4

Monitoring Plugged Wells

While Chapter 3 described the mechanisms that cause plugged wells to fail and the factors that can lead to such failures, this chapter examines monitoring a well or site for failure, including indications of leakages or other migrations at the surface or in the subsurface. Data collected during plug inspection and witnessing, as discussed in Chapter 3, may also inform post-plug risk assessment and monitoring, which assesses the long-term integrity of the plug and other well barriers as well as the presence or likelihood of failures.

Although this chapter focuses on post-plug monitoring and will not discuss pre-plug monitoring at length, pre-plug monitoring can help assess well integrity, determine the presence and rate of emissions, gather data for site reclamation, and locate other unknown wells in the area. Pre-plug monitoring data can establish baseline measurements of well integrity and the presence of contaminants (e.g., methane, hydrocarbons, salts), allowing for informed assessments during post-plug risk assessment and monitoring.

The chapter begins by discussing the potential for states and operators to develop prioritization frameworks for post-plug monitoring; it then discusses monitoring for contaminants as well as techniques and tools for surface and subsurface monitoring. The technologies described can be used to identify specific toxins, leakages, or other migrations at the surface or subsurface, including some fluid migrations that can begin in the subsurface and make their way to the surface. The chapter closes with additional discussion on post-plug monitoring costs, strategy, and uncertainty.

MONITORING IMPLEMENTATION AND RISK FACTORS

Post-plugging monitoring is not generally required by current regulations and is not consistently carried out among states. As noted in Chapter 3, during public

information-gathering sessions with the committee and Interstate Oil & Gas Compact Commission 2025 Annual Business Meeting participants on May 21st and 22nd, state representatives discussed that leakage from well failures are often known only after landowners or others have registered complaints about a well or well site. Because the potential risk posed to public and environmental health is greater for unplugged wells than for plugged wells¹ more funding and research has been directed toward well identification and plugging than to post-plug monitoring. More is known about the hazards that unplugged wells pose and their potential consequences.

As discussed in Chapter 2, many states have established frameworks for orphan well plugging prioritization and have developed quantitative assessment tools based on the risks posed by orphan wells (IOGCC 2023). For example, Louisiana’s prioritization scoring system assigns points to a given well based on over two dozen factors that may apply, such as if a well leaking natural gas is located within 300 feet of a public building or residence (the most points), or if a well is on land actively used for crops or forage (very few points; IOGCC 2023, p. 14). The points are then tallied for an overall score for prioritization. New Mexico assesses and scores over 30 factors when prioritizing wells for plugging (IOGCC 2023, p. 18). California uses a two-phase well screening and prioritization method, where the well is first screened and scored, and then a second screening step includes local government and feedback on the provisional rankings (IOGCC 2023, p. 6). States prioritize risks differently, and some states are farther along than others in establishing a prioritization framework. Some states explicitly take into account the cost of plugging the well. For example, Nevada considers whether bonding and other funds are available to plug the well properly. Illinois is still working on its ranking system: in its initial request for Infrastructure Investment and Jobs Act funds, it proposed a budget that included costs for ranking wells (IOGCC 2023, pp. 15, 17).

It is not feasible, nor is it necessary, to monitor every plugged well. Prioritization allows states, federal agencies, and Tribal governments to focus resources on plugged wells where monitoring would be most likely to yield significant environmental and public health and safety benefits. However, establishing analogous prioritization frameworks for post-plug monitoring remains challenging because of the lack of publicly available data about how wells are plugged, why plugs fail, and well monitoring. Detailed environmental risk analyses have been attempted, which have helped illustrate the range of possible problems, but the data available are not enough to fully understand and assess the risks (e.g., Kang et al. 2023; Meehan et al. 2025). Post-plug monitoring currently relies on a less detailed, sometimes qualitative, assessment of risk factors. For example, if a well was plugged with the required procedure, had no known prior issues, and encountered no problems when plugged, then the chances of plug failure are low and monitoring would not necessarily be required. But if gas migration was detected while setting plugs, or oil or water leakage was observed at the surface, a monitoring plan could be warranted.

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¹ Also noted in Chapter 3, studies have shown that plugged wells emit consistently lower rates of methane than unplugged wells (Kang et al. 2019; Riddick et al. 2019; Townsend-Small et al. 2016), and plugging wells is generally effective in reducing leaks and emissions (Kang et al. 2016; Raimi et al. 2021; Wisen et al. 2020).

