# Attachment I

Liquids Unloading

Environmental Defense Fund

Ramón Alvarez, Robert Harriss, David Lyon

### Oil and Natural Gas Sector: Liquids Unloading Proce sses

Peer Review Responses of Environmental Defense Fund

June 16, 2014

I. E XECUTIVE 

### S UMMARY

• Emissions from Liquids Unloading Activities are Sig nificant.

o Emissions Factors and Distribution. Data suggests liquids unloading emissions are highly skewed, with a small minority of wells c ontributing the majority of emissions. Approximately 65% of 2012 emissions rep orted to the Greenhouse Gas Reporting Program (GHGRP) were from 10% of well s.

o National Emissions. Several recent national emissions estimates suggest liquids unloading emissions are approximately 300 Gg CH 4 , making this activity a significant source of methane. Many of these estim ates are uncertain since inaccuracy of the prevalence or emissions of the hi ghest emitting wells could greatly impact national emissions estimates.

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

o Causes of Liquids Loading . At some point, most wells will experience liquid loading problems, which occurs when reservoir press ure is less than combined fluid and pipeline pressure and therefore unable to lift liquids out of the wellbore. Venting temporarily unloads liquids by reducing pip eline pressure to atmospheric pressure. However, venting depletes reservoir press ure and therefore exacerbates the problem and decreases ultimate hydrocarbon reco very and operational efficiency.

o Technologies. Plunger lifts can minimize venting during liquids u nloading by using a well’s own reservoir pressure to overcome p ressure differentials, but do not always lead to low emissions. Often, other sol utions are also available such as installing velocity tubing or using compressor engi nes to lower the operating pressure of the well and associated pipeline. When the aforementioned technologies are insufficient to lift liquids metho ds, artificial lift can successfully remove liquids from wells with little or zero emiss ions.

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 plung er lifts that do not vent. ICF projected that plunger lifts are capable of reducin g liquids unloading emissions by 95% at wells on which they are installed with a pos itive payback of $0.05/Mcf

methane reduced.E NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

2 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 operatio nal practices have been shown to reduce emissions by over 99% from these high emi ssion plunger lift equipped

wells.

o Mobile flaring technology is feasible and should be considered where other
technologies are inapplicable.E NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

3 

### I. Q UANTIFYING THE 

### P ROBLEM OF 

### M ETHANE 

### L EAKS FROM 

### L IQUIDS



### U NLOADING 

### A CTIVITIES 

### (Q UESTIONS 

### 1, 2, 3)

EPA’s Liquids Unloading White Paper presents data o n emissions factors from wells both with and without plunger lifts and likewise pr ovides 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 liqui ds unloading. Part II.B then assesses liquids unloading emissions factors, focusing both on the e missions factors reviewed in the White Paper, and, importantly, the distribution of emissions amo ng sources. Finally, Part II.C assesses national emissions estimates associated with liquids unloadi ng activities.

We conclude:

• Wells without plunger lifts have significant unload ing emissions, with an average emissions factor of approximately 2.9 metric tons m ethane per year (MT/y CH 4 ).

• Wells with plunger lifts can almost entirely elimin ate vented emissions from liquids unloading activities when operated optimally. Plung er lift equipped wells can vent, however, if the plunger becomes overloaded. Althoug h 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 liq uids unloading suggest these emissions are approximately 300,000 MT/y CH 4 . The skewed distribution of emissions leads to a high uncertainty of these esti mates since inaccurate estimates of emissions from the highest emitting wells could gre atly impact national emissions.



### A. Liquids Loading and Emissions Due to Unloading (Que stion 3)

Wells accumulate liquids when the reservoir gas pre ssure 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 p ressure declines as wells age, liquids loading eventually becomes an issue in most wells, but ther e is large variation in when and how often wells require liquids unloading. Sometimes operato rs 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 pr essure, then some of the liquids will be lifted out of the wellbore, temporarily restoring gas flow. H owever, gas will also be vented, which depletes reservoir pressure and therefore exacerbat es the problem in the long-term.E NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

4 Wells with plunger lifts can almost entirely elimin ate 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, ho wever, if the plunger becomes overloaded and fails to return to the top of the wellbore. Althoug h 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 Emission s Factors (Questions 1, 2)

EPA summarizes several sources for liquids unloadin g emission factors: Characterizing

Pivotal Sources of Methane Emissions from Natural G as 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, be low, summarize emissions factors from these sources, and other than GHGRP and ICF, reflect comb ined emission factors for venting wells with and without plunger lifts installed. The year ly emissions factor, in turn, is composed of two important drivers: the average amount of methane ve nted during each unloading event, and the number of times, on average, the well vents per yea r.

