The maximum strength of tremors induced by new fracking at Cuadrilla’s Preston New Road site has been estimated at 3.1ML, the company said in a document published today.
The figure came in the hydraulic fracturing plan for the second well at the shale gas site near Blackpool.
Cuadrilla fracked the first well at the site, PNR1Z, in autumn 2018, inducing more than small 50 seismic events.
In the fracking plan for the second well, PNR2, the company said:
“Cuadrilla is anticipating that the horizontal well bore, or the area intended to be hydraulically stimulated, will encounter a number of small local faults.
“Modelling a conservative assumption … the upper bound estimate for maximum magnitude possible would be 3.1 ML, which is considered to be a very low likelihood.”
This level of earth tremor would be significantly higher than any induced by fracking at PNR1Z.
Then, the strongest was 1.5ML (DrillOrDrop tremor tracker). Cuadrilla said there were six tremors with a magnitude of 0.5ML or more – the limit under the traffic light system regulations at which fracking should pause for 18 hours.
Cuadrilla has repeatedly called on the government to raise this threshold. It has pointed to limits of up to 4.5ML used in a few other countries. So far, ministers have declined to review the regulations.
Today’s plan said the 3.1ML maximum for PNR2 was based on worst case scenarios. They included direct injection into critically-stressed faults and using 2,000m3 injection stages. The plan said not all faults were critically stressed. The company also intended to use less than half the modelled volume of hydraulic fracturing fluid per stage.
A previous version of the hydraulic fracturing plan for PNR2 was written in August 2018 but was withdrawn by Cuadrilla. Today’s version is 12 pages longer and has some changes. It will now be reviewed by the Environment Agency and the Oil and Gas Authority.
The new version has changes to the procedures for responding to seismic events induced by fracking.
The previous plan required operations to verify well integrity after a trailing event – a tremor measuring 0.5ML or more that happened when fracking was not taking place.
In the revised version, operations can continue as normal if the centre of the tremor is outside the operational boundary. If it is inside, the operator must reduce well pressure, pause fracking for 18 hours and then verify well integrity.
Cuadrilla has also developed a ground motion prediction model in this version of the plan. This converts the magnitude of a seismic event at its origin into a predicted disturbance at the surface.
The plan said fluid volumes, rates, pressures and injection points would be adjusted if surface vibration caused by fracking was higher than 15mm/s PPV (peak particle velocity).
Modelling for the revised version takes into account higher viscosity frac fluid, which Cuadrilla is seeking permission to use at the site. The revised plan includes gel fluids as an additive in the fracking fluid. The previous version referred only to polyacrylamide.
There is also extra information about geological features. Cuadrilla said:
“The empirical evidence derived by microseismic observations of activated geological structures confirms that unidentified critically stressed local faults are present and are likely to slip.”
The company said five known faults (called Thistleton, Anna’s Road, PNR-1, SD3 and SD6) showed no activation during fracking of PNR1z. But a sixth fault, SD5, did show microseismic activity. This, however, did not lead to activity beyond the permit boundary, or any adverse seismicity, Cuadrilla said.
Reporting requirements have also changed. In the revised version, Cuadrilla said seismic events during fracking that measured above 0.0ML would be reported “as soon as possible” to the Oil & Gas Authority, Environment Agency and Health and Safety Executive. These regulators would receive a daily morning report during fracking operations only. The previous version said reports on the traffic light system status would “be reported in a timely manner” on Cuadrilla’s e-portal.
The plan confirms that Cuadrilla still intends to frack up to 45 stages of the PNR2 well. It proposes to use 765m3 of hydraulic fracturing fluid and 75 tonnes of proppant. The new version adds that there will be 47 sleeves used in fracking the well.
The company said it would take a “progressive stepped approach” using minifracs and “modest initial pumped volumes”, building up to the maximum pumped volumes.
Nick Mace, Cuadrilla environmental and permitting manager, said:
“We have already confirmed that both wells at our shale gas exploration site in Preston New Road, near Blackpool, Lancashire, remain candidates for hydraulic fracturing and each needs an approved hydraulic fracture plan in place before work can commence. This latest submission covers the second well.”
A spokesperson for Frack Free Lancashire said:
“We are disappointed to note that Cuadrilla have now published their latest fracking plan for Preston New Road.
“Considering they only managed to frack 5% of the first well and triggered 57 earth tremors, it is remarkable that they are pouring more resources into what we consider a doomed project.
“We also note that they now have permission to discharge into Carr Bridge Brook, against local wishes and representations.
“It seems extraordinary that, in spite of the recent declaration of a climate emergency, Cuadrilla are intent on continuing to explore for shale gas which we neither need nor want. This is completely incompatible with UK commitments on climate change.
“We will continue to campaign against fracking in particular and fossil fuels in general. This government should lead by example in transitioning to a cleaner and greener future.”