The risk factors to consider in post-plug monitoring decisions overlap significantly with those considered in plugging prioritization, but their associated impacts will differ. The risk factors associated with a plugged well include the following:

• Well construction, including quality of the original well design, drilling, and cementing (Kiran et al. 2017).
• Well age and condition (Atherton et al. 2017).
• Geological factors, including earthquakes, seismic activity, subsidence, corrosive fluids, and karst or sinkholes (Kang et al. 2019).
• Well proximity and/or the intersection of underground injection activity with the subsurface framework of nearby orphan wells (Murray et al. 2023; NASEM 2025).
• Leakages: A leaking well nearby can indicate a field-wide problem, while a single leaking well might be a more localized issue. The risk would depend on what substance is leaking (e.g., oil, methane, brine; DACC 2022).
• Proximity to surface water features: Wells located within or near wetlands, rivers, and lakes could contaminate water bodies if they leak (IOGCC 2023).
• Record-keeping: Poor or missing records due to changes in ownership can make it difficult to assess construction details (Boutot et al. 2022).
• Proximity to buildings that serve sensitive populations, such as schools or hospitals, or total population in the area (IOGCC 2023).

The committee members discussed how difficult it is to quantify the risks posed by plugged wells. However, based on their knowledge and experience they noted that there are many tools and techniques for post-plug monitoring, with general guidelines and associated use cases based on a broad assessment of risk factors. For example, wells farther from sensitive populations and with benign histories can be managed with periodic monitoring, using techniques such as surface casing vent flow (SCVF) and sustained casing pressure (SCP) checks, simple screening with optical gas imaging (OGI) or handheld detectors, and photo documentation. If the well has a history of prior integrity issues, is near sensitive populations, or was drilled into complex geology, monitoring might include semiannual SCVF and SCP measurements with telemetry, annual mobile cavity ring-down spectroscopy (CRDS) routes or soil gas and flux grids, and geophysical logging where migration is suspected. Wells near communities; water supplies; or carbon capture, utilization, and storage (CCUS) assets may warrant continuous telemetry on vents or annuli, frequent surface gas checks, methane sensors in buildings, and full pre- and post-plugging and abandonment downhole verification.

Although there is certainly a need for more post-plugging monitoring data to be collected, many states have considerable information about plugged wells and, as discussed above, several have established risk prioritization frameworks for plugging. With adequate and sustained funding, states could continue this work and advance development, testing, and implementation of post-plug monitoring prioritization systems or frameworks, including common tools or approaches that could be used across states.

These frameworks and associated data could also provide information to advance state regulations related to post-plug monitoring.

TYPES OF POTENTIAL CONTAMINATION

Monitoring techniques and practices differ by type of contaminant and whether contamination occurs in the air, at the surface, or below the surface. This section discusses air pollutants and surface water, groundwater, and soil contamination near orphan well sites, summarizing research findings on types of contamination that have been detected.

Air Pollutant Emissions

The most studied air pollutant emitted from orphan wells is methane. It is a precursor for ozone (an air pollutant) formation and a potent greenhouse gas with a global warming potential² of approximately 30 in a 100-year timeframe (IPCC 2023). Many interrelated engineering, geological, operational, and policy factors can lead to methane emissions from orphaned and abandoned wells. El Hachem and Kang (2023) compiled 38 factors from published literature, finding 15 that do not impact well leakage and eight that impact it inconsistently: age, depth, operator, density, corrosive environment, time since abandonment, production volume, and fluid type (i.e., whether the well was producing oil, gas, or both). The remaining 15 factors were consistently shown to be influential and included geographical area, wellbore deviation, and plugging status (El Hachem and Kang 2023). Generally, plugged wells emit orders of magnitude less methane than unplugged wells (Williams et al. 2021). Analyzing measurement data from about 500 non-producing wells (plugged, unplugged, and plugging status unknown) in Canada, Klotz et al. (2025) found interprovincial variation in emissions to be substantial but sub-provincial variation to be small.

Many technologies have been developed to detect methane emissions from producing wells, facilities, and pipelines; while discussed in the following sections, they are largely unsuitable and costly for the relatively low emissions rates from most leaking orphan wells. Other air pollutant emissions from plugged and unplugged wells, including orphan wells, that have been measured include hydrogen sulfide (El Hachem and Kang 2022; El Hachem et al. 2025) and volatile organic compounds (VOCs) (DiGiulio et al. 2023), such as benzene, a known carcinogen. However, large variations exist, with many wells emitting far below detection limits. Compared to methane emissions measurements, those available for hydrogen sulfide and VOCs are limited to a few regions and cannot be taken as representative of all orphan oil and gas wells.

