It's well known that water has been key to the shale oil and gas rush in the United States. But in one center of the hydraulic fracturing boom—North Dakota—authorities are finding that the initial blast of water to frack the wells is only the beginning.

The wells being drilled into the prairie to tap into the Bakken shale need "maintenance water"—lots of it—to keep the oil flowing. (See related photos: "Bakken Shale Boom Transforms North Dakota.")

So while the water first pumped down the hole to crack rock formations and release the underground oil and natural gas typically totals 2 million gallons (7.5 million liters) per well, each of North Dakota's wells is daily drinking down an average of more than 600 gallons (2,300 liters) in maintenance water, according to recent calculations by North Dakota's Department of Mineral Resources (DMR). (See related quiz: "What You Don't Know About Water and Energy.")

Without water, salt buildup forms and restricts the flow of oil.

Over the life of the well, which authorities presume will be 30 to 40 years, maintenance water needs could add up to 6.6 million to 8.8 million gallons (25 to 33.3 million liters)—or more than three to four times the water required for the initial fracking. (See related story: "Water Demand for Energy to Double by 2035.")

North Dakota DMR Director Lynn Helms addressed the maintenance water issue in a taped address earlier this fall at the annual meeting of the North Dakota Association of Oil and Gas Producing Counties. "What we're beginning to realize is that these . . . wells will need freshwater for maintenance over their life," Helms was quoted as saying in a report on the presentation in the local newspaper, the Dickinson Press.

Helms declined a request for an interview, but DMR geologist Richard Suggs confirmed that the department's calculations show that each well, over its 30- to 40-year lifespan, might end up consuming even more water for maintenance than for the fracking process originally used to create it.

Licking Salt

Why do North Dakota's wells have such a difficult-to-quench thirst? The naturally high salinity of the Bakken play's groundwater is the reason, according to Suggs.

"As they're producing the oil, they're also bringing up that water," Suggs said. "The salt precipitates in the well bore. It can restrict the flow of oil, and cause the pumping equipment to have problems as well."

That salt has to be flushed out by pumping fresh water down into the well bore, and then sucking it back up through the same tubing normally used for oil. "The salt basically dissolves in the water, the way a spoonful of table salt would in a glass of water," Suggs said.

The implications for water demand are significant in a state that gets less than 13 inches of rainfall each year in its driest areas, and ranks among the bottom 10 states for precipitation. In 2012, the Bakken oil industry used about 5.5 billion gallons (209 billion liters) of water—more than the amount used by the 110,000 inhabitants of Fargo, the state's biggest city. When the Bakken is fully developed in the next 10 to 20 years, the oil and gas play's 40,000 to 45,000 wells may need to consume roughly double that amount—as much as 10.2 billion gallons per year (28 million gallons each day)-in maintenance water to keep the oil flowing, according to a July 2013 DMR presentation to the North Dakota legislature. (See related story: "Oil Train Revival: North Dakota Relies on Rail to Deliver Its Crude.")

Recycling's Limits

One way to cut down on oil and gas industry water demand in North Dakota would be to recycle and reuse water from the fracking process. But that won't help in well maintenance. Water that's been used for fracking already has picked up too much salt from the underground geological formation to be used for flushing the wells of salt build-up, according to Suggs.

Instead, only freshwater—treated with a biocide to kill bacteria that might harm the oil deposit—can be used. After the maintenance water is withdrawn from the wells, it can then be recycled and used in the initial fracking of another well, or else disposed of in an underground saltwater storage facility.

Helms addressed recycling in his speech to the oil-producing counties group in September: "We're in the process of trying to find a way to recycle . . . produced water safely so that we can use produced water for fracking and save our freshwater resources for maintenance," Helms said, according to the Dickinson Press report.

The need for large amounts of maintenance water seems to be a unique problem in the Bakken play. Both energy industry and environmental groups contacted by National Geographic News said they had not even heard of the practice before. "I'm not familiar with it, and I've worked in different areas," said Andrew Patterson, executive vice president of technical and regulatory affairs for the Marcellus Coalition, a Pittsburgh-based group representing oil and natural gas companies active in developing Pennsylvania's Marcellus shale. "I'm pretty confident that it's not something that you would see in the Marcellus [play]. The brine in the Marcellus is not causing major [salt] precipitation." (See related blog post: "Fracking Water: It's Just So Hard to Clean.")

In North Dakota, Suggs said that the future price and availability of fresh water may well determine how long wells remain economical to operate. "If water becomes too expensive, that might potentially decrease the life of the well," he said.

The future price of water in North Dakota will depend on a number of factors beyond oil and gas industry demand, and the so-far much greater demand of the state's mainstay agricultural industry. North Dakota has a robust industry of private water suppliers who have developed a lucrative business selling water to the fracking industry. These private suppliers raised objections to a state government-backed co-op, the Western Area Water Supply Project, which is building $110 million pipeline, treatment facilities and water depots to boost northwestern North Dakota supplies with water taken from the Missouri River, which flows through the state.

One of the rationales for the project to bring fresh surface water to the region was the health risks of drinking water drawn from the state's groundwater, with its high sodium content. But the government-supported co-op also plans to sell about 20 percent of its water to the fracking industry, which will help keep prices low (and create competition for the private water suppliers).

Steve Mortensen, chairman of the North Dakota Independent Water Providers, the confederation of ranchers and farmers who are private water suppliers, said water availability isn't a problem in the state. "We have plenty of water to serve the industry," he said. "If anything, our business is more threatened by the new technology to reuse water for fracking." Mortensen said the Missouri River has the potential to provide vastly more water than oil drillers would ever need.

But not everyone agrees. Derrick Braaten, a Bismarck attorney who represents farmers and ranchers who are water buyers, said that many in the agriculture sector are worried about whether North Dakota's water supply will accommodate the growing demand. "There's concern about whether we are depleting the aquifers faster than they're going to be able to replenish," he said. (See related quiz: "What You Don't Know About Food, Water, and Energy.")