Fifty-stage frack jobs. Fifteen-foot cluster spacing. More than 2,000 pounds of proppant concentrate per foot. Top U.S. shale producers are pushing fracking technology to new extremes to get more oil out of their wells, as they weather lower-for-longer oil prices. While the impact of the techniques may be scarcely noticeable on current U.S. output with so few wells in operation, it could mean drillers are able to accelerate production more fiercely than ever once prices recover. The hunt for the next big technology to transform the process of fracking is still on, with companies looking at methods such as using carbon dioxide to coax more oil out of wells that have already been hydraulically fractured. Commentary from executives in recent weeks suggests they are doubling down on existing accomplishments and innovations to boost production. […]