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² The global warming potential is a measure of how much energy the emission of 1 ton of a gas will absorb over a given time relative to the emission of 1 ton of carbon dioxide (CO₂). The larger the number, the more a given gas warms the earth compared to CO₂ for the specified period (EPA n.d.).

Surface Water Quality

Orphan wells can impact both surface water and groundwater quality. However, available data are limited spatiotemporally, and attributing contamination events to specific wells is challenging (Kang et al. 2023). Surface water monitoring is one method for understanding whether an orphan well has leaked fluid. If evidence of leakage is present, detailed water quality monitoring can be applied. Assessing the impact includes collecting and analyzing water and biological samples from sources such as rivers, lakes, and streams. This can involve both manual sampling for laboratory analysis and continuous, real-time sensors to measure key parameters, including physical parameters (e.g., temperature, turbidity, and water level); chemical parameters, including pH, dissolved oxygen, and conductivity and biological nutrients (e.g., nitrogen, phosphorus); and algae and toxins. Alternatively, instead of attempting to measure the concentration of a contaminant, assessors can compare ratios of indicator elements, such as strontium, bromine, and barium in brines (Brantley et al. 2014). These data are used to identify impaired waterways, track trends, and assess resources.

A common issue with many orphan wells is surficial oil spills adjacent to the wellbore from storage tanks or directly from a leaking wellbore. Leaked or spilled hydrocarbons often form a thin film, or oil sheen, on the water surface that prevents oxygen exchange with water. Significant impacts may be seen on the chemical and physical environment and economic activities in those areas (Achang et al. 2020). Produced water, which often contains elevated levels of chloride and heavy metals (e.g., arsenic, cadmium, lead), can lead to potential environmental impacts adjacent to orphan wells whether spilled during operations or post plugging. And saltwater contamination, caused by subsurface leaks from poorly sealed boreholes, has been documented (Whittemore 2007).

Groundwater Quality

Orphan wells can raise serious environmental concern for groundwater quality (Woda et al. 2025). They can serve as pathways for contaminants such as hydrocarbons, produced wastewater, and brines, and chemicals such as methane, arsenic, benzene, and hydrogen sulfide to migrate from deeper geological formations into freshwater aquifers and thereby contaminate soils. Corroded casing or failed cement seals can allow the vertical movement of contaminants between strata. Aquifer contamination from orphan wells can require both monitoring and remediation.

Soil Quality

The soil profile near orphan wells can be impacted greatly by several factors, all of which can pose public and environmental health risks, including the leakage of hydrocarbons and brines to the surface. Adequate wellbore monitoring helps prevent potential pollution. Various studies have indicated that the cost of preventing soil or water pollution near well sites is far less than cleaning up that contamination later (Nanda and Pring 2013; Wells and Hester 2018).

Beyond immediate impacts on soil quality, longer-term effects may include restrictions on future surface use near contaminated well sites. Soil contamination can also cause the formation of certain soils into hardpan, a highly dense and solid layer formed by the precipitation of insoluble substances and pollutants, which can fuse and bind soil particles together. Hardpan can impede long-term water drainage and restrict plant root growth.

SURFACE MONITORING TECHNIQUES AND TOOLS

Regulatory testing at the wellhead forms the backbone of routine monitoring for plugged, producing wells; the committee noted that regulatory testing done at the wellhead for producing wells can serve as a starting point for developing a post-plug monitoring plan for orphan wells. Regulatory testing involves routing vent gas through a temporary stack with a flame arrester and a calibrated meter, if SCVF is allowed. If the vent is closed, pressure is monitored instead. Operators may collect gas samples for methane and other light hydrocarbons, carbon dioxide, hydrogen sulfide, and stable carbon and hydrogen isotopes to distinguish thermogenic from biogenic sources and support attribution where multiple wells exist (Micucci and Kang 2026). SCP is evaluated by bleeding the annulus to near zero, isolating it, and recording the rate and magnitude of pressure rebuild over time. The committee noted that repeating the test helps distinguish thermal effects from true well communication with formations.

