Oil needs to recover to $65 a barrel for U.S. drillers to tap a pent-up supply locked in shale wells and unleash more crude on markets than is produced by Libya.

Dipping into this “fracklog” would add an extra 500,000 barrels a day of oil into the market by the end of next year, Bloomberg Intelligence said in an analysis on Thursday. Producers in oil and gas fields from Texas to Pennsylvania have 4,731 idled wells at their disposal.

Prices are rebounding from a six-year low after drillers idled half the nation’s oil rigs, slowing the shale boom that boosted production to the highest in four decades. The number of wells waiting to be hydraulically fractured, known as the fracklog, has ballooned as companies wait for costs to drop. That could slow the recovery as firms quickly finish wells at the first sign of higher prices.

“Once service costs come down and drillers begin to work through their higher-than-normal backlog, the market should start to price in that supply coming online,” Andrew Cosgrove, an energy analyst for Bloomberg Intelligence in Princeton, New Jersey, said by phone. “It may act as a cap on prices.”

U.S. oil futures tumbled by more than $50 a barrel in the second half of last year amid a worldwide glut of crude. West Texas Intermediate for June delivery fell $1.16 to $56.58 a barrel at 11 a.m. on the New York Mercantile Exchange.

Oil production in the lower 48 states would rise to 7.67 million barrels a day in the fourth quarter of 2016 if drillers start shrinking their fracklogs by 125 wells a month in October and put some rigs back to work, Bloomberg Intelligence models show. The U.S. fracklog has more than tripled in the past year, with oil wells making up more than 80 percent of the total.

“One of the big reasons why production is finally falling is because of these fracklogs,” Phil Flynn, senior market analyst at the Price Futures Group in Chicago, said by phone on Thursday. “That’s an overhanging bearish fundamental.”

The Permian Basin, which covers parts of Texas and New Mexico, had the biggest collection of unfracked wells as of February, with 1,540 waiting to be completed. The count totaled 1,250 in Texas’s Eagle Ford formation and 632 in North Dakota’s Bakken shale.

Last week, Raoul LeBlanc, an oil analyst with Englewood, Colorado-based consultant IHS Inc., pegged the U.S. fracklog at around 3,000 wells. Halliburton Co., the world’s second-biggest provider of oilfield services, estimated there are about 4,000 uncompleted wells, citing “third party estimates.”

Fracklogs are growing faster in the fringe areas of plays where the wells are less productive, according to the Bloomberg Intelligence analysis. In the Eagle Ford, for example, counties at the edge, such as Lee and Lavaca, saw companies go from completing more than 60 percent of their wells in November to less than 20 percent in February.

Large independent producers from ConocoPhillips to EOG Resources Inc. hold a significant portion of the uncompleted wells.

Those companies are already seeing more incentive to start eating into their backlog as crude has risen by a third since mid-March. After-tax returns would be 5 to 10 percent higher than they were just two months ago when oil was at $45, Cosgrove said.

ConocoPhillips Chief Executive Officer Ryan Lance said at the IHS CERAWeek Energy conference in Houston on Monday that increased well completions may exacerbate the supply glut, depending on whether oil demand rises.

The Energy Information Administration estimates the world is oversupplied by more than 1 million barrels a day this year. That glut will almost disappear next year as demand climbs and lower prices stymie production, the agency said April 7.

The U.S. shale business has evolved over the years to produce a “shorter-term investment cycle” that will allow it to respond quicker to prices in the future, David Zusman, managing director at Talara Capital Management, which handles $400 million in energy investments, said by phone on Thursday.

“Shale will act as an accordion,” Zusman said. “It will expand and contract in much shorter cycles to meet the needs of global oil demand.”