UK seismicity report gives guarded nod to shale gas work

By OGJ editors

Hydraulic fracturing in the UK has received guarded approval in a government review of a study about seismic events last year around a Cuadrilla Resources Ltd. shale-gas well near Poulton-le-Fylde (OGJ Online, Nov. 3, 2011).

Cuadrilla commissioned the study under review, which found a high probability that hydraulic fracturing of its Preese Hall-1 well triggered seismic events last April and May. The well was completed in a Bowland basin shale, which Cuadrilla has said might hold 200 tcf of natural gas in place in its 437-sq-mile license area between Blackpool and Preston (OGJ Online, Sept. 22, 2011).

The Cuadrilla study said none of the seismic events caused structural damage. The largest event, in April 2011, had a magnitude of 2.3.

The study said the events resulted from a rare combination of geological conditions around the well.

Conducting the review for the UK Department of Energy and Climate Change were Christopher A. Green of G Frac Technologies, Peter Styles of Keele University, and Brian J. Baptie of the British Geological Survey.

“Based on the induced-seismicity analysis done by Cuadrilla and ourselves, together with the agreement to use more sensitive fracture-monitoring equipment and a DECC-agreed induced-seismic protocol for future operations, the authors of this report see no reason why Cuadrilla Resources Ltd. should not be allowed to proceed with their shale gas exploration activities and recommend cautious continuation of hydraulic fracture operations at the Preese Hall site,” they said.

Recommendations

The review authors recommend these measures for mitigation of the risk of future earthquakes in the Bowland basin:

• Hydraulic fracturing procedures should “invariably” include a smaller preinjection and monitoring stage before main injection.

• Hydraulic fracture growth and direction should be monitored with microseismic methods during future treatments.

• Future hydraulic fracturing work in the area should be subject to “an effective monitoring system that can provide automatic locations and magnitudes of any seismic events in near real time.”

• Operations should be halted and remedial action taken if events of magnitude 0.5 or above are detected.

The review group also recommended assessment of seismic hazards before shale gas work proceeds elsewhere in the UK. The assessment, it said, should include baseline seismic monitoring to establish background seismicity, characterization of any active faults based on all available geological and geophysical data, and application of ground-motion prediction models to assess potential effects of any induced earthquakes.

The DECC is accepting comments on the review through May 25. It said it won’t decide on whether to approve continuation of hydraulic fracturing until after the comment period.

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