Surface detection and quantification tools complement regulatory tests. For very high emissions from producing wells, OGI using infrared cameras can visually reveal hydrocarbon emissions at wellheads and in cellars (Zimmerle et al. 2020). Handheld flame ionization detectors enable parts per million (ppm) measurements of methane, have been widely used to detect methane leaks, and can perform rapid checks around wellheads. Photoionization detectors can provide VOC detection from soil samples across a broad range of concentrations (Ramaiyan et al. 2023).

Many other methane detection technologies are commercially available for monitoring emissions from oil- and gas-producing facilities and pipelines. However, these are generally incapable of detecting the low levels of emissions associated with orphan or improperly abandoned wells and include area-scale screening from the air and space. Satellite methane sensors flag regional anomalies at low or no cost for public data, commercial satellites provide targeted detection of larger plumes, and aircraft-mounted imaging spectrometers detect smaller releases across wide regions at low per-well cost when scaled (Sherwin et al. 2024). Barhole probing and shallow soil gas points allow direct sampling of methane, carbon dioxide, oxygen, and isotopes in the vadose zone, while closed-chamber flux domes measure surface methane flux. In areas with potable water receptors, operators collect baseline and periodic samples from nearby water wells for dissolved methane, isotopes, and major ions; where buildings are at risk, fixed methane sensors in basements or crawlspaces provide critical safety assurance. These measurements are relatively low cost compared with downhole diagnostics and most reliable when performed under calm weather with well-designed sampling grids.

SUBSURFACE MONITORING TECHNIQUES AND TOOLS

Subsurface, including groundwater, monitoring requires geophysical survey tools. Techniques include those used to monitor wellbore conditions such as cement integrity, fluid levels, and pressure and methods for geochemical monitoring. The techniques and tools described in this section are more often used with producing wells, but they can be applied in situations where orphan well plugs have failed and the wells are actively leaking. However, the committee also noted that their use is limited with plugged orphan wells, mainly due to high costs.

Geophysical Surveys

Subsurface and near-surface geophysical methods—such as ground-penetrating radar (GPR) or magnetometry—help find undocumented wells, map migration pathways, and focus remediation. In favorable soils, GPR helps locate shallow cellars and utilities. Ground or drone magnetics and electromagnetic surveys detect steel casing and flowlines quickly and non-invasively, enabling pre-development reconnaissance and well inventory cleanup at comparatively low cost per acre (Saneiyan and Mansourian 2023). Electrical resistivity tomography, which measures how a contaminant is changing the ground’s electrical resistance (Gasperikova et al. 2012), can detect changes in subsurface plumes through contrasts in the shallow subsurface; these contrasts can indicate gas-charged zones or disturbed backfill around suspect wells. Electrical resistivity tomography is effective for forensic mapping but requires careful data inversion and quality control (Godio and Naldi 2003). Self-potential methods can reveal seepage-related electrochemical anomalies but are less commonly applied in routine programs.

Subsurface Well Integrity Monitoring

Downhole diagnostics provide the most direct information about barrier conditions and leak pathways and are used selectively where risk or regulation warrants. These tests are performed during well construction or as part of the well characterization process. For wells where some infrastructure remains (e.g., wellheads), monitoring pressures (e.g., SCP) can uncover well integrity issues and potential leakage pathways. Cement evaluation begins with cement bond logs and variable density logs to assess bonding to casing and formation; advanced ultrasonic and radial scanning tools produce high-resolution maps that are better able to highlight micro-annuli and channels and are favored for high-consequence plugging decisions (Simmers et al. 2024).

Leak detection within the wellbore or behind casing may require multiple logging types. Temperature and noise logs can identify small flows. Pulsed neutron logs (e.g., carbon–oxygen or capture sigma measurements) can characterize fluids behind casing and confirm isolation across underground sources of drinking water. Production logging tools with spinners can quantify internal flow. Tracer injection studies, where permitted, help diagnose subtle paths (Lutfullin et al. 2014). Tubular integrity is evaluated with multi-arm calipers, ultrasonic thickness measurements, and magnetic flux leakage to quantify corrosion, ovality, and restrictions that could compromise barrier placement (Yang et al. 2022).

Acoustic fluid level analysis tools generate a sound wave (or a compressed carbon dioxide pulse) that travels down the casing annulus, reflecting off tubing collars and eventually the liquid surface, to determine fluid level depth (Andsager and Knapp 1967; Feder 2021). These analyses play an important role in understanding reservoir characteristics, indications of well integrity failure, and the increased risk of pollution to usable-quality water.

