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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460 

OFFICE OF 

 AIR AND RADIATION 

Petroleum and Natural Gas Systems (Subpart W) of the Greenhouse Gas Reporting Rule 

Docket: EPA-HQ-OAR-2009-0923 

Title: Equipment Threshold for Onshore Production Storage Tanks 

Summary: This memo discusses the development of a minimum throughput of oil production 

sent to onshore production field storage tanks, below which emissions could be calculated using 

emission factors rather than rule specified flash calculation models. 
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Equipment Threshold for Tanks 

This memo discusses the development of a minimum throughput of oil production sent to 

onshore production field storage tanks, below which emissions could be calculated using 

emission factors rather than rule specified flash calculation models. Industry commenters noted 

that many oil wells (e.g. stripper wells) have very small production rates and small gas evolution, 

for which application of the April 2010 proposed rule tank emissions model would be a 

substantial burden for little contribution to emissions. EPA determined that crude oil or gas 

condensate well pad production under 10 barrels per day (bbl/day) represents a large fraction of 

well pads while contributing a small fraction of total emissions from storage tanks receiving this 

oil. This memorandum describes the analysis that was conducted to determine this threshold. 

EPA examined existing regulations related to onshore production storage tanks for potential 

equipment thresholds. Subsequently, EPA developed an acceptable equipment threshold using 

aggregated data from the HPDI database. As part of the analysis, it estimated the number of 

separators and tanks that are above and below the threshold. 

Precedence for Onshore Production Storage Tanks Thresholds 

The National Emission Standards for Hazardous Air Pollutants (NESHAP) regulations require 

operators to follow standards on installation, operation, monitoring of control devices for major 

sources of hazardous air pollutants. For oil and natural gas production facilities, the standards are 

set forth in 40 CFR Part 63 Subpart HH1

. Under section §63.761, storage vessels with the 

potential for flashing emission are defined as: 

any storage vessel that contains a hydrocarbon liquid with a stock tank GOR equal to or 

greater than 0.31 cubic meters per liter [1740 cubic feet per barrel] and an API gravity 

equal to or greater than 40 degrees and an actual annual average hydrocarbon liquid 

throughput equal to or greater than 79,500 liters per day [500 barrels per day]. Flash 

emissions occur when dissolved hydrocarbons in the fluid evolve from solution when the 

fluid pressure is reduced. 

This definition sets a threshold of 79,500 liters per day (500 bbl/day), below which it is not 

considered a major source of HAP emissions. 

Quantitative Analysis of Threshold Options 

Applying the 80:20 rule criteria for selecting sources required to be reported in subpart W, EPA 

selected a threshold for onshore production storage tanks that should make the top emitters that 

produce 80% of the emissions report using a process simulator while allowing the remaining 

20% of emissions to be quantified using simpler methods. 

Methodology 

HPDI database contains oil and gas production data from thousands of reporting properties 

across the U.S. EPA aggregated to the reporting properties by field, then estimated the petroleum 

 

1

 EPA. 40 CFR Part 63 63.760 – 63.779. Subpart HH—National Emission Standards for Hazardous Air Pollutants 

From Oil and Natural Gas Production Facilities. http://ecfr.gpoaccess.gov/cgi/t/text/textidx?c=ecfr&sid=2c6a4334a98427e866966423764af7f6&rgn=div6&view=text&node=40:10.0.1.1.1.8&idno=40 
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and natural gas production rates and well counts for each field, and organized each field by the 

oil produced per well. 

The number of well pad separators in a particular field was estimated using the U.S Inventory of 

Greenhouse Gas Emissions and Sinks (U.S. Inventory). Industry experts approximated that for 

every four oil wells there is one field production storage tank. The number of tanks in each field 

was estimated using this ratio. The ratio of wells per separator was calculated by dividing the 

number of separators by the total number of wells. This ratio was used to calculate the number of 

separators in a particular field. 

To calculate the uncontrolled methane emissions from oil tanks in each field, an emission factor 

based on tank throughput was needed. The emission factor, valued at 5.28 scf per barrel, was 

obtained from the U.S. Inventory. This is an arithmetic average of the methane emissions per 

barrel from all the E&P Tank model default factors for oil less than 45°API gravity. A 

companion methane emission factor of 22.2 scf/bbl for E&P Tank oil equal or greater than 

45°API gravity is used in the U.S. Inventory for gas condensate storage tank emissions. Under 

the above referenced NESHAP regulation, operators of tanks with a throughput greater than 500 

bbl/day are required to reduce their hazardous air pollutants by 95%. As a result, to calculate the 

actual methane emissions, the uncontrolled methane emissions from storage tanks with a 

throughput greater than 500 bbl/day were reduced by 95%. 

Results

Exhibit 1 shows the results of quantitative analysis. EPA examined minimum throughputs (i.e. 

“floors”) of 5 and 10 bbl/day, and the NESHAP regulation floor of 500 bbl/day. At 5 bbl/day, 

10 % of all emissions, and 227,111 separators and 72,508 tanks would be calculated by the U.S. 

