Orphan wells

Orphan, orphaned, or abandoned wells are oil or gas wells that have been abandoned by fossil fuel extraction industries. These wells may have been deactivated because had become uneconomic, failure to transfer ownerships (especially at bankruptcy of companies), or neglect, and thus no longer have legal owners responsible for their care. Decommissioning wells effectively can be expensive, costing several thousands of dollars for a shallow land well to millions of dollars for an offshore one. Thus the burden may fall on government agencies or surface landowners when a business entity can no longer be held responsible.

Orphan wells are a potent contributor of greenhouse gas emissions, such as methane emissions, contributing to climate change. Much of this leakage can be attributed to failure to have them plugged properly or leaking plugs. A 2020 estimate of abandoned wells in the United States was that methane emissions released from abandoned wells produced greenhouse gas impacts equivalent to three weeks of US oil consumption each year. The scale of leaking abandoned wells is well understood in the US and Canada because of public data and regulation; however, a Reuters investigation in 2020 could not find good estimates for Russia, Saudi Arabia and China—the next biggest oil and gas producers. However, they estimate there are 29 million abandoned wells internationally.

Abandoned wells have the potential to contaminate land, air and water, potentially harming ecosystems, wildlife, livestock, and humans. For example, many wells in the United States are situated on farmland, and if not maintained could contaminate soil and groundwater with toxic contaminants.

Economic limits
A well is said to reach an "economic limit" when revenue from production does not cover the operating expenses, including taxes. If the economic limit is increased, the useful life of the well is shortened and proven oil reserves are lost. Conversely, when the financial limit is lowered, the life of the well is lengthened. When the economic limit is reached, the well becomes a liability if not abandoned.

At the economic limit, a significant amount of unrecoverable oil is often left in the reservoir. It might be tempting to defer physical abandonment for an extended period, hoping that the oil price will increase or that new supplemental recovery techniques will be perfected. In these cases, wells are merely shut in, or temporary plugs may be placed downhole. There are thousands of "temporarily abandoned" wells throughout North America, waiting to see what the market will do before permanent abandonment. However, lease provisions and governmental regulations often require quick abandonment; liability and tax concerns also may favor abandonment.

Theoretically, an abandoned well can be re-entered to restore production (or converted to injection service for supplemental recovery or downhole hydrocarbon storage), but reentry is often difficult mechanically and expensive. Traditionally elastomer and cement plugs have been used with varying degrees of success and reliability. Over time, they may deteriorate, particularly in corrosive environments, due to the materials from which they are manufactured. New tools have been developed that make re-entry easier; these tools offer higher expansion ratios than conventional bridge plugs and higher differential pressure ratings than inflatable packers, all while providing a V0-rated, gas-tight seal that cement cannot provide.

Reclaim and reuse
Some abandoned wells are subsequently plugged and the site is remediated; however, the cost of such efforts can be in the millions of dollars. In this process, tubing is removed from the well, and sections of wellbore are filled with cement to isolate the flow path between gas and water zones from each other, as well as from the surface. The wellhead is cut off, a cap is welded in place and then the stub is buried as the land contours are restored.

Plugging
The primary method of plugging wells is through elastomer and cement plugs. Government-led campaigns to plug wells are expensive but often facilitated by oil and gas taxes, bonds, or other fees applied to production. Environmental non-profit organizations, such as the Well Done Foundation, also carry out well-plugging projects and develop programs alongside government entities.

Plug bonds
Oil and gas companies on public land in the United States must post financial assurance to cover the cost of plugging wells if they go bankrupt or cannot plug the well themselves. The current financial assurance requirement, which has been in place for 60 years, is $10,000 per well. This is significantly less than the cost of plugging a well, ranging as high as $400,000. Thus many federal oil and gas leases have a bond that cannot cover the cost of cleanup.

New rules related to the Infrastructure Investment and Jobs Act will increase the financial assurance requirement to a minimum of $150,000 per well. This will help ensure that oil and gas companies have the financial resources to plug wells if they can no longer do so themselves.

CO2 injection
Unused wells, especially from natural gas might be used for carbon capture or storage. However, if not sealed properly, or the storage site is not sufficiently sealed, there is a possibility of leakage.

Geothermal generation
A 2014 study in China evaluated the use of abandoned wells for geothermal power generation. A similar study followed in 2019 for natural gas wells.

Hydraulic fracturing
Hydraulic fracturing, also known as fracture treating or fracking, is the process of fracturing bedrock with pressurized liquids. This creates cracks in rock formations that allow natural gas, petroleum, and brine to flow more effortlessly. When hydraulic fracturing is done in the vicinity of an orphaned well it can cause breaches of poorly sealed or unsealed abandoned wells that possibly can contaminate local ecosystems. These orphaned wells can allow gas and oil to contaminate groundwater due to improper sealing.