The ocean of oil from U.S. shale drove crude prices back toward six-year lows Friday, and American energy companies say they are poised to unleash a further flood that would keep prices from returning to lofty levels for a long time. The International Energy Agency reinforced the prospect of a prolonged slump in energy prices Friday, saying U.S. oil output was surprisingly strong in February and rapidly filling all available storage tanks. The Paris-based energy watchdog said this could lead to another sharp drop in crude prices, which fell by about 50% late last year. The report sent oil prices tumbling around the world, with the global benchmark Brent crude falling $2.41 to $54.67 a barrel. The U.S. benchmark West Texas Intermediate lost $2.21 to settle at $44.84, less than 40 cents above a six-year low it reached in late January. […]