Underground Injection Control: The Six Classes of Injection Wells and Their Regulation

 

     An injection well is a well that injects fluids underground to be stored in porous geologic formations. The injection zones may be deep sandstones or limestones or shallow soil layers. Injected fluids may be water, wastewater, brine (salt water), or water mixed with chemicals. For underground injection control (UIC) purposes a well may be defined as a drilled borehole or as a dug hole, an improved sinkhole, or a subsurface fluid distribution system. EPA notes:

“Injection well construction is based on the type and depth of the fluid injected. For example, wells that inject hazardous wastes or carbon dioxide (CO2) into deep isolated formations have sophisticated construction.  These wells are designed to provide multiple layers of protective casing and cement. In contrast, shallow wells are usually of simple construction.”

UIC wells are classified into six types based on depth, type of injection, and the risk to drinking water aquifers. Injection wells are used to store CO2, dispose of waste, enhance oil production, mine water disposal, solution mining, and prevent saltwater intrusion. In the 1930s the practice became widespread to dispose of oilfield brines. In the 1950s industrial wastewater from chemical companies began to be disposed of underground.

     EPA regulates UIC wells according to the 1974 Safe Drinking Water Act (SDWA). That law mandated them to develop safe practices and minimum standards for UIC wells in order to protect underground sources of drinking water (USDW) from contamination. The UIC program works in accordance with local and regional groundwater protection planning.

 

“The purpose of the UIC requirements is to ensure that either:

·        Injected fluids stay within the well and the intended injection zone

·        Fluids that are directly or indirectly injected into a USDW do not cause a public water system to violate drinking water standards or otherwise adversely affect public health.”        

 This is important because it is well known that fluids can migrate underground at vastly different rates depending on the geologic conditions. Injection well oversight is provided by states that are given primacy by the EPA or by the EPA through one of their regional offices. Currently, there are 33 states that have been given primacy to regulate underground injection. More than 740,000 injection wells were regulated by the UIC program in 2018. Some basic info and data is given below. The third graph below shows that Class II wells are inspected the most. The fourth graph shows that most Class V wells are not permitted but mostly documented older wells (if I am reading that right) so Class II wells make up the bulk of permits.

 

 

Class I:  Industrial and Municipal Waste Disposal Wells

     Class I wells can inject both hazardous (17%) and non-hazardous (53%) wastes. These wastewaters are injected into deep confined geologic reservoirs. Wastewater from petroleum refining, metal production, chemical production, pharmaceutical production, commercial disposal, food production, and municipal wastewater treatment are injected into Class I UIC wells. Wastes defined as hazardous are also regulated under the Resource Conservation and Recovery Act (RCRA). About 17% of Class I wells are hazardous waste disposal wells. Most are located at industrial facilities and dispose of waste generated onsite. They are operated in 10 states with the majority in Texas and Louisiana. Only a few commercial Class I wells accept hazardous waste generated offsite. There are about 800 class I injection wells in the U.S. Due to geological suitability, most of these are along the Gulf Coast or along the Great Lakes. There are non-hazardous class I UIC wells in 19 states. Most are in Texas, California, Louisiana, Kansas, and Wyoming. About 30% of class I UIC wells are municipal wastewater disposal wells located in Florida. Typical depth ranges for Class I wells are 1700 ft to greater than 10,000 ft.

“Every Class I well operates under a permit. Each permit is valid for up to 10 years. Owners and operators of Class I wells must meet specific requirements to obtain a permit. These requirements address the siting, construction, operation, monitoring and testing, reporting and record keeping, and closure of Class I wells.”

The sections below from the 2001 study of Class I UIC well risks summarize the construction, operation, monitoring and testing, reporting and record keeping, and closure requirements of Class I wells.

