Attachment I

Liquids Unloading

Environmental Defense Fund

Ramón Alvarez, Robert Harriss, David Lyon
Oil and Natural Gas Sector: Liquids Unloading Processes 

Peer Review Responses of Environmental Defense Fund

June 16, 2014 

I. EXECUTIVE SUMMARY

• Emissions from Liquids Unloading Activities are Significant. 

o Emissions Factors and Distribution. Data suggests liquids unloading emissions 

are highly skewed, with a small minority of wells contributing the majority of 

emissions. Approximately 65% of 2012 emissions reported to the Greenhouse 

Gas Reporting Program (GHGRP) were from 10% of wells. 

o National Emissions. Several recent national emissions estimates suggest liquids 

unloading emissions are approximately 300 Gg CH4, making this activity a 

significant source of methane. Many of these estimates are uncertain since 

inaccuracy of the prevalence or emissions of the highest emitting wells could 

greatly impact national emissions estimates. 

• Many different technologies can cost-effectively reduce liquids unloading emissions. 

o Causes of Liquids Loading. At some point, most wells will experience liquid

loading problems, which occurs when reservoir pressure is less than combined 

fluid and pipeline pressure and therefore unable to lift liquids out of the wellbore. 

Venting temporarily unloads liquids by reducing pipeline pressure to atmospheric 

pressure. However, venting depletes reservoir pressure and therefore exacerbates 

the problem and decreases ultimate hydrocarbon recovery and operational 

efficiency. 

o Technologies. Plunger lifts can minimize venting during liquids unloading by 

using a well’s own reservoir pressure to overcome pressure differentials, but do 

not always lead to low emissions. Often, other solutions are also available such as 

installing velocity tubing or using compressor engines to lower the operating 

pressure of the well and associated pipeline. When the aforementioned 

technologies are insufficient to lift liquids methods, artificial lift can successfully 

remove liquids from wells with little or zero emissions. 

o Plunger Lift Cost Effectiveness. GHGRP data suggests that the majority of wells 

with plunger lifts have lower emissions than wells without plunger lifts, especially 

after accounting for non-reporting wells with plunger lifts that do not vent. ICF 

projected that plunger lifts are capable of reducing liquids unloading emissions by 

95% at wells on which they are installed with a positive payback of $0.05/Mcf 

methane reduced. 
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o Operational Considerations. The effectiveness of plunger lifts can be optimized

by ensuring the technology is operated in a manner that minimizes venting. Smart 

automation of plunger lifts and changes in operational practices have been shown 

to reduce emissions by over 99% from these high emission plunger lift equipped 

wells. 

o Mobile flaring technology is feasible and should be considered where other 

technologies are inapplicable. 
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I. QUANTIFYING THE PROBLEM OF METHANE LEAKS FROM LIQUIDS 

UNLOADING ACTIVITIES (QUESTIONS 1, 2, 3) 

EPA’s Liquids Unloading White Paper presents data on emissions factors from wells 

both with and without plunger lifts and likewise provides estimates of national emissions due to 

liquids unloading. Before assessing these estimates, Part II.A describes the causes of liquids 

loading and associated methane emissions from liquids unloading. Part II.B then assesses liquids 

unloading emissions factors, focusing both on the emissions factors reviewed in the White Paper, 

and, importantly, the distribution of emissions among sources. Finally, Part II.C assesses national 

emissions estimates associated with liquids unloading activities. 

We conclude: 

• Wells without plunger lifts have significant unloading emissions, with an average 

emissions factor of approximately 2.9 metric tons methane per year (MT/y CH4). 

• Wells with plunger lifts can almost entirely eliminate vented emissions from liquids 

unloading activities when operated optimally. Plunger lift equipped wells can vent, 

however, if the plunger becomes overloaded. Although these emissions are usually 

lower than wells without plunger lifts, some of the highest emitting wells are 

equipped with plunger lifts. 

