Attachment G

Liquids Unloading

David Allen

University of Texas at Austin
Comments on 

 Oil and Natural Gas Sector Liquids Unloading Processes 

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 Liquids Unloading 

Processes” 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. Please comment on the national estimates of methane emissions and methane emission factors 

for liquids unloading presented in this paper. Please comment on regional variability and the 

factors that influence regional differences in VOC and methane emissions from liquids 

unloading. What factors influence frequency and duration of liquids unloading (e.g., regional 

geology)? 

Response: As noted in the Summary, on Page 25 of the white paper, “the majority of emissions 

from liquids unloading events come from a small percentage of wells”. In estimates reported by 

the American Petroleum Institute/America’s Natural Gas Alliance (American Petroleum Institute 

and America’s Natural Gas Alliance (API/ANGA) Characterizing Pivotal Sources of Methane 

Emissions from Natural Gas Production,: Summary and Analyses of API and ANGA Survey 

Responses, Final Report, updated September, 2012), three percent of wells without plunger lifts 

that had liquid unloading events accounted for half of emissions from this type of well and half of 

the wells accounted for more than 90% of emissions. In a limited number of measurements of 

methane emissions from manual unloadings of shale gas wells without plunger lifts, Allen, et al. 

(Allen, et al., Proceedings of the National Academy of Sciences, 110, 17768-17773, 2013) found 

that 95% of the emissions came from less than half of the wells. This dependence of total 

national emissions estimates on a small number of highest emitting wells means that the 

accuracy of national emission estimates will depend on understanding the small population of 

highest emitting wells. 

As noted in the Summary of the white paper on page 25, and on page 11 of white paper, Allen et 

al. (2013) concluded, based on a small number of measurements, that “the length of the liquids 

unloading event and the number of events per year are crucial factors in a well’s annual 

emissions from liquids unloading.” 

In addition to having the characteristics of high frequencies of events and long duration events, 

the highest emitting wells might be expected to occur in particular regions. Estimated emissions 

from liquid unloadings are spatially concentrated in Rocky Mountain production regions. Wells 

in the western United States account for more than half of estimated emissions from liquid 

unloadings reported in in the 2012/2013 GHGRP (U.S. Environmental Protection Agency 

Greenhouse Gas Reporting Program, Data for Reporting Year 2012, released in 2013). 

Temporal distributions also vary. Based on well unloading frequency data reported in the 

2012/2013 GHGRP, some wells unload several times per day while others may unload only once 

per year or once during the well’s production life cycle. 

2. Is there further information available on VOC or methane emissions from the various liquids 

unloading practices and technologies described in this paper? 

Allen et al. (2013) report gas composition data for each well unloading on which 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 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 a VOC emission rates for unloadings. An average VOC emission of 0.1 

Mg VOC for an unloading event with 1.1 Mg methane was estimated based on gas compositions 

in the Barnett Shale. 

3. Please comment on the types of wells that have the highest tendency to develop liquids 

loading. It is the EPA’s understanding that liquids loading becomes more likely as wells age and 

well pressure declines. Is this only a problem for wells further down their decline curve or can 

wells develop liquids loading problems relatively quickly under certain situations? Are certain 

wells (or wells in certain basins) more prone to developing liquids loading problems, such as 

hydraulically fractured wells versus conventional wells or horizontal wells versus vertical wells? 

Response: The data reported by Allen, et al. (2013) were all for relatively young wells. 

4. Did this paper capture the full range of feasible liquids unloading technologies and their 

associated emissions? Please comment on the costs of these technologies. Please comment on the 

emission reductions achieved by these technologies. How does the well’s life cycle affect the 

applicability of these technologies? 

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

5. Please provide any data or information you are aware of regarding the prevalence of these 

technologies in the field. 

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

6. In general, please comment on the ability of plunger lift systems to perform liquids unloading 

without any air emissions. Are there situations where plunger lifts have to vent to the 

atmosphere? Are these instances only due to operator error and malfunction or are there 

operational situations where it is necessary in order for the plunger lift to effectively remove the 

liquid buildup from the well tubing? 

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

7. Based on anecdotal experience provided by industry and vendors, the blowdown of a well 

removes about 15% of the liquid, while a plunger lift removes up to 100% (BP, 2006). Please 

discuss the efficacy of plunger lifts at removing liquids from wells and the conditions that may 

limit the efficacy. 

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

8. Please comment on the pros and cons of installing a plunger lift system during initial well 

construction versus later in the well’s life. Are there cost savings associated with installing the 

plunger lift system during initial well construction? 

Response: No response – this question is outside the scope of my expertise. 
9. Please comment on the pros and cons of installing a “smart” automation system as part of a 

plunger lift system. Do these technologies, in combination with customized control software, 

improve performance and reduce emissions? 

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

10. Please comment on the feasibility of the use of artificial lift systems during liquids unloading 

operations. Please be specific to the types of wells where artificial lift systems are feasible, as 

well as what situations or well characteristics discourage the use of artificial lift systems. Are 

there standard criteria that apply? 

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

11. The EPA is aware that in areas where the produced gas has a high H2S concentration 

combustion devices/flares are used during liquids unloading operations to control vented 

emissions as a safety precaution. However, the EPA is not aware of any instances where 

combustion devices/flares are used during liquids unloading operations to reduce VOC or 

methane emissions. Please comment on the feasibility of the use of combustion devices/flares 

during liquids unloading operations. Please be specific to the types of wells where combustion 

devices/flares are feasible. Are there operational or technical situations where combustion 

devices/flares could not be used? 

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

12. Given that liquids unloading may only be required intermittently at many wells, is the use of 

a mobile combustion device/flare feasible and potentially less costly than a permanent 

combustion device/flare? 

 

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

13. Given that there are multiple technologies, including plunger lifts, downhole pumps and 

velocity tubing that are more effective at removing liquids from the well tubing than blowdowns, 

why do owners and operators of wells choose to perform blowdowns instead of employing one 

of these technologies? Are there technical reasons other than cost that preclude the use of these 

technologies at certain wells? 

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

14. Are there ongoing or planned studies that will substantially improve the current 

understanding of VOC and methane emissions from liquids unloading events and available 

options for increased product recovery and emissions reductions? The EPA is aware of an 

additional stage of the Allen et al. study to be completed in partnership with the EDF and other 

partners that will directly meter the emissions from liquids unloading events. However, the EPA 

is not aware of any other ongoing or planned studies addressing this source of emissions. 

Response: Existing national estimates of emissions are based on the GHGRP, an API/ANGA 
survey, and from estimates in the NEI. There are only a few measurements available, and those 

may not be widely representative of all of the variations in unloading practices across the 

U.S. The EPA GHGRP provides some company self-reported measurement data, but only under 

one of three options for reporters. The few measurements available in the scientific literature, 

(Allen, et al., 2013) were only for shale gas wells. New measurements are needed to represent 

this practice across the U.S.; new measurements are expected to be published in the scientific 

literature, by a team led by the University of Texas, in mid-2014. 

Specific Comments 

Page 28: 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 ).