addressing uncertainty in oil and natural gas industry greenhouse

EXHIBIT 4-1: Uncertainty Example for a Simple Emission Estimation, continued
Uncertainty aggregation – The following calculations step through the aggregation of uncertainty for this
emission source. First, the uncertainty of the emission estimates for CH 4 and CO 2 are calculated by
applying the relative uncertainties to Equation 4-6.
Urel ( ) = U + U + U
2 2 2
Emission Estimate Number of Devices Composition Data Emission Factor
2
2 ⎛2.53
⎞
2
Urel ( )
CH
= 0 + 0.331 0.333 33.3%
4 ⎜ ⎟ + = =
⎝ 80 ⎠
2
2 ⎛0.158
⎞
2
Urel ( )
CO
= 0 + 0.331 0.333 33.3%
2 ⎜ ⎟ + = =
⎝ 5 ⎠
Next, the CH 4 emissions are converted to a CO 2 equivalent basis, since there are no additional emission
sources to sum for this example.
CO2e = ( 226 tonnes CH
4/yr × 21)
+ 38.8 tonnes CO
2/yr
= 4,740 + 38.8 = 4,780 tonnes CO e/yr
Note that the global warming potential (GWP) of CH 4 is treated as a constant.
The absolute uncertainty for the aggregated CO 2 equivalent emissions are based on Equation 4-5 since the
uncertainties for both CH 4 and CO 2 are perfectly correlated (r = 1).
( )
U abs U U r U U
2 2
( )
Correlated X + Y
=
X
+
Y
+ 2
X
×
Y
2 2
( ) ( ) r ( ) ( )
( )
2 2
( )
Correlated X + Y
= 1,580 + 12.9 + 2× 1× 1,580 × 12.9 = 1,590
2
( )
U ( abs) = 4,740× 0.333 + 38.8× 0.333 + 2 4,740 × 0.333 × 38.8×
0.333
U abs
Correlated X + Y
On a relative basis, this equates to 1,590/4,780 = 33.3%
The final emission estimate for this example is 4,780 tonnes CO 2 e/yr ± 33.3%
4.3.2 Fugitive Emission Estimation
This next example demonstrates the uncertainty calculation for estimating methane emissions resulting
from fugitive sources associated with an electric reciprocating compressor located in a conventional crude
production operation. The example is taken from a Canadian study on GHG emission uncertainty (CAPP,
2004).
Pilot Version, September 2009 4-10