Attachment K

Leaks

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 Quality Planning and Standards (OAQPS), dated April 2014. I have 

concentrated my review on my areas of expertise. My comments identify additional sources of 

data and describe some important features of the available data. The review is organized into 

two sections: responses to the charge questions for reviewers that appear in the white paper, and 

additional specific comments, organized by page number. 
Responses to charge questions

1. Did this paper appropriately characterize the different 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 exception. 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 source measurements, however, in their Supporting 

Information (Section S4, pages S-43 to S-58), Allen 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 effectiveness of 

mobile monitoring. 

In addition, the white paper describes tests done in a URS Corporation/University of Texas 

(URS/UT) study in describing acoustical 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 ability of the acoustic sensor to detect and quantify leaks. The results should 

not be generalized. 

2. Please comment on the approaches for quantifying 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 this 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 reports 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 methane emissions (333 Gg) are estimated for 

reporting year 2012, including emissions from wells, heaters, separators, dehydrators, and 

meters/piping. This is very similar to the total of 325 Gg reported for similar source 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 devices 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 emissions in the 2013 GHG NEI, with the range reflecting whether 

small compressors are included in the comparison (Allen et al., 2013, Supporting Information, 

Section S5). 

3. Are the emission estimating procedures and leak detection methods presented here equally 

applicable to both oil and gas production, processing, and transmission and storage sectors? 

Response: Many of the techniques described in the white paper have been applied in multiple 

sectors. Applicability may vary depending on the characteristics of the sources. 
4. Are there ongoing or planned studies that will substantially improve the current understanding 

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

Please list the additional studies 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 emission rates for equipment leaks. An average 

VOC emission rate of 0.062 Mg/yr per well VOC 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 sites, gathering and boosting stations, 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 number 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 full range of technologies available to identify leaks at oil and 

natural gas facilities? 

Response: Section 2.9 of the white paper describes mobile measurement studies conducted by 

EPA. This and other sections of the white paper capture the potential role of such mobile 

measurement studies in identifying locations of leaks, however, the variety of equipment 

configurations (e.g., solar occultation flux measurements vs. measuring methane in plumes at 

multiple heights above ground), data analysis methods (tracer release approaches vs. 

lagrangian modeling), and mobile platforms (low flying 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 detection technologies. Please 

discuss efficacy, cost and feasibility for various applications. 

Response: In the conclusions section, the white paper states that “ambient/mobile monitoring 

and OGI technology might be most effective when used in tandem”. While I agree with this 

assessment, I also note that the large number of small distributed sites in the natural gas 

production sector, and the travel time between sites, can make personnel costs associated with 

mobile monitoring relatively high. Mobile monitoring/OGI coupled with fixed monitoring 

networks may allow strategic deployment of the mobile monitoring. For example, measurements 

at fixed sites coupled with near-real time back trajectory tools (in use for more than a decade in 

the industrial regions of Houston and Corpus Christi, Texas) can guide the target areas for 

mobile monitoring, increasing probabilities of leak detection. An additional benefit 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 the different leak detection technologies at oil 

and gas facilities. Which technologies are the most commonly used? Does the type of facility 

(e.g., well site versus gathering and boosting station) affect which 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 costs and emission reductions achieved vary with 

different frequencies of revisit. 

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

10. Please comment on the potential for using ambient/mobile monitoring technologies in 

conjunction with OGI technology. This would be a two-phase approach where the 

ambient/mobile monitoring technology is used to detect the presence 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 response to Question 7, I agree that ambient/mobile monitoring 

coupled with OGI, may be very effective. I recommend a three-tiered, rather 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 personnel 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 SO2 emissions for more than a decade 

in the industrial regions of Houston 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 generally low costs of repair and the 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 correct that it is almost always economical to repair leaks once they are 

found, then how important is the quantification of emissions from leaks when implementing a 

program to detect and repair leaks? 
Response: There are near term and long term responses to this question. In the near term, it 

remains important to quantify leaks. 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 population of sources, (2) the difference between 

approaches based on ambient methane concentration measurements (top-down methods) and 

approaches based on direct measurement of emissions from individual sources (bottom-up 

methods) and (3) the difference in the extreme values 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 from Natural Gas Production and Use: Reconciling 

Bottom-Up and Top-Down Measurements, Current Opinion in Chemical Engineering, in press). 

Currently, top-down methods suggest that unidentified emission sources are necessary to 

account for total methane concentrations observed in the atmosphere, 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 remain 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 measurements indicate that small sub-populations of sources can 

have emissions much larger than overall population averages (characterized as “fat-tail” 

distributions). In the long term, simply identifying these sub-populations of high emitters, rather 

than determining whether they emit 5, 10 or 50 times the emissions of an average source, may be 

sufficient. 

There are currently a number of both “top-down” and “bottom-up” measurement studies 

underway, and these studies may help to reduce uncertainties. 

13. Please comment on the state of innovation in leak detection technologies. Are there new 

technologies under development that are not discussed in this paper? Are there significant 

advancements being made in the technologies that are not described in this paper? 

Response: There has been extensive innovation in methane detection instrumentation over the 

past decade. To cite one example, interband cascade (IBC) lasers are a disruptive technology 

for the identification, attribution and quantification of methane 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 emission sources. While not described 

explicitly in the white paper, these instruments are enabling the type of mobile 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 used, 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 information 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 sites associated with wells. 

Page 43: The white paper describes tests done in a URS Corporation/University of Texas 

(URS/UT) study in describing acoustical leak detection. It should be noted that the URS/UT 
study 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 addition 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 ).