A recent study by the British Geological Survey (Monaghan, 2014) stated that the significant volumes of potentially productive shale in the Midland Valley of Scotland represent a considerable oil and gas resource. In the Smith Commission Report of November 2014 for the further devolution of powers to the Scottish Government, the licensing of onshore oil and gas extraction for Scotland is to be devolved to the Scottish Government together with the mineral access rights for underground onshore extraction of oil and gas. In January 2015, the Scottish Government announced a moratorium on onshore oil and gas in Scotland and has adopted a cautious, considered and evidence based policy to unconventional oil and gas in Scotland. The aims of this project are to: (1) better understand the levels of induced seismic activity that could be associated with unconventional oil and gas activities in Scotland; and (2) better understand the robust regulatory and non-regulatory actions that can be taken to mitigate any noticeable effects on communities.
It is relatively well-known that anthropogenic activity can result in man-made or "induced" earthquakes. Although such events are generally small in comparison to natural earthquakes, they are often perceptible at the surface and some have been quite large. Davies et al. (2013) present a review of published examples of earthquakes induced by a variety of activities: underground mining, deep artificial water reservoirs, oil and gas extraction, geothermal power generation and waste disposal have all resulted in cases of induced seismicity. Furthermore, there are numerous examples of induced earthquakes in hydrocarbon fields related to oil and gas production ( e.g. Suckale, 2010). These are often a response to long-term production, where the extraction related subsidence is compensated by, for example, normal faulting on existing faults near or inside the reservoir (Van Eijs et al., 2006). For example, in 2001 a magnitude 4.1 Mw earthquake occurred in the Ekofisk field in the central North Sea (Ottemoller et al., 2005). The earthquake was thought to be related to the injection of around 1.9x10 6 m 3 of water.
Induced earthquakes with magnitudes as large as 3.5-4.0 ML are well documented in Enhanced Geothermal Systems ( EGS) ( e.g. Majer et al., 2007), in which injected fluids are heated by circulation through a hot fractured region of crystalline rocks and then brought back to the surface for power generation. Such crystalline rocks have different mechanical properties to sedimentary rocks, such as shales which may make induced seismicity associated with EGS more likely. There are examples of EGS in North America (Geysers, California), Central America (Berlin, El Salvador), Europe (Soultz, France; Basel, Switzerland; Rosemanowes, Cornwall) and Australia (Cooper Basin). Earthquakes with magnitudes of up to 4.6 and 4.4 ML have observed at Geysers and Berlin. The largest earthquakes observed at Soultz and Rosemanowes had magnitudes of 2.7 and 2.1 ML. A series of magnitude 3+ earthquakes induced during an EGS project in Basel, Switzerland resulted in the suspension of the project, which was ultimately abandoned almost 3 years later following further study and risk evaluation after these seismic events (Giardini, 2009).
In addition, several of the largest earthquakes in the U.S. midcontinent in 2011 and 2012 may have been triggered by nearby disposal wells ( e.g. Horton, 2012; Kim, 2013), suggesting that wastewater disposal by injection in to deep wells poses a significant seismic risk. The largest of these was a magnitude 5.7 event in central Oklahoma that destroyed 14 homes and injured two people (Kerenan et al., 2013).
The process of hydraulic fracturing in order to increase the permeability of reservoir formations and stimulate the recovery of hydrocarbons is also generally accompanied by microseismicity, commonly defined as earthquakes with magnitudes of less than 2.0 that are too small to be felt. Induced seismicity during hydraulic fracture operations has been discussed by a number of authors in the last few years, including Shetema et al. (2012), Warpinski et al. (2012) and Ellsworth (2013). A report by the National Research Council in the U.S. ( NAS, 2012) concluded that the process of hydraulic fracturing a well as presently implemented for shale gas recovery does not pose a high risk of inducing felt seismic events. Similarly, a Royal Society and Royal Academy of Engineering report in the UK (2012) concluded that "the health, safety and environmental risks associated with hydraulic fracturing (often termed 'fracking') as a means to extract shale gas can be managed effectively in the UK as long as operational best practices are implemented and enforced through regulation." However, more recently, there have also been a number of documented examples of rather larger felt earthquakes induced during hydraulic fracturing operations. These include: Horn River, Canada, 2009-2011 ( BC Oil and Gas Commission, 2012); Blackpool, UK, 2011 (de Pater and Baisch, 2011); Garvin County, Oklahoma, 2011 (Holland, 2013); Montney, Canada, 2014-2014 ( BC Oil and Gas Commission, 2014); Crooked Lake, Alberta, Canada, 2013-2014 (Schultz, 2015). The largest event had a magnitude of 4.4 Mw, which is, to date, the largest known earthquake suspected to have been induced by hydraulic fracturing in a hydrocarbon field anywhere in the world.