Table 1. Summary of Vented Emissions Factors for Un loading 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 rep orted

in 2012

GHGRP (non plunger lifts) 151,638 Average emission factor of 26,215 wells rep orted

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 wellsE NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

5 controlled with plunger lifts than wells without th ese 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, the n 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 cont rolled wells on average do indeed have fewer

emissions than uncontrolled wells.

In addition to these average emissions values, whic h are relatively similar across studies, the underlying data show substantial emissions vari ability among wells:

• GHG Inventory. The 2014 GHG Inventory regional emission factors, based on an EPA analysis of API/ANGA data reflect a 700x range (2,8 56 scfy/well for South West wells with plunger lifts to 1,998,082 scfy/well for Rocky Moun tain wells without plunger lifts).

• UT Study. The nine wells measured by the UT Study ranged fro m 1,900 to 1,337,000 scfy/well, with four of the nine events contributin g over 95% of total emissions.

• GHGRP Data. EDF has analyzed the GHGRP 2012 data to character ize 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 factor s 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 em issions per well from 2012 GHGRP data based on averages at the company/sub-basin lev el. Figure 1 is a cumulative probability function of emission factors, with the y-axis repre senting 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 plott ed on a logarithmic scale to more clearly display the distribution. For most of the distribut ion, wells without plunger lifts have higher average emissions than wells with plunger lifts, bu t for approximately the 60 th percentile and above, wells with a plunger lift have higher averag e 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 percenta ge 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 t op 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, Arko ma, and East Texas. For wells with plunger lifts, 54% of emissions are from a single b asin – the San Juan.E NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

6 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/co mpany level with the probability adjusted by the number of wells in each sub-basin/company. Emission factors are reported o n a logarithmic scale.E NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

7 Figure 2. The relationship of the percentage of wells and the percentage of cumulative emissions from GHGRP 2012 data for unloading wells that vent witho ut and with plunger lifts. Wells are ranked in ascending order of the average emissions per sub-ba sin/company with the probability adjusted by the number of wells in each sub-basin/company.

This combined discussion of average emissions facto rs 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 hav e no emissions. Moreover, for wells with plunger lifts that vent, GHGRP data shows that appr oximately 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 accur ately characterize the effectiveness of this control technology for eliminating emissions from t he majority of wells. Non-plunger lift equipped wells are a substantial source of emission s 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 s ources evaluated in the White Paper include national estimates of liquids unloading emi ssions, ranging from the 273,568 MT CH 4 inE NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

8 the 2014 GHG Inventory to 321,012 MT CH 4 in the ICF report. 1 Since the GHGRP only includes emissions from facilities above the 25,000 MT CO 2 e 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 o f 321,012 MT CH 4 is derived by scaling up an estimate derived from GHGRP data by 15%, based o n EPA’s estimate that 85 to 90% of emissions are covered by the GHGRP. The API/ANGA es timate of 319,664 MT CH 4 , which is similar to ICF’s estimate, was estimated using engi neering equations along with survey data.

All four of these national estimates are based on a nalyses of two underlying data sources: the GHGRP and API/ANGA survey, which estimated emis sions using engineering equations analogous to GHGRP Methods 2 & 3. The UT Study repo rted that Method 2 did not always accurately estimate emissions of the 9 measured man ual unloading events. Due to the small sample size of the UT study, it is premature to gen eralize the inaccuracy of the GHGRP methods for all unloading wells, but it does suggest that m ore empirical data is needed to test and possibly update the engineering equations. An additional rea son 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 tha t vent and methane emissions

Source Venting wells

without plunger lifts (#) Venting wells

with plunger lifts (#) Total venting

wells (#) Venting wells

without plunger lifts (Gg CH 4 ) Venting wells

with plunger lifts (Gg

CH 4 ) Total

National

Emissions (Gg CH 4 )
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 h ave been subject to some significant revisions in t he last several years. In 2011, EPA increased national est imates of emissions from liquids unloading in the U nited States Greenhouse Gas Inventory (GHG Inventory) to approxi mately 4,500,000 MT CH 4 , raising its estimated contribution to natural gas systems production sector emissions from 6% to 51%. Based on the above-described API/A NGA study, EPA subsequently revised this number to 256, 900 MT CH4 in 2013.E NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

