Stagnating rig productivity shows U.S. shale oil producers are running out of tricks to pump more with less in the face of crashing prices and points to a slide in output that should help rebalance global markets. Over the 16 months of the crude price rout, production from new wells drilled by each rig has risen about 30 percent as companies refined their techniques, idled slower rigs and shifted crews and high-speed rigs to “sweet spots” with the most oil. Such “high-grading” helped shale oil firms push U.S.