Oil and gas reservoirs are usually found in porous rocks, which also contain saltwater. This saltwater, which accompanies the oil and gas to the surface, can be disposed in two ways: 1) Returned by fluid injection into the reservoir where it originated for secondary or enhanced oil recovery; or 2) Injected into underground porous rock formations not productive of oil or gas, and sealed above and below by unbroken, impermeable strata.

Saltwater disposal wells use this second method to manage saltwater. Disposal wells inject fluid into an underground interval that is not productive of oil and gas. These wells may reach a depth of up to one mile below the earth’s surface, while fresh-water wells are usually only a few hundred feet.

Injection wells re-inject fluids into the same or similar reservoir, from which the fluids originated, for secondary recovery of oil. Operators use secondary recovery techniques when an oil field’s pressure has been depleted, and oil is no longer produced from a field’s natural pressure. Secondary recovery, sometimes known as waterflooding — generally injects produced saltwater into a reservoir to reestablish sufficient pressure to allow an operator to recover additional amounts of oil.