## Attachment K

L eaks

David Allen

University of Texas at Austin

### Comments on



## Oil and Natural Gas Sector Leaks

Draft dated April, 2014

Submitted to:

Carrie Richardson Fry Research Triangle Institute

Submitted by:

David Allen

June 16, 2014

Overview

I have reviewed the draft white paper titled “Oil and Natural Gas Sector Leaks” prepared by the U.S. EPA Office of Air Qualit y Planning and Standards (OAQPS), dated April 2014. I have concentrated my review on my areas of expertis e. My comments identify additional sources of data and describe some important features of th e available data. The review is organized into two sections: responses to the charge questions fo r reviewers that appear in the white paper, and additional specific comments, orga nized by page number.Responses to charge questions

1. Did this paper appropriately characterize the di fferent studies and data sources that quantify

VOC and methane emissions from leaks in the oil and natural gas sector?

Response: The white paper summarizes the data that I am aware of, that were available at the time the white paper was written, with one excep tion. That exception is related to data on emissions from well sites reported in Allen et al. (PNAS, 110, 17768-17773, referred to as Allen, et al. 2013 in this review). The white paper summarizes the data collected at 150 sites associated with gas wells using direct s ource measurements, however, in their Supporting Information (Section S4, pages S-43 to S-58), Alle n et al. (2013) also report emissions from 20 well sites using a downwind dual tracer method. These data are not used in the white paper. Allen, et al. (2013) compare the results from the dual tracer method to on-site measurements and estimates of emissions, and these data may be useful in assessing the potential e ffectiveness of mobile monitoring.

In addition, the white paper descri bes tests done in a URS Cor poration/University of Texas (URS/UT) study in describing acous tical leak detection (page 43). It should be noted that the URS/UT study was done at compressor stations and the vibrations associated with compression may influence the abil ity of the acoustic sensor to detect and quantify leaks. The results should not be generalized.

2. Please comment on the approaches for quantifyi ng emissions and on the emission factors used

in the data sources discussed. Please comment on the national estimates of emissions and emission factors for equipment leaks presented in th is paper. Please comment on the activity data used to calculate these emissions, both on the total national and regional equipment counts.

Response: Table 2-6 of the white paper re ports results from the 2014 EPA Greenhouse Gas National Emission Inventory (2014 GHG NEI) for the natural gas production sector. The white paper states, 332,662 metric tons of potential me thane emissions (333 Gg) are estimated for reporting year 2012, including emi ssions from wells, heaters, se parators, dehydrators, and meters/piping. This is very sim ilar to the total of 325 Gg reporte d for similar sour ce categories in the 2013 GHG NEI for reporting year 2011. Allen, et al, (2013) report a comparison of their measurements of equipment leaks for these devi ces to the 2013 GHG NEI. Their measurements averaged 1.23 ± 0.44 g methane per minute per well, as compared to 1.37-1.67 g methane per minute per well of potential emis sions in the 2013 GHG NEI, with the range reflecting whether small compressors are included in the compar ison (Allen et al., 2013, Supporting Information, Section S5).

3. Are the emission estimating procedures and l eak detection methods presented here equally applicable to both oil and ga s production, processing, and transm ission and storage sectors?

Response: Many of the technique s described in the white paper have been applied in multiple sectors. Applicability may vary dependi ng on the characteristics of the sources.4. Are there ongoing or planned studies that will s ubstantially improve the current understanding

of VOC and methane emissions from leaks and av ailable techniques for detecting those leaks?

Please list the additional st udies you are aware of.

Response: Some methane emission data can be used to estimate VOC emissions. Allen et al. (2013) report gas composition data for each well site on which methane emission measurements were made. The composition data are available in the data archives at the study web site maintained at the University of Texas ( http://dept.ceer.utexas.edu/methane/study ). Thus, while not explicitly reported, the methane emission data supplied by Allen, et al. (2013) can be used to calculate VOC emission rates. Zavala, et al. (Environmental Science & Technology, 48, 5314– 5321, 2014) use these data to estimate VOC emissi on rates for equipment leaks. An average VOC emission rate of 0.062 Mg/yr per well VO C for a controller with 0.646 Mg/yr per well

methane emissions was estimated based on gas compositions in the Barnett Shale.

