Injection and Disposal Wells

Disposal wells may be used to inject mineralized water produced with oil and gas into underground zones for the purpose of safely and efficiently disposing of the fluid. Typically, the underground interval is one that is not productive of oil and gas. In some cases, however, the disposal interval is a productive zone from which oil or natural gas has been produced or is currently produced. In either case, the disposal interval must be sealed above and below by unbroken, impermeable rock layers. Injection wells inject fluids into a reservoir for the purpose of enhanced oil recovery from the reservoir. The vast majority of wells in Texas are injection wells. Operators use injection wells to increase or maintain pressure in an oil field that has been depleted by oil production and also to displace or sweep more oil toward producing wells. This type of secondary recovery is sometimes referred to as water-flooding. Texas is the nation’s number one oil and gas producer with more than 315,618 active oil and gas wells statewide according to oil and gas well proration schedules (as of June 30, 2015). Injection and disposal wells are also located throughout the state to improve oil and gas recovery and to safely dispose of the produced water and hydraulic fracturing flowback fluid from oil and gas wells. Texas has more than 54,700 permitted oil and gas injection and disposal wells with approximately 34,200 currently active as of July 2015. Of these 34,200 active injection and disposal wells, about 8,100 are wells that are used for disposal, the remainder (about 26,100) are injection wells.

What chemicals are found in the fluid injected into injection and disposal wells?

The overwhelming majority of injected fluid is oilfield brine, which is also sometimes referred to as produced water. Oilfield brine is the water, with varying levels of salinity that is found in the same geologic formations that produce oil and gas. This produced water comes up simultaneously with the production of oil and gas. However, small quantities of substances used in the drilling, completion and production operations of a well may be mixed in this waste stream. Some of these materials that may enter into the oilfield brine waste stream are minor amounts of drilling mud, fracture fluids and well treatment fluids. Also, because the produced water is associated with crude oil and natural gas, small amounts of residual hydrocarbons may also be found in the produced water.

Categories of injection wells

EPA’s regulations group injection wells into six groups or “classes.” Classes I – IV and VI include wells with similar functions, construction, and operating features. This allows consistent technical requirements to be applied to these well classes.  Class V wells are those that do not meet the description of any other well class. Wells in Class V do not necessarily have similar functions, construction, or operating features.  In 2010, EPA finalized regulations for geologic sequestration of CO2. This final rule created a new class of wells, Class VI. Class VI wells are used solely for the purpose of long term storage of CO2.

Class I Industrial and Municipal Waste Disposal Wells

Class I wells are used to inject hazardous and non-hazardous wastes into deep, confined rock formations.  Class I wells are typically drilled thousands of feet below the lowermost underground source of drinking water (USDW). Approximately 800 operational Class I wells exist in the United States. The geologies of the Gulf Coast and the Great Lakes areas are best suited for these types of wells. Most Class I wells are found in there. Examples of industries that use Class I wells include:
  • Petroleum refining
  • Metal production
  • Chemical production
  • Pharmaceutical production
  • Commercial disposal
  • Food production
  • Municipal wastewater treatment
Based upon the characteristics of the fluids injected, Class I wells fall into one of four subcategories.
  • Hazardous waste disposal wells
  • Non-hazardous industrial waste disposal wells
  • Municipal wastewater disposal wells
  • Radioactive waste disposal wells
Some of the information was taken from the Texas Railroad Commission.