VOC Task Force – Development of the Bakken Emission Factors

In 2010, numerous Oil and Gas operators were tasked with attempting to develop an emission 

factor (EF) representative of the relatively volatile oil/condensate being produced from the Bakken 

formation in North Dakota. For the purpose of this study, the Bakken Pool was defined as the 

accumulation of oil and gas found in the interval 50 feet above the top of the Bakken formation to 

50 feet below the top of the Three Forks Formation. This was a collaborative effort that utilized 

an extensive array of sampling and modeling data from producing wells from this reservoir. The 

goal was to provide a conservative, yet reasonable EF associated with flashing, working and 

breathing VOC emissions from any crude oil produced from any reservoir where the Bakken 

Formation is the source rock. 

Ultimately, this study yielded an EF that has been used to drive permitting and control 

requirements for all production facilities in the Bakken and Three Forks. The control requirements 

and timelines have been established by the North Dakota Department of Health and have been 

posted in various regulatory guideline documents released in 2011. The overall result has been a 

well-documented, rigid control program that has dramatically lowered actual emissions from 

production facilities in these areas and established clear timelines for installing tank vapor controls 

both existing and new wells. It has also established definitive timelines for registering wells 

producing from the Bakken and Three Forks reservoirs. 

Emission Factor Development Process 

Numerous data were compiled from seven companies with wells producing from the Bakken 

formation, primarily in Mountrail and McKenzie County where the higher producing wells were 

initially clustered. A few different testing methods were utilized to estimate potential VOC 

emissions from oil tanks, all of which are used to assess tank emissions for a wide range of 

permitting applications. These methods include: 

1. Direct Measurements (Fox Flow Meter, etc.) 

2. E&P Tanks 2.0 in conjunction with pressured liquids samples upstream of tanks 

3. Flash gas analysis of liquids samples flashed in lab setting 

Companies Providing Emission Data: 

• Whiting Petroleum 

• EOG 

• Hess Corporation 

• Fidelity E&P 

• ConocoPhillips 

• Marathon Oil 

• Zenergy 

Sample Data Population Attributes: 

• All wells assessed in study are Bakken formation wells; 

• Vast majority from new Bakken wells (< 4 yrs old); 

• Sample data collected in warmer months of May-October when tank vapors are most 

volatile; 

• Primarily Mountrail County wells (others from Dunn, McKenzie and Williams Cty); 

• Separator pressure varied from 16 psig-60 psig; 

• API gravity of oil ranged from 42-48 for all wells 
Initial Tank Emissions Data: 

• Total of 112 data points were initially amassed as part of the EF development study 

• 103 were direct measurement using Fox Flow Meter or equivalent; 

• Remaining points were from liquids sampling data and subsequent E&P Tanks 2.0 

modeled data using the liquids analysis as input; 

• For all data points, tank vent vapor flow was either measured or estimated (E&P Tanks 

2.0) 

Final Tank Emissions Data:

• Final EF based on 87 direct measurement (DM) data points; 

• 15 of 103 direct measurement points were deemed outliers and not used in final EF 

development; 

• Data points amassed from 5 of 7 companies listed above; 

• No E&P Tanks 2.0 data was used as these were considered outliers and yielded much 

higher estimated VOCs than all of the direct measurement points; 

• Direct measurement data points all from 2009-2010 

 82 data points from Mountrail County 

 2 from McKenzie County 

 2 from Williams County 

 1 from Dunn County 

• Direct measurements sampled actual vapor flow rates (scf/bbl) 

• Direct measurements also provided vent gas composition by weight 

Final Emission Factor (EF) Development (see attached calculations): 

• Average flow rate from tank vent of 55.26 scf/bbl from 87 DM points; 

• 90th percentile of 97.91 scf/bbl; 

• Average VOC weight percent 69.57%; 

• 90th percentile of 79.83%; 

• Average HAP weight percent 1.26%; 

• 90th percentile of 2.26%; 

• Average tank vapor molecular weight 40.66 lb/lb-mol; 

• 90th percentile of 45.19 lb/lb-mol; 

• Using the average data yields average EF: 

 4.12 lbs/bbl VOC 

 0.07 lb/bbl HAP 

• Using the 90th percentile data yields 90th percentile EF: 

 9.31 lbs/bbl VOC 

 0.26 lb/bbl HAP 

Why did VOC Task Force decide on using 90th percentile for EF?

• Intended as a “screening” tool to trigger tank vapor control requirements;

• Using this EF in conjunction with NDDH Guidance Document (May 2, 2011), any 

Bakken/Three Forks well that produces >=12 bopd would require controls with 98% DRE;

• ALL Bakken/Three Forks wells produce well above this even after years of active 

production;

• As such, using a more stringent percentile (>90th) is inconsequential and ineffective in 

terms of control requirements in the Bakken/Three Forks.