An injection well is a device that places fluid deep underground into porous rock formations, such as sandstone or limestone, or into or below the shallow soil layer. The fluid may be water, wastewater, brine, or water mixed with industrial chemical waste.

Produced water is a term used in the oil industry or geothermal industry to describe water that is produced as a byproduct during the extraction of oil and natural gas, or used as a medium for heat extraction. Water that is produced along with the hydrocarbons is generally brackish and saline water in nature. Oil and gas reservoirs often have water as well as hydrocarbons, sometimes in a zone that lies under the hydrocarbons, and sometimes in the same zone with the oil and gas. In geothermal plants, the produced water is usually hot. It contains steam with dissolved solutes and gases, providing important information on the geological, chemical, and hydrological characteristics of geothermal systems. Oil wells sometimes produce large volumes of water with the oil, while gas wells tend to produce water in smaller proportions.