In this study we have addressed five specific research questions:
1. What are the characteristics of Scottish geology in areas assessed by the British Geological Survey for their prospectivity for shale oil and gas and CBM, with a focus on the characteristics most relevant to the different stages and techniques used in unconventional oil and gas developments?
2. What is the international and UK experience of induced seismic activity arising from hydraulic fracturing with an emphasis on sites with similar geological characteristics to Scotland?
3. What are the international experiences and lessons from statutory and non-statutory frameworks for monitoring and mitigating induced seismic activity from unconventional oil and gas developments, and from other applicable industries such as onshore oil exploration and quarrying?
4. What are the main international lessons and experiences from seismic monitoring and mitigation at unconventional oil and gas development sites? What does best practice look like and how relevant is the Scottish Government's existing planning advice on controlling blasting at quarries to unconventional oil and gas developments?
5. What lessons are there from the research as a whole in relation to monitoring and regulating induced seismic activity arising from unconventional oil and gas developments, including consideration of the DECC 'traffic light system' and the requirement for an associated Hydraulic Fracturing Plan, which currently applies across the UK? How could these lessons be interpreted and applied in a Scottish context to support robust regulation and industry best practice?
Each question is addressed in separate section of the report.
In Section 2 we review the geological and seismic characteristics of the Midland Valley of Scotland. This includes a description of key features of the prospective Carboniferous strata in the study region such as the timing and style of pre-existing faulting. Catalogues of natural and man-made seismicity are used to examine the spatial and temporal characteristics of earthquake activity in the study area and establish robust estimates of earthquake activity rates that represent a numerical expression of the expected future seismicity of the region. Mining-induced earthquakes are removed from the catalogue using a spatial filter to ensure that our estimates of earthquake activity rate are not contaminated by induced seismicity. Published earthquake focal mechanisms along with smoothed stress orientations are used to evaluate the current state of crustal stress in the study area. The resulting estimates of stress direction are compared with orientation of known faults to estimate fault reactivation potential and highlight those fault that may present an elevated seismic hazard for any UOG operations.
In section 3 we present a review of induced seismicity in UOG operations. Limited data is available globally, and we focus on three geographic areas: central and eastern United States; the West Canada Sedimentary Basin; and Blackpool, UK. We also examine seismicity in the Raton Basin, an active coal-bed methane field in the Colorado-New Mexico border region, known for several notable earthquakes, including a magnitude 5.3 event in August 2011. Given the limited data available, we also discuss examples of recent seismicity related to waste water disposal in the eastern United States.
Section 4 provides a detailed overview of the regulatory frameworks that are in place in both Europe and North America for monitoring and mitigating induced seismic activity from unconventional oil and gas developments. We compare the regulatory framework to address possible seismic activity associated with future UOG operations that is in place in the UK with those that are in place in the USA and Canada. We also draw on experience of monitoring induced seismic activity in Enhanced Geothermal Systems ( EGS), which has led to the development of "traffic light systems" linked to real-time monitoring of seismic activity.
In section 5 we discuss some of the lessons that can be learned from previous experiences of seismic monitoring and mitigation of induced seismicity in UOG operations. Examples are used to highlight some of the problems that may result from lack of monitoring before, during and after operations. We also examine the detection capability of existing seismic monitoring networks in the study area. We use a stochastic modelling approach to simulate possible ground motions for small to moderate earthquakes that might be related to activities such as hydraulic fracturing and compare these with the limits for levels of ground vibrations caused by blasting set out in the British Standards BS 6472-2 and BS 7385-2.
In section 6 we review some of the key lessons that can be learned from the research.