A recent study on the potential migration of contaminants from hydraulically fractured gas shales through overlying, naturally fractured rock formations, has stirred some debate on the risk of groundwater contamination. While acknowledging uncertainties, the study suggests that "transport times [from hydraulically fractured shale to near surface aquifers] could be decreased from geologic time scales to as few as tens of years. Preferential fllow through natural fractures or fracking-induced fractures could further decrease the travel times to as little as just a few years."

The full paper, Potential Contaminant Pathways from Hydraulically Fractured Shale to Aquifers, by Tom Myers, is available by paid subscription only, from the Wiley Online Library, although the abstract is accessible without charge.

Reactions

While the study was covered heavily by groups opposing shale gas development, several scientists criticized the modeling study and the assumptions used. Additionally, different results on contaminant migration have been published in a recent report on the risks of hydraulic fracturing.

"If fluids could flow as quickly as Myers asserts, fracking wouldn't be necessary to open up the gas deposits.” says Terry Engelder, a professor of geosciences at Penn State University (U.S.) who has been a proponent of shale development. "This would be a huge fracture porosity," Engelder said. "So I read this and I say, 'Golly, does this guy really understand anything about what these shales look like?' The concern then arises from using a model rather than observations." Engelder quotes are re-published from a ProPublica article.

Don Siegel, Professor of Hydrogeology at Syracuse University (U.S.), reasoned in more detail over badly-chosen assumptions in the study and concluded that "Myers has developed an implausible model that predictably leads to implausible, and in my judgment, completely wrong results — from simple first principles of geologic and hydrologic understanding, let alone acceptable model development." For the full comment of Don Siegel, see Energy In Depth.

Other results

Whether or not contaminants from hydraulic fracturing could migrate into usable groundwater from below was recently investigated by a German expert group as part of the "Risikostudie Fracking" (in German). Their models were based on conservative assumptions, especially that:

1. individual induced fractures extend more than 300 m, beyond the shale formation into overlying strata, and connect directly to a pre-existing fault;
2. faults may have a length of more than 1 km and liquids may migrate much faster through these faults. 

The experts concluded that even with these conservative assumptions, Frac-fluids pumped into the underground could migrate upwards only 50 m. Fracturing fluids could migrate only as long as the fracturing-pressure is maintained. This means that contaminants cannot get into usable groundwater via this route.

There is one exemption; if the pressure of deep formation water is significantly above and of shallow groundwater significantly below the hydrostatic pressure, and if continuous and permeable faults are present, fracturing fluid may migrate upwards. The researchers pointed out that these specific geo-hydrologic settings can be detected before drilling operations start. Research on this part of the risk study was led by Prof. Helmig of Stuttgart University, Germany.