Four days after the largest earthquake in Oklahoma state history struck Pawnee County, the U.S. Environmental Protection Agency (EPA) ordered the shutdown of 17 disposal wells in neighboring Osage County.
Meanwhile, the U.S. Geological Survey (USGS) said it has updated the official magnitude of last Saturday's quake from 5.6 to 5.8, and was deploying additional seismic equipment to the region.
EPA said it began coordinating actions with the Oklahoma Corporation Commission (OCC) after the record temblor (see Shale Daily, Sept. 6). OCC's Oil and Gas Conservation Division (OGCD) ordered 37 disposal wells targeting the Arbuckle formation to cease operations in response to the quake.
Although all 37 of the wells ordered shut down by OCC lie within a 725-square-mile area of the state, an additional 211 square miles of that zone are in Osage County and are part of the Osage Nation Mineral Reserve, where EPA is the permitting authority for disposal wells under the Safe Drinking Water Act.
"EPA has contacted the operators of the 17 permitted disposal wells injecting into the Arbuckle formation within this portion of Osage County," said EPA's Region 6, which is based in Dallas. "Injection well operators agreed to shut down their wells consistent with the actions in state jurisdiction, and some well operators had already shut down.
"EPA is committed to coordinating closely with officials from the Osage Nation, Osage Minerals Council, state of Oklahoma, the Bureau of Indian Affairs and the USGS to determine further actions."
On Wednesday, the USGS said it had revised the magnitude of last Saturday's earthquake after conducting further in-depth analysis of seismic recordings.
"Changes in estimated magnitude for an earthquake are common in the hours to days following the event, as more data are analyzed in greater detail than is possible in the first minutes after the earthquake occurs," USGS said.
Up until last Saturday, the largest recorded earthquake in Oklahoma history struck the Prague area on Nov. 6, 2011. USGS said it was also updating the official magnitude of the Prague quake from 5.6 to 5.7 magnitude, again based on further analysis.
"USGS analyses indicate that the two earthquakes are very similar in size, to within typically-cited uncertainties of 0.1 magnitude units," said USGS research geophysicist Gavin Hayes. "However, the 2016 Pawnee event is slightly larger than the Prague earthquake in 2011.
"While the difference in size between the two events is less than 0.1 magnitude units, rounding magnitudes to one decimal place means that the magnitude of the Prague earthquake is 5.7, and the Pawnee earthquake is 5.8."
USGS spokeswoman Heidi Koontz confirmed to NGI's Shale Daily on Thursday that the agency would deploy eight new seismic instruments to the Pawnee area to gather additional data.
Since the beginning of the year, OCC has ordered operators of wastewater injection wells targeting the Arbuckle Formation to cease or curtail their operations.
Last February, OGCD unveiled its Western Regional Reduction Plan, which called for a nearly 500,000 b/d reduction in wastewater injection volumes (see Shale Daily, Feb. 17). One month later, the agency released its Central Oklahoma Volume Reduction Plan, which called for a 300,000 b/d reduction in injection volumes (see Shale Daily, March 7).