Farm Progress

Zipper-fracking of Utica natural gas well laterals has been tied to deep earthquakes over bedrock faults.

February 27, 2017

2 Min Read
DEEP-DOWN SHAKES: Zipper-fracking over fault lines may trigger mini-quakes.Spencer Platt, Getty Images

Remember last April’s earthquakes in western Pennsylvania? Most people didn’t — until they saw it in the news. The April 1.8 to 2.3 Richter “shakes,” some 7,900 feet down in the Utica shale formation, were detectable only by seismograph.

A report issued last week by the Pennsylvania Department of Environmental Protection confirmed that the microseismic events in Lawrence County were related to shale fracking. The tremors were also related to what’s called zipper fracking by Hilcorp in northeast Ohio two years earlier.

DEP’s report outlined procedures now used “to reduce seismic risks going forward,” noted DEP Acting Secretary Patrick McDonnell. The series of low-magnitude earthquakes showed a “marked temporal/spatial relationship” to hydraulic fracturing activities at Hilcorp’s North Beaver township well pad. 

The Hilcorp pad includes four horizontal laterals drilled into the Utica formation and lies within a 5-mile radius of the reported quake epicenters. Hilcorp employed “zipper fracturing,” hydraulic fracturing at two horizontal wellbores parallel and adjacent to each other.

Fracking-related quakes are extremely rare. Conditions must be just right — fracking immediately over the top of a fault in the rock layers.

When contacted by DEP, Hilcorp voluntarily stopped activities and later reported it had discontinued hydraulic fracturing and stimulation operations at the well pad indefinitely.

Zipper-fracking recommendations
The report advised discontinuation of zipper fracking when there’s less than a quarter mile between lateral portions of adjacent wellbores. It’s still allowed for greater distances.

Hilcorp would maintain its own seismic network for accurate detection of local, low-magnitude events within the affected townships and follow a reporting schedule for such events within 6 miles of the wellbore path.

Seismic events of 1.5 Richter or more within a 3-mile radius of that path would warrant a shutdown of stimulation operations and developing a plan to modify them.

What’s zipper fracking?
In brief, it’s when operators drill two horizontal laterals side by side, then frack them at the same time. The fractures form a zipper pattern that cracks the rocks more deeply and efficiently than in a single well. The process allows both wells to produce more oil and gas. In one before-and-after Marcellus Shale study in Pennsylvania, output of both wells increased 168%.

Source: Marcellus Drilling News and Pennsylvania DEP

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