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- 0122 MEMORANDUM

DATE: July 28 , 2011

SUBJECT: Composition of Natural Gas for use in the Oil and Natural Gas Sector

Rulemaking

FROM: Heather P. Brown, P.E.

TO: Bruce Moore, EPA/OAQPS/SPPD

The purpose of this memorandum is to document the development of a representative

natural gas composition for use in the oil and natural gas sector rulemaking

. This composition

will be used to determine hazardous air pollutant ( HAP ) and volatile organic compound ( VOC )

emissions from several segments of the oil and natural gas sector.

Gas composition data was compiled from several sources across the industry

. Th e

following is a list of the sources of data used for this analysis:

 CENRAP database

. “Recommendations for Improvements to the CENRAP States’ Oil

and Gas Emissions Inventory”, November 13, 2008. Covers the following States: Texas,

Louisiana, Arkansas, Ok lahoma, Kansas, Nebraska, Missouri, Iowa, and Minnesota

 GTI Database

. “GTI’s Gas Resource Database, Second Edition

– August 2001”

 TX Barnett Shale

. “Emissions from Natural Gas Production in the Barnett Shale Area and

Opportunities for Cost

- Effective Improv ements”, January 26, 2009

 INGAA/API Compendium

. “Greenhouse Gas Emission Estimation Guidelines f or

Natural Gas Transmission a nd Storage Volume 1

– GHG Emission Estimation

Methodologies and Procedures” September 28, 2005

 GOADS Offshore

. “Year 2005 Gulfwide Emission Inventory Study” December 2007

 NREL LCA

. “Life Cycle Assessment of a Natural Gas Combined

- Cycle Power

Generation System” September 2000

 Union Gas

. Chemical Composition of Natural Gas found online at

http://www.uniongas.com/aboutus/aboutng/composition.asp

 Marcellus

. “Supplemental Generic Environmental Impact Statement On The Oil, Gas and

Solution Mining Regulatory Program

- Well Permit Issuance for Horizontal Drilling And

High

- Volume Hydraulic Fracturing to Develop the Marcellus Shale and Other Low

Permeability Gas Reservoirs” September 2009

 Wyoming DEQ

. Speciation of Natural Gas and Condensate

. Courtesy of Cynthia

Madison, Wyoming DEQ2

Table s 1 and 2 present a summary of the m ethane, VOC, and HAP contents provided in

the above data sources for the production and transmission sectors, respectively, along with an

identification of the basins/areas of the country covered by the gas composition.

In addition to the above, ga s composition d ata were collected from the industry in 1995

during the development of the original maximum achievable control technology ( MACT )

standards for this sector

. These data are presented in Tables 3 and 4 for production and

transmission, respectiv ely. 1 This 1995 GRI data represents gas samples from across the United

States.

Gas Composition for Pneumatics, Equipment Leaks, and Compressors

Tables 1 and 2 also present a comparison of the 1995 GRI data to the other data sources

. For production, the 1995 GRI data is well within the ranges of the other data sources which

range from 1.19 to 11.6 percent for VOC by volume

. The 1995 GRI data is also within the 95

percent confidence interval of the production data which range from 2.81 to 7.82 percent vol ume

for VOC . O f the data sources that provide data on HAP emissions, the GRI data represent gas

compositions across the United States, while the CENRAP, TX Barnett, and Marcellus data are

specific to the regions specified in Tables 1 and 2 . In addition, it can be expected that the gas

composition for pneumatic controllers, equipment leaks, and compressors associated with these

emissions units are associated with gas from oil wells and gas wells making the range of VOC

composition widely varied. Therefore, i t was determined that the 1995 GRI data was appropriate

to use to develop a representative gas composition for pneumatic controllers, equipment leaks,

and compressors

.

For the transmission sector, the average 1995 GRI VOC concentration of 0.89 percent

vol ume was compared to other data sources and was found to be in the range of the VOC

composition, which ranged from 0.29 to 6.84 percent VOC by volume

. It was determined that

the 1995 GRI gas composition would be used to represent the average composition of natural gas

in the transmission sector, because the other data sources represented natural gas compositions

outside the U.S

. i

The gas compositions from the 1995 GRI data were then converted to weight percents

. First, because the average volume percent w as not equal to 100, the volume percents were

normalized for each component

. Then the weight of each component present in the gas was

calculated using the molecular weight (MW) for each component in pounds per pound mole

(lb/lbmol) and an assumed gas volum e of 385 cubic feet ( ft 3 ) , which represents one pound mole

of gas

. Finally, relative weight percents for each component were calculated

. These weight

percents are presented in Table 5

.

