State and federal regulators said 64 wastewater disposal wells targeting the Arbuckle Formation in Oklahoma should be either shut in or have reduced operations, after a Tuesday evening earthquake that measured 4.5 on the Richter scale struck the same area as the state’s largest temblor recorded in September.

Meanwhile, the head of the Oklahoma Geological Survey (OGS) said Tuesday’s quake was not an aftershock of the 5.8-magnitude temblor that struck on Sept. 3 (see Shale Daily, Sept. 6).

According to the U.S. Geological Survey (USGS), the most recent earthquake struck at 11:26 p.m. CDT Tuesday. It was centered 14 kilometers (8.7 miles) east-southwest of Pawnee, OK, at a depth of 2.6 kilometers (1.6 miles).

In an advisory issued Thursday, the Oklahoma Corporation Commission (OCC) said it had developed a plan to address the latest Pawnee quake after cooperation with the OGS and the U.S. Environmental Protection Agency (EPA). The plan will affect 38 disposal wells under jurisdiction of the OCC’s Oil and Gas Conservation Division (OCGD), and 26 wells under the dominion of the EPA.

The plan creates three concentric zones around the epicenter of the latest quake, at radii of six, 10 and 15 miles. 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.

OCC spokesman Matt Skinner told NGI’s Shale Daily that 15 of the 38 disposal wells included in the latest directive have already been shut in; they were included in a separate directive in September, immediately following the record quake (see Shale Daily, Sept. 13). Skinner said four wells will be shut in under the latest directive, three of which had their volumes curtailed by 25% in the September action. He said another 10 wells within the 10-mile zone would have their volumes reduced by 25%, two of which were already curtailed in September.

Skinner said the remaining nine wells are within the 15-mile zone, eight of which had no previous action taken against them but one well had already been directed to reduce their volumes by 25% in September. He said the one well would keep the 25% limit.

Under the latest plan, the EPA will direct operators of 20 disposal wells to limit their intake volumes to their last 30-day average. Six additional wells will be directed to reduce their volumes by 25% of their last 30-day average. The EPA has sole jurisdiction over disposal wells in Osage County because it lies within the Osage Nation Mineral Reserve.

“When it first started, it sounded like it was going to be an aftershock,” OGS Director Jeremy Boak told NGI’s Shale Daily on Thursday. “But it’s actually lined up on a very small fault segment that we had previously mapped. It’s on our fault map.

“I wouldn’t be surprised if there isn’t some relationship between the two earthquakes — that one may have triggered the other. We’ve been waiting for a magnitude-4.0 aftershock from the [record] Pawnee earthquake. That was an unusual earthquake, in that there doesn’t seem to have been any substantial foreshocks, and we haven’t yet had an aftershock.”

USGS data indicated that the temblor was felt in southern Kansas, southwest Missouri and northwest Arkansas.

Last month, it was discovered that the EPA had recommended that Oklahoma regulators enact a moratorium on oil and natural gas disposal wells in sensitive areas of the Arbuckle (see Shale Daily, Oct. 11). The agency said it was concerned that the state’s response to injection-induced seismicity is both inconsistent and ineffective.

Since the beginning of the year, OCC has ordered operators of about 700 wastewater injection wells targeting the Arbuckle to cease or curtail their operations.

OGCD unveiled its Western Regional Reduction Plan, which called for a nearly 500,000 b/d reduction in wastewater injection volumes, in February (see Shale Daily, Feb. 17). It released its Central Oklahoma Volume Reduction Plan, which called for a 300,000 b/d reduction in injection volumes, one month later.