Energy companies and oil trading firms that teamed up to build several mini-refineries that convert a swelling surplus of ultra-light U.S. crude into fuels for export seemed like a pretty safe investment bet for a while. Now, as U.S. oil output reverses its five-year rise and after lawmakers ended the 40-year-old export ban this month, oil executives and analysts question the wisdom of nearly $1 billion worth of so-called condensate splitters built over the past year, and the future of another $1.2 billion planned.