New research published by the Bulletin of the Seismological Society of America reaffirms conclusions reached by Ohio regulators that horizontal hydraulic fracturing operations at a Hilcorp Energy Co. site in March activated a previously unknown fault line and triggered a series of earthquakes.
A series of five recorded earthquakes were reported, with one, a 3.0-magnitude quake felt by the public. They ranged in size from 2.1- to 3.0-magnitude and were recorded within about a half-mile of Hilcorp's operations at the Carbon Limestone Landfill in Poland Township, OH -- about 10 miles southeast of Youngstown -- where Hilcorp had been stimulating one of six wells at a pad there leading up to the seismic events (see Shale Daily, March 11).
"These earthquakes near Poland Township occurred in the Precambrian basement, a very old layer of rock where there are likely to be many pre-existing faults," said study co-author Robert Skoumal of Miami University in Ohio. "This activity did not create a new fault, rather it activated one that we didn't know about prior to seismic activity."
Using a technique called template matching, the researchers went through seismic data recorded by a network of seismic stations looking for repeating signals similar to the known Poland Township earthquakes with magnitudes from 1.0 to 3.0 that occurred between March 4 and 12 in the area. The researchers compared the identified earthquakes to well stimulation reports released by the Ohio Department of Natural Resources (ODNR). They found the earthquakes coincided with hydraulic fracturing at specific stages of stimulation.
The seismic activity, researchers said, outlined a vertical, east-west fault within roughly a half mile of the well. Other nearby wells produced no seismicity, which suggested the fault had a limited extent.
"Because earthquakes were identified at only the northeastern extent of the operation, it appears that a relatively small portion of the operation is responsible for the events," Skoumal said. "We just don't know where all the faults are located. It makes sense to have close cooperation among government, industry and the scientific community as hydraulic fracturing operations expand in areas where there's the potential for unknown pre-existing faults."
A Hilcorp spokesman said the company would not be commenting about the study until it has had a chance to review it.
Speaking at an industry conference last month, Tom Tomastik, who formerly served as lead geologist of ODNR's Underground Injection Control Program, said Hilcorp had been conducting zipper fracks on two wells at the site at the time of the earthquake (see Shale Daily, Dec. 4, 2014; March 12, 2014). Regulators working with Hilcorp, he said, learned that a Precambrian fault line did run beneath the company's well pad. But Tomastik said the company had no idea until it went back to examine its 2-D and 3-D seismic data.
ODNR halted operations at the Hilcorp site after the earthquakes, a precautionary move the company agreed with at the time, saying “public safety is of paramount importance.” The agency also eventually issued more stringent permitting standards for wells near known fault lines (see Shale Daily, April 11, 2014).