5. Are there types of wells s ites, gathering and boosting stat ions, processing plants, and transmission and storage stations that are more prone to leaks than others? Some factors that could affect the potential for leaks are the numbe r and types of equipment, the maintenance of that equipment, and the age of the equipment, as well as factors that relate to the local geology.

Please discuss these factors and others that you believe to be important.

Response: No response – this question is outside the scope of my expertise.

6. Did this paper capture the fu ll range of technologies availabl e to identify le aks at oil and natural gas facilities?

Response: Section 2.9 of the white paper descri bes mobile measuremen t studies conducted by EPA. This and other sections of the white paper capture the pot ential role of such mobile measurement studies in identifying locations of leaks, however, the variety of equipment configurations (e.g., solar occultation flux measur ements vs. measuring methane in plumes at multiple heights above ground), data analysis methods (tracer release approaches vs. lagrangian modeling), and mobile platforms (low fly ing aircraft vs. ground vehicles) that can be

used in mobile measurement studies is not reflected the white paper.

7. Please comment on the pros and cons of the different leak detectio n technologies. Please discuss efficacy, cost and feasib ility for various applications.

Response: In the conclusions section, the white paper states that “ambie nt/mobile monitoring and OGI technology might be most effective when used in tandem”. Wh ile I agree with this assessment, I also note that th e large number of small distribut ed sites in the natural gas production sector, and the travel time between sites, can make pe rsonnel costs associated with mobile monitoring relatively high. Mobile monitoring/OGI coupled with fixed monitoring networks may allow strategic deployment of th e mobile monitoring. Fo r example, measurements at fixed sites coupled with near-real time back tr ajectory tools (in use for more than a decade in the industrial regions of Housto n and Corpus Christi, Texas) c an guide the target areas for mobile monitoring, increasing probab ilities of leak detection. An additional bene fit of coupled fixed ambient monitoring/mobile monitoring/OGI could be the detection of intermittent sources(e.g., various blow-down activities) . Intermittent sources can be difficult to detect since the probability of a drive-by monitor being at the location of an event, without guidance from fixed networks, can be very low.

8. Please comment on the prevalence of the use of th e different leak detecti on technologies at oil and gas facilities. Which technologies are the mo st commonly used? Does the type of facility (e.g., well site versus gathering and boosting station) affect whic h leak detection technology is

used?

Response: No response – this question is outside the scope of my expertise.

9. Please provide information on current frequencies of revisit of existing voluntary leak detection programs in industry and how the cost s and emission reductions achieved vary with different frequencie s of revisit.

Response: No response – this question is outside the scope of my expertise.

10. Please comment on the poten tial for using ambient/mobile monitoring technologies in conjunction with OGI technology. This w ould be a two-phase approach where the ambient/mobile monitoring technology is used to detect the pres ence of a leak and the OGI technology is used to identify the leaking component. Please discuss efficacy, cost and feasibility.

Response: As noted in the res ponse to Question 7, I agree that ambient/ mobile monitoring coupled with OGI, may be very effective. I r ecommend a three-tiered, ra ther than a two tiered approach. The large number of small distributed sites in the natural gas production sector, and the travel time between sites, can make personne l costs associated with mobile monitoring relatively high, unless the mobile monitoring is guided by additional information regarding regions that, on a daily basis, may have a high probability for leak detection. The approach of using fixed ambient measurement sites, coupled with near-real time back trajectory tools, has been in use for identifying potential sources of VOC and SO 2 emissions for more than a decade in the industrial regions of H ouston and Corpus Christi, Texas. For natural gas production regions, fixed networks could guide the target areas for mobile monitoring, increasing probabilities of leak detection.

11. Please comment on the cost of detecting a leak when compared to the cost to repair a leak. Multiple studies described in this paper suggest that detecting leaks is far more costly than repairing leaks and, due to genera lly low costs of repair and th e subsequent product recovery, it is almost always economical to repair leaks once they are found. Please comment on this overall

conclusion.

