U.S. E&Ps have raised more from bond sales in 2017 than in any year since the slump in oil prices began in 2014, shoring up their finances to support drilling and production growth.

Equity financing for the US E&P industry, which boomed in 2016, has slumped this year, but the debt market has been robust, accounting for the largest share of oil and gas E&P company fundraisings since 2009.

Companies in the sector have raised just under $60 billionin bond sales so far this year, already a 28% jump from 2016, according to Dealogic. Over the past three weeks, Whiting Petroleum, Continental Resources and Endeavor Energy Resources have each raised $1 billion in debt.

The bond sales have helped finance increased activity in U.S. shale reserves. The number of rigs drilling the horizontal wells used for shale oil production has more than doubled from its low of 248 in May last year to 652 last week, according to Baker Hughes, the oilfield services group controlled by General Electric.

“It is the resurgence in the commodity, that’s the bottom line,” said Tom Stolberg, a portfolio manager with Loomis Sayles.

“The commodity trending better has brought out a slew of new issuance. The fringier [companies], lower rated, more stressed balance sheets, it benefits them a whole lot more because it gets them over the hump of not making it at $50 or $45 oil.”

U.S. crude prices have risen 22% over the past six months to about $57 a barrel, helped by healthy global demand growth and agreements among OPEC and leading non-OPEC producers to extend their curbs on production to the end of next year.

Artem Abramov, vice-president of analysis at Rystad Energy, said although investors were looking for better returns from US E&Ps, the industry as a whole was likely to continue outspending its cash flows next year.

A few leading E&P companies including ConocoPhillips and EOG Resources are generating enough cash to pay for their spending and dividends, and some others are close to that position, but most of the industry is likely to continue to rely on a sustained inflow of capital, Abramov added.

Vance Scott, leader for oil and gas transactions in the Americas for EY, said companies were taking advantage of the receptive markets to lock in low interest rates with long-dated debt.

Continental Resources this month raised $1 billion from a sale of notes maturing in 2028, which it is using to pay off bank debt in the form of a term loan and a revolving credit facility.

Companies have also been stabilising their financial positions by stepping up hedging of revenues using derivatives. By the end of September, a sample of 43 exploration and production companies had hedged 38% of their expected 2018 oil output, up from 23% at the end of June, according to Energy Aspects, a research firm.

Phillip Tamplin, head of energy high-yield capital markets at Credit Suisse, said hedging was a key issue for investors looking at exploration and production companies’ debt.

“Many of the companies have hedged, which removes the potential volatility of next year’s earnings,” he said.

Whiting Petroleum, which has hedged 52% of next year’s expected oil production, on Tuesday announced a planned sale of $750 million of senior notes maturing in 2026. Later that same day, it increased the sale to $1 billion, following strong demand from investors, two portfolio managers following the offering said. The company priced the new notes with a yield of 6.625%, lower than initially expected.

Tamplin said he expected 2018 to be “a good year” with plenty of refinancings if oil stayed at its present levels. He added: “You’ll probably see an increase in use of proceeds for drilling dollars.”

Abramov said Rystad expected enough drilling activity to drive U.S. oil production from about 10m barrels per day at the end of this year to about 11 MMbbl/d at the end of 2018.

Other forecasters are expecting slower growth. The U.S. government’s Energy Information Administration has projected an increase from 9.77 MMbbl/d in December 2017 to 10.34 MMbbl/d in December 2018. But even that more cautious estimate implies an extra 570,000 bbl/d coming on to world markets during the year.