An article to be published in an upcoming issue of Groundwater includes analysis suggesting that the rapid upward migration of brine and hydraulic fracturing fluids, which has been predicted to occur as a result of increased hydraulic fracturing activity, is not physically plausible due to the following characteristics of sedimentary basins in which black shales are located:
- The rock layers above black shale are typically dominated by layers with inherently low permeability, such as shales, siltstones, and mud-stones. The permeability of these layers is further reduced by the high-effective stress at depth, cementation, and partial saturation.
- The overburden is much thicker than the layer of rock affected by hydraulic fracturing, and the properties of the bedrock limit pressure propagation at depth, meaning that elevated pressures from hydraulic fracturing are short-lived and localized to the fracture network. Upward migration of hydraulic fracturing fluid or brine is therefore limited by the thick layer of low permeability bedrock below the shallow groundwater.
- Natural vertical head gradients are typical, and are driven by topography or overpressure at depth, leading to low flow rates and long timescales for transport that are often over one million years.
The article consists of a review of the literature on the hydrology of sedimentary basins, in order to evaluate recent studies conducted by Rozell and Reaven, Myers, and Warner et al. that posited that a hydraulic connection between black shales and shallow groundwater may be either natural or induced by hydraulic fracturing, and may lead to rapid upward migration of hydraulic fracturing fluids or brine. The article was written by Samuel Flewelling, Ph.D., and Manu Sharma, M.S., P.E., scientists with Gradient Corp., an environmental and risk science consulting firm.