## 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 revi ew 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 tw o sections: responses to the char ge questions for reviewers that appear in the white paper, and additional specific comments, orga nized by page number.Responses to charge questions

1. Please comment on the national estimates of me thane emissions and methane emission factors for liquids unloading presented in this paper. Please comment on regiona l variability and the factors that influence regional differences in VOC and methane emissions from liquids unloading. What factors influen ce frequency and duration of li quids unloading (e.g., regional geology)?

Response: As noted in the Summa ry, on Page 25 of the white paper, “the majority of emissions from liquids unloading events come from a small pe rcentage of wells”. In estimates reported by

the American Petroleum Institute/America’s Natu ral Gas Alliance (American Petroleum Institute

and America’s Natural Gas Alli ance (API/ANGA) Characterizi ng Pivotal Sources of Methane Emissions from Natural Gas Production,: Su mmary 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 t han 90% of emissions. In a lim ited number of measurements of methane emissions from manual unloadings of shal e 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 me asurements, 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 liqui ds unloading.”

In addition to having the characte ristics 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 concentrat ed in Rocky Mountain production regions. Wells in the western United States account for more than half of estimate d 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 we ll’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 fo r 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) usethese 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 ha ve 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 fo r wells further down their decline curve or can wells develop liquids loading problems relative ly 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 a ll for relatively young wells.

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

associated emissions? Please comment on the cost s of these technologies. Please comment on the emission reductions achieved by these technologies. How does the well’s life cycle affect the applicability of th ese technologies?

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

5. Please provide any data or information you ar e aware of regarding th e 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 pl unger lift systems to pe rform 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 malf unction or are there operational situations where it is necessary in order for the plunge r 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 experi ence provided by industry and ve ndors, the blowdown of a well removes about 15% of the liqui d, while a plunger lift remove s up to 100% (BP, 2006). Please discuss the efficacy of plunger lif ts 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 instal ling a plunger lift system during initial well

construction versus later in the well’s life. Are there cost savings associ ated with installing the plunger lift system during init ial well construction?

Response: No response – this question is outside the scope of my expertise.9. Please comment on the pros and cons of instal ling a “smart” automation system as part of a plunger lift system. Do these tech nologies, 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 system s 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 th e produced gas has a high H2S concentration combustion devices/flares are used during liqui ds unloading operations to control vented emissions as a safety precaution. However, th e EPA is not aware of any instances where combustion devices/flares are used during li quids unloading operations to reduce VOC or

methane emissions. Please comment on the feasib ility of the use of co mbustion devices/flares during liquids unloading operations . Please be specific to the t ypes of wells where combustion devices/flares are feasible. Are there operation al or technical situations where combustion

devices/flares coul d not be used?

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

12. Given that liquids unloading may only be require d 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 re moving 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 te chnical 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 fr om liquids unloading events and available options for increased product recovery and emi ssions 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 em issions are based on the GHGRP, an API/ANGAsurvey, and from estimates in the NEI. There are only a few measuremen ts available, and those may not be widely representativ e of all of the vari ations 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 availabl e 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 publis hed 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 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 ).