i It should be noted that the GRI data contains a statement that the BTEX data are “skewed toward high BTEX and

VOC content gases....” However, the 1995 GRI data are within the ranges of the other data and very close to the

average of other data identified. Therefore, these data were determined t o be appropriate to use to develop a

representative gas composition for pneumatics, equipment leaks and compressors.3 Table 1

. Gas Composition (volume %) for Production Sector

Data Source a Source of Natural Gas Area Covered Volume %

Methane VOC HAP

CENRAP b Conventional Gas Wells 11 Basins: Louisiana Mississippi Salt,

Southern Oklahoma, Nemaha Uplift,

Arkoma, Cambridge Arch Central Kansas

Uplift, Fort Worth, Cherokee Platform,

Permian, East TExas, Western Gulf, and

Anadarko 87.8 3.50 0.019

GTI Database c Gas Wells Nationwide, proven reserves, and

undiscovered reserves data from 462

basins/formations 82.8 3.61 n/a

INGAA Unprocessed Natural

Gas Unknown 80.0 5.00 n/a

NREL LCA d Gas Well Worldwide 65.7 5.66 n/a

MARCELLUS e Gas Well Marcellus 97.2 2.02 0.03345

WYOMING

DEQ b Gas Well Wyoming 92.4 1.19 0.08

Minimum 65.7 1.2 0.0

Maximum 97.2 5.7 0.1

Average 84.3 3.50 0.0

Gas

Composition Production Nationwide 83.1 3.66 0.164

n/a = not available

a Data from the Barnett Shale database was not speciated and therefore not included in this analysis.

b HAP data contains BTEX and n

- Hexane

c HAP Speciation not provided; hexan e s reported as Hexanes Plus

d Data provided were ranges for each pollutant (min and max). These values represent normalized averages of these

values and may not be valid representations

e HAP data only reported for hexane4 Table 2. Gas Composition (volume %) for Transmission Sector

Data Source Source of Natural Gas Area Covered Volume %

Methane VOC HAP

INGAA Pipeline Gas Unknown 91.9 6.84 n/a

GOADS

Offshore a Sales Gas Offshore Gas in the Gulf of Mexico 94.5 1.27 0.099

NREL LCA Pipeline Gas Worldwide 94.4 0.90 n/a

Union Gas Pipeline Gas United States, Western Canada, and Ontario 95.2 0.29 n/a

Minimum 91.9 0.3 0.099

Maximum 95.2 6.8 0.099

Average 94.0 2.3 0.099

GRI

- MACT Transmission/Unknown Nationwide 92.7 0.89 0.014

n/a = not available

a HAP data contains BTEX and n

- Hexane5 Table 3

. 1995 MACT Correspondence with GRI & EC/R

- Production Da ta

Sector Production

Site GRI1 GRI2 GRI3 GRI4 GRI5 GRI6 GRI7 GRI8 GRI9 GRI10 GRI11 GRI12

Mole %

Nitrogen 2.72 0.44 0.78 0.46 0.79 1.52 1.18 1.74 1.90 1.30 0.52 6.81

Carbon Dioxide 0.04 0.90 0.29 3.37 1.00 0.38 1.67 0.68 0.00 0.47 0.54 8.12

Methane 95.60 93.26 90.62 56.62 80.40 78.38 79.55 74.67 83.90 91.93 88.40 79.83

Ethane 1.04 3.16 4.31 10.87 10.41 10.88 10.40 12.57 7.90 3.80 7.25 2.89

Propane 0.33 1.14 1.90 13.90 4.25 5.41 4.15 5.98 3.86 1.23 1.53 0.94

Butanes 0.16 0.64 1.15 8.59 1.65 2.10 1.74 2.55 1.70 0.70 0.90 0.54

Pentanes 0.07 0.22 0.51 3.61 0.65 0.77 0.69 1.21 0.49 0.24 0.36 0.30

Hexanes+ 0.03 0.20 0.37 2.03 0.60 0.36 0.43 0.35 0.17 0.24 0.42 0.52

ppmv

n

- Hexane 88.7 277 664 2783 965 1173 937 2125 517 307 510 681

Isooctane 8.0 31.5 63.5 1552 151 145 112 103 52.0 49.6 32.0 87.0

Benzene 4.9 257 218 328 294 74.4 294 102 57.9 143 617 196

Toluene 2.9 108 117 251 468 92.4 263 31.4 45.6 142 222 213

Ethylbenzene 0 19.7 6.7 27.3 14.5 4.3 3.3 0.8 1.2 11.2 9.0 10.4

m,p

- Xylenes 0 34.0 26.6 26.0 87.9 21.7 16.7 1.7 7.3 56.6 45.0 66.0

o

- Xylene 0 19.9 5.0 6.2 16.1 3.2 2.4 0.3 0.6 16.9 10.0 16.4

NR = Not Reported6 Table 4

. 1995 MACT Correspondence with GRI & EC/R (Transmission Data)

