A-165 

3.5. Methodology for Estimating CH4 and CO2 Emissions from Petroleum 

Systems 

The methodology for estimating CH4 and non-combustion CO2 emissions from petroleum systems is based on 

the 1999 EPA draft report, Estimates of Methane Emissions from the U.S. Oil Industry (EPA 1999) and the study, Methane 

Emissions from the U.S. Petroleum Industry (EPA/GRI 1996). Sixty-four activities that emit CH4 and thirty activities that 

emit non-combustion CO2 from petroleum systems were examined from these reports. Most of the activities analyzed 

involve crude oil production field operations, which accounted for about 98 percent of total oil industry CH4 emissions. 

Crude transportation and refining accounted for the remaining CH4 emissions of less than one half and about one and a 

half percent, respectively. Non-combustion CO2 emissions were analyzed for production operations and asphalt blowing 

in refining operations. Non-combustion CO2 emissions from transportation operations are not included because they are 

negligible. The following steps were taken to estimate CH4 and CO2 emissions from petroleum systems. 

Step 1: Determine Emission Factors for all Activities 

The CH4 emission factors for the majority of the activities for 1995 are taken from the 1999 EPA draft report, 

which contained the most recent and comprehensive determination of CH4 emission factors for the 64 CH4-emitting 

activities in the oil industry at that time. Emission factors for pneumatic devices in the production sector were recalculated

in 2002 using emissions data in the EPA/GRI 1996c study. The gas engine emission factor is taken from the EPA/GRI 

1996b study. The oil tank venting emission factor is taken from the API E&P Tank Calc weighted average for API gravity 

less than 45 API degrees with the distribution of gravities taken from a sample of production data from the HPDI database. 

Offshore emissions from shallow water and deep water oil platforms are taken from analysis of the Gulf-wide Offshore 

Activity Data System (GOADS) report (EPA 2005, BOEMRE 2004). The emission factors determined for 1995 were 

assumed to be representative of emissions from each source type over the period 1990 through 2009. Therefore, the same 

emission factors are used for each year throughout this period. 

The CO2 emission factors were derived from the corresponding source CH4 emission factors. The amount of CO2

in the crude oil stream changes as it passes through various equipment in petroleum production operations. As a result, 

four distinct stages/streams with varying CO2 contents exist. The four streams that are used to estimate the emissions 

factors are the associated gas stream separated from crude oil, hydrocarbons flashed out from crude oil (such as in storage 

tanks), whole crude oil itself when it leaks downstream, and gas emissions from offshore oil platforms. The standard 

approach used to estimate CO2 emission factors was to use the existing CH4 emissions factors and multiply them by a 

conversion factor, which is the ratio of CO2 content to methane content for the particular stream. Ratios of CO2 to CH4

volume in emissions are presented in Table A- 5. The two exceptions are the emissions factor for storage tanks, which is 

estimated using API E&P Tank Calc simulation runs of tank emissions for crude oil of different gravities less than 45 API 

degrees; and the emissions factor for uncontrolled asphalt blowing, which is estimated using the data and methods 

provided by API (2009). 

Step 2: Determine Activity Levels for Each Year 

Activity levels change from year to year. Some factors change in proportion to crude oil rates: production, 

transportation, refinery runs. Some change in proportion to the number of facilities: oil wells, petroleum refineries. Some 

factors change proportional to both the rate and number of facilities. 

For most sources, activity levels found in the EPA/GRI 1996 for the 1995 base year are extrapolated to other 

years using publicly-available data sources. For the remaining sources, the activity levels are obtained directly from 

publicly available data and are not extrapolated from the 1995 base year. 

For both sets of data, a determination was made on a case-by-case basis as to which measure of petroleum 

industry activity best reflects the change in annual activity. Publicly-reported data from the Bureau of Ocean Energy 

Management, Regulation and Enforcement (BOEMRE), Energy Information Administration (EIA), American Petroleum 

Institute (API), the Oil & Gas Journal (O&GJ), the Interstate Oil and Gas Compact Commission (IOGCC), and the U.S 

Army Corps of Engineers (USACE) were used to extrapolate the activity levels from the base year to each year between 

1990 and 2009. Data used include total domestic crude oil production, number of domestic crude oil wells, total imports 

and exports of crude oil, total petroleum refinery crude runs, and number of oil-producing offshore platforms. The activity 

data for the total crude transported in the transportation sector is not available. In this case, all the crude oil that was 

transported was assumed to go to refineries. Therefore, the activity data for the refining sector was used also for the 

transportation sector. In the few cases where no data was located, oil industry data based on expert judgment was used. In 

the case of non-combustion CO2 emission sources, the activity factors remain the same as for CH4 emission sources. 
A-166 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009 

Step 3: Estimate Methane and Carbon Dioxide Emissions for Each Activity for Each Year 

Annual CH4 emissions from each of the 64 petroleum system activities and CO2 emissions from the 30 petroleum 

system activities were estimated by multiplying the activity data for each year by the corresponding emission factor. 