Inventory emission factor. At 10 bbl/day, 21% of total emissions, and 285,409 separators and 

91,121 tanks would be calculated by an emission factor. If the floor is set at 10 bbl/day, 18% of 

all storage tanks, which represents the top emitters, would report 79% of emissions using a 

process simulator. A 500 bbl/day threshold would require 99.8% of storage tanks to calculate 

emissions using the inventory emission factor. 

Exhibit 1: Options for threshold on oil throughput per tank. 

Percentage of 

Population 

Emissions 

Covered by 

Population 

Emission Factor 

Estimated 

Population 

Emissions 

Covered by 

Population 

Emission Factor 

(scf/yr) 

Oil Throughput 

(bbl/day) 

Number of tanks 

with an Oil 

throughput below 

the Equipment 

threshold 

(assuming 4 wells 

per tank) 

Number of 

Separators with 

an Oil throughput 

below the 

Equipment 

threshold 

10% 744,466,420 5.00 72,508 227,111 

21% 1,471,541,143 10.00 91,121 285,409 

96% 6,909,737,129 500.00 111,489 349,204 

Total 7,169,918,328 111,697 349,859 

Emission Factor for Tanks 

EPA investigated several sources of emission factors to determine which would be suitable for 

calculating emissions from tanks that receive hydrocarbon liquids from wellhead separators or 
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wells with less than 10 bbl/day of average production. EPA determined that the emission factors 

for oil and condensate storage tanks that are currently used in the U.S. Inventory are the best 

publicly available emission factors for these tanks. These factors provide the volume of methane 

and carbon dioxide that is emitted per barrel of throughput. 

EPA investigated how to further reduce burden on reporters while providing the necessary 

quality of data to inform policy, by developing factors that would not require reporters to collect 

information on the throughput of each individual tank. The U.S. Inventory provides the average 

production rate of marginal wells in the U.S. at 2.2 bbl/day. Since this is the production 

collected in the storage tanks that will be estimated using emission factors, EPA assumed that 

each of these tanks has an average throughput of 2.2 bbl/day so that reporters would only be 

required to know the number of wells flowing directly to tanks or wellhead separators that 

produce less than 10 bbl/day to storage tanks. This calculation and the results are provided in 

Table 2. 

Exhibit 2. Development of Emission Factor for Onshore Production Storage Tanks 

Receiving Hydrocarbon Liquids from Wells or Separators with Less Than 10 bbl/day 

Output. 

Methane Emissions scf/bbl2 x bbl/day2 x days/year = scf/year 

Crude Oil Tanks 5.36 x 2.2 x 365 = 4,305 

Gas Condensate Tanks 22.2 x 2.2 x 365 = 17,832 

Carbon Dioxide Emissions scf/bbl2 x bbl/day2 x days/year = scf/year 

Crude Oil Tanks 3.58 x 2.2 x 365 = 2,877 

Gas Condensate Tanks 3.58 x 2.2 x 365 = 2,877 

Note: Emission factor from the U.S. Inventory are adjusted from 60 ̊F and 14.7 psia to 68 ̊F and 14.7 psia. Each 

column is rounded independently for the purposes of displaying in this Exhibit; thus, the final emission factors may 

differ slightly from using the rounded parameters above.

The resulting emission factors for crude oil tanks receiving wellhead production less than 10 

bbl/day are 4,305 scf/year of methane and 2,877 scf/year of carbon dioxide. The resulting 

emission factors for gas condensate tanks receiving wellhead production less than 10 bbl/day are 

17,832 scf/year of methane and 2,877 scf/year of carbon dioxide. These factors would be 

multiplied by the number of separators or wellheads with average production less than 10 

bbl/day sent to storage tanks. 

Conclusion 

If the equipment threshold is set at 5 bbl/day, over 122,000 separators, 35%, will report 90% of 

the emissions from onshore production storage using rigorous model calculation methodologies. 

This threshold was deemed overly burdensome on reporters. At 500 bbl/day, approximately 

100% of the separators will report emissions using the U.S. Inventory emission factor. Although 

the burden on industry is minimized, the data gathered would be no better than the U.S. National 

Inventory, and inadequate to inform future policy. A 10 bbl/day equipment threshold on 

 

2

 EPA. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2008. April 2010, U.S. EPA # 430-R-10-

006. <http://epa.gov/climatechange/emissions/usinventoryreport.html>. 
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separators and wells flowing directly to tanks satisfies the significant contribution criteria for 

subpart W by requiring the 19% of largest oil throughput separators to report approximately 79% 

of the total estimated production tank emissions using the more rigorous tank flashing model 

methodologies. EPA has determined that the most appropriate threshold for onshore production 

storage tanks would be 10 bbl/day for the purposes of subpart W. 

EPA developed emission factors for both methane and carbon dioxide emitted from these tanks 

for both crude oil and gas condensate storage. In order to minimize reporting burden for these 

smaller sources, EPA developed annual tank emission factors per separator or well pad flowing 

directly to tanks by assuming the average marginal well production rate as a throughput. This 

enabled EPA to develop emission factors that would not require reporters to monitor the 

throughput of each small production separator or well pad.