• For wells with and without plunger lifts, emissions exhibit a highly-skewed 

distribution, with a relatively small percentage of wells accounting for a substantial 

percentage of overall emissions. 

• Several recent national emissions estimates for liquids unloading suggest these 

emissions are approximately 300,000 MT/y CH4. The skewed distribution of 

emissions leads to a high uncertainty of these estimates since inaccurate estimates of 

emissions from the highest emitting wells could greatly impact national emissions. 

A. Liquids Loading and Emissions Due to Unloading (Question 3) 

Wells accumulate liquids when the reservoir gas pressure is insufficient for lifting liquids 

up the wellbore. The liquids settle at the bottom of the well tubing, therefore obstructing gas 

flow and inhibiting production. Since reservoir pressure declines as wells age, liquids loading 

eventually becomes an issue in most wells, but there is large variation in when and how often 

wells require liquids unloading. Sometimes operators remove these liquids by venting a well, 

which reduces the downward pressure on the liquids from pipeline to atmospheric pressure. If 

the reservoir pressure is higher than the liquid pressure, then some of the liquids will be lifted out

of the wellbore, temporarily restoring gas flow. However, gas will also be vented, which 

depletes reservoir pressure and therefore exacerbates the problem in the long-term. 
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Wells with plunger lifts can almost entirely eliminate vented emissions from liquids 

unloading activities when operated optimally, which we discuss in more detail in Part III, below. 

Plunger lift equipped wells can require venting, however, if the plunger becomes overloaded and 

fails to return to the top of the wellbore. Although these emissions are usually lower than wells 

without plunger lifts, some of the highest emitting wells are equipped with plunger lifts. 

B. Analysis and Critique of Liquids Unloading Emissions Factors (Questions 

1, 2) 

EPA summarizes several sources for liquids unloading emission factors: Characterizing 

Pivotal Sources of Methane Emissions from Natural Gas Production: Summary and Analysis of 

API and ANGA Survey Responses (API/ANGA), Inventory of Greenhouse Gas Emissions and 

Sinks: 1990-2012 (2014 GHG Inventory), Allen et al, 2013 (UT Study), and Economic Analysis 

of Methane Emission Reduction Opportunities in the U.S. Onshore Oil and Natural Gas 

Industries (ICF Report). The values in Table 1, below, summarize emissions factors from these 

sources, and other than GHGRP and ICF, reflect combined emission factors for venting wells 

with and without plunger lifts installed. The yearly emissions factor, in turn, is composed of two 

important drivers: the average amount of methane vented during each unloading event, and the 

number of times, on average, the well vents per year. 

Table 1. Summary of Vented Emissions Factors for Unloading Wells 

Emissions 

Factor (scf 

CH4/ yr) 

Source 

API / ANGA 254,409 Survey data for 5,327 wells combined with 

Subpart W calculations 

2014 GHG 

Inventory 

233,579 Average emission factor of 60,810 wells 

GHGRP 

(plunger 

lifts) 

319,732 Average emission factor of 32,448 wells reported 

in 2012 

GHGRP 

(non plunger 

lifts) 

151,638 Average emission factor of 26,215 wells reported 

in 2012 

UT Study* 300,000 Direct measurement of single events from 9 wells 

with company reported venting frequency 

ICF (plunger 

lifts) 

277,000 Analysis of GHGRP 2011 & 2012 data 

ICF (non 

plunger lifts) 

163,000 Analysis of GHGRP 2011 & 2012 data 

* only manual unloading wells without plunger lifts were measured 

 

Presenting a combined emissions factor for plunger and non-plunger lift wells may be 

helpful for calculating national emissions, though, to understand the particular control strategies 

we discuss below, it is important also to separate emission from these wells. The ICF Report 

addresses these emissions separately, showing that the average emission factor is higher for wells 
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controlled with plunger lifts than wells without these controls. We emphasize, however, that the 

GHGRP only requires reporting of wells with plunger lifts that vent, so the emission factor is not 

representative of all wells with plunger lifts. If it is assumed that 21.1% of wells with plunger 

lifts vent, as reported in the API/ANGA survey, then the average emission factor for all wells 

with plunger lifts would be 58,447 scfy/well, less than half of the emission factor for wells 

without plunger lifts, underscoring that these controlled wells on average do indeed have fewer 

emissions than uncontrolled wells. 

