An injection well is a vertical pipe in the ground into which water, other liquids, or gases are pumped or allowed to flow.
Injection wells are used for many purposes.
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One application is waste water disposal, in which treated waste water is injected into the ground between impermeable layers of rocks to avoid polluting fresh water supplies or adversely affecting quality of receiving waters. Injection wells are usually constructed of solid walled pipe to a deep elevation in order to prevent injectate from mixing with the surrounding environment.
Injection wells are widely considered[by whom?] to be the best method for disposal of treated waste water. Unlike outfalls or other direct disposal techniques, injection wells utilize the earth as a filter to further clean the treated wastewater before it reaches the receiving water. This method of waste water disposal also serves to spread the injectate over a wide area, further decreasing environmental impacts.
Critics of waste water injection wells[who?] cite concerns relating to the injectate polluting receiving waters. Most environmental engineering professionals,[who?] however, consider waste water treatment followed by disposal through injection wells to be the most cost effective and environmentally responsible method of waste water treatment. The only known alternatives to injection wells are direct discharge of treated waste water to receiving waters or utilization of the treated water for irrigation. Direct discharge does not disperse the water over a wide area; the environmental impact is focused on a particular segment of a river and its downstream reaches or on a coastal waterbody. Extensive irrigation is often prohibitively expensive and requires ongoing maintenance and large electricity usage.
Oil and gas production
Another use of injection wells is in natural gas and petroleum production. Steam, carbon dioxide, water, and other substances can be injected into an oil-producing unit in order to maintain reservoir pressure, heat the oil or lower its viscosity, allowing it to flow to a producing well nearby. See also Enhanced oil recovery and Hydraulic fracturing.
Waste site remediation
Yet another use for injection wells is in environmental remediation, for cleanup of either soil or groundwater contamination. Injection wells can insert clean water into an aquifer, thereby changing the direction and speed of groundwater flow, perhaps towards extraction wells downgradient, which could then more speedily and efficiently remove the contaminated groundwater. Injection wells can also be used in cleanup of soil contamination, for example by use of an ozonation system. Complex hydrocarbons and other contaminants trapped in soil and otherwise inaccessible can be broken down by ozone, a highly reactive gas, often with greater cost-effectiveness than could be had by digging out the affected area. Such systems are particularly useful in built-up urban environments where digging may be impractical due to overlying buildings.
Recently the option of refilling natural aquifers with injection or percolation has become more important, particularly in the driest region of the world, the MENA region (Middle East and North Africa).
Surface runoff can also be recharged into dry wells, or simply barren wells that have been modified to functions as cisterns. These hybrid stormwater management systems, called recharge wells, have the advantage of aquifer recharge and instantaneous supply of potable water at the same time. They can utilize existing infrastructure and require very little effort for the modification and operation. The activation can be as simple as inserting a polymer cover (foil) into the well shaft. Vertical pipes for conduction of the overflow to the bottom can enhance performance. The area around the well acts as funnel. If this area is maintained well the water will require little purification before it enters the cistern.
In the United States, injection well activity is regulated by the United States Environmental Protection Agency (EPA) and state governments under the Safe Drinking Water Act (SDWA). EPA has issued Underground Injection Control (UIC) regulations in order to protect drinking water sources.
The EPA has defined six classes of injection wells. Class I wells are used for the injection of municipal and industrial wastes beneath underground sources of drinking water. Class II wells are used for the injection of fluids associated with oil and gas production, excluding hydraulic fracturing. Class III wells are used for the injection of fluids used in mineral solution mining beneath underground sources of drinking water. Class IV wells, like Class I wells, are used for the injection of hazardous wastes but inject waste into or above underground sources of drinking water instead of below. Class V wells are those used for all non-hazardous injections that are not covered by Classes I through IV. Examples of Class V wells include stormwater drainage wells and septic system leach fields. Finally, Class VI wells are used for the injection of carbon dioxide for sequestration, or long term storage. Currently, there are no Class VI wells in operation, but 6 to 10 wells are expected to be in use by 2016.
- U.S. Environmental Protection Agency (EPA). Washington, DC. "Basic Information about Injection Wells." Updated 2010-01-22.
- EPA. Washington, DC. "Oil and Gas Related Injection Wells (Class II)." Updated 2010-01-22.
- EPA. New York, NY (2003-04-17). "EPA Announces Cleanup Plan for Contaminated Soil and Ground Water at Central Islip Superfund Site." Example of use of ozonation wells for remediation in situ.
- H2O magazine (2010-10-16). "Strategic reserve" by Anoop K Menon
- H2O magazine (2011-05-03). "Recharging dry wells." by Nicol-André Berdellé
- Prototype-Creation (2011-04-20). "Recharge wells and ASR." by Nicol-André Berdellé
- EPA. Washington, DC. "Underground Injection Control Program: Regulations." Updated 2010-01-22.
- EPA. Washington, DC (July 2001). "Technical Program Overview: Underground Injection Control Regulations." Document no. EPA 816-R-02-025.
- Congress excluded hydraulic fracturing from coverage under the SDWA. Energy Policy Act of 2005, (Pub.L. 109–58), approved 2005-08-08. Amended SDWA § 1421(d). See 42 U.S.C. § 300h.
- EPA. "Underground Injection Control: Classes of Wells." Updated 2012-03-06.
- US Army Environmental Center. Aberdeen Proving Ground, MD (2002). "Deep Well Injection." Remediation Technologies Screening Matrix and Reference Guide. 4th ed. Report no. SFIM-AEC-ET-CR-97053.