the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Apr 2025
Orphaned Oil & Gas Well Methane Emission Rates Quantified with Gaussian Plume Inversions of Ambient Observations
Abstract. Annually, ~ 3.6 million abandoned oil and gas wells in the U.S. emit a combined ~ 2.6 Tg methane (CH4) adversely affecting climate and regional air quality. However, these estimates depend on emission factors derived from measuring sub-populations of wells that vary by orders of magnitude due to very limited field sampling and poorly characterized distributions. Currently, U.S. protocols to remediate orphaned wells lacks standardized quantification methods needed to both prioritize plugging and account for emission reductions. Therefore, sensitive, reliable, affordable, and scalable CH4 flux quantification methods are needed. We report the use of a simple Gaussian plume method where the dispersion parameters are constrained by in situ ground measurements of CH4 concentration at four locations 7.5 – 49 m downwind of the orphan well as well as local winds to estimate the leak rate from an orphan well. We derive a flux of 10.53 ± 1.16 kg CH4 h-1 during a venting procedure in April 2023 that agrees with the directly measured volumetric flow rate of 9.00 ± 0.25 kg CH4 h-1. This is 71 % greater than the 5.3 kg CH4 h-1 flux measured 7-months prior. Additionally, we discovered a secondary leak through the surface-casing inferred as 0.43–0.67 kg CH4 h-1 by both our ground Gaussian analysis and by transecting the plume with an uncrewed aerial system (UAS). We show that in situ determination of the dispersion parameters used in our Gaussian inversions allows us to measure methane emissions to 15 % accuracy significantly reducing errors when compared to standard practice of assuming stability class. Our results help develop simpler methods and protocols for robust orphan well emission quantification that can be used for reporting.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Supplement
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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Supplement
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-344', Anonymous Referee #2, 29 Apr 2025
Great paper! Here are a few recommendations:
• Scalability – Can authors briefly discuss the scalability of the proposed method, and based on their judgment, if this approach can be a recommended method for measuring and quantifying methane from orphaned O&G wells? In other words, what else needs to be done to be able to consider this approach (using one measurement unit, or multiple) as a recommended method?
• Downwind distances – can authors elaborate on the details related to the design of the experiment, including (i) a discussion on the reason for selecting 7.5m, 15m, 22.5m, and 47m and selected downwind distances for monitors, (ii) employing measurement units from two different manufacturers, which may introduce a level of uncertainty, and (iii) the reason for deploying Aeris Pico at a height of 0.8m.
• Neighboring sources – I see some neighboring sources, including a small (orphaned?) site approximately 300 m to the north of this site, and traces of a site ~850 m to the south. This is a sort of ideal case with minimal impact from off-site sources. To be able to generalize the application of this method, I was wondering if authors could briefly explain their thoughts on how to account for the impact of neighboring sources (specifically upwind sources) with similar gas composition (from O&G reservoirs), and with potential fugitive leaks.
• Complex terrain impact – Can you also discuss the impact of complex terrain, given a large number of orphaned wells exist in regions with more complex terrain?
• Baseline – I was wondering if the estimated baseline (which is ~2 ppm above atm background) is observed upwind of this specific site. I wonder how much of it is the contribution of the Foster 1S fugitive leak. In other words, what is the baseline when the wind blows from a different direction and Foster 1S is not upwind of the monitors?
• Line 32 – There is an undefined character that needs to be removed
• Ethane measurements – can authors clarify why the ethane measurements from the monitor placed at the distance of 22.5m from the source is not presented here?
• Line 201: Authors stated that “The UAS instrument suite is designed to measure instantaneous (1 Hz) point methane fluxes…” is it to measure point-in-space methane concentrations or fluxes?
• Wind speed – Can you please discuss the impact of wind speed and potential correlations between wind speed and plume spread?
• The numbering of tables and figures needs to be updated
• For Figure 10, can you please include timeseries of wind speed and wind direction, as well?
• Regarding line 290: “This could be related to the velocity of gas emitted from the pipe (~0.6 m s-1) in comparison to the ambient wind speed of 6.6 m s-1 and the direction of emission across the wind. Low plume dispersion was observed farther away from the well which is more typical of more laminar flow.” Can you clarify the release direction relative to the predominant wind direction?
• Line 325: Please elaborate on the following statement “Other iterations of our empirically constrained dispersion model, where plume rise or ground reflection are excluded, result in estimates from 5.6 - 12.8 kg CH4 h-1...”
