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 twenty-nine 

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 97.8 percent of total oil industry CH4 

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

two 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 1996 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 average for API gravity less than 

44 API degrees. Offshore emissions from shallow water and deep water oil platforms are taken from analysis of the Gulfwide Offshore Activity Data System (GOADS) report (EPA 2005, MMS 2004). The emission factors determined for 1995 

were assumed to be representative of emissions from each source type over the period 1990 through 2008. 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 44 API 

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

provided by API (2004). 

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 Minerals Management Service 

(MMS), 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 (UCACE) were used to 

extrapolate the activity levels from the base year to each year between 1990 and 2008. 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-158 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008 
 

 

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- 134, Table A- 135, Table A- 136, and Table A- 139 provide 2008 activity factors, emission factors, and 

emission estimates and Table A- 137 and Table A- 140 provide a summary of emission estimates for the years 1990 

through 2008. 

Table A- 134: 2008 CH4 Emissions from Petroleum Production Field Operations 

Activity/Equipment 

Emission 

Factor Units 

Activity 

Factor Units 

Emissions 

(Bcf/yr) 

Emissions 

(Gg/yr) 

Vented Emissions 63.977 1,230 

Oil Tanks 

Pneumatic Devices, High Bleed 

5.28 scf of CH4/bbl crude 

330 scfd CH4/device 

1,326 MMbbl/yr (non stripper 

wells) 

138,471 No. of high-bleed devices 

7.001 

16.70 

134.6 

321.1 

Pneumatic Devices, Low Bleed 52 scfd CH4/device 257,160 No. of low-bleed devices 4.881 93.86 

Chemical Injection Pumps 248 scfd CH4/pump 28,113 No. of pumps 2.545 48.95 

Vessel Blowdowns 78 scfy CH4/vessel 180,628 No. of vessels 0.014 0.271 

Compressor Blowdowns 3,775 scf/yr of CH4/compressor 2,444 No. of compressors 0.009 0.177 

Compressor Starts 8,443 scf/yr. of CH4/compressor 2,444 No. of compressors 0.021 0.397 

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 17,929 Oil well completions 0.013 0.253 

Well Workovers 96 scf CH4/workover 39,375 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 

Offshore Platforms, Deepwater oil, 

fugitive, vented and combusted 

54,795 scfd CH4/platform 

260,274 scfd CH4/platform 

1,479 No. of shallow water oil 

platforms 

26 No. of deep water oil 

platforms 

29.58 

2.470 

568.8 

47.50 

Fugitive Emissions 2.516 48 

Oil Wellheads (heavy crude) 0.13 scfd/well 14,648 No. of hvy. crude wells 0.001 0.013 

Oil Wellheads (light crude) 16.6 scfd/well 193,123 No. of lt. crude wells 1.172 22.54 

Separators (heavy crude) 0.15 scfd CH4/separator 10,643 No. of hvy. crude seps. 0.001 0.011 

Separators (light crude) 14 scfd CH4/separator 96,859 No. of lt. crude seps. 0.490 9.418 

Heater/Treaters (light crude) 19 scfd CH4/heater 73,127 No. of heater treaters 0.512 9.852 

Headers (heavy crude)1 0.08 scfd CH4/header 13,695 No. of hvy. crude hdrs. 0.000 0.007 

Headers (light crude) 11 scfd CH4/header 42,455 No. of lt. crude hdrs. 0.168 3.235 

Floating Roof Tanks 

Compressors 

338,306 scf CH4/floating roof 

tank/yr. 

100 scfd CH4/compressor 

24 No. of floating roof tanks 

2,444 No. of compressors 

0.008 

0.089 

0.159 

1.715 

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

Sales Areas 41 scf CH4/loading 1,505,384 Loadings/year 0.061 1.174 

Pipelines 

Well Drilling 

NE scfd of CH4/mile of 

pipeline 

NE scfd of CH4/oil well drilled 

11,694 Miles of gathering line 

19,864 No. of oil wells drilled 

NE 

NE 

NE 

NE 

Battery Pumps 0.24 scfd of CH4/pump 157,500 No. of battery pumps 0.014 0.265 

Combustion Emissions 3.748 72.07 

Gas Engines 0.24 scf CH4/HP-hr 15,396 MMHP-hr 3.695 71.06 

Heaters 0.52 scf CH4/bbl 1,807 MMbbl/yr 0.001 0.018 

Well Drilling 2,453 scf CH4/well drilled 19,864 Oil wells drilled 0.049 0.937 

Flares 20 scf CH4/Mcf flared 154,014 Mcf flared/yr 0.003 0.059 

Process Upset Emissions 0.171 3.284 

Pressure Relief Valves 35 scf/yr/PR valve 150,918 No. of PR valves 0.005 0.101 

Well Blowouts Onshore 2.5 MMscf/blowout 66.2 No. of blowouts/yr 0.166 3.183 

Total 70.41 1,354 

- 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 

A-159 
 

 

Table A- 135: 2008 CH4 Emissions from Petroleum Transportation 

Emission Activity Emissions Emissions 

Activity/Equipment Factor Units Factor Units (Bcf/yr) (Gg/yr) 

