Preese Hall Shale Gas Fracturing Review & Recommendations for Induced Seismic Mitigation

SUMMARY A series of studies were commissioned by Cuadrilla Resources Ltd to examine the possible relationship between hydraulic fracture operations at the Preese Hall well, near Blackpool, and a number of earthquakes which occurred in April and May 2011, the largest of which had a magnitude of 2.3 M L. The reports from these studies conclude that the earthquake activity was caused by direct fluid injection into an adjacent fault zone during the treatments, but that the probability of further earthquake activity is low. The reports analyse the earthquake activity and use available geological and geophysical data, including background geology, well logs and core samples, along with fracture treatment data, to develop a conceptual geomechanical model. A numerical model consisting of a single fault plane in a rock matrix was used to simulate the induced seismicity, compare with observations, and estimate maximum magnitudes for induced earthquakes. A critical magnitude at which damaging ground motions might occur was estimated using the German DIN4150 standard, and a simple relationship suggested for ground motions as a function of magnitude and distance. Finally, a protocol for controlling operational activity is proposed. This builds on extensive Enhanced Geothermal System experience and uses a traffic light system based on real-time monitoring of seismic activity. We have been asked by DECC to review these reports, and the further studies and information provided by Cuadrilla; and to make appropriate recommendations for the mitigation of seismic risks in the conduct of future hydraulic fracture operations for shale gas. We agree with the conclusion that the observed seismicity was induced by the hydraulic fracture treatments at Preese Hall. However, we are not convinced by the projected low probability of other earthquakes during future treatments. We believe it is not possible to state categorically that no further earthquakes will be experienced during a similar treatment in a nearby well. The analyses failed to identify a causative fault, and detailed knowledge of faulting in the basin is poor. In the present state of knowledge it is entirely possible that there are critically stressed faults elsewhere in the basin. It is possible that a 3-D seismic reflection survey could help better characterize faulting within the basin. We also consider that the use of the numerical simulations to estimate maximum likely magnitude of any further earthquake should be treated with some caution, mainly because the model is necessarily simplistic due to lack of data …