Before final abandonment, isolated interval pressure tests using retrievable packers validate cement and plug performance by pressure testing selected zones. Distributed fiber optic sensing, including temperature and acoustic, can provide continuous leak and acoustic monitoring in new or sentinel wells, but retrofitting legacy wells is rarely cost-effective (Ashry et al. 2021). Based on committee members’ experience, these subsurface services typically range from tens of thousands of dollars to over $100,000 per run when a rig and multiple tool suites are required.

Geochemical Monitoring

Various leakage pathways for gases and fluids can develop over a well’s life cycle. Isotopic analysis can help identify the location of specific zonal contributors to methane leaks. This technique can determine the composition of gases such as methane and noble gases sampled from a well annulus outside the production casing or a nearby water well. Isotopic analyses can also help trace the path of fluid and gas migration (Zazzeri et al. 2015). Another key component in methane analysis is using its isotopic composition to differentiate between microbial and thermogenic gas, which could provide insight into subsurface gas migration pathways. As an example, in western Canada, gases in shallow aquifers, coalbed methane plays, and shale gas reservoirs often have distinct dryness values and stable carbon isotope values of methane (Mayer 2025).

Isotopic analysis can also characterize contaminated water leaking from wells. As an example, Warner et al. (2012) used strontium isotopes to distinguish between deep produced water contamination and shallow water sources in the Marcellus Shale formation in the Appalachian Basin. The integration of chemical data and isotopic ratios suggested that mixing of shallow groundwater and deep formation-produced water could lead to salinization of groundwater (Warner et al. 2012). Noble gases, such as helium, neon, and argon, are utilized as highly effective, inert tracers to detect and identify leaking orphan wells, distinguishing their emissions from natural gas sources. Because they are non-reactive and have well-characterized concentrations, they provide a unique “fingerprint” of deep thermogenic gases leaking from wells (Darrah et al. 2014).

Artificial tracers (e.g., fluorescent dyes) can also be used to determine the extent of any potential leakage pathway within a wellbore. For example, studies have demonstrated the efficacy of artificial tracers to establish the connection between an abandoned well and a nearby spring, and to estimate groundwater travel time (Magal et al. 2008).

Groundwater Monitoring

Addressing groundwater contamination risks from orphan wells requires a multipronged approach combining various monitoring techniques (NASEM 2025; Yang et al. 2019), including water quality and level monitoring methods, well integrity assessments, and advanced technologies such as sensors and machine learning. A comprehensive understanding of a site’s hydrogeological conditions is crucial for identifying contamination sources accurately and developing effective remediation plans (NASEM 2025). Traditional methods involve periodic physical inspections, including soil surface methane testing and gas detection surveys. However, these approaches are often time consuming, expensive, and limited in their ability to provide continuous, real-time data. To overcome these limitations, advanced technologies are increasingly employed (Dhapre et al. 2025).

Regular sampling and analysis of groundwater from surrounding wells can detect the presence and concentrations of contaminants potentially originating from orphan wells. Real-time sensor technologies can provide continuous monitoring of water quality parameters, including levels of methane, benzene, and other potential pollutants; these sensors are increasingly inexpensive and easy to install (Li et al. 2017). Monitoring changes in groundwater levels can indicate the influence of nearby pumping wells or cascading water within the orphan wellbore. Electronic pressure transducers offer continuous data logging and help assess water level trends and potential interference effects from nearby pumping or water bodies (Taylor and Alley 2001).

Well integrity assessment methods include visual inspections and geophysical techniques. Visual inspection of wellhead infrastructure and the surrounding ground for signs of deterioration, such as rust or subsidence, is crucial (Simmers et al. 2024). Geophysical methods such as GPR and electrical resistivity tomography can help evaluate subsurface conditions and detect compromised well casings or potential leakage pathways (Gasperikova et al. 2012). Remotely operated vehicles and closed-circuit television can be used to inspect casing and grout integrity in deeper wells (Wang et al. 2024).

Advanced technologies, such as sensors and machine learning, can be used for more efficient and effective monitoring. Soil gas surveys can be used to detect methane and other VOCs emanating from leaking wells (El Hachem and Kang 2023). Numerical modeling can help predict the migration and transformation of pollutants, identify pollution sources and optimize remediation efforts (Liu et al. 2024). Machine learning algorithms can be trained to predict the location of undocumented wells and prioritize monitoring efforts by identifying regions with higher risk factors, such as proximity to aquifers or sensitive populations (Dhapre et al. 2025).

