Behind a low-slung building in Zelienople, Pa., stands a towering mechanical structure where Deep Well Services trains employees in the highly specialized work of completing fracking wells for natural gas. It is well-paying and, until recently, growing work. The company went from 25 employees in 2008 to 450 last year. Then, the pandemic clobbered demand for oil and gas, drilling slowed to a crawl, and Chief Executive Mark Marmo had to lay off about 100 people. Whether growth resumes, he said, depends on the pandemic, and next…