The geology of western North Dakota provides an ideal place to dispose of waste saltwater from oil production, according to Ed Murphy, state geologist for North Dakota.
Induced seismic activity, earthquakes brought about by human actions, has made news in Oklahoma recently. Oklahoma documented 907 earthquakes in 2015 with a magnitude of 3 or greater .A website maintained by the state of Oklahoma said, "Seismologists have documented the relationship between wastewater disposal and triggered seismic activity." Oklahoma averaged less than 2 earthquakes per year prior to 2000. The website earthquaketrack.com reported 2,144 earthquakes of more than 1.5 magnitude during the last 12-month period and 28 in the past seven days.
Allison Ritter, public affairs officer for the North Dakota Department of Mineral Resources, said the saltwater disposal injection wells differ from what is commonly called fracking in which water and chemicals are injected into an oil well to open the rock formation and allow the oil to flow.
"Fracking is a method to bring in a well economically," she said.
Oil companies use disposal wells to get rid of the saltwater that is pumped to the surface with the oil.
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North Dakota has 518 disposal wells injecting about 1 million barrels of water into the ground each day.
Murphy said the saltwater is injected into a layer of sandstone known as the Inyan Kara. This rock formation acts like a sponge to absorb the salt water.
In the Oil Patch, useable water extends down to a depth of about 2,000 feet below the surface. The Inyan Kara is about 3,500 feet below the water level and about 8,000 feet above the granite layer where earthquakes would originate.
"We need to stay between the water and the granite," Murphy said.
Murphy said in Oklahoma, the wastewater is injected at or near the granite layer. This causes increases in pressure near fault lines that can lead to earthquakes.
Research into the geology of the North Dakota Oil Patch caused the Interstate Oil and Gas Compact Commission to rate North Dakota at a very low risk for induced seismic activity.
Despite the relatively safe position of the Inyan Kara, research continues and the permitting process for additional disposal wells is strict, Murphy said.
Ritter said disposal wells must be monitored monthly and constructed with a concrete sleeve around the well casing through the area where water quality could be impacted by a leak. Companies applying to drill a disposal well are required to map any known or suspected geologic faults in the area and may be required to place seismic monitors in the area after the well is operational.
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"As long as we have oil and gas production this is the only option," Ritter said. "Recycling is not economical because the water is very salty."
