The state Department of Environmental Protection said Friday minor earthquakes that occurred in Lawrence County one day last April were likely correlated to natural gas hydraulic fracturing by Hilcorp Energy Co.
The DEP said in a news release the quakes, recorded April 25 in western Lawrence County, were less than a mile from a natural gas well owned by Houston-based Hilcorp.
The quakes were too weak to be felt by humans, and no damage was reported. Hilcorp ceased fracking at the well pad after the quakes occurred.
Fracking is a method that extracts gas or oil from underground shale rock. It has been tied to earthquakes in Ohio and other states, but never before in Pennsylvania, the nation’s No. 2 natural gas-producing state.
According to DEP’s report, a series of low-magnitude earthquakes that began at 4:17 a.m. April 25 in North Beaver, Union and Mahoning townships showed a “marked temporal/spatial relationship” to hydraulic activities at Hilcorp’s North Beaver NC Development well pad.
The Pennsylvania Seismic Network registered four earthquakes. The final magnitude of the events ranged from 1.8 to 2.3 on the Richter scale, putting them in the category of microseismic events, which are likely to go unnoticed by humans and are only recorded by sensitive seismic monitors.
DEP said the Hilcorp pad, which includes four wells drilled into the Utica Shale formation, lies within a five-mile radius of the reported seismic epicenters. The agency said hydraulic fracturing activities began at the pad March 30.
Hilcorp was using a technique known as “zipper fracturing,” hydraulic fracturing operations that are carried out concurrently at two horizontal wellbores that are parallel and adjacent to each other.
Dave Yoxtheimer, a hydrogeologist with the Penn State Marcellus Center for Outreach and Research in State College, said Friday that zipper fracturing has become more common among drillers who work with longer laterals and want to reduce downtime by moving equipment back and forth between parallel well sites that are being hydraulically fractured.
He acknowledged that the western portion of Lawrence County, much like neighboring Ohio, is in an area where the Utica is closer to the surface of the Earth and also has crystalline bedrock closer to the shale strata.
“The closer you get to the crystalline bedrock, the more critically stressed the earth gets,” he said, adding that when fracking fluid is applied under high pressure for several hours it can cause the rock to slip, creating seismic events.
He said a similar effect was noted in Ohio when drilling occurred near underground storage wells, sometimes causing seismic events.
“It’s the geology and the geologic setting,” he said of the circumstances that can create the heightened possibility of seismic occurrences from hydraulic fracturing.
DEP said that when it contacted Hilcorp on April 25, it voluntarily stopped activities and later said it would discontinue hydraulic fracturing and stimulation operations at the well pad indefinitely.
DEP recommended that zipper fracturing should be discontinued during any future completions when there is less than a quarter-mile between lateral portions of adjacent wellbores.
In addition, it recommended that Hilcorp maintain operation of its own seismic network within the townships where the tremors occurred, to allow for the accurate detection of local, low-magnitude events.
It also outlined a seismic event reporting schedule and operator response plan for Hilcorp, has also recommended that the terms apply to any new permits requested by Hilcorp and recommended that other operators follow similar plans within the referenced townships.
In November, DEP approved a seismic monitoring plan submitted by Hilcorp implementing the recommendations set forth by the agency.
Dave Spigelmyer, president of the Marcellus Shale Coalition, said Friday the Lawrence County seismic events represented a single incident in the Utica formation, noting that there are more than 7,000 unconventional wells drilled, fractured and safely in production.
“Pennsylvania’s natural gas producers hold the highest commitment to community, workforce and environmental safety. The isolated and exceptionally rare nature of this single occurrence reflects the fact that industry-leading best practices are working as designed,” he said in a statement.