Financiers keep pouring cash into the shale oil sector, providing producers with a path to keep U.S. output rising through the middle of the next decade. The United States is on track to deliver up to 80 percent of the world’s oil production gains through 2025, the International Energy Agency estimates, increases fueled in part by easy access to capital. Rising U.S. production is undermining OPEC’s attempts to curb global supply and boost prices, forcing the oil cartel to continue restraining output through the end of 2018. Hedge funds and private equity firms have given producers a range of new and traditional financial levers they can pull as needed to keep shale rigs drilling, according to interviews with more than a dozen financiers, advisers and executives. […]