Oil drillers expecting prices to rebound after the biggest drop in six years have come up with an alternative to storing their crude in tanks: They’re keeping it in the ground. It’s a new twist on an old oil-trading technique, known as a contango storage play, in which a trader buys cheap crude in an oversupplied market and saves it to lock in profits at higher future prices. Drillers who have spent millions boring holes through petroleum-rich shale rock are just waiting for prices to go up before turning on the spigot. From North Dakota to Texas, there are more than 3,000 wells that have been drilled but not tapped, based on estimates from Wood Mackenzie Ltd. and RBC Capital Markets LLC. Waiting gives producers such as Apache Corp. and EOG Resources Inc. a better chance of receiving a higher price. It could also delay a recovery by attracting more supply every time prices rise. “Effectively, the rock is the storage,” Troy Cook, an analyst with the Energy Information Administration in Washington D.C., said by phone. “If you can afford to hang on to it, you could certainly choose to wait until the price goes up.”