Class II

Class II wells include all injection wells associated with oil and gas production operations. However, the UIC program does not include any wells that are used exclusively for production. In 2018, there were 181,431 Class II wells which comprised ~25% of all UIC wells. Inspections were completed on ~45% of those wells.

2018 Injection Wells by Class

Injection Well ClassNumber of Injection WellsInjection Wells Inspected
Class I830453
Class II181,43184,317
Class III28,327406
Class IV12227
Class V531,1764,038
Class VI20

Source: EPA 816F19005 April 2020

States (tribes and territories) have the option of requesting primacy for Class II wells under either Section 1422 or Section 1425 of the SDWA.1

Under Section 1422 states must meet EPA’s minimum requirements for UIC programs. Programs authorized under section 1422 must include well owner and operator requirements for:

  • Construction
  • Operation
  • Monitoring and testing
  • Reporting
  • Closure requirements

Under Section 1422 enhanced recovery wells may either be issued permits or be authorized by rule. Disposal wells are issued permits.  The owners or operators of the wells must meet all applicable requirements, including strict construction and conversion standards and regular testing and inspection.

Under Section 1425 states must demonstrate that their existing standards are effective in preventing endangerment of USDWs. These programs must include requirements for:

  • Permitting
  • Inspections
  • Monitoring
  • Record-keeping
  • Reporting

Class II wells are subdivided into three broad classes: disposal, enhanced oil recovery, and hydrocarbon storage.


During oil and gas production, water associated with the producing formation rises to the surface with hydrocarbons. This produced water may be called brine, as it may contain high levels of salts or other dissolved solids. Operators separate produced water from the oil and gas at the surface and return it to subsurface formation for disposal using injection wells. Operators also use Class II injection wells to inject flowback water from hydraulic fracturing. Class II wells may also receive other forms of exploration and production waste such as gas plant waste cooling tower blowdown. Class II disposal wells make up about 20% of the total number of Class II wells.1

Enhanced Oil Recovery

While the EPA does not regulate oil and gas production wells, some enhanced oil recovery techniques inject fluids using wells that fall under these regulations. Operators use enhanced oil recovery techniques in cases of heavy oil, and in some cases tight oil and gas scenarios. Enhanced recovery wells account for approximately 80% of all Class II wells.1  

Hydrocarbon Storage

After production liquid hydrocarbons may be stored underground. For example, the United States maintains a massive underground Strategic Petroleum Reserve (SPR) with a total capacity of 727 million barrels of crude oil. Over 100 Class II liquid hydrocarbon storage wells operate in the United States.1

Hydraulic Fracturing

The Energy Policy Act of 2005 exempted hydraulic fracturing activities on wells drilled for production operations from the UIC  provisions of the SDWA. As a result, injection for hydraulic fracturing is not covered by UIC regulations except in one specific case. If operators plan to stimulate a formation with fracturing fluids that contain any amount of diesel fuel, the well falls under Class II regulations, including the appropriate permitting and program requirements.

Diesel fuel when used as a component of drill mud, pipe joint compounds used in well construction or used in motorized equipment on the surface is not subject to UIC Class II permitting requirements in certain cases.

The EPA has provided UIC Guidance 84 regarding Oil and Gas Hydraulic Fracturing Activities Using Diesel Fuels. 

What is a diesel fuel according to UIC Guidance 84?

A hydraulic fracturing activity is subject to UIC Class II permitting requirements under the SDWA if any portion of the injectate contains “diesel fuels.” The EPA interprets this statutory term to mean any of the following five CASRNs (Chemical Abstracts Service Registry Number):

  • CASRNs: 68334-30-5 Primary Name: Fuels, diesel Common Synonyms: Automotive diesel oil; Diesel fuel; Diesel oil (petroleum); Diesel oils; Diesel test fuel; Diesel fuels; Diesel fuel No. 1; Diesel fuel [United Nations-North America (UN/NA) number 1993]; Diesel fuel oil; European Inventory of Existing Commercial Chemical Substances (EINECS) 269-822-7.
  • CASRNs: 68476-34-6 Primary Name: Fuels, diesel, No. 2 Common Synonyms: Diesel fuel No. 2; Diesel fuels No. 2; EINECS 270-676-1; No. 2 Diesel fuel. •
  • CASRNs: 68476-30-2 Primary Name: Fuel oil No. 2 Common Synonyms: Diesel fuel; Gas oil or diesel fuel or heating oil, light [UN1202] No. 2 Home heating oils; API No. 2 fuel oil; EINECS 270-671-4; Fuel oil No. 2; Home heating oil No. 2; No. 2 burner fuel; Distillate fuel oils, light; Fuel No. 2; Fuel oil (No. 10 US EPA,
  • CASRNs: 68476-31-3 Primary Name: Fuel oil, No. 4 Common Synonyms: Caswell No.14 333AB; Cat cracker feed stock; EINECS 270-673-5; EPA Pesticide Chemical Code 063514; Fuel oil No. 4; Diesel fuel No. 4.
  • CASRNs: 8008-20-6 Primary Name: Kerosene Common Synonyms: JP-5 navy fuel/marine diesel fuel; Deodorized kerosene; JP5 Jet fuel; AF 100 (pesticide); Caswell No. 517; EINECS 232-366-4; EPA Pesticide Chemical Code 063501; Fuel oil No. 1; Fuels, kerosine; Shell 140; Shellsol 2046; Distillate fuel oils, light; Kerosene, straight run; Kerosine, (petroleum); Several Others.

Class II EPA UIC Wells
  1. USEPA. (n.d.). Class II oil and gas related injection wells. Retrieved 11/9/2020 from
  2. USEPA (2014). Permitting guidance for oil and gas hydraulic fracturing activities using diesel fuels: Underground Injection Control Program Guidance #84.

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