Hydraulic fracturing or “fracking” in shale gas formations has revolutionized the energy industry in the U.S. But critics say the national gas boom poses environmental risks of polluted drinking water and induced seismicity. Haiying Huang, assistant professor in the School of Civil and Environmental Engineering at Georgia Tech, explains that hydraulic fracturing is only one stage in the entire life of a well. She says fracking by itself has relatively low risks of contaminating drinking water and triggering earthquakes, if properly designed and executed.
Hydraulic fracturing as a reservoir stimulation technique generally involves first isolating a certain interval of the well, pressurizing the isolated interval with a fluid to initiate and propagate the fractures and finally injecting a mixture of fluid and proppants (usually sands) to keep the fractures open after fluid injection stops and reservoir production begins.
Hydraulic fracturing increases well productivity for low permeability formations since the propped fractures reduce the resistance for the hydrocarbon to flow from the reservoir to the well. At large depth, the growth of the fractures and leakoff of the fracturing fluids in the vertical direction are constrained by the rock layers above and below the production zone since the reservoirs are generally horizontally layered. Current development in the Marcellus shale has mostly been below 5000 ft while the aquifer is located within 1000 ft from the surface.
Hydraulic fracturing is only one stage in the entire life of a well, which also includes exploration, drilling, well completion, fracturing fluid flow back and production. If properly designed and executed, the hydraulic fracturing stage is likely to have the lowest risks of polluting drinking water and induced seismicity for the deep wells since the duration of the fracturing treatment is relatively short, typically about half an hour to a few hours, and the fracturing fluids usually do not get into contact with the formations before reaching the bottomhole. The potential of drinking water contamination during drilling and completion as well as the flow back and production stages is probably higher.
The short fracturing treatment duration also means that hydraulic fracturing triggered seismic events are extremely rare. Instead, it is the subsurface disposal of injecting the large volume of high salinity formation water produced together with natural gas that poses a realistic seismic risk of small-to-moderate earthquakes. Nevertheless, the connection between fluid injection and induced seismicity has been well known since 1960’s and the use of injection wells to dispose brine has been a common practice for many decades in the U.S. Through proper planning and careful monitoring, the potential of injection induced seismicity could be minimized and managed.
Two ongoing DOE/NETL led studies were reported at the Congressional briefing hosted by the U.S. Geological Survey and the American Geophysical Union earlier in June. The studies are being carried out at the Washington county site and the Greene County site in Pennsylvania. The study at the Washington county site documents the baseline conditions of air, land and water resources and monitors the changes as the wells develop and natural gas production ensues in the next a few years.
The study at the Greene County site addresses the possibility of hydraulic fracture breaking into upper formations by monitoring the microseimicity and well performance in the upper Devonian formation, which is around 4000 ft below the surface, midway between the Marcellus formation and the surface. Tracers will also be added to the fracturing fluid in order to investigate the potential of upward fluid migration. Studies such as those could encourage responsible development in energy production and bring transparency to the general public.
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