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addressing uncertainty in oil and natural gas industry greenhouse

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EXHIBIT 4-2: Uncerta<strong>in</strong>ty Example for Fugitive Emissions Estimation, cont<strong>in</strong>ued<br />

Methane Emissions<br />

Table 4-3. Methane Weight Fractions for Production Operations by Service<br />

Operations Wt. Fraction<br />

Lower<br />

Uncerta<strong>in</strong>ty<br />

(%)<br />

Upper<br />

Uncerta<strong>in</strong>ty<br />

(%)<br />

Conventional Oil: 0.523 5% 5%<br />

Gas Service<br />

Conventional Oil: 0.0363 5% 5%<br />

Light Liquid Service<br />

Source: Picard, D. J., B. D. Ross, <strong>and</strong> D. W. H. Koon. A Detailed Inventory of CH 4<br />

<strong>and</strong> VOC Emissions from Upstream Oil <strong>and</strong> Gas Operations <strong>in</strong> Alberta, Volume II<br />

Development of the Inventory, Canadian Petroleum Association, March 1992,<br />

Tables 12 through 15.<br />

The methane emissions are estimated by the follow<strong>in</strong>g equation.<br />

Methane Emissions = Component Count × Emissions Factor × Weight Fraction<br />

Uncerta<strong>in</strong>ty Estimate:<br />

o To estimate the <strong>uncerta<strong>in</strong>ty</strong> of the emissions for the <strong>in</strong>dividual components, use Equation 4-6, the<br />

equation for the <strong>uncerta<strong>in</strong>ty</strong> of a product. This applies the relative uncerta<strong>in</strong>ties for uncerta<strong>in</strong>ties<br />

that are <strong>in</strong>dependent. For this example, the equation is written as:<br />

Urel U U U<br />

2 2 2<br />

( ) component<br />

= count<br />

+ emission factor<br />

+ weight fraction<br />

For example, the total methane emissions for compressor seals are:<br />

0.713 kg 8760hr tonnes<br />

Emissions = 2 seals× × 0.523 wt. fraction × × = 6.53 tonnes/yr<br />

hr-source year 1000kg<br />

The lower <strong>uncerta<strong>in</strong>ty</strong> of this estimate is calculated as follows:<br />

Urel U U U<br />

2 2 2 2 2 2<br />

( )<br />

component<br />

=<br />

count<br />

+<br />

EF<br />

+<br />

wt fraction<br />

= 0.75 + 0.36 + 0.05 = 83.3%<br />

o<br />

Note that <strong>in</strong> the emissions formula, there are two constants. Multiplication by a constant does not<br />

change the relative <strong>uncerta<strong>in</strong>ty</strong> of an estimate. The total methane emissions are the sum of the<br />

methane emissions for all of the components. The total <strong>uncerta<strong>in</strong>ty</strong> for methane emissions result<strong>in</strong>g<br />

from fugitive sources associated with the electric reciprocat<strong>in</strong>g compressor are calculated us<strong>in</strong>g<br />

Equation 4-4, which applies the absolute uncerta<strong>in</strong>ties for the <strong>uncerta<strong>in</strong>ty</strong> of a sum.<br />

Pilot Version, September 2009 4-12

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