MONITORING COSTS, UNCERTAINTY, AND STRATEGIC GUIDANCE

As discussed in previous chapters, data on plugged wells are incomplete and collected inconsistently, and orphan wells are found in a broad range of geologic and geographic regions. With the great variety of orphan wells to be plugged, the committee

could not recommend specific guidelines for monitoring plugged wells beyond the discussion of prioritization frameworks outlined earlier in this chapter. Other aspects of monitoring that need to be considered are discussed below.

Monitoring costs span a wide range. For example, methane monitoring alone can cost about $5,000 per well; for example, Texas reported an average cost of $2,200–$5,500 per well (Webb 2024). Based on committee members’ experience, they noted that visual inspections, SCVF and SCP checks, handheld detectors, and barhole probing typically cost in the hundreds to low thousands of dollars per well. OGI surveys, isotopic analysis, mobile CRDS routes, telemetry, and flux chambers generally range from several thousand to tens of thousands of dollars per site or campaign. Downhole interventions—including cement evaluation—pulsed neutron logging, tracer studies, isolated interval tests, and rig work often range from tens of thousands of dollars to over $100,000 per well.

Monitoring results have important limits and failure modes—emissions can be episodic and intermittent (so one-off or infrequent surveys may miss events); near-surface oxidation and diffusion can attenuate methane signals and bias concentrations; source attribution can be uncertain without isotopic/tracer data in mixed-source areas; performance is weather dependent (wind, stability, temperature inversions, precipitation/snow cover, soil moisture, and barometric swings can affect detectability); and, in low-emitting settings, detection limits and background variability can yield false negatives. These constraints underscore the need for repeated measurements and conservative interpretation of low or non-detectable results. For example, when decisions are potentially highly consequential or cement conditions atypical, multiple downhole logs can be acquired to cross-validate interpretations. Additionally, managing uncertainty requires combining complementary methods. For example, pairing OGI with quantification, or continuous SCVF telemetry with periodic flux grids, greatly improves detection confidence, and gas isotopes substantially reduce source-attribution uncertainty.

Matching strategy to context helps allocate monitoring and plugging resources efficiently (King and King 2013). The committee discussed the possibilities for post-plug monitoring and the range of strategies that could be implemented. For example, in orphan well fields with sparse records, one cost-effective approach may be to start with desktop risk factor ranking; using magnetics to locate undocumented wells; applying airborne methane imaging to flag super-emitters; and using OGI or CRDS to transition the highest-risk wells to plugging and abandonment activities, while reserving downhole logs for design and verification where needed. Idle wells near sensitive populations can benefit from continuous vent or annulus telemetry, quarterly surface gas checks, and annual water sampling; these routines can be escalated to include downhole diagnostics if sustained pressure or migration persists. In storage fields, depleted reservoirs, or CCUS projects with higher-risk well profiles, upfront barrier evaluation, sentinel monitoring points, and routine interferometric synthetic aperture radar review can be justified.

However, monitoring methods developed primarily for larger emissions from oil and gas production sites and surface facilities may not be applicable to low-volume emissions from orphan wells. Continuous SCVF and SCP telemetry, coupled with

disciplined data analytics and transparent stakeholder engagement, may have sufficient resolution and cost-effectiveness to enable detection, targeted remediation, and demonstrable regulatory compliance (Watson and Bachu 2009).

It is useful for monitoring to follow the well life cycle. For example, before abandonment, baselines can be established with SCVF and SCP testing; OGI or handheld screening; soil gas and water sampling where receptors exist; and targeted logging to inform plug design. During temporary abandonment, one would maintain a hydrostatic head of inhibited fluid, perform periodic SCVF and SCP checks with corrosion control, and use telemetry where feasible. After plugging and abandonment, monitoring is based on risk factor analysis of well and geographic location at set intervals using repeat vent checks, soil gas or flux, and water sampling to demonstrate low or non-detectable emissions. Based on known practices, monitoring intervals could be 30–90 days post-plug and again within 6–12 months, with additional checks at 3–5 years (if required by regulation).

CONCLUSION AND RECOMMENDATION

Conclusion 4-1: Pre-plugging prioritization rankings provide a baseline to determine potential risks associated with orphan wells. Post-plug monitoring prioritization and planning could use existing pre-plug prioritization frameworks as models.

Recommendation 4-1: States should work toward establishing risk factor-based monitoring prioritization frameworks to determine which wells and what type of post-plug monitoring should be prioritized.