells,” the report said. […]