Abstract
Shale oil and gas exploitation by hydraulic fracturing experienced a strong development worldwide over the last years, accompanied by a substantial increase of related induced seismicity, either consequence of fracturing or wastewater injection. In Europe, unconventional hydrocarbon resources remain underdeveloped and their exploitation controversial. In UK, fracturing operations were stopped after the Mw 2.3 Blackpool induced earthquake; in Poland, operations were halted in 2017 due to adverse oil market conditions. One of the last operated well at Wysin, Poland, was monitored independently in the framework of the EU project SHEER, through a multidisciplinary system including seismic, water and air quality monitoring. The hybrid seismic network combines surface mini-arrays, broadband and shallow borehole sensors. This paper summarizes the outcomes of the seismological analysis of these data. Shallow artificial seismic noise sources were detected and located at the wellhead active during the fracturing stages. Local microseismicity was also detected, located and characterised, culminating in two events of Mw 1.0 and 0.5, occurring days after the stimulation in the vicinity of the operational well, but at very shallow depths. A sharp methane peak was detected ~19 hours after the Mw 0.5 event. No correlation was observed between injected volumes, seismicity and groundwater parameters.
Highlights
Hydraulic fracturing (HF), or fracking, is a technique designed to recover gas and oil from so-called unconventional reservoirs, which correspond to tight sands, coal beds or shale formations
The most numerous and recent cases of induced seismicity which have been directly associated to HF, with a highly correlation in time and space with fracturing wells, were located in the Western Canada Sedimentary Basin (WCSB)[8]
In south-central Oklahoma, earthquakes ranging in local magnitude from ML 0.6 to 2.9 were identified in January 2011, which were likely triggered by HF operations[13]
Summary
Hydraulic fracturing (HF), or fracking, is a technique designed to recover gas and oil from so-called unconventional reservoirs, which correspond to tight sands, coal beds or shale formations. The main concerns to HF are the potential contaminate of groundwater at the fracking site due to the injection of proppants, air pollution resulting by HF operations, and induced seismicity. The largest event ever related to HF operations occurred on August 17th, 2015, near Fort St. John, British Columbia, with a Mw 4.611; we note that magnitudes up to Mw 4.7 have been reported in the Sichuan Basin (China) involving injection-induced fault reactivation[12]. Recent works studied the seismicity associated with the fracking of 53 wells and initiation of wastewater injection over a 3-month period in 2010 in the Guy-Greenbrier, Arkansas area[16] Their results showed that only about half of the stimulated wells induced seismicity at a detection threshold below ML 0. There is substantial variability in the seismic response to fracking, both regionally and within a single field
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