Vented Emissions 0.215 4.139 

Tanks 0.021 scf CH4/yr/bbl of crude delivered to refineries 5,361 MMbbl crude feed/yr 0.110 2.124 

Truck Loading 0.520 scf CH4/yr/bbl of crude transported by truck 69.3 MMbbl trans. by truck 0.036 0.693 

Marine Loading 2.544 scf CH4/1000 gal. crude marine loadings 20,508,086 1,000 gal./yr loaded 0.052 1.003 

Rail Loading 0.520 scf CH4/yr/bbl of crude transported by rail 4.3 MMbbl. Crude by rail/yr 0.002 0.043 

Pump Station Maintenance1 36.80 scf CH4/station/yr 502 No. of pump stations 0.000 0.000 

Pipeline Pigging 39 scfd of CH4/pig station 1,004 No. of pig stations 0.014 0.275 

Fugitive Emissions 0.050 0.959 

Pump Stations 25 scf CH4/mile/yr. 50,214 No. of miles of crude p/l 0.001 0.024 

Pipelines NE scf CH4/bbl crude transported by pipeline 6,858 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 0.265 5.097 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- 136: 2008 CH4 Emissions from Petroleum Refining 

Emission Activity Emissions Emissions 

Activity/Equipment Factor Units Factor Units (Bcf/yr) (Gg/yr) 

Vented Emissions 1.131 21.75

 Tanks 20.6 scf CH4/Mbbl 1,885 Mbbl/cd heavy crude feed 0.014 0.273

 System Blowdowns 137 scf CH4/Mbbl 14,688 Mbbl/cd refinery feed 0.733 14.11

 Asphalt Blowing 2,555 scf CH4/Mbbl 411 Mbbl/cd production 0.383 7.370 

Fugitive Emissions 0.089 1.712

 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 14,688 Mbbl/cd refinery feed 0.010 0.194

 Cooling Towers 2.36 scf CH4/Mbbl 14,688 Mbbl/cd refinery feed 0.013 0.243 

Combustion Emissions 0.091 1.712

 Atmospheric Distillation 3.61 scf CH4/Mbbl 15,027 Mbbl/cd refinery feed 0.020 0.381

 Vacuum Distillation 3.61 scf CH4/Mbbl 6,585 Mbbl/cd feed 0.009 0.167

 Thermal Operations 6.01 scf CH4/Mbbl 2,103 Mbbl/cd feed 0.005 0.089

 Catalytic Cracking 5.17 scf CH4/Mbbl 4,934 Mbbl/cd feed 0.009 0.179

 Catalytic Reforming 7.22 scf CH4/Mbbl 3,006 Mbbl/cd feed 0.008 0.152

 Catalytic Hydrocracking 7.22 scf CH4/Mbbl 1,359 Mbbl/cd feed 0.004 0.069

 Hydrorefining 2.17 scf CH4/Mbbl 2,097 Mbbl/cd feed 0.002 0.032

 Hydrotreating 6.50 scf CH4/Mbbl 9,619 Mbbl/cd feed 0.023 0.439

 Alkylation/Polymerization 12.6 scf CH4/Mbbl 1,053 Mbbl/cd feed 0.005 0.093

 Aromatics/Isomeration 1.80 scf CH4/Mbbl 1,003 Mbbl/cd feed 0.001 0.013

 Lube Oil Processing 0.00 scf CH4/Mbbl 163 Mbbl/cd feed 0.000 0.000

 Engines 0.006 scf CH4/hp-hr 1,143 MMhp-hr/yr 0.007 0.125

 Flares 0.189 scf CH4/Mbbl 14,688 Mbbl/cd refinery feed 0.001 0.019
 

Total 1.311 25.22 1 Emissions are not actually 0, but too small to show at this level of precision. 

Table A- 137: Summary of CH4 Emissions from Petroleum Systems (Gg) 

Activity 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008
 

Production Field Operations 1,581 1,593 1,552 1,514 1,506 1,492 1,490 1,481 1,469 1,427 1,406 1,403 1,388 1,355 1,334 1,312 1,311 1,340 1,354
 

Pneumatic device venting 489 498 482 472 469 463 462 461 452 437 428 425 424 412 408 397 396 398 415 

Tank venting 179 179 173 166 162 161 161 165 163 153 153 154 150 149 142 134 134 137 135 

Combustion & process upsets 88 90 86 84 83 82 82 83 80 77 76 75 75 73 72 71 71 72 75 

Misc. venting & fugitives 799 799 786 768 767 762 759 747 748 735 726 726 716 699 691 690 693 714 707 

Wellhead fugitives 26 26 24 24 25 25 25 26 26 24 22 22 23 22 21 19 17 20 23 

Crude Oil Transportation 7 6 6 6 6 6 6 6 6 6 5 5 5 5 5 5 5 5 5 

Refining 25 24 24 25 25 26 26 27 27 27 28 27 27 27 28 28 28 27 25 

Total 1,613 1,624 1,582 1,545 1,537 1,524 1,521 1,513 1,501 1,460 1,439 1,435 1,420 1,387 1,367 1,344 1,344 1,372 1,384 

