The insurance protecting shale drillers against plummeting prices has become so crucial that for one company, SandRidge Energy Inc., payments from the hedges accounted for a stunning 64 percent of first-quarter revenue. The insurance that producers bought before the collapse in oil — much of which guaranteed minimum prices of $90 a barrel or more — is expiring. “A year ago, you could hedge at $85 to $90, and now it’s in the low $60s,” said Chris Lang, a senior vice president with Asset Risk Management, a hedging adviser for more than 100 exploration and production companies.