In addition to these average emissions values, which are relatively similar across studies, 

the underlying data show substantial emissions variability among wells: 

• GHG Inventory. The 2014 GHG Inventory regional emission factors, based on an EPA 

analysis of API/ANGA data reflect a 700x range (2,856 scfy/well for South West wells with 

plunger lifts to 1,998,082 scfy/well for Rocky Mountain wells without plunger lifts). 

• UT Study. The nine wells measured by the UT Study ranged from 1,900 to 1,337,000 

scfy/well, with four of the nine events contributing over 95% of total emissions. 

• GHGRP Data. EDF has analyzed the GHGRP 2012 data to characterize the distribution of 

average emissions per well at the company/sub-basin level. For wells without plunger lifts, 

emissions range from 3 to 14,762,527 scfy/well with an average of 151,638 scfy/well. For 

wells with plunger lifts that vent, emission factors range from 3 to 12,123,078 scfy/well with 

an average of 319,732 scfy/well. 

Figures 1 and 2, below, show the distribution of emissions per well from 2012 GHGRP 

data based on averages at the company/sub-basin level. Figure 1 is a cumulative probability 

function of emission factors, with the y-axis representing the probability that the emission factor 

is equal or less than the value on the x-axis. The emissions of both wells with and without 

plunger lifts are highly skewed and therefore plotted on a logarithmic scale to more clearly 

display the distribution. For most of the distribution, wells without plunger lifts have higher 

average emissions than wells with plunger lifts, but for approximately the 60th percentile and 

above, wells with a plunger lift have higher average emissions. 

Figure 2 plots the percentage of wells, ranked from lowest to highest average emissions 

per well at the company/sub-basin level, versus the percentage of total emissions. As expected 

with a highly skewed distribution, a small percentage of wells contribute a large percentage of 

total emissions. Over half the emissions from wells with and without plunger lifts are contributed 

by 8% and 7% of wells, respectively. Overall, the top 10% highest emitting wells contribute 65% 

of total emissions. 

There also are some geologic basins that contribute a large proportion of national 

emissions. For wells without plunger lifts, 55% of emissions are from four basins – the 

Anadarko, Appalachian Eastern Overthrust Belt, Arkoma, and East Texas. For wells with 

plunger lifts, 54% of emissions are from a single basin – the San Juan. 
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Figure 1. Cumulative probability function of GHGRP 2012 data for unloading wells that vent without 

and with plunger lifts. Emissions per well are averaged at the sub-basin/company level with the 

probability adjusted by the number of wells in each sub-basin/company. Emission factors are reported on 

a logarithmic scale.
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Figure 2. The relationship of the percentage of wells and the percentage of cumulative emissions from 

GHGRP 2012 data for unloading wells that vent without and with plunger lifts. Wells are ranked in 

ascending order of the average emissions per sub-basin/company with the probability adjusted by the 

number of wells in each sub-basin/company.

This combined discussion of average emissions factors and the distribution of emissions 

is important for several reasons. First, for wells with plunger lifts, we emphasize the conclusion 

from API’s data that approximately 79% of wells have no emissions. Moreover, for wells with 

plunger lifts that vent, GHGRP data shows that approximately 60% of wells in the lower end of 

the distribution have lower average emissions than non-plunger lift wells. We discuss more about 

the effectiveness of plunger lift technology, below, but here underscore that an average emission 

factor based solely on venting wells does not accurately characterize the effectiveness of this 

control technology for eliminating emissions from the majority of wells. Non-plunger lift 

equipped wells are a substantial source of emissions and represent an important opportunity for 

mitigation, but there also is opportunity to reduce emissions from the highest emitting wells 

already equipped with plunger lifts. 