A-160 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008 
 

 

Table A- 138: 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- 139: 2008 CO2 Emissions from Petroleum Production Field Operations and Petroleum Refining 

Activity/Equipment 

Emission 

Factor Units 

Activity 

Factor Units 

Emissions 

(Bcf/yr) 

Emissions 

(Gg/yr) 

Vented Emissions 5.393 285.2 

Oil Tanks 

Pneumatic Devices, High Bleed 

3.53 scf of CO2/bbl crude 

6.704 scfd CO2/device 

1,326 MMbbl/yr (non stripper 

wells) 

138,471 No. of high-bleed devices 

4.678 

0.339 

247.4 

17.92 

Pneumatic Devices, Low Bleed 1.055 scfd CO2/device 257,160 No. of low-bleed devices 0.099 5.238 

Chemical Injection Pumps 5.033 scfd CO2/pump 28,113 No. of pumps 0.052 2.731 

Vessel Blowdowns1 1.583 scfy CO2/vessel 180,628 No. of vessels 0.000 0.015 

Compressor Blowdowns1 77 scf/yr of CO2/compressor 2,444 No. of compressors 0.000 0.010 

Compressor Starts1 171 scf/yr. of CO2/compressor 2,444 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 0.798 

Well Completion Venting1 14.87 scf/completion 17,929 Oil well completions 0.000 0.014 

Well Workovers1 1.95 scf CO2/workover 39,375 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

Offshore Platforms, Deepwater oil, 

fugitive, vented and combusted

 358 scfd CO2/platform 

1,701 scfd CO2/platform 

1,479 No. of shallow water oil 

platforms 

26 No. of deep water oil 

platforms 

0.193 

0.016 

10.22 

0.854 

Fugitive Emissions 0.052 2.738 

Oil Wellheads (heavy crude)1 0.003 scfd/well 14,648 No. of hvy. crude wells 0.000 0.001 

Oil Wellheads (light crude) 0.337 scfd/well 193,123 No. of lt. crude wells 0.024 1.258 

Separators (heavy crude)1 0.003 scfd CO2/separator 10,643 No. of hvy. crude seps. 0.000 0.001 

Separators (light crude) 0.281 scfd CO2/separator 96,859 No. of lt. crude seps. 0.010 0.526 

Heater/Treaters (light crude) 0.319 scfd CO2/heater 73,127 No. of heater treaters 0.009 0.450 

Headers (heavy crude)1 0.002 scfd CO2/header 13,695 No. of hvy. crude hdrs. 0.000 0.000 

Headers (light crude) 0.220 scfd CO2/header 42,455 No. of lt. crude hdrs. 0.003 0.181 

Floating Roof Tanks1

Compressors 

17,490 scf CO2/floating roof 

tank/yr. 

2.029 scfd CO2/compressor 

24 No. of floating roof tanks 

2,444 No. of compressors 

0.000 

0.002 

0.023 

0.096 

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

Sales Areas 2.096 scf CO2/loading 1,505,384 Loadings/year 0.003 0.167 

Pipelines 

Well Drilling 

NE scfd of CO2/mile of 

pipeline 

NE scfd of CO2/oil well drilled 

11,694 Miles of gathering line 

19,864 No. of oil wells drilled 

NE 

NE 

NE 

NE 

Battery Pumps 0.012 scfd of CO2/pump 157,500 No. of battery pumps 0.001 0.038 

Process Upset Emissions 0.004 0.192 

Pressure Relief Valves1 1.794 scf/yr/PR valve 150,918 No. of PR valves 0.000 0.014 

Well Blowouts Onshore 0.051 MMscf/blowout 66 No. of blowouts/yr 0.003 0.178 

Refining Emissions 3.111 164.5

 Asphalt Blowing2 20,736 scf CO2/Mbbl 411 Mbbl/cd production 3.111 164.5 

Total 8.559 453.1 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 

A-161 
 

 

 

 

Table A- 140: Summary of CO2 Emissions from Petroleum Systems (Gg) 

Activity 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 

Production Field 

Operations

Pneumatic device venting 

Tank venting 

Misc. venting & fugitives 

Wellhead fugitives 

Refining 

Asphalt Blowing 

376 

27 

328 

18 

1 

180 

377 

28 

330 

18 

1 

172 

365 

27 

319 

18 

1 

168 

350 

26 

304 

18 

1 

180 

342 

26 

297 

18 

1 

181 

341 

26 

296 

18 

1 

187 

341 

26 

296 

17 

1 

184 

348 

26 

303 

17 

1 

194 

343 

25 

299 

17 

1 

199 

325 

24 

282 

17 

1 

202 

323 

24 

281 

17 

1 

211 

325 

24

283 

17

1 

194 

318 

24 

276 

16 

1 

197 

314 

23

273 

16

1 

199 

302 

23 

262 

16 

1 

204 

285 

22 

246 

16 

1 

205 

285 

22 

246 

16 

1 

203 

292 

22 

252 

16 

1 

182 

288 

23 

247 

16 

1 

165 

Total 555 549 533 530 523 528 525 542 543 527 534 519 515 512 505 490 488 474 453 

A-162 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008