Shale oil producers from West Texas and North Dakota have harvested enough crude to overwhelm the global oil market and force Saudi Arabia's oil cartel to play offense on the world's energy stage. But U.S. producers have recovered only a small fraction of the oil that's trapped in those rocks, and though the oil-market crash has put the nation's energy boom on hold, some oil-technology companies are pursuing what they say will be a second American shale revolution. That belief lies partially in re-fracking - giving oil shale deposits a second blast of water, chemicals and sand - to get...