United States shale oil output is set to surge over the next five years as drillers recover rapidly from a three-year slump, the International Energy Agency said, sharply upgrading its previous growth forecasts. A landmark deal in 2017 between OPEC and other oil producers including Russia to curb output in order to battle a global glut materially improved the outlook for other producers as oil prices rose sharply throughout the year, the IEA said.