C. Analysis and Critique of National Liquids Unloading Emissions Estimate 

(Charge Questions #1, 2) 

With the exception of the UT Study, the remaining sources evaluated in the White Paper 

include national estimates of liquids unloading emissions, ranging from the 273,568 MT CH4 in 
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the 2014 GHG Inventory to 321,012 MT CH4 in the ICF report.1 Since the GHGRP only 

includes emissions from facilities above the 25,000 MT CO2e reporting threshold, it should not 

be considered a comprehensive estimate of national emissions. The 2014 GHG Inventory 

estimate is lower than the total emissions reported to the GHGRP, so it also clearly 

underestimates national emissions. ICF’s estimate of 321,012 MT CH4 is derived by scaling up 

an estimate derived from GHGRP data by 15%, based on EPA’s estimate that 85 to 90% of 

emissions are covered by the GHGRP. The API/ANGA estimate of 319,664 MT CH4, which is 

similar to ICF’s estimate, was estimated using engineering equations along with survey data. 

All four of these national estimates are based on analyses of two underlying data sources: 

the GHGRP and API/ANGA survey, which estimated emissions using engineering equations 

analogous to GHGRP Methods 2 & 3. The UT Study reported that Method 2 did not always 

accurately estimate emissions of the 9 measured manual unloading events. Due to the small 

sample size of the UT study, it is premature to generalize the inaccuracy of the GHGRP methods 

for all unloading wells, but it does suggest that more empirical data is needed to test and possibly 

update the engineering equations. An additional reason for the uncertainty of the national 

estimates is the highly skewed distribution of the emissions, since inaccuracy in the estimates of 

the highest emitting well could have a large impact on national emissions. For instance, if 

highest-emitting wells are more prevalent than the existing data suggest, national unloadings 

emissions could be substantially higher. At minimum, however, it is clear that liquids unloadings 

are a substantial source of methane emissions. 

Table 2. National estimates of number of unloading wells that vent and methane emissions 

Source 

Venting wells 

without 

plunger lifts 

(#) 

Venting wells 

with plunger 

lifts (#) 

Total venting 

wells (#) 

Venting wells 

without 

plunger lifts 

(Gg CH4) 

Venting wells 

with plunger 

lifts (Gg 

CH4) 

Total 

National 

Emissions 

(Gg CH4) 

GHGRP 26,411 32,252 58,663 76.6 199.8 276.4 

2014 GHG 

Inventory 37,307 23,503 60,810 154.7 118.9 273.6 

API/ANGA 28,863 36,806 65,669 NA NA 319.7 

ICF Report 31,113 44,286 75,399 287.8 35.3 321.0 

 

 

1 We note that national liquids unloading emissions have been subject to some significant revisions in the last 

several years. In 2011, EPA increased national estimates of emissions from liquids unloading in the United States 

Greenhouse Gas Inventory (GHG Inventory) to approximately 4,500,000 MT CH4, raising its estimated contribution 

to natural gas systems production sector emissions from 6% to 51%. Based on the above-described API/ANGA 

study, EPA subsequently revised this number to 256,900 MT CH4 in 2013. 
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II. TECHNOLOGY, EFFECTIVENESS & COST 

We generally believe that EPA’s White Paper accurately presents information on various 

mitigation technologies, including cost and effectiveness. This section emphasizes a few aspects 

of plunger lift technologies, briefly discusses alternative technologies, and finally addresses 

EPA’s charge questions on flaring. We conclude that primary techniques (plunger lifts and 

artificial lift) alone, or in combination with remedial techniques, can often entirely eliminate 

emissions associated with liquids unloading. 