• I wonder if the estimated sigma z corresponds to the plume shape/edges from drone observations
• Line 383 – please elaborate on the discussion related to narrow plumes and sensor “blind-time”, by including further analyses on the %time when this sensor placement configuration results in direct source-to-receptor/sensor pollutant transport (direct signal). For a similar analysis, I refer you to a recent preprint that we recently published, analyzing the blind time of CMS sensor networks with 3 sensors: https://chemrxiv.org/engage/chemrxiv/article-details/66fee87bcec5d6c142103149
• I recommend revising the conclusion to focus solely on the findings and moving the discussion to a dedicated section.Citation: https://doi.org/10.5194/egusphere-2025-344-RC1 - AC1: 'Reply on RC1', Emily Follansbee, 15 May 2025
- AC4: 'Reply on RC1', Emily Follansbee, 16 May 2025
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RC2: 'Comment on egusphere-2025-344', Anonymous Referee #1, 02 May 2025
The authors present an inverse Gaussian plume analysis of ground-based spectroscopic methane observations downwind of a venting abandoned oil well to quantify emissions. They compare their results with separate emission estimates based on UAS and flow-meter observations. Emission estimates are consistent between platforms, and a key finding is that direct characterization of the atmospheric stability class based on the observed variability of wind direction and methane enhancement dramatically improves the accuracy of the emission estimate compared to using standard stability classes. The manuscript is well written and a good fit for AMT. I recommend accepting it for publication subject to the following comments/questions:
- Line 79: Some satellites can detect down to 100 kg/h (GHGSat) or even below (Worldview-3) in favorable conditions. Bridger aircraft instruments can detect below 10 kg/h. Perhaps better to describe these detection limits as orders of magnitude (1–10 kg/h, 100 kg/h).
- Line 126: Some punctuation is missing here.
- Line 183: The wrong figure is referenced here.
- Section 2.2.3: Why were the UAS flights done before the venting operation and not during?
- Line 238: When were these enhancements observed? They’re different from the value reported at 15-m downwind in line 244 (27.2 ppm).
- Line 240: Could nearby wells not have similar methane-ethane ratios?
- Figure 4: The time axis does not appear to be in UTC, inconsistent with text.
- Equation 3: y should be subscript in exponent.
- Line 323-325: Was the wind speed averaged over the full course of the release to obtain this estimate? Would it be possible to estimate the time-varying release rate with your methodology?
- Line 336: It would be useful to report the range of values/assumptions used for the error analysis.
- Discussion/conclusions sections: It would be helpful to articulate any practical advantages of the downwind Gaussian plume approach compared to the flow meter. E.g., could it be easier to deploy at scale? How would that be done? The paper evaluates a methodology but it is unclear how the approach would be operationalized.
Citation: https://doi.org/10.5194/egusphere-2025-344-RC2 - AC2: 'Reply on RC2', Emily Follansbee, 15 May 2025
- AC3: 'Reply on RC2', Emily Follansbee, 16 May 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-344', Anonymous Referee #2, 29 Apr 2025
Great paper! Here are a few recommendations:
• Scalability – Can authors briefly discuss the scalability of the proposed method, and based on their judgment, if this approach can be a recommended method for measuring and quantifying methane from orphaned O&G wells? In other words, what else needs to be done to be able to consider this approach (using one measurement unit, or multiple) as a recommended method?
• Downwind distances – can authors elaborate on the details related to the design of the experiment, including (i) a discussion on the reason for selecting 7.5m, 15m, 22.5m, and 47m and selected downwind distances for monitors, (ii) employing measurement units from two different manufacturers, which may introduce a level of uncertainty, and (iii) the reason for deploying Aeris Pico at a height of 0.8m.
• Neighboring sources – I see some neighboring sources, including a small (orphaned?) site approximately 300 m to the north of this site, and traces of a site ~850 m to the south. This is a sort of ideal case with minimal impact from off-site sources. To be able to generalize the application of this method, I was wondering if authors could briefly explain their thoughts on how to account for the impact of neighboring sources (specifically upwind sources) with similar gas composition (from O&G reservoirs), and with potential fugitive leaks.
• Complex terrain impact – Can you also discuss the impact of complex terrain, given a large number of orphaned wells exist in regions with more complex terrain?
• Baseline – I was wondering if the estimated baseline (which is ~2 ppm above atm background) is observed upwind of this specific site. I wonder how much of it is the contribution of the Foster 1S fugitive leak. In other words, what is the baseline when the wind blows from a different direction and Foster 1S is not upwind of the monitors?