These annual emissions for each activity were then summed to estimate the total annual CH4 and CO2 emissions, 

respectively. Table A- 140 provides the CO2 content in natural gas for equipment in different crude streams to estimate 

CO2 emission factors using CH4 emission factors. 

Table A-136, Table A-137, Table A-138, and Table A-141 provide 2009 activity factors, emission factors, and 

emission estimates and Table A-139 and Table A-142 provide a summary of emission estimates for the years 1990, 1995, 

2000, and 2005 through 2009. Table A- 140 provides the CO2 content in natural gas for equipment in different crude 

streams to estimate CO2 emission factors using CH4 emission factors. 

Table A-136: 2009 CH4 Emissions from Petroleum Production Field Operations 

Activity/Equipment 

2009 EPA Inventory Values

Emission Factor Activity Factor Emissions

(Bcf/yr)

Emissions

(Gg/yr)

Vented Emissions 67.625 1,300

Oil Tanks 7.39 scf of CH4/bbl crude 1,490 MMbbl/yr (non stripper 

wells) 

11.016 211.8

Pneumatic Devices, High Bleed 330 scfd CH4/device 139,797 No. of high-bleed 

devices 

16.859 324.2

Pneumatic Devices, Low Bleed 52 scfd CH4/device 259,624 No. of low-bleed 

devices 

4.928 94.76

Chemical Injection Pumps 248 scfd CH4/pump 28,166 No. of pumps 2.550 49.04

Vessel Blowdowns 78 scfy CH4/vessel 182,451 No. of vessels 0.014 0.274

Compressor Blowdowns 3,775 scf/yr of CH4/compressor 2,473 No. of compressors 0.009 0.179

Compressor Starts 8,443 scf/yr. of CH4/compressor 2,473 No. of compressors 0.021 0.401

Stripper wells 2,345 scf/yr of CH4/stripper well 317,230 No. of stripper wells 

vented 

0.744 14.31

Well Completion Venting 733 scf/completion 11,804 Oil well completions 0.009 0.166

Well Workovers 96 scf CH4/workover 39,450 Oil well workovers 0.004 0.073

Pipeline Pigging 2.40 scfd of CH4/pig station 0 No. of crude pig 

stations 

- -

Offshore Platforms, Shallow water 

Oil, fugitive, vented and combusted 

54,795 scfd CH4/platform 1,441 No. of shallow water oil 

platforms 

28.823 554.3

Offshore Platforms, Deepwater oil, 

fugitive, vented and combusted 

260,274 scfd CH4/platform 28 No. of deep water oil 

platforms 

2.649 50.93

Fugitive Emissions 2.539 49

Oil Wellheads (heavy crude) 0.13 scfd/well 14,718 No. of hvy. crude wells 0.001 0.013

Oil Wellheads (light crude) 16.6 scfd/well 194,052 No. of lt. crude wells 1.178 22.65

Separators (heavy crude) 0.15 scfd CH4/separator 10,740 No. of hvy. crude seps. 0.001 0.012

Separators (light crude) 14 scfd CH4/separator 97,745 No. of lt. crude seps. 0.494 9.504

Heater/Treaters (light crude) 19 scfd CH4/heater 73,966 No. of heater treaters 0.518 9.965

Headers (heavy crude)1 0.08 scfd CH4/header 13,721 No. of hvy. crude hdrs. 0.000 0.007

Headers (light crude) 11 scfd CH4/header 42,536 No. of lt. crude hdrs. 0.169 3.241

Floating Roof Tanks 338,306 scf CH4/floating roof 

tank/yr. 

