What is a disposal well?

As defined on the EPA websiteAn injection well is used to place fluid underground into porous geologic formations. These underground formations may range from deep sandstone or limestone, to a shallow soil layer. Injected fluids may include water, wastewater, brine (salt water), water mixed with chemicals.  The definition of a well is codified in the UIC regulations at 40 CFR 144.3.  Well means:  A bored, drilled, or driven shaft whose depth is greater than the largest surface dimension; or, a dug hole whose depth is greater than the largest surface dimension; or, an improved sinkhole; or, a subsurface fluid distribution system. 

The EPA underground injection control (UIC) program defines six different classes of injection wells based on type and depth of injection activity.  Class II UIC wells are wells used only to inject fluids associated with oil and natural gas production.  Class II UIC wells fall into one of three different categories, enhanced recovery (injection) wells, disposal wells, and hydrocarbon storage wells.

When oil and gas is extracted, water (which is commonly salty) that is also present in the production zone is brought to the surface. In order to dispose of this byproduct in an environmentally safe and efficient manner, the water, known as produced water, at times will be injected into underground geologic formations through a disposal well.  Wastewater from hydraulic fracturing activities can also be injected into Class II wells. Class II disposal wells are typically thousands of feet below the surface in formations isolated from underground sources of drinking water preventing soil and water contamination.

Certain geological characteristics are considered when researching the location of the disposal well. The formation where the produced water will be injected may be the same one from which the water came or one similar in characteristics. The receiving formation should be porous in order to absorb the produced water and surrounded by impermeable rock that will act as a confining zone.  Other characteristics examined include the geologic structure such as for the potential presence of faults or fractures and the water quality in the disposal formation. 

Additionally, the well bore through which the produced water will be injected is isolated by encasing it with multiple layers of steel casing and special cement. Proper injection pressure limits are set so that fracturing of the disposal zone or confining units does not occur. 

Who regulates disposal wells?

The United States Environmental Protection Agency (EPA) regulates the UIC program through the Safe Drinking Water Act.  States have the option to request primacy from EPA in order to manage the UIC program.  To be granted primacy, state regulations must meet EPA’s minimum requirements for UIC programs by demonstrating that their standards are effective in preventing endangerment of USDWs in all of the following areas:

            ·         Permitting

·         Inspections

·         Monitoring

·         Record-keeping

·         Reporting

Wyoming was granted primacy in 1982 and through the Wyoming Oil and Gas Conservation Commission (WOGCC) successfully regulates the state’s Class II UIC wells by meeting EPA’s minimum requirements for construction, operation, monitoring and testing, reporting and closure requirements.  In Wyoming, other classes of UIC wells are regulated by the Department of Environmental Quality (DEQ).

Class II disposal well permits in Wyoming are issued through a rigorous application process. All applicable requirements must be met, including strict construction standards and regular testing and inspection.

Nationwide, disposal wells make up about 20 percent of the total number of Class II wells. Other categories of Class II wells are enhanced recovery wells and hydrocarbon storage wells. Approximately 180,000 total Class II wells are in operation in the United States with most of them in Texas, California, Oklahoma, and Kansas. 

In Wyoming, there are approximately 7,218 Class II wells with 606 of those being disposal wells for the oil and gas industry. 

Aquifer Exemption

An aquifer is a geologic formation, group of formations, or part of a formation that is capable of yielding a significant amount of water to a well or spring.  Based on certain criteria, UIC regulations allow the state or EPA to exempt aquifers to be used by energy and mining companies for oil or mineral extraction or disposal purposes in compliance with the state’s or EPA’s UIC regulations.

Every exemption is subject to public input.  When reviewing an aquifer exemption application either states that are granted primacy, such as Wyoming, or the EPA must provide a notice, take public comments, and hold a public hearing. 

According to the WOGCC’s rules, which are substantially the same as EPA’s regulations, an aquifer which contains fresh and potable water may be exempt if the WOGCC determines any of the following criteria exists: 

1.    It is mineral, hydrocarbon, or geothermal energy producing

2.    It is situated at a depth or location which makes recovery of fresh and potable water economically or technologically impractical

3.    It is so contaminated that it would be economically or technologically impractical to render the water fit for use as fresh and potable water

4.    It is located over a mining area subject to subsidence or catastrophic collapse, or

5.    It has a total dissolved solids (TDS) of more than five thousand and less than ten thousand milligrams per liter and is not reasonably expected to be used as fresh or potable water. 


Furthermore, the upper and lower boundaries of the aquifer exemption proposed by the well owner/operator are also evaluated so that nearby drinking water sources remain protected.


The approval process includes the EPA, who receives information about the aquifer proposed for exemption from the WOGCC.  Unless the TDS of the disposal zone is greater than 10,000 milligrams per liter, injection of fluids can begin only after the EPA reviews and concurs with the state’s decision on an aquifer exemption and an underground injection control permit is granted. 

EPA’s Nationwide Aquifer Exemptions Map

Next steps

All disposal wells must pass an initial mechanical integrity test (MIT), which demonstrates that the casing, tubing, and cement will withstand the necessary pressure of injection without losing its integrity.  An MIT is required at least every five years thereafter. In addition to MITs, regular inspections are conducted and monthly reports of injected volumes and pressures are provided to the WOGCC to ensure proper operations.    

For more information on UIC Class II wells, please visit EPA’s website on UIC Class II wells