A. Plunger Lifts (Questions 6, 7, 8, 9) 

In general, EPA has accurately presented existing information on plunger lift efficacy and 

cost. Here, we simply emphasize that the agency should consider the cost-reducing aspects of 

plunger lifts that it identified in the White Papers, including additional captured gas and 

improved productivity. And, in response to charge question 8, we note that the ICF report 

provides further support for the conclusion that venting a well removes only a portion of the 

liquids as compared to utilizing a plunger lift.2

 

Below, we provide additional information in support of EPA’s assessment that plunger 

lifts, when operated optimally, can almost entirely eliminate emissions associated with liquids 

unloading. We also briefly discuss national reduction potential. 

1. Plunger Lifts can Eliminate Emissions (Questions 6, 9) 

We agree with the agency’s assessment that plunger lifts can eliminate emissions if 

reservoir pressure is sufficient to lift the plunger and accumulated liquids to the top of the 

wellbore. The API/ANGA survey indicated that 78.9% of wells with plunger lifts do not vent 

and the ICF report notes that plunger lifts can be 100% effective. 

However, if reservoir pressure is insufficient then the plunger will become stuck in the 

wellbore. Typically, this situation is resolved by venting the well to atmosphere, which removes 

the pressure from the pipeline. Even in situations where reservoir pressure is insufficient, 

though, the ICF Report finds that plunger lifts usually reduce emissions by 90% when compared 

to non-plunger lift equipped wells that vent.3

 

Even so, the GHGRP data indicates that certain wells with plunger lifts have very high 

emissions. EPA requests comment on the reason for these emissions, and we believe they are 

 

2

ICF International, Economic Analysis of Methane Emission Reduction Opportunities in the U.S. Onshore Oil and 

Natural Gas Industries 5-7 (Mar. 2014) [hereinafter “ICF Report”], available at 

http://www.edf.org/sites/default/files/methane_cost_curve_report.pdf. (“However, studies have shown that simply 

venting wells to the atmosphere only removes about 15% of the accumulated fluids in the well. This results in the 

need to frequently vent the well, as well as diminished overall production from the well. Of course it also results in 

significant methane emissions and lost gas revenue.”). 

3

 ICF Report at 5-7 to 5-8.
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frequently due to plunger lifts that are not operated in an optimal fashion or are malfunctioning. 

The ICF report provides an explanation: 

For example, plunger lifts are generally used to plunge the liquids in the gas well to the 

sales line. This requires that the pressures and flowrates in the well be closely monitored 

to ensure that sufficient energy exists to bring the plunger up during its cycle. If the 

pressure is allowed to fall too low, the well will need to be opened to the atmosphere to 

increase the pressure gradient and allow the plunger to bring the liquids to the surface. 

Allowing this to occur will reduce the potential emission reductions. Further, since the 

well has been cleared of liquids, the gas will be producing at or near its maximum rate. If 

the well remains open to the atmosphere after plunger, which is common, the increased 

gas flow rate may negate or possibly even exceed emissions from venting a non‐plunger 

well to the atmosphere.4

Smart automation systems can minimize emissions from wells with plunger lifts that are 

operating in an inefficient manner or malfunctioning. In particular, these computerized programs 

can monitor pressures in the tubing and casing, identifying changes in these values that 

determine when the plunger lift should be deployed. Using improved algorithms for optimizing 

plunger lift performance, BP has reduced 4 Bcf/y of liquids unloading emissions in the San Juan 

Basin by over 99%.5 In addition to improving automation, BP notes the importance of training 

personnel to view venting as a last resort rather than first option for restoring production to wells 

impacted by liquids loading. Ultimately, we believe available data support EPA’s conclusion that 

smart automation systems can substantially reduce emissions from plunger lift systems that are 

operated manually or on predetermined schedules. 