24 No. of floating roof 

tanks 

0.008 0.159

Compressors 100 scfd CH4/compressor 2,473 No. of compressors 0.090 1.736

Large Compressors 16,360 scfd CH4/compressor 0 No. of large comprs. - -

Sales Areas 41 scf CH4/loading 1,630,377 Loadings/year 0.066 1.271

Pipelines NE scfd of CH4/mile of 

pipeline 

11,249 Miles of gathering line NE NE

Well Drilling NE scfd of CH4/oil well 

drilled 

13,255 No. of oil wells drilled NE NE

Battery Pumps 0.24 scfd of CH4/pump 157,800 No. of battery pumps 0.014 0.266

Combustion Emissions 4.7926 92.22

Gas Engines 0.24 scf CH4/HP-hr 15,577 MMHP-hr 3.739 71.89

Heaters 0.52 scf CH4/bbl 1956.8 MMbbl/yr 1.020 19.61

Well Drilling 2,453 scf CH4/well drilled 13,255 Oil wells drilled 0.033 0.625
A-167 

Activity/Equipment 

2009 EPA Inventory Values

Emission Factor Activity Factor Emissions

(Bcf/yr)

Emissions

(Gg/yr)

Flares 20 scf CH4/Mcf flared 242,355 Mcf flared/yr 0.005 0.932

Process Upset Emissions 0.116 2.233

Pressure Relief Valves 35 scf/yr/PR valve 163,449 No. of PR valves 0.006 0.109

Well Blowouts Onshore 2.5 MMscf/blowout 44.2 No. of blowouts/yr 0.110 2.124

Total 75.08 1,444

- Zero Emissions 

1

 Emissions are not actually 0, but too small to show at this level of precision. 

 NE: Not estimated for lack of data 

Table A-137: 2009 CH4 Emissions from Petroleum Transportation 

Activity/Equipment 

Emission 

Factor Units 

Activity

Factor Units 

Emissions

(Bcf/yr)

Emissions

(Gg/yr)

Vented Emissions 0.209 4.024

Tanks 0.021 scf CH4/yr/bbl of crude delivered to 

refineries 

5,233 MMbbl crude feed/yr 

0.108 2.073

Truck Loading 0.520 scf CH4/yr/bbl of crude transported by 

truck 

67.8 MMbbl trans. by 

truck 0.035 0.678

Marine Loading 2.544 scf CH4/1000 gal. crude marine loadings 19,609,664 1,000 gal./yr loaded 0.050 0.959

Rail Loading 0.520 scf CH4/yr/bbl of crude transported by 

rail 

4.2 MMbbl Crude by 

rail/yr 0.002 0.042

Pump Station 

Maintenance1 

36.80 scf CH4/station/yr 496 No. of pump stations 

0.000 0.000

Pipeline Pigging 39 scfd of CH4/pig station 992 No. of pig stations 0.014 0.271

Fugitive Emissions 0.050 0.958

Pump Stations 25 scf CH4/mile/yr. 49,585 No. of miles of crude 

p/l 

0.001 0.024

Pipelines NE scf CH4/bbl crude transported by 

pipeline 

6,431 MMbbl crude piped NE NE

Floating Roof Tanks 58,965 scf CH4/floating roof tank/yr. 824 No. of floating roof 

tanks 

0.049 0.934

Combustion Emissions NE NE

Pump Engine Drivers 0.24 scf CH4/hp-hr NE No. of hp-hrs NE NE

Heaters 0.521 scf CH4/bbl burned NE No. of bbl Burned NE NE

Total 0.259 4.982 1

 Emissions are not actually 0, but too small to show at this level of precision. 

 NE: Not estimated for lack of data 

Table A-138: 2009 CH4 Emissions from Petroleum Refining 

Activity/Equipment 

2009 EPA Inventory Values

Emission Factor Activity Factor Emissions

(Bcf/yr)

Emissions

(Gg/yr)

Vented Emissions 1.065 20.48

 Tanks 20.6 scf CH4/Mbbl 1,839 Mbbl/calendar day heavy 

crude feed 0.014 0.266

 System Blowdowns 137 scf CH4/Mbbl 14,336 Mbbl/calendar day refinery 

feed 0.716 13.77

 Asphalt Blowing 2,555 scf CH4/Mbbl 359 Mbbl/calendar day 

production 0.335 6.445

Fugitive Emissions 0.088 1.702

 Fuel Gas System 439 Mscf CH4/refinery/yr 150 Refineries 0.066 1.266

 Floating Roof Tanks1 587 scf CH4/floating roof 

tank/yr. 

