Drillers for Pennsylvania’s natural gas are facing a growing threat—from their own productivity. Hydraulic fracturing and other new drilling techniques have caused the amount of natural gas flowing out of the state’s Marcellus Shale to soar in recent years. Production is expected to hit 16 billion cubic feet a day next month, nearly double what it was two years ago, according to a federal-government estimate. The fracking surge has overwhelmed pipelines, creating a local glut that has caused gas prices to crater and forced drillers to re-evaluate operations. The situation became more pressing this summer, when federal regulators delayed construction of a pipeline meant to help producers move more gas by the end of next year. Some analysts say they don’t expect enough new pipeline capacity to be available until 2017.