A $2 billion private-equity fund that borrowed heavily to buy oil and gas wells before energy prices plunged is now worth essentially nothing, an unusual debacle that is wiping out investments by major pensions, endowments and charitable foundations. EnerVest Ltd., a Houston private-equity firm that focuses on energy investments, manages the fund. The firm raised and started investing money in 2013, when oil was trading at more than double the current price of about $45 a barrel. But the fund added $1.3 billion of…