Tumbling oil prices have exposed a weakness in the insurance that some U.S. shale drillers bought to protect themselves against a crash. At least six companies, including Pioneer Natural Resources Co. (PXD) and Noble Energy Inc. (NBL) , used a strategy known as a three-way collar that doesn’t guarantee a minimum price if crude falls below a certain level, according to company filings. While three-ways can be cheaper than other hedges, they can leave drillers exposed to steep declines. “Producers are inherently bullish,” said Mike Corley, the founder of Mercatus Energy Advisors, a Houston-based firm that advises companies on hedging strategies. “It’s just the nature of the business. You’re not going to go drill holes in the ground if you think prices are going down.” The three-way hedges risk exacerbating a cash squeeze for companies trying to cope with the biggest plunge in oil prices this decade. West Texas […]