Refracking, the practice of fracking an oil and gas well a second time, is still too unpredictable to rely on as a way to slash costs and increase output during the oil price slump, top U.S. shale oil executives said on Tuesday. Oilfield service companies, including Schlumberger NV and Baker Hughes Inc., have touted refracking as a cheap way to revive output from existing shale wells. Output from existing wells, measured in barrels per day, normally drops as much as 70 percent in the year of operation. Also, some wells were not thoroughly fracked the first time. But executives from producers say the refracking technology, while promising, remains tricky. “We have not tried any refracks. Our outlook on that is that it is really technical,” said Bill Thomas, CEO of EOG Resources Inc., widely regarded as one of the most efficient U.S. shale oil producers. […]