National Geographic has an article on the water demands of North Dakota's shale oil wells - North Dakota's Salty Fracked Wells Drink More Water to Keep Oil Flowing.
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.
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). Without water, salt buildup forms and restricts the flow of oil.