It is “highly probable” that the controversial process of hydraulic fracturing to create shale gas off the coast of Blackpool triggered two earthquakes earlier this year, a report revealed today.
Two seismic events — one of magnitude 2.3 in April and another of 1.5 in May — were the focus of the report but another 48 weaker seismic events were also detected by the British Geological Survey following activities by developer Cuadrilla to extract shale gas. These water injection operations were taking place over 3km below the earth’s surface and around 3.5km east of the outer limits of Blackpool.
Cuadrilla suspended its drilling operations — known as fracking — at Preese Hall-1 well near Blackpool in early June ahead of commissioning the report.
An “unusual combination of factors” caused the seismic events (see below), including the geology of the well site coupled with the pressure exerted by water injection, it was reported. The report concluded that this combination of factors was “unlikely to occur” again at future well sites but that if it did then local geology would mean seismic events of around magnitude 3 could occur as a worst case scenario.
The report recommended that Cuadrilla install an early detection system to mitigate the escalation of seismic events and in turn, the chance of any future seismic event exceeding safe limits — it said that based on internationally accepted German standards a maximum seismic magnitude of 2.6 should be used.
“We are ready to put in place the early detection system that has been proposed in the report so that we can provide additional confidence and security to the local community,” said Cuadrilla Resources chief executive Mark Miller. “Cuadrilla is working with the relevant local and national authorities to implement the report’s recommendations so we may safely resume our operations.”
The report has been submitted to the Department of Energy and Climate Change (Decc) and the British Geological Survey, the latter acting in its capacity as adviser to Decc.
An ‘unusual combination of factors’
The report indicates that a number of factors coincided to cause the seismic events:
- The Preese Hall-1 well encountered a pre-existing critically stressed fault
- The fault was transmissible so it accepted large quantities of fluid
- The fault was brittle enough to fail seismically
- The repeated seismicity was most likely induced by repeated direct injection of fluid into the same fault zone
- The strongest events took place around 10 hours after the injection because the pressure spread out over a larger area
- It is unlikely that the actual opening of the hydraulic fractures induced the seismic events because there is a delay of many hours between the injection of fluid and the strongest seismic event. Fluid pressure on the fault, however, has a natural time scale which fits with the observed delay
- The chance of any one of these factors occurring is small, therefore the probability of a repeat occurrence of a fracture-induced seismic event with similar magnitude in the Bowland basin is very low