By Dr. Bernd Wiese

The paper by Cai & Ofterdinger (2014) aims to identify the conditions under which hydraulic fracturing of the Bowland Shale in central Britain could contaminate groundwater in St. Bees Sandstone (600-1000 m below ground level). This research is carried out using a numerical model of 5x3 km horizontal extent. The model has a vertical extent of 3000 m and comprises the ten main geological layers, mainly alternating high and low conductivity*1; the Bowland Shale is located at a depth between 2000 and 2500 m. The hydraulic properties of the model are based on field measurements. The deep groundwater is over-pressured, i.e. it flows upwards and transports hazardous substances of natural or anthropogenic origin if a conductive pathway*2 exists. During hydraulic fracturing additional over-pressure is applied and upward directed hydraulic pathways are created. The model investigates, how these processes affect the water quality of aquifers that are above the Bowland Shale.

Under natural conditions the aquifers are well separated and there is no significant fluid flux between them. Hydraulic fracturing is then applied to the model. Generally it is concluded that contaminants can migrate as high as the fractures extent, therefore the hydraulic fracture height is the critical parameter to consider. Different scenarios for the fracture heights are applied. Some fractures have a vertical extent of 1850 m, while the highest known fractures have a length of 1100 m (Davies et al., 2012). For the simulated fracture heights, contamination of the St. Bees Sandstone is possible. The authors consider the variability of hydraulic properties due to the uncertainty of the measured hydraulic conductivity. Low hydraulic conductivities increase the vulnerability of the shallower groundwater. This may appear as a paradox, but it occurs because a high conductivity is correlated with a high horizontal groundwater flow velocity. Here, the horizontal groundwater flow carries contaminants away from the fracture, and natural water from the respective aquifer flows upwards. The authors conclude that high hydraulic conductivities prevent upward flow of contaminants from the Bowland Shale horizon.

It has to be emphasized that the model simulates the impact of just one well that is hydraulically fractured. The results cannot be directly transferred to commercial field exploitation with multiple wells. Fluid injection by multiple wells could increase general pressure in the Bowland shale (Keranen et al., 2014) and therefore enhance the upward fluid flux.

I have objections to this paper in respect to the scenarios where Collyhurst Sandstone at a depth of 1100 to 1250 m is penetrated by hydraulic fractures. All three permeability scenarios use an implausible combination of hydraulic conductivity and hydraulic gradient. This results in implausibly high flow velocities that therefore act as a barrier for upwelling contaminants. The barrier function is significantly lower if the flow velocity is reduced to realistic values. Currently, I investigate this issue.

Note 1 conductivity: A high conductivity level means water (and other fluids) can flow easily through the rock, a low conductivity level means that a rock can be impermeable to water and other fluids. 

Note 2 conductive pathway: a region with high permeability, but low spatial extent, such as an open fault or fracture.

Refernces:
Davies, R.; Mathias, S.; Moss, J.; Hustoft, S. & Newport, L. (2012), 'Hydraulic fractures: How far can they go?', Marine and Petroleum Geology 37(1), 1-6.

Keranen, K.; Weingarten, M.; Abers, G. c.; Bekins, B. & Ge, S. (2014), 'Sharp increase in central Oklahoma seismicity since 2008 induced by massive wastewater injection', Science 345(6195), 448-451.