Complex geology in one of the UK’s leading shale gas regions places a major constraint on exploration and production, scientists have concluded.
A research paper from Heriot-Watt University found that faulting in the Bowland shale in the Fylde area of Lancashire was more complex than previously thought.
This would limit suitable sites for well pads and the opportunity to drill long lateral boreholes needed to maximise production, the paper concluded. It also reduced the size of the shale gas resource and affected well productivity.
The research, published in the latest edition of the journal, Petroleum Geoscience, focussed on the exploration licence PEDL165, covering the area around Blackpool.
This has been explored by Cuadrilla for the past 10 years. It is the only part of the UK where shale gas companies have carried out high volume hydraulic fracturing.
The company has described the Bowland Shale in the Fylde as “a huge natural gas resource of the highest quality”.
But fracks at Cuadrilla’s sites in the area at Preese Hall in 2011 and Preston New Road in 2018 and 2019 induced earth tremors. The government responded by imposing moratoriums on fracking. The most recent, which began in November 2019, is still in place.
The researchers used Cuadrilla’s seismic data to recreate 3D images of a 100km2 of the Fylde.
They said the data revealed complex underlying geology and a series of fault lines. Their mapping showed that faulting was denser and more complex than apparent from geological mapping of the surface outcrop.
They said the area was folded and crossed by several large faults, dividing the area into fault-bound compartments, the maximum width of which was 3.2km.
This restricted the connected shale resource, the researchers said. It also limited the length over which long lateral boreholes can remain in a productive horizon. It placed an “important constraint on optimal well positioning, reducing the size of the shale gas resource and affecting well productivity”, the researchers concluded.
A press release from Heriot-Watt said the researchers also identified one significant fault line which was said to have been “misrepresented in Cuadrilla’s previously published material”.
They said this was significant because faulting increases the risk of induced earth tremors during fracking and may have contributed to seismicity at Preese Hall in 2011.
“Accurate mapping of faults needed before drilling”
The lead author, Iain Anderson, said in a Heriot-Watt article:
“Our mapping has demonstrated the severity of seismically resolvable faulting underneath the Fylde region which would limit the locations in which hydraulic fracturing could occur.
“The occurrence, size and dimensions of the compartments between the faults place limits on shale resources, the optimal location of well sites and on horizontal well bore lengths.
“The accurate mapping of faults prior to drilling and hydraulic fracturing of wells may, in the future, help reduce the risk of fault reactivation and induced seismicity.”
John Underhill, professor of exploration geoscience at Heriot-Watt, said:
“We believe our research demonstrates the need for detailed, forensic mapping of the subsurface to be carried out as a prerequisite for shale gas exploration, not just in Lancashire, but in any area where shale gas extraction may be attempted.
“This will allow the subsurface geology to be fully understood and the risk of seismic hazard evaluated in advance of drilling. It is clear that there is much more structural complexity and many more faults in the subsurface than evident at the surface.
“The understanding, identification and accurate mapping of faults is vital in any shale resource assessment and when seeking to evaluate the risk of intersecting critically stressed fractures, upon which seismicity might be induced.
“Our structural interpretations suggest that this particular shale gas reserve, known as the Bowland Shale gas play, remains highly challenged with significant uncertainty in its resource estimates, the planning of well site locations, horizontal wellbore pathways and risk of induced seismicity on faults that are seismically resolvable and those that are sub-seismic scale.”
A leading expert on induced seismicity has applauded the research.
Peter Styles, emeritus professor of geophysics at Keele University, advised the former prime minister, David Cameron, on the Preese Hall earth tremors. He told DrillOrDrop in April 2018 that fracking near geological faults in former coal mining areas could trigger earthquakes and should not take place without careful assessment of the available geological data.
“This is a very significant and impactful paper in the context of UK unconventional gas exploitation.
“As the authors ably demonstrate, this was an area which had significant geological complexity and that shale gas development would pose extraordinary challenges.”
Professor Styles published a report in May 2018 which concluded that fracking companies had failed to use all available geological data.
“Despite the ensuing problems of seismicity, the sites in the Lancashire Bowland Basin are probably simpler than many of the other UK PEDLs which mostly lie beneath areas which have had significant coal mining with its associated subsidence and mining-induced earthquakes.
“This paper demonstrates, through robust forensic analysis and interpretation, the issues which geology poses for any future attempts to exploit onshore UK shale resources in a profitable and environmentally (and sociologically) acceptable manner. It should be read carefully and the lessons learned before any future projects are proposed.”
“We always suspected the geology was unsuitable”
Barbara Richardson, chair of Roseacre Awareness Group which successfully campaigned against Cuadrilla’s plans to frack at Roseacre in the Fylde, said local people had long had doubts about the area’s geology:
“The communities of the Fylde have had to endure much stress and anxiety these past few years living with earthquakes at both the sites Cuadrilla have attempted to frack for shale gas.
“We always suspected the geology was unsuitable and that Cuadrilla’s 3d data was lacking in detail. This study highlights the inherent issues with the region’s geology. It is time Cuadrilla admit defeat and restore PNR to the green fields it once was and let the residents of the Fylde get on with their lives without the threat of fracking hanging over them.
“This country is far too densely populated, and far too geologically faulted, to make fracking for shale gas a viable opportunity in the Fylde or anywhere for that matter.
“We do not believe fracking is safe or sustainable and it is certainly not supported by local communities”.
A spokesperson for Cuadrilla said:
“Cuadrilla strongly refutes the claim made in the Heriot-Watt University press release that it “misrepresented” a fault line and will take this matter up directly with the University.
“We see nothing particularly new in the Herriot Watt interpretation of our Bowland 3D seismic data and the interpretative approach taken appears rigorous.
“By contrast the subsequent statement of what that 3D seismic assessment “suggests” in respect of Bowland resource estimation, well placement or development is not informed by any detailed production analysis or development planning. We have no other comment to make on this suggestion.”
- Updated 5/5/2020 with Cuadrilla’s response to the article