• Line 32 – There is an undefined character that needs to be removed
• Ethane measurements – can authors clarify why the ethane measurements from the monitor placed at the distance of 22.5m from the source is not presented here?
• Line 201: Authors stated that “The UAS instrument suite is designed to measure instantaneous (1 Hz) point methane fluxes…” is it to measure point-in-space methane concentrations or fluxes?
• Wind speed – Can you please discuss the impact of wind speed and potential correlations between wind speed and plume spread?
• The numbering of tables and figures needs to be updated
• For Figure 10, can you please include timeseries of wind speed and wind direction, as well?
• Regarding line 290: “This could be related to the velocity of gas emitted from the pipe (~0.6 m s-1) in comparison to the ambient wind speed of 6.6 m s-1 and the direction of emission across the wind. Low plume dispersion was observed farther away from the well which is more typical of more laminar flow.” Can you clarify the release direction relative to the predominant wind direction?
• Line 325: Please elaborate on the following statement “Other iterations of our empirically constrained dispersion model, where plume rise or ground reflection are excluded, result in estimates from 5.6 - 12.8 kg CH4 h-1...”
• I wonder if the estimated sigma z corresponds to the plume shape/edges from drone observations
• Line 383 – please elaborate on the discussion related to narrow plumes and sensor “blind-time”, by including further analyses on the %time when this sensor placement configuration results in direct source-to-receptor/sensor pollutant transport (direct signal). For a similar analysis, I refer you to a recent preprint that we recently published, analyzing the blind time of CMS sensor networks with 3 sensors: https://chemrxiv.org/engage/chemrxiv/article-details/66fee87bcec5d6c142103149
• I recommend revising the conclusion to focus solely on the findings and moving the discussion to a dedicated section.Citation: https://doi.org/10.5194/egusphere-2025-344-RC1 - AC1: 'Reply on RC1', Emily Follansbee, 15 May 2025
- AC4: 'Reply on RC1', Emily Follansbee, 16 May 2025
-
RC2: 'Comment on egusphere-2025-344', Anonymous Referee #1, 02 May 2025
The authors present an inverse Gaussian plume analysis of ground-based spectroscopic methane observations downwind of a venting abandoned oil well to quantify emissions. They compare their results with separate emission estimates based on UAS and flow-meter observations. Emission estimates are consistent between platforms, and a key finding is that direct characterization of the atmospheric stability class based on the observed variability of wind direction and methane enhancement dramatically improves the accuracy of the emission estimate compared to using standard stability classes. The manuscript is well written and a good fit for AMT. I recommend accepting it for publication subject to the following comments/questions:
- Line 79: Some satellites can detect down to 100 kg/h (GHGSat) or even below (Worldview-3) in favorable conditions. Bridger aircraft instruments can detect below 10 kg/h. Perhaps better to describe these detection limits as orders of magnitude (1–10 kg/h, 100 kg/h).
- Line 126: Some punctuation is missing here.
- Line 183: The wrong figure is referenced here.
- Section 2.2.3: Why were the UAS flights done before the venting operation and not during?
- Line 238: When were these enhancements observed? They’re different from the value reported at 15-m downwind in line 244 (27.2 ppm).
- Line 240: Could nearby wells not have similar methane-ethane ratios?
- Figure 4: The time axis does not appear to be in UTC, inconsistent with text.
- Equation 3: y should be subscript in exponent.
- Line 323-325: Was the wind speed averaged over the full course of the release to obtain this estimate? Would it be possible to estimate the time-varying release rate with your methodology?
- Line 336: It would be useful to report the range of values/assumptions used for the error analysis.
- Discussion/conclusions sections: It would be helpful to articulate any practical advantages of the downwind Gaussian plume approach compared to the flow meter. E.g., could it be easier to deploy at scale? How would that be done? The paper evaluates a methodology but it is unclear how the approach would be operationalized.
Citation: https://doi.org/10.5194/egusphere-2025-344-RC2 - AC2: 'Reply on RC2', Emily Follansbee, 15 May 2025
- AC3: 'Reply on RC2', Emily Follansbee, 16 May 2025
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Orphaned Oil & Gas Well Methane Emission Rates Quantified with Gaussian Plume Inversions of Ambient Observations Emily Follansbee https://doi.org/10.17632/3p598pbhz6.2
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Emily Follansbee
Mohit L. Dubey
Jonathan F. Dooley
Curtis Shuck
Ken Minschwaner
Andre Santos
Sebastien C. Biraud
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(5007 KB) - Metadata XML
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Supplement
(968 KB) - BibTeX
- EndNote
- Final revised paper