High-Resolution Imaging of the ML 2.9 August 2019 Earthquake in Lancashire, United Kingdom, Induced by Hydraulic Fracturing during Preston New Road PNR-2 Operations

Abstract

Hydraulic fracturing (HF) at Preston New Road (PNR), Lancashire, United Kingdom, in August 2019, induced a number of felt earthquakes. The largest event (ML 2.9) occurred on 26 August 2019, approximately three days after HF operations at the site had stopped. Following this, in November 2019, the United Kingdom Government announced a moratorium on HF for shale gas in England. Here we provide an analysis of the microseismic observations made during this case of HF-induced fault activation. More than 55,000 microseismic events were detected during operations using a downhole array, the vast majority measuring less than Mw 0. Event locations revealed the growth of hydraulic fractures and their interaction with several preexisting structures. The spatiotemporal distribution of events suggests that a hydraulic pathway was created between the injection points and a nearby northwest–southeast-striking fault, on which the largest events occurred. The aftershocks of the ML 2.9 event clearly delineate the rupture plane, with their spatial distribution forming a halo of activity around the mainshock rupture area. Across clusters of events, the magnitude distributions are distinctly bimodal, with a lower Gutenberg–Richter b-value for events above Mw 0, suggesting a break in scaling between events associated with hydraulic fracture propagation, and events associated with activation of the fault. This poses a challenge for mitigation strategies that rely on extrapolating microseismicity observed during injection to forecast future behavior. The activated fault was well oriented for failure in the regional stress field, significantly more so than the fault activated during previous operations at PNR in 2018. The differing orientations within the stress field likely explain why this PNR-2 fault produced larger events compared with the 2018 sequence, despite receiving a smaller volume of injected fluid. This indicates that fault orientation and in situ stress conditions play a key role in controlling the severity of seismicity induced by HF.

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“…This was the case at the Preston New Road wells, where the red light was triggered on multiple occasions. Most notably, during PNR-2 the red-light threshold was reached with an M 1.6 event on the 21st August 2019, but further injection took place on the 23rd August 2019, after which the M 2.9 event occurred (Kettlety et al 2020). Evidently, the pause in injection did little to reduce the cumulative impact of injection, with event magnitudes continuing to rise after injection re-started.…”

“…In the UK, hydraulic fracturing of the Preese Hall well in 2011 generated an M 2.3 event: this was the first widely reported case of HF-IS (Clarke et al, 2014). In December 2018, stimulation of the Preston New Road PNR-1 well produced an M 1.5 event that was felt by nearby residents (Clarke et al, 2019), while stimulation of the adjacent PNR-2 well in August 2019 produced an M 2.9 event that was felt in the nearby towns of Blackpool and Preston (Kettlety et al 2020;Cremen et al, 2020). While the magnitudes of events in the UK have not been as large as those in the WCSB or the Sichuan Basin, all 3 stimulated wells in the Bowland Shale have produced HF-IS of sufficient magnitude to be felt by nearby residents, although it should be noted that these three wells are within 5 km of each other, and so may not be representative of the Bowland as a whole.…”

“…Moreover, in many cases, the post-injection seismicity has continued to increase in magnitude (e.g. Majer et al, 2007;Häring et al, 2008;Clarke et al, 2014Clarke et al, , 2019Bao and Eaton, 2016;Kao et al, 2018;Ellsworth et al, 2019;Cremen et al, 2020;Kettlety et al, 2020). Such earthquakes are often referred to as "trailing events".…”

“…The first was the Preese Hall well in Lancashire in 2011, which produced an M 2.3 event (Clarke et al, 2014). In 2018, the Preston New Road PNR-1z well was stimulated, approximately 4 km from the Preese Hall site, producing an M 1.5 event (Clarke et al, 2019), and in 2019 the adjacent PNR-2 well was stimulated, producing an M 2.9 event (Cremen et al, 2020;Kettlety et al, 2020).…”

“…As an example, at the Preston New Road site in the UK, Kettlety et al (2020) identified that the different orientations of the two reactivated faults may account for their different responses. The PNR-1z fault, which was moderately well orientated in the in situ stress field, saw a gradual increase in magnitudes over time, whereas the PNR-2 fault, which was very well orientated in the in situ stress field, reactivated after a much smaller volume had been injected, and produced larger magnitude jumps and trailing events.…”