 767 No. of floating roof tanks 0.000

0.009

 Wastewater Treating 1.88 scf CH4/Mbbl 14,336 Mbbl/calendar day refinery 

feed 

0.010

0.189

 Cooling Towers 2.36 scf CH4/Mbbl 14,336 Mbbl/calendar day refinery 

feed 

0.012

0.237

Combustion Emissions 0.090 1.723

 Atmospheric Distillation 3.61 scf CH4/Mbbl 14,659 Mbbl/calendar day refinery 

feed 0.019 0.371

 Vacuum Distillation 3.61 scf CH4/Mbbl 6,503 Mbbl/calendar day feed 0.009 0.165
A-169 

Activity/Equipment 

2009 EPA Inventory Values

Emission Factor Activity Factor Emissions

(Bcf/yr)

Emissions

(Gg/yr)

fugitive, vented and combusted platforms 

Fugitive Emissions 0.052 2.776

Oil Wellheads (heavy crude)1 0.003 scfd/well 14,718 No. of hvy. crude wells 0.000 0.001

Oil Wellheads (light crude) 0.337 scfd/well 194,052 No. of lt. crude wells 0.024 1.267

Separators (heavy crude)1 0.003 scfd CO2/separator 10,740 No. of hvy. crude seps. 0.000 0.001

Separators (light crude) 0.281 scfd CO2/separator 97,745 No. of lt. crude seps. 0.010 0.531

Heater/Treaters (light crude) 0.319 scfd CO2/heater 73,966 No. of heater treaters 0.009 0.456

Headers (heavy crude)1 0.002 scfd CO2/header 13,721 No. of hvy. crude hdrs. 0.000 0.000

Headers (light crude) 0.220 scfd CO2/header 42,536 No. of lt. crude hdrs. 0.003 0.181

Floating Roof Tanks1 17,490 scf CO2/floating roof 

tank/yr. 

24 No. of floating roof tanks 0.000 0.023

Compressors 2.029 scfd CO2/compressor 2,473 No. of compressors 0.002 0.097

Large Compressors 332 scfd CO2/compressor 0 No. of large comprs. 0.000 0.000

Sales Areas 2.096 scf CO2/loading 1,630,377 Loadings/year 0.003 0.181

Pipelines NE scfd of CO2/mile of 

pipeline 

11,249 Miles of gathering line NE NE

Well Drilling NE scfd of CO2/oil well 

drilled 

13,255 No. of oil wells drilled NE NE

Battery Pumps 0.012 scfd of CO2/pump 157,800 No. of battery pumps 0.001 0.038

Process Upset Emissions 0.003 0.134

Pressure Relief Valves1 1.794 scf/yr/PR valve 163,499 No. of PR valves 0.000 0.016

Well Blowouts Onshore 0.051 MMscf/blowout 44 No. of blowouts/yr 0.002 0.119

Refining Emissions 2.720 144.2

 Asphalt Blowing2 20,736 scf CO2/Mbbl 359 Mbbl/calendar day 

production 

2.720 144.2

Total 8.747 463.6 1

 Emissions are not actually 0, but too small to show at this level of precision. 2 Asphalt Blowing emissions are the only significant vented emissions from the refining sector; other sources are too small to show at this level of 

precision. 

 NE: Not estimated for lack of data 

Energy use CO2 emissions not estimated to avoid double counting with fossil fuel combustion 

Table A-142: Summary of CO2 Emissions from Petroleum Systems (Gg) 

Activity 1990 1995 2000 2005 2006 2007 2008 2009 

Production Field 

Operations 376 341 323 285 285 292 288 319 

Pneumatic device venting 27 26 24 22 22 22 23 23 

Tank venting 328 296 281 246 246 252 247 278 

Misc. venting & fugitives 18 18 17 16 16 16 16 16 

Wellhead fugitives 1 1 1 111 1 1 

Refining 

Asphalt Blowing 180 187 211 205 203 182 165 144 

Total 555 528 534 490 488 474 453 463 
A-168 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009 

Activity/Equipment 

2009 EPA Inventory Values

Emission Factor Activity Factor Emissions

(Bcf/yr)

Emissions

(Gg/yr)

