Hydraulic fracturing, also widely known as fracking, is a technique used by the petroleum industry to recover more oil or natural gas from strata that might otherwise be difficult to tap. Mixtures of sand, water, and various combinations of chemicals can be injected under high pressure into deep underground formations of sandstone or shale containing fossil fuels. The pressurized fluid splits these formations by widening any tiny fractures that are present and thereby frees the oil or gas they contain, which can then move up the wellbore for collection. The process is credited with increasing yields from oil and gas fields that have already been drilled extensively or are otherwise hard to exploit through traditional drilling. See also: Coalbed methane; Oil and gas field exploitation; Oil and gas well completion; Oil and gas well drilling
Commercial use of hydraulic fracturing dates back to 1949, and the technology has been used on oilfields around the world since then. In recent years it has also come to prominence as a way of obtaining the massive amounts of natural gas found inside formations such as the Marcellus Shale extending beneath much of the Appalachian Basin in North America. See also: High methane in drinking water near fracking sites; Natural gas wells leakier than believed
Nevertheless, fracking has also become highly controversial because of the possibility that it might endanger the local environment and human health. Some of the worries are that methane in the liberated natural gas, along with radioactive elements leached from the underground rocks and chemicals in the fracking fluids themselves, could contaminate aquifers and imperil drinking water. Videos shot in homes near fracking sites have sometimes shown tap water that could be ignited and burned as it emerged from the faucet because of high levels of contained methane or other combustible vapors in it. Leakage of chemicals and fracking fluid from on-site storage tanks is a concern, particularly because the identities of the chemicals used are often guarded as company secrets.
Injecting fluids down into underground rock formations also carries the potential to destabilize them seismically: The U.S. Geological Survey and other organizations have linked the disposal of hydraulic fracturing wastewater to some small earthquakes and tremors, and a 4.0-magnitude earthquake near Youngstown, Ohio, was specifically tied to the practice by scientists at Columbia University’s Lamont-Doherty Earth Observatory.
How serious these dangers may be in practice and how specifically they can be blamed on fracking are still hotly debated, however. Gas drilling companies have argued, for example, that fracking fluids are injected at depths too far beneath aquifers for contamination to be possible; any methane entering water supplies, they say, is a consequence of drilling and not of the fracking. Similarly, the seismic tremors seem to be caused by the disposal of wastewater from the drilling back down the wellholes, but this practice is common in mining and not limited to hydraulic fracturing. The National Research Council of the U.S. National Academy of Sciences has stated that shale gas fracking is unlikely to cause earthquakes normally perceptible to humans.