 

Class II: Oil and Gas Related Injection Wells

     Class II UIC wells primarily inject oil and gas brines. It is estimated that over 2 billion gallons of oil and gas wastewater are injected into Class II wells daily. In this case, the waste came from the ground, being produced along with oil and gas and so it is basically being put back into the ground, though many times in a different geological formation and depth than from where it originated. Texas, California, Kansas, and Oklahoma have the most Class II injection wells. There are about 180,000 Class II wells in the U.S. There are three basic types: 1) disposal wells, 2) enhanced recovery wells, and 3) hydrocarbon storage wells. Brine disposal wells make up about 20% of Class II wells. Enhanced recovery wells, where water is injected to help recover oil from reservoirs, make up nearly 80% of Class II wells. There are about 100 hydrocarbon storage wells in the U.S. Most of these are in the Strategic Petroleum Reserve.

     Oil and gas brines are toxic due to high salinities, often greater than seawater, and the presence of toxic heavy metals and low-level radioactivity. As with Class I wells there are requirements for permitting, construction, conversion, operation, monitoring and testing, inspections, reporting, record keeping and closure.

     Well operators are required to submit maps, area of review plans, corrective action plans, a schematic of the proposed well, landowner information, geological data, a formation testing plan, construction and conversion procedures, an operating and monitoring plan, a plugging and abandonment plan, financial assurance, a site security plan, manifest requirements, and existing EPA permits.

     Modern oil and gas wastewater injection wells may inject high volumes at high pressures. This is determined based on geology and mechanical reservoir properties. This has led to some serious issues with oil and gas wastewater injection. There are three major problems: 1) induced seismicity, 2) migration of injected fluids into nearby oil and gas reservoirs, and 3) migration via surface spills or casing leaks that may affect drinking water aquifers. Injecting wastewater into reservoirs that are connected to fault systems can lead to induced seismicity, or earthquakes, by helping known and unknown faults to slip. These types of shallow earthquakes are rarely damaging but there are some cases of property damage and lawsuits. Number 2, the migration of injected fluids into nearby oil and gas reservoirs, is also accelerated by high injection volumes and pressures. There are some cases in Southeastern Ohio where this is documented. The issue is mainly economic as nearby gas producers say the water, which was injected in the gas reservoir, was chemically identified and tied to the injection wells, is lowering the gas production in their wells. In that case, water migrating faster through fracture systems is a likely scenario. Number 3, migration via surface spills or casing leaks above an aquifer, is the highest risk scenario. Some Class II disposal wells handle a high volume of wastewater. More water means more opportunities for spills. Often old wells are converted into Class II disposal wells. Sometimes, they may develop well integrity issues. This is what happened in a gas storage well at Aliso Canyon on California that experienced a dangerous gas leak in 2015. The well was drilled in the 1950s and later converted into a gas storage field production well. It was later determined that there was a shallow casing leak that leaked the gas. Since Class II wells are usually much deeper than any freshwater aquifers there is little possibility of gas migrating up into an aquifer from below.  Contamination is far more likely from a surface spill or if there is a casing leak above the aquifer. Casing leaks are rare but less so with older wells. The presence of H2S inside or outside the borehole can corrode the casing over time. Some water is corrosive. Early in the Marcellus shale gas play in Northeastern Pennsylvania, there were wells that would find sustained casing pressure in the annulus, indicative of “stray gas” migration. Some thought the gas had migrated up from deep reservoirs such as the Marcellus, but it was chemically proven that the stray gas came from gas zones just below the aquifer. Better well cementing has led to far fewer stray gas incidents. The biggest danger from the gas is its flammability. It must be vented for safety if it is above the threshold concentration. There is generally no contamination risk.       

     The use of diesel fuel in hydraulic fluids also requires Class II well permitting. I believe this is currently very rare to non-existent although it was practiced in the past. In the Energy Policy Act of 2005, hydraulic fracturing received a broad exemption under the SDWA, with the exception of diesel fuel.

 

Class III: Injection Wells for Solution Mining

 

     In Class III UIC wells fluids are injected to dissolve and extract minerals. Solution mining involves injection wells and production wells. Wells that produce brine for surface extraction are not regulated under the UIC program, only wells that inject. Many wells do both and are regulated as Class III wells. Uranium, salt, copper, and sulfur are produced through solution mining. More than half of all salt mining and 80% of uranium mining is done through Class III UIC wells.

