CERAWEEK-U.S. fracking costs falling fast, may keep fields in play HOUSTON, April 22 (Reuters) - U.S. oil and natural gas companies have pushed down costs of fracking a shale well faster than expected, and if the trend holds up it could allow producers to keep working in oilfields that just months ago looked uncompetitive after the oil price crash.A more than 50 percent fall in the price of crude oil since June has left oil and gas producers insisting on steep price cuts from oilfield service companies that provide everything from drilling rigs to hydraulic fracturing.Oil is trading...