Goldman Sachs is set to acquire a 12.5% stake in Riverstone, a private equity firm, for $500m. The deal represents a vote of confidence in Riverstone, which suffered when oil prices collapsed in late 2014 but has recovered as energy prices stabilized, according to securities filings. While some of its big investments, such as oil explorers EP Energy and Fieldwood Energy, remain underwater or flat, Riverstone has had big profits in others, particularly companies that drill in the Permian Basin in West Texas.