     Most Class III wells in the U.S. are uranium in-situ leaching (ISL). The leaching process involves injecting a solution known as a lixiviant. The lixiviant dissolves the uranium ore after sufficient contact time. Then the resulting fluid is produced to surface, the lixiviant is separated from the uranium and reinjected to dissolve more ore.

     Salt solution mining wells inject water to dissolve the salts with the resulting brine being produced to the surface. There are two main methods: 1) injecting through the tubing and producing through the annular space between the tubing and the production casing. This is a single well method. 2) if the salt occurs in a dome a single well is used but if the salt occurs in separated layers, then multiple wells injection wells are used. Copper solution mining occurs in a few states where a sulfuric acid solution is injected to dissolve the copper. Sulfur solution mining wells may occur where super-heated steam makes a sulfur solution that can be produced through the Frasch process. Currently, there are no wells injecting for sulfur solution mining. In solution mining more fluid is extracted than injected, according to EPA, apparently to prevent fluid migration that could impact drinking water. It is not uncommon for benign fluids to be injected into underground sources of drinking water (USDW). This is often true for uranium ISL which taps sandstone groundwater aquifers. Solution mining, especially salt solution mining, can cause subsidence in overlying aquifers. Additional requirements include tubing that can accommodate the injected fluids, cementing to prevent fluid migration into a USDW, pre-injection pressure testing, monitoring flow rate and pressure, casing testing every five years, and adequate plugging and abandonment.

 

 

 

Class IV: Shallow Hazardous and Radioactive Injection Wells

 

     The EPA explains Class IV UIC wells:

“Class IV wells are shallow wells used to dispose hazardous or radioactive wastes into or above a geologic formation that contains an underground source of drinking water (USDW). In 1984, EPA banned the use of Class IV injection wells. These wells may only operate as part of an EPA- or state-authorized ground water clean-up action. Less than 32 waste clean-up sites with Class IV wells exist in the United States.”

Both Class IV and Class V wells inject fluids into or above USDW. If a Class V well injects hazardous fluids, it becomes a Class IV well. Both classes may include septic systems or dry wells. Class IV wells are wells that are used to clean up groundwater contaminated by hazardous chemicals. A common method of treatment is the pump-and-treat method where groundwater is pumped to the surface for treatment and then injected back into the aquifer over and over until the contaminant level drops to acceptable levels or until no more treatment is possible. Non-hazardous waste wells also use this method of groundwater remediation.

 

Class V: Wells for Injection of Non-Hazardous Fluids into or Above Underground Sources of Drinking Water

     Class V UIC wells are wells that are left over from the other classifications and may include aquifer storage and recovery wells, geothermal electric power wells, or deep injection wells for salinity control. Class V wells make up the bulk of UIC wells at up to 650,000 and well over 70% of UIC wells.

     EPA explains:

“Most Class V wells are "low-tech" and depend on gravity to drain fluids directly below the land surface. Dry wells, cesspools, and septic system leach fields are examples of simple Class V wells. Because their construction often provides little or no pretreatment and these fluids are injected directly into or above an underground source of drinking water, proper management is important.”

“More sophisticated Class V wells may rely on gravity or use pressure systems for fluid injection. Some sophisticated systems include advanced wastewater disposal systems used by industry, experimental wells used to test new or unproven technologies, and systems used to inject and store water for later reuse.”

A 1999 EPA study involving 23 Class V well types estimated that there are more than 650,000 of these wells in use in the U.S. This study looked at 22 different kinds of Class V injection wells in terms of fluids injected and developed recommendations for regulation. Some of the well types are agricultural drainage wells, stormwater drainage wells, car washes, large capacity septic systems, food processing disposal wells, sewage treatment effluent wells, mine backfill wells, aquifer remediation wells, some geothermal wells, saline intrusion barrier wells, aquifer recharge/recovery wells, and subsidence control wells.

