Hydraulic fracturing (HF), colloquially known as “fracking,” refers to a set of technologies used to increase the extraction of natural gas and oil from low-permeability geological formations and typically involves vertical and horizontal drilling at depths as much as 3 kilometers beneath the surface. It was first employed as an experiment in 1947 and within the past 15 years has become the source of dramatic increases in gas and petroleum production in the United States.
The EPA estimates that 25,000 to 30,000 new wells were drilled and hydraulically fractured each year between 2011 and 2014, requiring more than 40 billion gallons of water per year.
In a conventional gas or oil field, the deposits lie in relatively porous, permeable rock. But many gas and oil deposits are trapped by the downward pressure of largely solid rock (often shale) overhead. To extract these deposits, it is necessary to inject enough “fracturing fluid” at high pressure to crack the rock and allow the gas and oil to flow up the well. Fluid composition varies, but in general consists of 90% water and more than 9% sand, with the remainder made up of various chemicals: lubricants, gels, foams, surfactants, etc., as appropriate to a specific site.
Depending on depth and other factors, the U.S. Geological Survey reports that it can take as much as 36,000 cubic meters of water—about 10 million gallons—per well to cause the desired fracture. The national average is around 1.5 million gallons per well, according to the U.S. Environmental Protection Agency (EPA). (Some fraction of that is recovered for wastewater treatment and/or re-used.) The sand serves to hold the cracked areas open.
In the United States, one successful drilling rig can extract about 500 barrels of oil per day or more than 6 billion cubic feet of gas per day. And there are many HF wells. The EPA estimates that 25,000 to 30,000 new wells were drilled and hydraulically fractured each year between 2011 and 2014, requiring more than 40 billion gallons of water per year.
The nature and pace of HF activity have raised significant questions about its environmental and public health effects. Some injections contain toxic chemicals that may escape into the air with or without a well blowout or remain in the earth, posing a potential threat to nearby aquifers. Unexpected seismic events threaten contamination of aquifers and surface water and natural gas may find its way into tap water or into the atmosphere where it serves as a greenhouse gas. So far, such events have been relatively few and confined to local areas.
A draft assessment of the impact of HF on U.S. drinking water released by the EPA in 2015 identified numerous potential mechanisms by which hydraulic fracturing could affect drinking water resources. Above ground mechanisms can affect surface and groundwater resources and include water withdrawals at times, or in locations of low water availability, spills of hydraulic fracturing fluid and contaminated water, and inadequate treatment and discharge of hydraulic fracturing wastewater. Below ground mechanisms include movement of liquids and gases via the production well into underground drinking water resources and movement of liquids and gases from the fracture zone to these resources via pathways in subsurface rock formations.
But the EPA report concluded that there was no evidence that “these mechanisms have led to widespread, systemic impacts on drinking water resources in the United States,” and that the “number of identified cases where drinking water resources were impacted are small relative to the number of hydraulically fractured wells.”
- Chemistry and Engineering of Shale Gas and Tight Oil Development: A Workshop in Brief (2015)
- Health Impact Assessment of Shale Gas Extraction (2014)
- Characterization, Modeling, Monitoring, and Remediation of Fractured Rock (2015)
- Induced Seismicity Potential in Energy Technologies (2013)
- Risks and Risk Governance in Shale Gas Development (2014)