2. National Mitigation Potential 

Though EPA does not specifically address national mitigation potential, we do so briefly 

here. The ICF Report projects 2018 emissions of 17 Bcf methane and a mitigation potential of 

1.6 Bcf. We think this is very conservative for two reasons: 

• First, for wells without plunger lifts, ICF assumes that plunger lift technology can 

only be applied to 30% of total emissions. For this subset of emissions, ICF 

applies a 95% mitigation potential. Though we agree that a 95% mitigation 

potential is appropriate, it is likely that the 30% number significantly understates 

the percentage of emissions (as opposed to wells) that controls can be applied to. 

As we describe above, emissions from non-plunger lift wells are highly skewed, 

with the top 10% of wells accounting for approximately 56% of emissions. Welldesigned policies can broadly and cost-effectively prevent emissions from this 

source category. 

 

4

 ICF Report at 5-9. 

5

 Reid Smith, BP. Managing Venting for Liquids Unloading. Natural Gas STAR Production Technology Transfer 

Workshop (February 11, 2014). http://www.epa.gov/gasstar/documents/workshops/denver2014/Managing_Venting.pdf 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014

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• Second, as ICF notes in its report, “there may be an opportunity for significant 

emission reduction through optimization of plunger lifts, which is not included [in 

the report] and would be additional to the reduction estimates this analysis 

provides for installation of new plunger lifts.”6 

 

B. Additional Mitigation Technologies (Question 4) 

In addition to plunger lifts, there are several options for temporarily mitigating liquids 

unloading, which EPA refers to as remedial technologies. These include installing velocity 

tubing strings to increase gas velocity or injecting surfactants to lower liquid surface tension and 

density. Both of these technologies can remove liquids with zero emissions. However, installing 

velocity tubing requires a workover rig and surfactants do not work well with hydrocarbon 

liquids, so these technologies may not always be applicable. Finally, EPA reviews artificial lift 

as a primary solution to liquids unloading. Artificial lift is necessary to maintain production in 

wells in which the reservoir production is no longer sufficient to lift liquids even with a plunger 

lift or remedial technologies. External energy is applied using equipment such as a beam pump 

to remove liquids from the wellbore. These technologies eliminate venting but may have 

associated emissions from the equipment providing energy such as gas powered engines or 

turbines. 

There are several additional technologies for liquids unloading that were not evaluated in 

the white paper. The Artificial Lift Research & Development Council (ALRDC) maintains a 

website with extensive information on existing and developing technologies for well 

deliquification.7

 It is outside the scope of this review to comprehensively summarize the 

technologies presented by ALRDC, but there are a few technologies that bear mentioning due to 

their potential wide-scale applicability. One example is installing compressor engines at well 

pads or gathering stations to decrease pipeline pressure at the well head, which can reduce 

liquids loading by increasing the pressure differential between the reservoir and pipeline. 

Another example is intermittently shutting in a well to allow reservoir pressure to increase and 

force liquids back into the formation – although this method is not appropriate for formations 

that can be damaged by re-injecting water, it can be an inexpensive option for many wells in 

early stages of liquids loading.8

C. Flaring Liquids Unloading Emissions (Questions 11, 12) 

Liquids unloading emissions are intermittent and therefore a permanent flare is unsuitable 

for controlling emissions without a separate fuel supply to maintain a pilot flame. However, 

mobile flares may be a cost-effective solution for controlling emissions. A consultant prepared a 

analysis for EDF summarizing the available technologies and cost effectiveness of mobile flares 

for controlling liquids unloading emissions. Trailer and skid-mounted flare systems designed to 

 

6

 ICF Report at 3-18 

7

 http://www.alrdc.com/ 

8

 Artificial Lift Research & Development Council. Guidelines and Recommended Practices for Selection of

Artificial Lift for Gas Well Deliquification, available at 

http://www.alrdc.com/recommendations/gas%20well%20deliquification/artificial%20lift%20selection.htm 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014

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handle flow rates of 1 MMcf/day are available to rent for approximately $250/day or to purchase 

for $30,000. The cost-effectiveness of mobile flares, discounting labor cost since staff typically 

also would be required for uncontrolled manual unloadings, is estimated to be $3 to $12/Mcf.