Induced Seismicity and its Relationship to Shale Gas Hydrofracturing

Shale Gas exploration in the UK began in 2010 with five stages of hydraulic fracturing of the Bowland Shale. Hydofracturing in Cuadrilla Preese Hall-1 used 5 couplets of mini- and main-fracs with c. 10,000 bbl per stage, hydraulically isolated from each other during injection. An earthquake of 2.3 ML, a 1.5ML and a series of smaller events occurred from 31st March through May 2011 when work ceased for detailed analysis. Seismicity commenced after stages 2, 4 and 5; the largest occurring 10 hours after stage-2 shut-in. Seismological analysis using observations from seismometers emplaced after the 2.3 event, and regional data detected 55 seismic events from ML-2 to ML2, with 14 between ML 0.2 to M2.3, indicating a low B-value of 0.8 (+/- 0.3) suggesting an unusually small number of weaker events. The timing was highly correlated with injection with the largest events (stages 2 and 4) preceded by weaker events (ML0 and ML1.4) all located near the Preese Hall well, c. 500 meters south of the injection interval. The observed seismicity is almost certainly induced by hydraulic fracturing of the Preese Hall well. Future mitigation based on microseismic monitoring during hydrofracturing should reduce the likelihood of inducing further felt earthquakes.