In August this year Lancashire experienced the largest fracking-induced earthquake recorded in the UK. Fracking was suspended. Legislators discussed a complete ban. As a result, serious questions are being asked about the effectiveness of the safety regulations, given the operating company, Cuadrilla, predicted there was a “low-likelihood” of such events occurring. And given the UK’s “traffic light system” (TLS) is the most stringent TLS in the world, how did it fail to foresee and prevent the series of earthquakes in August. Miles Wilson, Richard Davies and Jon Gluyas at ReFINE explain that for predictions and regulations to be effective a much deeper study into injection volumes and pressures, and local geology, need to be done. Before their conclusions, they run through the sequence and details of the event.
Hydraulic fracturing (otherwise known as fracking) for shale gas revolutionised natural gas (and associated hydrocarbon liquids) production from the United States (US), turning the US from an importer to an exporter during the last decade. Other countries have looked to follow suit with commercial production also occurring in Canada, China and Argentina. Elsewhere, including in the United Kingdom (UK), shale gas resources have been identified and licenses issued, but only a few wells drilled.
In the UK hydraulic fracturing for shale gas has attracted strong public opposition, in part because of felt, induced earthquakes associated with the first shale gas well ever drilled in England. In 2011, fluid injection at Preese Hall, Lancashire, induced two felt earthquakes and this resulted in a temporary ban and seven-year hiatus of the practice.
By October 2018, hydraulic fracturing resumed at the nearby Preston New Road site with stimulation of the PNR-1z well. As expected small magnitude earthquakes accompanied the stimulation and again raised public concern.
Public concern: earthquakes felt by locals
The British Geological Survey (BGS) detected a total of 57 earthquakes associated with the PNR-1z stimulation, of which two were felt by local residents. The largest magnitude earthquake associated with the operation was a local magnitude (ML) 1.5 on 11th December 2018. This was smaller than the largest earthquake (ML 2.3) induced at Preese Hall.
Hydraulic fracturing of the second well (PNR-2) at the Preston New Road site began on 15th August 2019 and has so far resulted in a further 128 earthquakes, including a ML 2.9 earthquake on 26th August. This earthquake was felt widely across the surrounding local area and given an intensity of six by the BGS (Table 1).
Table 1: European Macroseismic Intensity Scale.
The ML 2.9 earthquake is now the largest fracking-induced earthquake recorded in the UK and has resulted in the suspension of operations until the licensing regulator, the Oil and Gas Authority (OGA), has investigated why an earthquake of this magnitude occurred.
Such an investigation is needed because the earthquake magnitude was comparable to the “low-likelihood” expected maximum magnitude. Moreover, the earthquake occurred during a period of suspended fluid injection which was taking place because of the risk mitigation strategy used for fracking-induced earthquakes in the UK.
To date the largest proposed fracking-induced earthquake worldwide is a ML 5.7 earthquake in the Sichuan Province, China. The occurrence and magnitude of induced earthquakes are likely a combination of natural geological factors and anthropogenic operational factors. Consequently, the occurrence and magnitudes of induced earthquakes may differ across the world.
In the UK the maximum observed magnitude induced earthquakes documented in the human-induced earthquake database are those associated with coal mining, for example a body wave magnitude (mb) 3.4 earthquake in North Staffordshire on 15th July 1975 and a ML 3.2 earthquake in the Nottinghamshire coalfield area on 22nd March 1984.
Likelihood of fracking earthquakes underestimated
The Environmental Statement prepared by the operator for the Environment Agency (the environmental regulatory body in England) for hydraulic fracturing at Preston New Road estimated that the expected maximum magnitude earthquake that could be induced by the operations would be similar to those induced by coal mining. This was consistent with a commissioned report (following the induced seismicity at Preese Hall) that the maximum magnitude earthquake that could be induced by hydraulic fracturing using stage injection volumes of ~2000 m3 was ~ML 3.1.
Injection volumes for Preston New Road are in fact much smaller than 2000 m3 and it was stated in the Environmental Statement that “as such the maximum magnitude that could be induced will be lower”, and “the likelihood of a 3.1 ML event is considered to be very low (i.e. rarely encountered, never reported, or highly unlikely)”.
The operator announced recently that only 17 of the 41 planned injection stages in PNR-1z underwent fluid injection and, of these, only two were injected with the planned 400 m3 of fluid and 50 tons of proppant. The same fluid volumes and proppant mass were also planned for stages in PNR-2.
It therefore appears that the magnitude of fracking-induced earthquakes at Preston New Road are nearing both the maximum observed magnitudes of induced seismicity in the UK and the expected maximum magnitude estimated for this site using hydraulic fracturing operations (Fig. 1). However, the expected maximum magnitude was based on models using larger volumes of fluid than have actually been used.
Is the Traffic Light System working?
The traffic light system (TLS) used by the OGA to regulate induced seismicity associated with hydraulic fracturing in the UK currently has a red-light threshold of ML 0.5. On reaching an earthquake of this magnitude the operator must suspend injection, reduce pressure and monitor seismicity and ground motion for 18 hours for any further earthquakes before potentially resuming injection.
The main purpose of the TLS is to reduce the risk of trailing earthquakes, whereby earthquake magnitudes continue to increase after injection has been suspended.
Arguably the UK TLS is the most stringent TLS employed worldwide and by setting the red-light magnitude so low compared to other TLS, it is hoped that the risk of trailing earthquakes is further reduced. Nevertheless, there has been industry pressure to relax the red-limit magnitude.
The ML 2.9 earthquake that occurred on 26th August 2019 followed red-light earthquakes on all preceding days from the 21st August, bar the 25th (Fig. 1). Fluid injection operations had been suspended at least two days before the ML 2.9 earthquake, defining the earthquake as a trailing earthquake. As such, at the time of the earthquake there was no injection to be suspended and fluid pressures had presumably already been reduced in response to preceding red-light earthquakes.
Figure 1: BGS detected earthquakes associated with the stimulation of PNR-2. Red-light earthquakes (ML ≥ 0.5) are coloured red.
This raises challenging questions regarding the currently employed TLS. How effective is the existing TLS at reducing earthquake risk given that the largest fracking-induced earthquake in the UK occurred as a trailing earthquake several days after any fluid injection? Did the suspension of fluid injection and reduction of pressures in the preceding days actually reduce the magnitude of the earthquake on the 26th, i.e. did the TLS fulfil its purpose? And ultimately, is the current TLS still the most appropriate method for mitigating the risk of induced seismicity?
Investigations by the OGA are underway to establish why an earthquake of this magnitude occurred. These investigations will likely focus on subsurface fault maps, operational parameters such as injected fluid volumes and pressures, and microseismic data recorded during the stimulation.
However, further research is needed to investigate why hydraulic fracturing of the Bowland Shale in Lancashire seems prone to producing earthquakes capable of being felt, and if anything further can be done, prior to fluid injection, to identify geological faults not currently identifiable using existing seismic reflection data.
Miles Wilson is a research affiliate at ReFINE
Prof. Richard Davies is Pro-Vice-Chancellor of Engagement and Internationalisation at Newcastle University, UK
Prof Jon Gluyas is Executive Director, Durham Energy Institute, UK
About ReFINE: ReFINE is the leading international research consortium on hydraulic fracturing. Based in the UK and led jointly by Newcastle and Durham Universities, ReFINE works closely with a global network of leading scientists and institutions to research the potential environmental and social impacts of shale gas exploitation.