 

Class VI - Wells used for Geologic Sequestration of Carbon Dioxide

     Class VI UIC wells inject CO2 underground for sequestration. This is an emerging type of injection well that will become more common as CCUS projects proceed according to development plans. CO2 is injected as a supercritical fluid under pressure. These wells must account for CO2 properties such as buoyancy, subsurface mobility, corrosivity in the presence of water, and large injection volumes. CO2 is more likely to leak due to its buoyancy. CO2 injection wells also require monitoring, including comparison of pre-injection and post-injection reflection seismic surveys.  

     Requirements include site investigation, geologic modeling, reservoir characterization, and other planning requirements consistent with other injection well classes. The EPA established rules for Class VI wells in December 2010. In August 2023 the EPA established environmental justice provisions for Class VI wells. These call for site selection and continual public engagement through time that considers potentially affected EJ communities. Outreach in developing emergency response plans is another requirement. The phases of a Class VI well are shown below.   

      The first phase is the pre-permitting phase where the operator applies for the permit and meets with the permitting authority to discuss the permit. EPA also encourages public engagement and consideration of potential environmental justice issues. The second phase is the pre-construction phase includes review of the permit application, public posting of a draft permit, a 30-day public comment period, with comments considered before the final permit is issued. The pre-operation phase involves construction and pre-operation testing, which is reviewed before permission is given to operate the well. Any new information from the drilling of the well is incorporated and plans and/or permits altered as needed. Authorization to inject is then given. The injection phase is the operating phase of the well. Required ongoing monitoring and testing results are reported to the permitting authority and through them to the public. The Area of Review is also re-evaluated. The post-injection phase involves plugging the well, and monitoring the CO2 plume and pressure front, and the site is closed pending no identified risks.

     The Class VI well-permitting process includes a completeness review where the regulator makes sure there are no deficiencies. If there is, a notice of deficiency (NOD) is returned to the operator to rectify. Without an NOD a completeness review takes about 30 days. The technical review is next. It involves dialogue between operators and regulators so that the details of the project are understood, and environmental protection is assured. This is when a request for additional information (RAI) may be formally requested. A draft permit is then issued for public comment. There may be certain conditions relative to the well added to the permit. Then the final permit is issued. EPA hopes to be able to issue Class VI permits within 24 months of the application but that is without any delays due to incompleteness or lacking information. I think this could be speeded up to a year when more of these permits are reviewed and issued. The current timeframe is too slow.

     EPA has a Class VI permit tracker. It gives permit progress information. The screenshot below includes the latest permit stats. The vast majority of wells are in the technical review process. Only 4 well applications are in the Prepare Final Permit Decision phase.

 

 

References:

Protecting Underground Sources of Drinking Water from Underground Injection (UIC). U.S. EPA. Protecting Underground Sources of Drinking Water from Underground Injection (UIC) | US EPA

Underground Injection Control Program (Factsheet). U.S. EPA. UIC Fact Sheet (epa.gov)

Class I Underground Injection Control Program: Study of the Risks Associated with Class I Underground Injection Wells. U.S. EPA. March 2001. Class I Underground Injection Control Program: Study of the Risks Associated with Class I Underground Injection Wells, March 2001 (epa.gov)

United States Environmental Protection Agency Underground Injection Control (UIC) Program Class II Permit Application Completeness Review Checklist. August 2018. UIC Class II Permit Application Checklist (epa.gov)

Class V Underground Injection Control Study. U.S. EPA. Class V Underground Injection Control Study | US EPA

Environmental Justice Guidance for UIC Class VI Permitting and Primacy. U.S. EPA. August 17, 2023. Memo and Environmental Justice Guidance for UIC Class VI Permitting and Primacy - August 2023 (epa.gov)

Underground Injection Control (UIC) Class VI Permit Tracker. UIC Class VI Wells Permit Tracker Dashboard (epa.gov)

Injection Wells. Ohio Department of Natural Resources. Injection Wells | Ohio Department of Natural Resources (ohiodnr.gov)