 Thermal Operations 6.01 scf CH4/Mbbl 2,034 Mbbl/calendar day feed 0.004 0.086

 Catalytic Cracking 5.17 scf CH4/Mbbl 4,694 Mbbl/calendar day feed 0.009 0.170

 Catalytic Reforming 7.22 scf CH4/Mbbl 2,970 Mbbl/calendar day feed 0.008 0.150

 Catalytic Hydrocracking 7.22 scf CH4/Mbbl 1,393 Mbbl/calendar day feed 0.004 0.071

 Hydrorefining 2.17 scf CH4/Mbbl 2,067 Mbbl/calendar day feed 0.002 0.031

 Hydrotreating 6.50 scf CH4/Mbbl 9,480 Mbbl/calendar day feed 0.022 0.432

 Alkylation/Polymerization 12.6 scf CH4/Mbbl 1,027 Mbbl/calendar day feed 0.005 0.091

 Aromatics/Isomeration 1.80 scf CH4/Mbbl 956 Mbbl/calendar day feed 0.001 0.012

 Lube Oil Processing 0.00 scf CH4/Mbbl 165 Mbbl/calendar day feed 0.000 0.000

 Engines 0.006 scf CH4/hp-hr 1,128 MMhp-hr/yr 0.006 0.124

 Flares 0.189 scf CH4/Mbbl 14,336 Mbbl/calendar day refinery 

feed 0.001 0.019

Total 1.243 23.90

1

 Emissions are not actually 0, but too small to show at this level of precision. 

Table A-139: Summary of CH4 Emissions from Petroleum Systems (Gg) 

Activity 1990 1995 2000 2005 2006 2007 2008 2009 

Production Field 

Operations 1,653 1,557 1,468 1,366 1,365 1,396 1,409 1,444 

Pneumatic device venting 489 463 428 397 396 398 416 419 

Tank venting 250 226 214 187 188 192 189 212 

Combustion & process 

upsets 88 82 76 71 71 72 75 94 

Misc. venting & fugitives 799 762 727 691 693 714 707 696 

Wellhead fugitives 26 25 22 19 17 20 23 23 

Crude Oil Transportation 7 6 5 5 5 5 5 5 

Refining 25 26 28 28 28 27 25 24 

Total 1,685 1,589 1,501 1,398 1,398 1,427 1,439 1,473 

Note: Totals may not sum due to independent rounding. 

Table A-140: Ratios of CO2 to CH4 Volume in Emissions from Petroleum Production Field Operations 

Whole Crude, 

Post-Separator Associated Gas Tank Flash Gas Offshore 

Ratio %CO2 / %CH4 0.052 0.020 0.017 0.004 

Table A-141: 2009 CO2 Emissions from Petroleum Production Field Operations and Petroleum Refining 

Activity/Equipment 

2009 EPA Inventory Values

Emission Factor Activity Factor Emissions

(Bcf/yr)

Emissions

(Gg/yr)

Vented Emissions 5.972 316.5

Oil Tanks 3.53 scf of CO2/bbl crude 1,490 MMbbl/yr (non stripper 

wells) 

5.256 278.6

Pneumatic Devices, High Bleed 6.704 scfd CO2/device 139,797 No. of high-bleed 

devices 

0.342 18.13

Pneumatic Devices, Low Bleed 1.055 scfd CO2/device 259,624 No. of low-bleed devices 0.100 5.299

Chemical Injection Pumps 5.033 scfd CO2/pump 28,166 No. of pumps 0.052 2.742

Vessel Blowdowns1 1.583 scfy CO2/vessel 182,451 No. of vessels 0.000 0.015

Compressor Blowdowns1 77 scf/yr of CO2/compressor 2,473 No. of compressors 0.000 0.010

Compressor Starts1 171 scf/yr. of CO2/compressor 2,473 No. of compressors 0.000 0.022

Stripper wells 48 scf/yr of CO2/stripper well 317,230 No. of stripper wells 

vented 

0.015 0.800

Well Completion Venting1 14.87 scf/completion 11,804 Oil well completions 0.000 0.009

Well Workovers1 1.95 scf CO2/workover 39,450 Oil well workovers 0.000 0.004

Pipeline Pigging NE scfd of CO2/pig station NE No. of crude pig stations NE NE

Offshore Platforms, Shallow water 

Oil, fugitive, vented and combusted 

 358 scfd CO2/platform 1,441 No. of shallow water oil 

platforms 

0.188 9.984

Offshore Platforms, Deepwater oil, 1,701 scfd CO2/platform 28 No. of deep water oil 0.017 0.917