Well-known firms have pumped billions of dollars back into management teams that have already scored in the Permian and elsewhere. Some firms are also shoving money into public companies, combining strength in the Stack and elsewhere. E. Murphy Markham of EnCap Investments, Garry A. Tanner of Quantum Energy Partners, and Charles Cherington of Argus Energy Managers decode PE firms' secrets of success and the unique challenges they face in today's marketplace.