How can we be sure fracking will not pollute aquifers? Lessons from a major longwall coal mining analogue (Selby, Yorkshire, UK)

Abstract

Development of shale gas by hydraulic fracturing ('fracking') is opposed by campaigners who propose (inter alia) that freshwater aquifers could be polluted by upward migration of fractures and any fluids they contain. Prima facie hydrogeological analysis of this proposition has been undertaken. For it to occur, two conditions must be satisfied:(i) sufficient hydraulic interconnection (i.e., a continuous permeable pathway); and (ii) a sustained driving head, oriented upwards.With regard to (i), shale gas developers have a major vested interest in avoiding creating such hydraulic connection, as it would result in uneconomically excessive amounts of water needing to be pumped from their wells to achieve gas production. In relation to (ii), nominal upward hydraulic gradients will typically only be developed during fracking for periods of a few hours, which is far too brief to achieve solute transport over vertical intervals of one or more kilometres; thereafter, depressurisation of wells to allow gas to flow will result in downward hydraulic gradients being maintained for many years. The proposition is therefore found to be unsupportable. Albeit for contrasting motivations, developers and environmental guardians turn out to have a strong common interest in avoiding inter-connection to aquifers.A powerful illustration of the potential long-term effects of fracking is provided by the hydrogeological history of underground coal mining in the UK. Where large-scale mining proceeded from the surface downwards, major hydraulic inter-connection of shallow and deep zones resulted in widespread water pollution. However, where new mines were developed at depth without connections to shallow old workings (as in the Selby Coalfield, Yorkshire), complete hydraulic isolation from the near-surface hydrogeological environment was successfully maintained. This was despite far greater stratal disruption and induced seismicity than shale gas fracking could ever produce. The lesson is clear: without hydrogeological connectivity to shallow aquifers, shale gas fracking per se cannot contaminate shallow ground water.

Seismicity induced by longwall coal mining at the Thoresby Colliery, Nottinghamshire, U.K.

“…The 84 collapsed, brecciated roof material filling this void is known as goaf (e.g. Younger, 2016). At …”

A method for screening groundwater vulnerability from subsurface hydrocarbon extraction practices

“…In addition, the area of subsurface voids for coal mines in the UK can be up to 200,000 m 2 . Mine collapse can create bed-parallel fractures up to 20 m above the roof of a mined coal seam (Younger, 2016) and increased permeability up to 160-200 m above and 40-70 m below worked seams (Jones et al, 2004), providing multiple pathways for contaminants (Ward et al, 2015;Monaghan, 2017). In the UK, the statutory stand-off interval between longwall workings and an aquifer is 105 m, reducing to 45 m for supported methods of mining (Younger, 2016).…”

Integrating induced seismicity with rock mechanics: a conceptual model for the 2011 Preese Hall fracture development and induced seismicity

“…The above considerations bear upon the potential for vertical growth of hydraulic fractures to cause environmental pollution, as a result of breaching upward into sediments that are used for water supply (e.g., Younger, 2016a). It is evident that if such a breach were to occur then, rather than pollution entering shallower groundwater zones, the result would be for shallower groundwater to flood downward into the fracture, preventing shale gas production, so shale operators have a strong incentive to ensure that this does not happen (e.g., Younger, 2016b).…”