Sector Transmission Unknown a Transmission Unknown a Transmission

Site GRI13 GRI14 GRI15 GRI16 GRI17 GRI18 GRI19 GRI20 GRI21 GRI22 GRI23 GRI24

Mole %

Nitrogen 9.89 8.68 2.96 2.55 0.22 1.25 1.16 1.1 1.15 1.12 0.3 1.85

Carbon Dioxide 0.28 0.40 0.58 0.54 0.35 2.62 0.15 0.12 0.07 1.06 1.36 0.66

Methane 81.97 82.61 91.8 92.7 97.4 95.4 98.5 88.2 81.1 94.6 95.8 93

Ethane 6.84 7.06 3.68 3.35 1.94 0.31 0.09 9.69 11.8 2.81 2.03 3.13

Propane 0.78 0.99 0.59 0.52 0.042 0.075 0.005 0.67 3.95 0.155 0.4 0.8

Butanes 0.14 0.17 0.159 0.148 <0.006 0.059 <0.006 0.035 1.189 0.116 0.075 0.314

Pentanes 0.04 0.05 0.045 0.042 <0.003 0.039 <0.003 <0.003 0.341 0.039 0.014 0.132

Hexanes+ 0.04 0.03 0.042 0.042 0.004 0.202 <0.002 <0.002 0.226 0.129 0.015 0.103

ppmv

n

- Hexane 63.2 66.9 NR NR NR NR NR NR NR NR NR NR

Isooctane 17.5 14.5 NR NR NR NR NR NR NR NR NR NR

Benzene 5.0 7.9 51 36 <0.2 471 <0.2 <0.2 10 <0.2 4.5 15

Toluene 5.1 8.1 16 13 <0.1 100 <0.1 <0.1 13 <0.1 3.7 14

Ethylbenzene 0.5 0.6 3 3 <0.1 15 <0.1 < 0.1 9 <0.1 0.1 1

m,p

- Xylenes [1] 1.4 2.2 12 7 <0.1 11 <0.1 <0.1 1 <0.1 0.6 3

o

- Xylene [1] 0.4 0.4

[1] Sites 15

- 36 reported only a total xylene result that includes all xylene isomers.

NR = Not Reported

a Based on the high methane content (greater than 90 percent) of this datapoint, it was assumed that they were samples from the transmission

segment.7 Table 4

. 1995 MACT Correspondence with GRI & EC/R

- Transmission Data

(Continued)

Sector Transmission Unknown a

Site GRI25 GRI26 GRI27 GRI28 GRI29 GRI30 GRI31

Mole %

Nitrogen 1.24 1.75 1.02 1.04 0.49 0.42 0.54

Carbon Dioxide 0.3 0.13 0.44 0.65 1.76 0.87 0.92

Methane 90.2 97.8 96.6 96.1 95.5 96 95.7

Ethane 7.02 0.26 1.78 1.86 1.74 2 2.12

Propane 1 0.014 0.091 0.213 0.351 0.413 0.414

Butanes 0.146 <0.006 0.025 0.06 0.093 0.181 0.175

Pentanes 0.03 0.0015 0.0089 0.0218 0.0354 0.0675 0.0665

Hexanes+ 0.021 0.0037 0.0052 0.0219 0.0322 0.073 0.069

ppmv

n

- Hexane NR NR NR NR NR NR NR

Isooctane NR NR NR NR NR NR NR

Benzene 9 1.2 0.8 6 7 59 58

Toluene 13 0.4 <0.4 6 6 23 26

Ethylbenzene <0.3 0.3 <0.1 0.3 0.5 1.8 2

m,p

- Xylenes [1] 4 0.2 <0.1 1 1.5 7 5

o

- Xylene [1]

[1] Sites 15

- 36 reported only a total xylene result that includes all xylene isomers.