9 

### II. T ECHNOLOGY 

### , E FFECTIVENESS 

### & C OST

We generally believe that EPA’s White Paper accurat ely presents information on various mitigation technologies, including cost and effecti veness. This section emphasizes a few aspects of plunger lift technologies, briefly discusses alt ernative technologies, and finally addresses EPA’s charge questions on flaring. We conclude tha t primary techniques (plunger lifts and artificial lift) alone, or in combination with reme dial 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 i nformation on plunger lift efficacy and cost. Here, we simply emphasize that the agency sh ould consider the cost-reducing aspects of plunger lifts that it identified in the White Paper s, 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 ve nting 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 reduct ion 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 plunge r 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 usu ally reduce emissions by 90% when compared to non-plunger lift equipped wells that vent. 3

Even so, the GHGRP data indicates that certain well s with plunger lifts have very high emissions. EPA requests comment on the reason for t hese emissions, and we believe they are

2 ICF International, Economic Analysis of Methane Emi ssion Reduction Opportunities in the U.S. Onshore O il 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 th at simply venting wells to the atmosphere only removes about 15% of the accumulated fluids in the well. This res ults in the need to frequently vent the well, as well as dimini shed 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.E NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

10 frequently due to plunger lifts that are not operat ed in an optimal fashion or are malfunctioning. The ICF report provides an explanation:

For example, plunger lifts are generally used to pl unge the liquids in the gas well to the sales line. This requires that the pressures and fl owrates in the well be closely monitored to ensure that sufficient energy exists to bring th e 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 plunge r to bring the liquids to the surface. Allowing this to occur will reduce the potential em ission reductions. Further, since the well has been cleared of liquids, the gas will be p roducing at or near its maximum rate. If the well remains open to the atmosphere after plung er, which is common, the increased gas flow rate may negate or possibly even exceed em issions from venting a non

‐ plunger well to the atmosphere. 4

Smart automation systems can minimize emissions fro m wells with plunger lifts that are operating in an inefficient manner or malfunctionin g. In particular, these computerized programs can monitor pressures in the tubing and casing, ide ntifying 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 t han 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 e missions from plunger lift systems that are operated manually or on predetermined schedules.

2. National Mitigation Potential

Though EPA does not specifically address national m itigation 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 tw o 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) t hat controls can be applied to. As we describe above, emissions from non-plunger li ft wells are highly skewed, with the top 10% of wells accounting for approximat ely 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/ga sstar/documents/workshops/denver2014/Managing_Venting.pdfE NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

11 • Second, as ICF notes in its report, “there may be a n opportunity for significant emission reduction through optimization of plunger lifts, which is not included [in the report] and would be additional to the reductio n 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 opt ions for temporarily mitigating liquids unloading, which EPA refers to as remedial technolo gies. These include installing velocity tubing strings to increase gas velocity or injectin g surfactants to lower liquid surface tension and density. Both of these technologies can remove liq uids with zero emissions. However, installing velocity tubing requires a workover rig and surfact ants do not work well with hydrocarbon liquids, so these technologies may not always be ap plicable. Finally, EPA reviews artificial lift as a primary solution to liquids unloading. Artifi cial lift is necessary to maintain production in wells in which the reservoir production is no longe r 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 technol ogies eliminate venting but may have associated emissions from the equipment providing e nergy such as gas powered engines or

turbines.

There are several additional technologies for liqui ds unloading that were not evaluated in the white paper. The Artificial Lift Research & Dev elopment 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 comprehe nsively summarize the technologies presented by ALRDC, but there are a fe w technologies that bear mentioning due to their potential wide-scale applicability. One exam ple is installing compressor engines at well pads or gathering stations to decrease pipeline pre ssure at the well head, which can reduce liquids loading by increasing the pressure differen tial between the reservoir and pipeline. Another example is intermittently shutting in a wel l to allow reservoir pressure to increase and force liquids back into the formation – although th is method is not appropriate for formations that can be damaged by re-injecting water, it can b e 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 th erefore a permanent flare is unsuitable for controlling emissions without a separate fuel s upply 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 technolo gies and cost effectiveness of mobile flares for controlling liquids unloading emissions. Trail er and skid-mounted flare systems designed to

6 ICF Report at 3-18

7 http://www.alrdc.com/

8 Artificial Lift Research & Development Council. Gu idelines and Recommended Practices for Selection of Artificial Lift for Gas Well Deliquification, available at http://www.alrdc.com/recommendations/gas%20well%20d eliquification/artificial%20lift%20selection.htmE NVIRONMENTAL D EFENSE F UND • J UNE 16, 2014

12 handle flow rates of 1 MMcf/day are available to re nt for approximately $250/day or to purchase for $30,000. The cost-effectiveness of mobile flare s, discounting labor cost since staff typically also would be required for uncontrolled manual unlo adings, is estimated to be $3 to $12/Mcf.