Marginal Conventional Wells

Marginal conventional wells are vertically drilled or slightly deviated oil or natural gas wells with a known owner/operator that produced less than or equal to 15 barrels of oil equivalent per day and/or 90 thousand cubic feet of natural gas per day over the prior 12-month period (U.S. DOE et al., 2024). Although these wells account for a low percentage of total oil and gas production, research indicates that low production well sites account for about 50% of oil and gas methane emissions (Bowers, 2022; Omara et al., 2022).

Newly drilled oil and natural gas wells typically achieve peak production rates within the first months or year of production. As the reservoir is depleted, reservoir pressure gradually drops leading to a decline in production rates. There are unique characteristics for oil wells versus gas wells during the decline in production:

• Oil reservoirs decline in pressure over time to a point where oil will not flow unassisted up the well. When this happens, well operators often use a down-hole pump, also called a pump-jack, to resume production. As pressure declines further, the production of associated gas (i.e., natural gas that is produced along with the crude oil) also decreases to the point where capturing and transporting this gas to sales may not be economically justified. When associated gas is not captured, it tends to accumulate in the well casing and gather around the downhole pump, eventually stopping oil production. To prevent this from happening, the gas is commonly vented (i.e., intentionally released to the atmosphere) or flared (i.e., burned off) at the wellhead. Over time, some wells may require additional maintenance, commonly referred to as well workovers, to maintain production.
• Gas reservoirs decline in pressure and productivity over years of production. As the reservoir pressure declines, the underlying water and liquid condensate (i.e., a lighter hydrocarbon oil within the gas reservoir) eventually rise in the reservoir where it is entrained in the gas flow up the well tubing. Accumulation of liquids (i.e., water and condensate) in a mature gas well can slow down and sometimes halt gas production. A common method of restoring gas flow is to open the wellhead to the atmosphere and blow the liquids out of the tubing, a process known as liquids unloading or well blowdown. Once the reservoir pressure has declined to the point where liquids unloading is necessary, this process must be performed routinely and results in the venting of an increasingly large amount of methane rich natural gas into the air as production declines. 

Marginal conventional wells, also known as stripper wells, are low-producing wells that often have disproportionately high methane emissions. Studies indicate that approximately 70% of the more than 900,000 onshore oil and gas wells in the U.S. were low-producing in 2021. These wells accounted for only 7% of 2021 U.S. oil and gas production but were responsible for approximately 60% of the U.S. natural gas production emissions and about 40% of the U.S. oil production emissions (Bowers, 2022). 

Marginal conventional wells may operate for 25 years or more and, in the absence of mitigation measures, which may be beyond the resources of small well site owner/operators, these wells may continue to emit large amounts of methane for years. In addition, marginal conventional wells that are idle or permanently inactive are at high risk of being improperly abandoned and falling into the category of “orphaned wells” (i.e., wells that are not used for an authorized purpose and for which there is no responsible operator to plug and abandon the well) (U.S. DOI).

Methane emissions from marginal conventional well sites occur at sources common to all well sites, regardless of production rate. Figure 1 depicts some of these emissions sources, including:

• Equipment leaks, also known as fugitive emissions, which are characterized as unintentional emissions and may occur from equipment components (e.g., connectors, valves, open-ended lines, pressure relief valves, and storage tank thief hatches). Equipment leaks are generally more prevalent at marginal conventional well sites and generally associated with deteriorated site conditions and neglected maintenance on well site infrastructure (e.g., corrosion-related integrity issues to wellhead components).
• Vented emissions, which are designed to occur or are caused by intentional operations and maintenance activities. These emissions occur from specific sources (e.g., gas powered pneumatic controllers, dehydrators, engines, and compressors) and from operations performed at the well to maintain or restore production (e.g., liquids unloading, well workovers).

Figure 1. Schematic showing sources and locations of intentional (vented) and unintentional (fugitive) methane emissions at general well sites (top) and marginal conventional well sites (bottom) (U.S. DOE et al., 2024).

In addition to methane, marginal conventional well sites can emit volatile organic compounds, Hazardous Air Pollutants including benzene, toluene, ethylbenzene, and xylene, and, in some cases, hydrogen sulfide.

Bowers, R. (2022). Quantification of Methane Emissions from Marginal (Low Production Rate) Oil and Natural Gas Wells (DOE-GSI-31702). https://doi.org/10.2172/1865859.

Omara, M., Zavala-Araiza, D., Lyon, D., Hmiel, B., Roberts, K., & Hamburg, S. (2022). Methane emissions from US low production oil and natural gas well sites. Nature Communications, 13 (2085). https://www.nature.com/articles/s41467-022-29709-3.

U.S. DOE, NETL. (2024). Methane Measurement Guidelines for Marginal Conventional Wells. https://netl.doe.gov/sites/default/files/2024-06/DOE-NETL%20Methane%20Measurement%20Guidelines%20for%20Marginal%20Conventional%20Wells%20April%202024.pdf (pdf) (1.2 MB).

U.S. DOE, NETL, FECM, and U.S. EPA. (2023). Inflation Reduction Act (IRA)-Mitigating Emissions from Marginal Conventional Wells (Administrative and Legal Requirements Document for DE FOA 0003109). https://www.grants.gov/search-results-detail/350045.

U.S. DOI. (n.d.). Orphaned Wells. https://www.doi.gov/orphanedwells.