In situations in which treatment of flowback and produced water is not appropriate or affordable, operators dispose of water permanently. This is most commonly achieved through the use of injection wells. Injection wells for the permanent disposal of water associated with oil and gas production are regulated by the U.S. Environmental Protection Agency under its Underground Injection Control (UIC) Program: UIC Class II protocols.
Locating a site appropriate for wastewater disposal requires the analysis of a variety of factors. The site should be close to production but far from residential areas. The geology of the injection zone is crucial and requires isolated permeability to allow the water not only to enter the pressurized subsurface formation but also to stay there without migrating to the surface or to underground sources of groundwater. Because of the high pressures and extended timelines of injection, well integrity must be maintained at all times.
Most UIC Class II wells are located in Texas, California, Oklahoma and Kansas. As of 2016, there were approximately 180,000 permitted Class II wells across the country. About 20% of these wells are used exclusively for underground wastewater disposal with the balance used for enhanced oil recovery.
Induced seismicity relating to oil and gas production has entered the conversation about fracturing, production and disposal. Contemporary research supports a link between disposal wells and felt earthquakes in some areas, especially where critically stressed faults occur nearby or where fluids reach basement rock. Felt earthquakes have also been linked to hydraulic fracturing in a limited number of events, but remains very uncommon.
Citations
Graphic: Top Energy Training
Images: “Illustration” by Top Energy Training
