The US shale revolution that propelled the country to energy dominance was backed in large part by private-equity firms. Now, many of those firms may be at risk as the price of oil reaches 20-year lows and fuel demand evaporates. Small to midsize firms that specialize in oil and gas will have trouble raising funds in the future, while larger firms with diverse portfolios will be shielded, according to nearly a dozen experts. That puts the future expansion of America's oil and gas industry into uncertain territory. Visit BI Prime for more stories.