NR = Not Reported

a Based on the high methane content (greater than 90 percent) of this datapoint, it was

assumed that they were samples from the transmission segment.8 Table 5

. Gas Composition Conversion to Weight Percent

Component MW

(lb/lbmol) Production Transmission

Avg

Vol

% b Normalized

Vol % Weight per

385 ft 3 Gas

(lbs) Weight

% Avg

Vol

% b Normalized

Vol % Weight per

385 ft 3 Gas

(lbs) Weight

%

Carbon Dioxide 44.01 1.46 1.5% 0.002 3.2% 0.70 0.70% 0.001 1.8%

Nitrogen 28.02 1.68 1.7% 0.001 2.3% 2.04 2.0% 0.001 3.3%

Methane 16.04 82.76 82.9% 0.035 65.7% 92.68 92.8% 0.039 86.2%

Ethane 30.07 7.12 7.1% 0.006 10.6% 3.66 3.7% 0.003 6.4%

Propane 44.09 3.72 3.7% 0.004 8.1% 0.60 0.60% 0.001 1.5%

Butane 58.12 1.87 1.9% 0.003 5.4% 0.16 0.16% 0.000 0.55%

Pentane 72.15 0.76 0.76% 0.001 2.7% 0.05 0.052% 0.000 0.22%

n

- Hexane 86.17 0.09 0.092% 0.000 0.39% 0.01 0.0065% 0.000 0.032%

Other hexanes 86.17 0.32 0.32% 0.001 1.4% 0.001 0.00086% 0.000 0.0043%

Isooctane

- a 114.23 0.02 0.020% 0.000 0.11% 0.002 0.0016% 0.000 0.011%

Benzene 78.11 0.02 0.022% 0.000 0.083% 0.004 0.0039% 0.000 0.018%

Toluene 92.14 0.02 0.016% 0.000 0.074% 0.001 0.0013% 0.000 0.0070%

Ethylbenzene 106.17 0.001 0.00090% 0.000 0.0047% 0.0002 0.00020% 0.000 0.0012%

Xylene 106.17 0.004 0.0041% 0.000 0.021% 0.0003 0.00030% 0.000 0.0019%

Total 99.85 100.0% 0.053 100.0% 99.91 100.0% 0.045 100.0%

a

- Isooctane = 2,2,4, Trimethylpentane

b

- Average of all gas compositions presented in Tables 1 and 2 for production and transmission, respectively.9

Once the weight percents were calculated for each natural gas component, relative ratios were

calculated for methane:total organic compounds (TOC), VOC:TOC, HAP:TOC, VOC:Methane,

HAP:Methane, BTEX:Methane, HAP:VOC, and BTEX:VOC. These relative ratios are p resented

in Table 6.

Natural Gas Composition for Completions and Recompletions

The gas composition for completions and recompletions from gas wells were determined

by performing a sensitivity analysis on the compositions of the gas well data using a lar ger

sample size which included data from hydraulically fractured wells. The results of this analysis

are shown in Table 7. A mean of 3.63 percent VOC with a 95 percent confidence interval that

ranges from 3.30 to 3.96 percent VOC by volume was determined. Based on the summary

statistics, these data appear to be reasonable for use in developing an average natural gas

composition to use for completions and recompletions of gas wells.

Once it was determined that this data was appropriate, the average gas co mposition was

calculated and then normalized so that the total volume percent equaled 100. This average gas

composition is presented in Table 8. The gas composition data was then converted to weight

percent by normalizing the volume percent for each compo nent, then calculating the weight of

each component using the MW for each component in lb/lbmol and a standard gas volume of

385 ft 3 . Finally, relative weight percents for each component were calculated. Once the weight

percents were calculated for each na tural gas component, relative ratios were calculated for

methane:total organic compounds (TOC), VOC:TOC, HAP:TOC, VOC:Methane,

HAP:Methane, BTEX:Methane, HAP:VOC, and BTEX:VOC. These relative ratios are presented

in Table 9.

A similar analysis was perfor med for completions and recompletions from oil wells

. The

results of this analysis are presented in Table 10. The average VOC composition was

11.62 percent by volume, with a 95 percent confidence interval that ranges from 6.73 to

16.5 percent VOC by volume

. As was done for gas wells, the average composition was

normalized. The gas composition used for completions and recompletions for oil wells is

presented in Table 8. The gas composition data was converted to weight percent using the same

approach detail ed for gas wells and are presented in Table 9.10 Table 6

. Weight Ratios to Use in Estimating Emissions

Production Transmission

Methane:TOC a 0.695 0.908

VOC b :TOC a 0.193 0.0251

HAP:TOC a 0.00728 0.000746

VOC b :Methane 0.278 0.0277

HAP:Methane 0.0105 0.000822

BTEX:Methane 0.00280 0.000322

HAP:VOC b 0.0377 0.0297

BTEX:VOC b 0.0101 0.0116

a TOC = all organic compounds listed in Table 3.

b VOC = all organic compounds listed in Table 3, except ethane and methane.

