Thrifty drillers have found a new use for the glut of natural gas that’s sent prices for the fuel below zero in America’s biggest shale patch: Use it to power fracking operations. For decades, explorers have used massive diesel engines mounted on tractor-trailers to shoot a mixture of water, sand and chemicals down wells and blast open layers of oil-soaked shale rock. That’s changing now that soaring output has crushed gas prices, especially in West Texas’ Permian Basin, where the fuel is a byproduct of crude oil extraction.