Well Plugging

Well plugging is the process of placing plugs at specific depths in a wellbore to seal off the oil and gas producing zones and prevent migration of fluids into aquifers. Marginal or low-producing oil and gas wells, and the supporting production equipment, are a significant source of methane emissions and other air pollutants. Permanently plugging these oil and gas wells can prevent gas from leaking into the atmosphere and prevent air and water pollution.

The well plugging process generally involves the removal of the production tubing and then the placement of plugs at the depths of the oil and gas producing zones and water aquifers through which the wellbore passes. These plugs create a barrier and stop any oil and gas in the producing zones from entering and escaping from the well. Well plugging operations typically involve the following steps and activities, but can also present unique circumstances and hazards (Occupational Safety and Health Administration):

1. Preparation of the well site. The well site is inspected and prepared for plugging. Inspections may include, but are not limited to, assessments to ensure safe site conditions, verification of soil stability, and identification and repair of well integrity issues. 
2. Removal of well bore casing and other associated equipment or infrastructure. A well bore is a hole drilled into the ground to extract oil and natural gas. The casing is the pipe put into the well bore to maintain the well opening and prevent leakage of oil or gas out of the well. For cased holes, if the casing is of poor quality or there is no annular ring-shaped section of cement present inside the wellbore, then the casing is removed before placing the plug (Khalifeh & Saasen, 2020; Vrålstad et al., 2019). In addition, any other production tubing or debris in the well are removed.
3. Placement of cement plugs in the borehole at the appropriate depths to seal off the oil and gas producing zones. Strong plugging materials with low permeability, such as cement, are commonly used to create an effective seal. A variety of additives can be mixed with the cement to modify setting time, density, water loss, and other properties. Drilling mud and bentonite are often used to fill spaces between the cement plugs. Special cement mixtures are required to plug specific types of wells, including moderate to high pressure gas wells, horizontal wells, salt zones, and deep wells. To place the plug, cement and/or other approved plugging materials are pumped into the wellbore at the required depth intervals. Mechanical plugs can also be used with cement plugs to provide a foundation and reduce the amount of plugging cement required (National Petroleum Council, 2011). Once a well has been plugged and the cement is set, pressure testing is conducted to ensure that the plug fully sealed the well (Khalifeh & Saasen, 2020). 
4. Excavation around well head and capping of the well prior to surface restoration, as required. If plugs are installed inside casing, the casing is cut off below the surface. Production equipment (e.g., separator, tank) is removed and the well is capped by covering the borehole to prevent exposure at the well surface.
5. Reclamation activities. The well site is restored to its original condition and to match its environment. Reclamation activities may be subject to federal and state environmental regulations.

The Bureau of Land Management establishes minimum plugging and abandonment requirements for onshore oil and gas wells on Federal and Indian lands (Bureau of Land Management, 1988). State regulations specify well plugging and abandonment requirements on private land, and may include specific requirements, such as the allowed plugging materials and depth intervals in the well to be plugged. The Bureau of Safety and Environmental Enforcement establishes requirements for plugging offshore wells.

Methane emissions can be measured before and after well plugging to verify that plugged wells are no longer emitting methane and quantity the emissions reductions. Given the variety of marginal conventional wells across the U.S., the measurement approaches for the detection and measurement/quantification of methane emissions approaches may vary.

Determining whether to plug a well involves multiple considerations by the operating company. Production volume and economics of a well along with a company’s overall goals and plans (e.g., reduce greenhouse gas emissions, workover strategy for a depleted field) play a part. The following are considerations to determine which marginal wells may be good candidates for plugging: 

• Wells that, at current prices, produce insufficient oil and gas to generate revenue to recover the costs of operation (e.g., produced water disposal, energy for pump jacks, chemicals for hydrate suppression). 
• Wells in reservoirs that are not suitable for workovers utilizing hydraulic fracturing. 
• Wells in reservoirs that are suitable for workovers utilizing hydraulic fracturing, but that do not fit in the workover strategy for a depleted reservoir (e.g., workover certain wells and plug others) to extend a well’s life.
• Gas wells in depleted reservoirs with insufficient shut-in pressure to expel liquids, which stops gas production. 
• Wells located in close proximity to communities and sensitive receptors (e.g., hospitals, schools) that may pose health and/or safety risks.

The most appropriate well plugging technique is determined by the specific well conditions and in consideration of the applicable state and federal requirements.

Methane emissions reductions can be estimated by taking the difference in emissions from the well site before and after well plugging. The Department of Energy National Energy Technology Laboratory provides guidance for states and operators/well owners to adequately measure methane emissions prior to and following the plugging of any marginal conventional well (U.S. DOE et al., 2024). The measurement approaches taken to quantify pre- and post-plugging emissions will vary depending on several factors. The measurement approaches can be categorized into direct source emissions measurements, near-field measurements, and remote sensing. Selection criteria for which approach(es) should be used are dependent upon desired performance metrics, such as cost, wellpad area coverage, time, and the skill required, as well as the marginal conventional well site characteristics (e.g., site configuration and status, number of potential leak/venting points, accessibility, access to electricity). Any selected measurement approach should have a current, accurate, and traceable calibration. Quantitative approaches should also have a minimum detection limit of less than 100 grams/hour and relatively high accuracy, so mitigated emission estimates are accurate (U.S. DOE et al., 2024).

In addition to reducing emissions of methane, well plugging activities may:

• Reduce local air pollution: Reduces emissions of volatile organic compounds, hazardous air pollutants (including benzene, toluene, ethylbenzene, and xylene), and, in some cases, hydrogen sulfide.  
• Provide other health-related benefits: Improves ambient air, surface, and groundwater quality as marginal conventional wells often leak liquids that may include oil and brine. The liquids pose risks to local environments and safety concerns to local communities.
• Reduce fire hazards: Reduces emissions of explosive gases, therefore reducing the risk of fires and explosions.

Bureau of Land Management (BLM). (1988, December 19). Onshore Oil and Gas Operations; Federal and Indian Oil and Gas Leases; Onshore Oil and Gas Order No. 2, Drilling Operations. https://www.blm.gov/sites/blm.gov/files/energy_onshoreorder2.pdf (pdf) (281 KB).

Khalifeh, M., Saasen, A. (2020). Fundamentals of Plug Placement. In: Introduction to Permanent Plug and Abandonment of Wells. Ocean Engineering & Oceanography, vol 12. Springer, Cham. https://doi.org/10.1007/978-3-030-39970-2_7.

National Petroleum Council (NPC). (2011, September 15). Plugging and Abandonment of Oil and Gas Wells. https://www.npc.org/Prudent_Development-Topic_Papers/2-25_Well_Plugging_and_Abandonment_Paper.pdf (pdf) (610 KB).

Occupational Safety and Health Administration. (n.d.). Oil and Gas Drilling and Servicing: Plugging and Abandoning Oil and Gas Wells. https://www.osha.gov/etools/oil-and-gas/abandoning-well.

U.S. DOE, NETL, and FECM. (2024, April 17). 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).

Vrålstad, T., Saasen A., Fjær E., Øia T., Ytrehus J. D. & Khalifeh M. (2019). Plug & abandonment of offshore wells: Ensuring long-term well integrity and cost-efficiency. Journal of Petroleum Science and Engineering, 173, 478-491. https://doi.org/10.1016/j.petrol.2018.10.049.