Table 7

. Summary Statistics of Sensitivity Analysis on Gas Composition for Gas Well and

Hydraulically Fractured Wells

Methane VOC

Mean 83.238 Mean 3.630

Standard Error 0.709 Standard Error 0.170

Median 86.581 Median 3.104

Mode 0 Mode 0.000

Standard Deviation 15.207 Standard Deviation 3.626

Sample Variance 231.244 Sample Variance 13.149

Kurtosis 12.943 Kurtosis 9.258

Skewness

- 3.08 Skewness 2.262

Range 99.75 Range 29.560

Minimum 0 Minimum 0.000

Maximum 99.748 Maximum 29.560

Sum 38289.387 Sum 1655.427

Count 460 Count 456.000

Confidence Level(95.0%) 1.393 Confidence Level(95.0%) 0.334

Volume

Percent Volume

Percent

(Lower of 95% conf interval) 81.844 (Lower of 95% conf interval) 3.297

Methane 83.238 VOC 3.630

(Higher of 95% conf interval) 84.631 (Higher of 95% conf interval) 3.96411

Table 8. Average Gas Composition for Completions and Recompletions of Gas and Oil

Wells

Average Volume Percent

Pollutant Gas Wells Oil Wells

Carbon dioxide (CO2) 1.631 1.00162

Nitrogen (N2) 4.455 29.19

Methane (C1) 83.081 46.73

Ethane (C2) 4.924 10.17

Propane (C3) 2.144 6.62

i

- Butane (i

- C4) 0.348 1.067004

n

- Butane (n

- C4) 0.643 2.136346

i

- Pentane (iC5) 0.095 0.550849

n

- Pentane (nC5) 0.119 0.515798

Cyclopentane 0.005 0.001091

n

- Hexane (n

- C6) 0.155 0.005182

Hexanes (C6) 0.000

-

Cyclohexane 0.001 0.001455

Other Hexanes 0.010 0.007636

Methylcyclohexane 0.002 0.001818

C6+ Heavies 0.114

-

Heptanes (C7) 0.009 0.697080

n

- Heptanes (C7) 0.000 0.001909

C8+ Heavies 0.004 0.005182

Benzene 0.005 0.006182

Toluene 0.003 0.000223

Ethylbenzene 0.000 0.000445

Xylenes 0.001

-

2,2,4

- Trimethylpentane 0.000 0.000223

Helium 0.140

-

Oxygen 0.084

-

Hydrogen 0.001 0.575909

Hydrogen disulfide (H2S) 2.027 0.709092

Total 100 100

VOC 3.66 11.6212

Table 9

. Weight Ratios to Use in Estimating Emissions for Completion and Recompletions

Gas Wells Oil Wells

Methane:TOC a 0.796 0.4453

VOC b :TOC a 0.116 0.3729

HAP:TOC a 0.0084 0.0006

VOC b :Methane 0.146 0.8374

HAP:Methane 0.0106 0.0001

BTEX:Methane 0.0006 0.0007

HAP:VOC b 0.0726 0.0016

BTEX:VOC b 0.0040 0.0009

a TOC = all organic compounds listed in Table 3.

b VOC = all organic compounds listed in Table 3, except ethane and methane.

Table 10

. Summary Statistics of Sensitivity Analysis on Gas Composition for Oil Wells

Methane VOC

Mean 46.73157 Mean 11.61755

Standard Error 4.196101 Standard Error 2.193276

Median 49.63115 Median 9.697621

Mode 49.63115 Mode #N/A

Standard Deviation 19.68146 Standard Deviation 7.274275

Sample Variance 387.3598 Sample Variance 52.91508

Kurtosis 1.385922 Kurtosis 1.438744

Skewness

- 1.15094 Skewness 1.127773

Range 71.93094 Range 25.91599

Minimum 0.156 Minimum 1.381007

Maximum 72.08694 Maximum 27.297

Sum 1028.095 Sum 127.793

Count 22 Count 11

Confidence Level(95.0%) 8.72627 Confidence Level(95.0%) 4.886924

(Lower of 95% Conf interval) 38.0053 (Lower of 95% Conf interval) 6.730621

Methane 46.73157 VOC 11.61755

(Higher of 95% Conf. Interval) 55.45784 (Higher of 95% Conf. Interval) 16.5044713

REFERENCES

1

. Letter and Attachments from Evans, J. M., Gas Research Institute, to G. Viconovic, EC/R

Incorporated. Natural Gas BTEX Content. April 19, 1005. Legacy Docket Number A

94

- 04, Item II

- D

- 35.