Abstract
Over significant areas of the UK and western Europe, anthropogenic alteration of the subsurface by mining of coal has occurred beneath highly populated areas which are now considering a multiplicity of ‘low carbon’ unconventional energy resources including shale gas and oil, coal bed methane, geothermal energy and energy storage. To enable decision making on the 3D planning, licensing and extraction of these resources requires reduced uncertainty around complex geology and hydrogeological and geomechanical processes.An exemplar from the Carboniferous of central Scotland, UK, illustrates how, in areas lacking hydrocarbon well production data and 3D seismic surveys, legacy coal mine plans and associated boreholes provide valuable data that can be used to reduce the uncertainty around geometry and faulting of subsurface energy resources. However, legacy coal mines also limit unconventional resource volumes since mines and associated shafts alter the stress and hydrogeochemical state of the subsurface, commonly forming pathways to the surface. To reduce the risk of subsurface connections between energy resources, an example of an adapted methodology is described for shale gas/oil resource estimation to include a vertical separation or ‘stand-off’ zone between the deepest mine workings, to ensure the hydraulic fracturing required for shale resource production would not intersect legacy coal mines. Whilst the size of such separation zones requires further work, developing the concept of 3D spatial separation and planning is key to utilising the crowded subsurface energy system, whilst mitigating against resource sterilisation and environmental impacts, and could play a role in positively informing public and policy debate.
Highlights
During the 19th to mid-20th Century, extensive subsurface mining of Carboniferous-aged coals drove co-incident industrialisation of large areas of the UK
Critical data for UK Carboniferous shales is not publically available resulting in the range of in-place resource estimation values (Andrews, 2013; Monaghan, 2014)
In the central Scotland exemplar, abandoned coal mines are widespread across the areas underlain by strata with shale resource potential (Fig. 1)
Summary
During the 19th to mid-20th Century, extensive subsurface mining of Carboniferous-aged coals drove co-incident industrialisation of large areas of the UK. Geothermal energy resources and opportunities for energy storage are being considered within that same rock volume (Campbell et al, 2010; Gillespie et al, 2013; Younger, 2016b) In these areas, legacy deep coal mines occur above, and in some cases at similar burial depths, to the unconventional energy resources Conflicts can exist between the various subsurface uses For both effective resource exploitation and to mitigate against environmental impacts, exploration for unconventional oil or gas would strongly avoid hydraulic connection with aquifers (Younger, 2016a) and pathways to the ground surface. Given the spatial extent where coal mining legacy could overlie unconventional resources (Fig. 1), and the relative paucity of published literature on potential subsurface connections and separation zones between deep (100s m to kilometres) energy resources, further investigation is required. Potential positive impacts in informing public and policy debate are considered
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