Response: No response – the costs of repairing leaks is outside the scope of my expertise.

12. If the conclusion is co rrect that it is almost always economical to re pair leaks once they are found, then how important is the quantification of emissions from leaks when implementing a program to detect a nd repair leaks?Response: There are near term and long term responses to this question. In the near term, it remains important to quantify le aks. Estimates of emissions along the natural gas supply chain have varied widely in the scientific literature. Reasons for the uncertainty in emissions from the natural gas supply chain include (1) the large p opulation of sources, (2) the difference between approaches based on ambient methane concentration measurements (top-down methods) and approaches based on direct meas urement of emissions from i ndividual sources (bottom-up methods) and (3) the difference in the extreme va lues of emission rates, compared to mean emission rates, from many of the emission sources in the natural gas supply chain (a “fat-tail” distribution) (see Allen, Methane Emissions fr om Natural Gas Production and Use: Reconciling Bottom-Up and Top-Down Measurements, Current Opin ion in Chemical Engineering, in press).

Currently, top-down methods suggest that unide ntified emission sources are necessary to account for total methane concen trations observed in the atmo sphere, particularly in some natural gas production regions (see reviews by Miller, et al., Proc. Natl. Acad. Sci.

, 110, 20018– 20022, 2013; and Brandt, et al. Science, 343, 733-735, 2014). Until bottom-up approaches can account for these unidentified sources, it will re main important to quantify leaks to the extent possible.

In the longer term, once the overall magnitude of various sources is understood, and top-down and bottom-up approaches lead to consistent emission estimates, the situation will be different.

Current direct source emission me asurements indicate that small sub-populations of sources can have emissions much larger than overall popul ation averages (characterized as “fat-tail” distributions). In the long term, simply identify ing these sub-populations of high emitters, rather than determining whether they emit 5, 10 or 50 ti mes the emissions of an average source, may be sufficient.

There are currently a number of both “top-dow n” and “bottom-up” measurement studies underway, and these studies may help to reduce uncertainties.

13. Please comment on the state of innovation in leak detecti on technologies. Are there new technologies under development that are not disc ussed in this paper? Are there significant advancements being made in the technologies that are not describe d in this paper?

Response: There has been extensive innovation in methane detection inst rumentation over the past decade. To cite one examp le, interband cascade (IBC) lasers are a disruptive technology for the identification, at tribution and quantification of met hane sources. These instruments employ continuous, quantitative spectroscopic ethane and methane measurements in the fingerprint mid-IR with extreme precision and speed; the 1-second detection sensitivities are 400 and 150 parts per trillion for methane and ethane respectively. These instruments are transforming the identification and attribution of methane emissi on sources. While not described explicitly in the white paper, these instruments ar e enabling the type of m obile monitoring that is described in the white paper.Specific Comments

Page 6: The white paper states, “Although methane emissions from oil and natural gas production operations have been measured, analyzed and reported in studies spanning the past few decades, VOC emissions from these operations are not as well represented”. As noted in the response to Question 4, in some cases methane emissions can be use d, together with gas composition data, to estimate VOC emissions (see Zavala, et al., 2014, for example).

Page 26: Data from the Fort Worth study are used to calculate the faction of sites on which leaks were detected. Similar informa tion can be extracted from Allen, et al. (2013), using information available in the data archives at the study web site maintained at the University of Texas ( http://dept.ceer.utexas.edu/methane/study ). The Figures below summarize the distribution of

emissions observed at 150 site s associated with wells.

Page 43: The white paper describes tests done in a URS Corporation/University of Texas (URS/UT) study in describing acou stical leak detection. It s hould be noted that the URS/UTstudy was done at compressor stations and the vibrations associated with compression may influence the ability of the acoustic sensor to detect and quantify leaks. The results should not be generalized.

Page 57: For the Allen, et al (2013) reference, in add ition to citing the material published by the Proceedings of the National Academy of Sciences (which is open access), the white paper should cite the study web site, which provides supplementary data. ( http://dept.ceer.utexas.edu/methane/study ).