A report this week from the U.S. Geological Survey finds that six states are at increased risk from earthquakes caused by oil and gas wastewater injection wells.
The practice of injecting wastewater in deep wells is at cause, not hydraulic fracking, likely because of the pressurized volumes of the liquid, the report says.
North Dakota is not on the list, despite hundreds of wastewater injection wells and billion-plus barrels injected underground. In fact, a supplemental report by the Seismological Society of America says that earthquakes are rarer near injection wells in the Williston Basin than in oil basins in Texas and Oklahoma.
“The reason why Williston Basin earthquakes are so scarce is unclear,” the report says. The authors suggest that in Oklahoma, with most seismic activity and thousands of wells, injection well volumes are higher than in North Dakota.
This is the first time the USGS has included human-caused hazards in its report and provided a one-year outlook. It finds that 7 million people in Oklahoma, Kansas, Texas, Colorado, New Mexico and Arkansas — ranked in order of concern — are at risk for damaging ground-shaking events this year. The survey recorded 1,010 earthquakes in those states in 2015, compared to an average of 24 from 1973-2008. The largest earthquake near active injection wells was a 5.6 magnitude quake in Oklahoma.
The overall number of injection wells and volume has increased significantly with oil and gas production on the rise.
North Dakota has 439 active disposal wells in the oil patch. Last year, a record 441 million barrels of saltwater were injected in wells throughout the Bakken region. That compares to 136 million barrels injected in 2010 and 70 million barrels in 2000, according to the Department of Mineral Resources.
Saltwater is a byproduct of oil and is produced at roughly a one-to-one ratio, meaning in excess of 1 million barrels of salty wastewater comes from Bakken production daily.
One reason why earthquakes are rare in North Dakota — a total of 13 reported between 1915 and 2012 — may have to do with the luck of geology.
State geologist Ed Murphy says injection wells are required to be drilled into the Dakota Group zone, a layer about 5,000 feet down where the Inyan Kara sandstone formation provides a porous container for the liquid.
Murphy said the Inyan Kara sandstone can run as much as 200 feet thick and is well below fresh water aquifers and well above igneous and metamorphic basement rock. Additionally, North Dakota is not underlain by deep fault lines where the wastewater injection could increase instability, Murphy said.
“We’re blessed with our geology here and may be better suited than other basins,” he said.
Alison Ritter, spokeswoman for the Department of Mineral Resources, said injection wells are permitted for specific pressure and closely monitored.
“In theory, the formation can take almost anything (in volume), and injection wells are plugged based mostly on well bore integrity,” she said.
Murphy said his department is using drilling logs to create maps that show the thickness of the Inyan Kara sandstone so injection wells can be best placed. So far, it’s mapped the northern half of McKenzie County and is adding to the data across the Bakken.
“We’ve gotten a lot of positive feedback, and companies are asking us to hurry and do the rest of the state. We can get a good handle on the volume of sandstone and then get a handle on the most desirable locations,” he said.
The Department of Mineral Resources has information about the injection well program and earthquakes at www.dmr.nd.gov/oilgas/undergroundfaq.asp#mr10.