6 Lessons from the Research
Hydraulic fracturing to recover hydrocarbons is generally accompanied by earthquakes with magnitudes of less than 2 ML that are too small to be felt. In the United States, the large number of hydraulic fracturing operations (1.8 million) and the small number of felt earthquakes directly linked to them (3) suggests that the probability of induced earthquakes that can be felt is small. In western Canada, there are more examples of induced earthquakes with magnitudes larger than 3 than in the United States. These include a magnitude 4.4 earthquake, which is the largest earthquake linked to hydraulic fracturing in the world. However, given the large number of hydraulically fractured wells (>12,000), the probability of induced earthquakes that are large enough to be felt also appears to be small.
Well-documented increases in earthquake rates and significant earthquakes in many areas of the Central and Eastern United States have been linked to wastewater injection in deep disposal wells rather than hydraulic fracturing, and seismic hazard forecasts for these areas now include contributions from both induced and natural earthquakes. These forecasts show increases in earthquake hazard by a factor of 3 or more in some areas of induced earthquake activity. However, it is important to note that most wastewater injection wells are not associated with any earthquake activity. Additionally, the nature of the wastewater injected into deep wells varies: while some comes from hydraulic fracturing used in unconventional oil and gas production, many wastewater injection wells are used to dispose of produced water from conventional hydrocarbon production.
Studies of earthquake activity in the Raton Basin an area that has produced coal-bed methane since 1994, provides strong evidence that a this activity is related to the subsequent disposal of wastewater from the coal-bed methane extraction process by injection into deep wells, rather than from the extraction process itself. Literature was not located concerning induced seismicity and coal-bed methane extraction in Canada, Australia or other parts of the USA, suggesting that this is not a major issue in those areas.
A detailed examination of existing earthquake catalogues has shown that seismic activity in Scotland is low. Historical records of earthquakes in Scotland date back to the 16th century, and show that despite many accounts of earthquakes felt by people, damaging earthquakes are relatively rare. The largest recorded earthquake in Scotland had a magnitude of 5.2 ML and only two other earthquakes with magnitudes of 5.0 ML or greater have been observed in the last 400 years. As a result, the risk of damaging earthquakes is low.
Most earthquake activity in Scotland lies north of the Highland Boundary Fault. Earthquake activity in the Midland Valley of Scotland is lower and most of the recorded earthquakes in this area in the 1970's, 1980's and 1990's were induced by coal-mining. Since the decline of the coal-mining industry in the 1990's, very few mining-induced earthquakes have been recorded. Most of the mining induced earthquakes are small and the largest mining-induced earthquakes in Scotland had a magnitude of 2.6 ML. Earthquake activity rates calculated for the Midland Valley are lower than north of the Highland Boundary Fault, suggesting that earthquake risk is even lower here than elsewhere in Scotland. However, the small number of observed earthquakes for this area means the values have large uncertainties.
Existing catalogues of earthquake activity in Scotland are incomplete at magnitudes below 2 ML, from 1970 to present, and for higher magnitudes prior to this. This is due to the detection capability of the networks of seismometers that have operated in the study area over the last few decades. This, together with the low background activity rates limits our ability to identify any areas that might present an elevated seismic hazard for any Unconventional Oil and Gas operations based on seismic data alone. Better earthquake catalogues will be needed to rectify this and provide reliable estimates of background activity rates and that allow the discrimination and forecasting of induced seismic activity. This will require denser arrays of seismic instrumentation that currently deployed. These dense arrays are also required to provide high-quality, real-time earthquake locations, which are required as part of any traffic light system for mitigating risk.
Earthquake focal mechanisms provide both fault geometries and principal stress directions that can be used to constrain our understanding of the driving forces of current deformation. The observed orientation of the maximum and minimum compressive stresses means that the reactivation potential of faults that strike either NE- SW or NW- SE are interpreted, based on existing data, to be highest, with the former optimally oriented for left-lateral strike-slip motion. This means that fault systems that follow the widely observed Caledonian trend may have a higher reactivation potential than other orientations. However, the reactivation potential for E-W or ENE- WSW striking faults in the Midland Valley region is interpreted to be low based on current data. This seems in keeping with the low levels of observed seismicity in this part of Scotland. However, the limited number of focal mechanisms for earthquakes in Scotland, along with other reliable stress indicators such as borehole breakouts means that it is not possible to identify any particular parts of the study area where the fault reactivation potential is higher or that may present an elevated seismic hazard for any UOG operations.
In addition, our knowledge of fault systems in the sub-surface is generally limited to areas where detailed geophysical surveys have been carried out. For example in the case of the Blackpool earthquake activity, the existing geophysical data was insufficient to identify any faulting close to the Preese Hall well prior to hydraulic fracturing operations, although Clarke et al. (2014) subsequently identified a possible causative fault close to the well using data from a later detailed 3-D seismic reflection study. Improved understanding of the hazard from induced earthquakes and the successful implementation of regulatory measures to mitigate the risk of induced seismicity are likely to require new geological and geophysical data that can be used to map sub-surface fault systems in high resolution as well as more measurements of the orientation of magnitude of the sub-surface stress field.
Regulatory measures for the mitigation of induced seismicity ( DECC, 2013) include: avoiding faults during hydraulic fracturing; assessing baseline earthquake activity; monitoring seismic activity during and after fracturing; and a 'traffic light' system that controls whether injection can proceed or not, based on that seismic activity. These are similar to regulatory measures that are in place in the US and Canada. In the US, much of this regulation is aimed at induced seismicity related to wastewater disposal in deep wells, although this is also relevant to induced seismicity from hydraulic fracturing.
In the UK, the magnitude limit for the cessation of hydraulic fracturing operations (0.5 ML) is considerably less than the limits in California (2.7 ML) and Illinois, Alberta and British Columbia (4.0 ML), and may be considered a conservative threshold. Local monitoring systems that are capable of reliable measurement of earthquakes with very small magnitudes will be required to implement the UK limit successfully. A magnitude 4.0 ML earthquake in an area of high population density, such as the Midland Valley of Scotland, would be strongly felt by many people and may even cause some superficial damage.
British Standards BS 6472-2 and BS 7385-2 define limits for acceptable levels of ground vibrations caused by blasting and quarrying and the limits for vibrations caused by blasting, above which cosmetic damage could take place. A comparison of modelled ground motions for a range of earthquake magnitudes with these limits suggests that earthquakes with magnitudes of 3.0 or less are unlikely to exceed the limits above which cosmetic damage may occur, as set out in BS 7385-2, except at distances of less than a few kilometres. This seems reasonably consistent with observations that the largest mining-induced earthquakes, with magnitudes of around 3.0 ML, caused some superficial damage (Westbrook et al., 1980; Redmayne, 1998) including, minor cracks in plaster and harling. Smaller earthquakes may also exceed the limits for vibration set out in BS 6472-2, but again only at small distances of less than a few kilometres. An alternative traffic light system could use these vibration limits as well as, the current magnitude thresholds.
Improved understanding of the hazard from induced earthquakes and the successful implementation of regulatory measures to mitigate the risk of induced seismicity is likely to require industrial data from hydraulic fracturing operations such as injection rates and volumes, along with downhole pressures, in addition to seismic, geological and geophysical data.