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Saltwater disposal well

Abbreviation
SWD
Also known as
SWD, salt water disposal well, disposal well

A saltwater disposal (SWD) well is a well used to inject produced water and other oilfield brines deep underground into a formation that can accept and permanently hold them. In the United States these are Class II injection wells, regulated under the Safe Drinking Water Act.

What is a saltwater disposal well?

Oil and gas wells produce large volumes of salty water along with the hydrocarbons, often several barrels of water for every barrel of oil. A saltwater disposal well, usually shortened to SWD, is a well dedicated to getting rid of that water safely by injecting it into a deep, porous rock formation that is isolated from drinking water sources by layers of impermeable rock.

The injected fluid is mostly produced water, along with flowback from hydraulic fracturing and other oilfield wastewater that cannot be economically treated or reused.

How SWD wells are regulated

In the United States, wells that inject fluids associated with oil and gas production are Class II wells under the Environmental Protection Agency’s Underground Injection Control program, authorized by the Safe Drinking Water Act. Class II wells include enhanced recovery injection wells as well as disposal wells; see Class II injection well. Many states, including major producing states, administer the program themselves. Permits set limits on injection pressure, volume and the formations that may be used, and require testing to confirm that the well’s casing and cement keep fluids where they belong.

How an SWD well works

Water from producing wells arrives by pipeline or truck, is usually passed through tanks and filters to remove oil and solids, and is pumped into the well. The well is completed in a disposal zone with enough porosity, permeability and capacity to take fluid at acceptable pressures. Operators track the rate at which the well accepts fluid for a given pressure, its injectivity, because the economics of a disposal well depend on moving as much water as possible within its permitted pressure.

Why SWD wells lose injectivity

Over time, the rock near the wellbore can become plugged, forcing operators to raise pressure or accept lower rates. Common causes include:

  • Suspended solids and fines carried in the water.
  • Oil and organic deposits, including paraffin, carried over from inadequate separation.
  • Mineral scale and iron sulfide formed when incompatible waters mix or conditions change.
  • Bacterial growth and corrosion products.
  • Emulsions that restrict flow into the rock.

Remedies include better water treatment at the surface, acid treatments, and chemical treatments that disperse deposits and restore flow.

Disposal and induced seismicity

The U.S. Geological Survey has linked large volumes of wastewater injection to increased earthquake activity in parts of the central United States, including Oklahoma. Regulators in affected areas have responded with limits on injection volumes and depths. This is one reason disposal capacity is closely managed in some basins.

SWD wells at Locus

Locus’s FloBoost is a biosurfactant injectivity aid for disposal and injection wells, which Locus describes as dispersing solids and organic deposits and reducing backpressure to restore injection performance.

Frequently asked questions

Both are Class II wells. A disposal well injects water simply to get rid of it. An enhanced recovery injection well injects water, gas or chemicals into a producing reservoir to push more oil toward producing wells.

It depends on the geology. Disposal zones must be below and isolated from underground sources of drinking water, and suitable formations are commonly thousands of feet deep.

Because solids, oil, scale, iron sulfide and bacterial growth accumulate in the rock near the wellbore and in the well itself, reducing the rate at which the formation accepts fluid at a given pressure.

  • Produced water

    Produced water is the water that comes to the surface along with oil and gas from a well. It includes water naturally present in the reservoir, injected water and returning fracturing fluid, and it is usually salty, with dissolved minerals, traces of oil and production chemicals.

  • Injectivity

    Injectivity is how readily a well and the rock around it accept injected fluid, usually expressed as the injection rate achieved per unit of pressure above reservoir pressure. Falling injectivity means more pressure is needed to inject the same volume.

  • Class II injection well

    A Class II injection well is a well used to inject fluids associated with oil and natural gas production, such as produced water for disposal or water for enhanced oil recovery. Class II wells are regulated under the Underground Injection Control program of the Safe Drinking Water Act.

  • Total dissolved solids TDS

    Total dissolved solids (TDS) is the total concentration of minerals, salts and other substances dissolved in water, usually reported in milligrams per liter. It is the standard measure of how salty or mineralized a water is, from drinking water to oilfield brine.

Sources

  1. Class II oil and gas related injection wells, U.S. Environmental Protection Agency, Underground Injection Control program
  2. Induced earthquakes, U.S. Geological Survey
  3. injectivity; injection well, SLB Energy Glossary