One day after two earthquakes struck Oklahoma, including one believed to be the third-largest in the state’s history, regulators with the Oklahoma Corporation Commission (OCC) scrambled to take hold of the situation, ordering four wastewater disposal wells shut in and another seven to curtail their operations.

According to the OCC and the U.S. Geological Survey (USGS), an earthquake that measured 4.7 magnitude on the Richter scale struck at 1:42 a.m. CST Thursday eight miles southwest of Cherokee, OK (see Shale Daily, Nov. 19), followed by a 4.0-magnitude earthquake at 8:24 a.m. CST Thursday, its epicenter 15 kilometers (9.3 miles) north of Crescent, OK. The first quake was the third-largest earthquake in state history.

On Friday, the OCC’s Oil and Gas Conservation Division (OGCD) ordered four wells within a three-mile radius of the Crescent quake’s epicenter to shut down on or before Nov. 26. Those wells are Stephens Energy Group LLC’s Kittenbrink 1-36 SWD; Dorado E&P Partners LLC’s Vonda SWD 1-6; and Devon Energy Production Co. LP’s Adkisson 1-33 SWD and Fuxa 25-19-4W 1SWD.

The agency also ordered another seven wells within a three- to six-mile radius of the Crescent quake to reduce their disposal volumes by 50% — 25% on or before Dec. 3, and an additional 25% on or before Dec. 17. Collectively, the disposal volumes were to decrease 29%, from 47,783 b/d to 34,123 b/d.

The OCC said operators of another 10 disposal wells within a six- to 10-mile radius of Crescent were told that they could maintain their current disposal levels at this time, but should future seismic activity occur in the region they may be ordered to reduce their injection volumes as well.

On Thursday, the OCC ordered two wells to shut down and another 23 to reduce their injection volumes after the first quake struck southwest of Cherokee.

The largest earthquake in Oklahoma history was a 5.6-magnitude temblor that struck the Prague area on Nov. 6, 2011. It spawned a 4.8-magnitude aftershock two days later.

Thursday’s two earthquakes bring to 29 the total number of earthquakes of 4.0-magnitude or higher to hit Oklahoma so far this year.

For months, the OCC has been taking action to reduce the number and intensity of earthquakes in the state, ordering some disposal wells to shut down operations and others to reduce the volume of wastewater they accept. The watershed moment came in April, when the OGCD issued new rules for injection well operators working in “areas of interest” that inject into the Arbuckle Formation, the state’s deepest formation (see Shale Daily, April 2).

The OCC shut down two wells and ordered volumes to be cut at another three in September (see Shale Daily, Sept. 21), and ordered 13 wells to change operations in October (see Shale Daily, Oct. 19). The operators of two wells voluntarily agreed to shut down their operations in July, and a third agreed to cut intake volumes by half (see Shale Daily, July 29).

Last March, the OGCD issued a directive to operators of 347 disposal wells targeting the Arbuckle formation to prove that they are not injecting oil and gas waste into basement rock below it. The directive was expanded to include an additional 211 disposal wells earlier this month. The wells are all within “areas of interest,” which include 21 of the state’s 77 counties that have seen increased seismicity (see Shale Daily, July 20; April 2).

Scientists with the Oklahoma Geological Survey (OGS) reported in April that the state was being hit by earthquakes at a rate about 600 times greater than historic background data (see Shale Daily, April 22). Seismicity increased from an average of one-and-a-half earthquakes of 3.0 magnitude or greater to about two-and-a-half such earthquakes every day in 2014.

According to the OGS, the earthquakes are primarily occurring within the crystalline basement. The majority of the state’s approximately 900 injection wells target the Arbuckle formation, which closely overlies the crystalline basement.

Last fall, Oklahoma regulators shut in a disposal well that was thought to have been drilled too deep following a spate of seismic activity in the area (see Shale Daily, Oct. 31, 2014).

A recently released report by Energy In Depth (EID), an industry-backed national shale gas education initiative, said that less than 1% of wastewater disposal wells in the United States used by the oil and gas industry have a potential link to induced seismicity (see Shale Daily, Nov. 19).