Oklahoma regulators have directed operators of more than 50 wastewater injection wells near Cushing to either cease or scale back their operations, in response to a magnitude-5.0 earthquake that struck near the crude storage and transportation hub last weekend.
In an advisory, the Oklahoma Corporation Commission's (OCC) Oil and Gas Conservation Division (OGCD) said it had created a plan establishing three concentric zones around the epicenter of the latest quake, at radii of six, 10 and 15 miles. The OGCD created a similar plan, with matching zones, around the epicenter of another quake, which measured 4.5 in magnitude and struck near Pawnee, OK, on Nov. 1.
The OGCD directed that all disposal wells within the six-mile zone be shut in, and that all disposal wells within the 10-mile zone reduce their intake volumes by 25% of their last 30-day average. Disposal wells within the 15-mile zone will have their volumes limited to their last 30-day average.
Although the latest plan will cover 58 disposal wells targeting the Arbuckle formation, four wells have already been shut-in under a previous directive from October 2015. Fifty-four wells will require action under the latest plan.
"It is important to note that this plan is an initial response, and operators are being warned that work is underway on a broader plan that will encompass a greater area and more Arbuckle disposal wells," the OGCD said in a statement. "Work is expected to take several weeks."
Under the latest directive, seven wells will be shut-in, 16 wells will reduce their intake volumes by 25% of their last 30-day average, and 31 wells will have their volumes limited to their last 30-day average. The volume reductions for the latter two groups, 47 wells in all, will be addition to 40% volume reductions regulators imposed earlier this year.
The deadline for shut-in compliance is Monday, while the deadline for volume limit compliance is Nov. 21.
According to U.S. Geological Survey (USGS) data, the 5.0-magnitude earthquake struck 1.2 miles west of Cushing at 7:34 p.m. CST on Nov. 6, at a depth of 3.1 miles. USGS picked up an aftershock measuring 2.6-magnitude less than an hour later, at 8:23 p.m., centered 1.2 miles north-northwest of Cushing and at a depth of 3.2 miles. No damage was reported at the Cushing oil terminal.
The Oklahoma Geological Survey (OGS) said the fault that caused the most recent temblor and aftershocks in the Cushing area is the same fault responsible for a series of minor quakes that struck the city more than a year ago.
The OGCD began ordering operators to either shut down or curtail intake volumes at injection wells in March 2015, shortly before scientists with the OGS attributed the increase in seismic activity to injection well stargeting the Arbuckle formation, which closely overlies the crystalline basement. The OGS said the disposal of extremely salty water, a byproduct of oil and gas production, not the mostly freshwater used for hydraulic fracturing, is responsible for the quakes.