More than two dozen quakes have been recorded in far north central Oklahoma in past four days.
State regulators on Tuesday directed five companies to limit operations at nine disposal wells north of Medford after more than two dozen earthquakes shook the area over the past four days.
The strongest recent tremor in the area was a magnitude 4.1 that struck Saturday morning.
"When we had the (magnitude) 4 (earthquake), we did our standard area review," said Tim Baker, director of the Oklahoma Corporation Commission's Oil and Gas Conservation Division. "We isolated the wells closest to where the 4 occurred."
The commission has directed that volumes at six nearby disposal wells be cut back 25 percent; that volumes at two wells be cut in half; and that one well be shut down and reduced in depth before volume is reduced by 50 percent. The directives were sent to well operators SandRidge Exploration Production LLC; Kirkpatrick Oil Co. Inc.; Primexx Operating Corp.; Union Valley Petroleum Corp.; and Special Energy Corp.
The agency said its plan is based on how close the wells are to the center of the seismic activity: Wells within three miles of the center of recent earthquake activity must reduce wastewater disposal volumes by half; those from three to six miles must cut volumes 25 percent, and all others within 10 miles should be "on notice to prepare for possible future changes."
Oil and gas industry representatives supported the commission's actions Tuesday.
"It's a good, measured response," said Chad Warmington, president of the Oklahoma Oil and Gas Association. "As long as it's being driven by good data of where the earthquakes are occurring, taking in the injection well data, looking at where the injections are and trying to balance out the injections, I think it's the right approach."
Medford is in Grant County in north-central Oklahoma, just south of the Kansas state line. The area until the past few days had seen a decline in seismic activity over the past three months, said Jeremy Boak, director of the Oklahoma Geological Survey.
"Since July, we've had about a 20 percent reduction in the frequency of earthquakes," Boak said during a panel at the Governor's Energy Conference on Tuesday.
"Is this a lull that ended last weekend, is it part of long-term activity in response to actions the Corporation Commission has asked for, or is it that a wells are being shut in because they are not economical? We still need to look at the area where the reduction is happening and see what's going on."
Overall, seismic activity in Oklahoma has slowed since July, but the reduction has not been uniform, he said. The state averaged 4.8 earthquakes of magnitude 2.8 or greater per day in July. That number has decreased to 3.5 per day. While the overall rate has dropped, isolated areas — including Cushing and parts of Logan County — have seen increases. The biggest reduction has been in Grant and Alfalfa counties, Boak said.
Grant, Alfalfa and Logan counties have been the most seismically active this year, representing most of the state's 28 earthquakes of 4.0 or greater.
Stanford University professor Mark Zoback said scientists and regulators now have a much better understanding of what is causing the quakes, but that they still need more information to determine how to reduce the shaking.
“We understand the process now,” he said. “Large amounts of water being injected is driving the process. There are faults causing the earthquakes that are unmapped and that we don't know about. The question is how to move forward in a way that sustains economic development and protects the public.”
So far, regulators have focused on reducing disposal volumes, making sure disposal wells are not drilled too deep and gaining a better understanding of the natural faults that underlie the state.
“The solution will involve less injection in areas where the fluid has direct contact with faults,” Zoback said. “The challenge is we don't know where all the faults are.”
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