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MEMORANDUM

DATE: August 23, 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. The

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 followi ng States: Texas,

Louisiana, Arkansas, Oklahoma, 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 Improvements”, 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, Wyo ming DEQ2

Table s 1 and 2 present a summary of the Methane, 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 g as composition.

In addition to the above, gas 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, respectively. The original submission from GRI is contained in Attachment 1 of

this memorandum. This 1995 GRI data represents gas samples from across the United States.

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

As shown in the tables, the 1995 GRI data is well within the ranges of the other data sources and

in fact for Production, the VOC volume percent is equal to the avera ge of the other data sources.

Further, of 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 Tabl es 1 and 2 . Therefore, it was determined that the 1995

GRI data was appropriate to use to develop a representative gas composition. It should be noted

that the attached memorandum in Attachment 1 contains a statement that the BTEX data are

“skewed toward high BTEX and VOC content gases....” However, as stated previously, Tables

1 and 2 show that the 1995 GRI data are within the ranges and very close to the average of other

data identified. Therefore, these data were determined to be appropriate to use to develop a

representative gas composition

.

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

First, because the average volume percent was 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 volume of 385 ft 3 , which represents one mole of gas. Finally,

relative weight percents for each component were calculated. These weight percents are

presented in Table 5

.

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,

VO C:Methane, HAP:Methane, BTEX:Methane, HAP:VOC, and BTEX:VOC. These relative

ratios are presented in Table 6

.3 Table 1. Gas Composition (volume %) for Production Sector4 Table 2. Gas Composition (volume %) for Transmission Sector5 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 * Transmission Unknown * 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

* 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 *

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

* 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 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.

### ATTACHMENT 1

Original data submitted by GRI