Abstract
Hydrofracturing, used for shale gas exploitation, may induce felt, even damaging earthquakes. On 15 June 2019, an Mw2.8 earthquake occurred, spatially correlated with the location of earlier exploratory hydrofracturing operations for shale gas in Wysin in Poland. However, this earthquake was atypical. Hydrofracturing-triggered seismicity mainly occurs during stimulation; occasionally, it continues a few months after completion of the stimulation. In Wysin, there were only two weaker events during two-month hydrofracturing and then 35 months of seismic silence until the mentioned earthquake occurred. The Wysin site is in Gdańsk Pomerania broader region, located on the very weakly seismically active Precambrian Platform. The historical documents, covering 1000 years, report no natural earthquakes in Gdańsk Pomerania. We conclude, therefore, that despite the never observed before that long lag time after stimulation, the Mw2.8 earthquake was triggered by hydrofracturing. It is possible that its unusually late occurrence in relation to the time of its triggering technological activity was caused by changes in stresses due to time-dependent deformation of reservoir shales. The Wysin earthquake determines a new time horizon for the effect of HF on the stress state, which can lead to triggering earthquakes. Time-dependent deformation and its induced stress changes should be considered in shall gas reservoir exploitation plans.
Highlights
Hydraulic fracturing (HF) is a high-pressure liquid injection technique to increase rocks’ matrix permeability
We present and discuss the case of the Mw2.8 earthquake in Wysin, Poland, which was most likely triggered by HF exploration operations
In light of the gathered facts, we conclude that earlier hydrofracturing operations triggered the Wysin earthquake
Summary
Hydraulic fracturing (HF) is a high-pressure liquid injection technique to increase rocks’ matrix permeability. It is routinely used to exploit unconventional hydrocarbon reservoirs, often located in shale formations (e.g., [1]). Like any technics used for go-resources exploitation, HF can have adverse effects. It is well known that shale gas extraction based on HF may induce or trigger seismic events. A comprehensive summary of cases from Canada, the United Kingdom, the United States, and China can be found in [3,4]. As the authors of the cited publications indicate, only a very small fraction of HF wells are associated with induced seismic events. The events like, e.g., the two M5+ (2018, 2019) earthquakes in Sichuan province, China [5,6], and the M4+ earthquakes in Canada and the US ([3], Table 2), exemplify a possible destructive potential of the HF-induced seismicity and evoke understandable public concern
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