addressing uncertainty in oil and natural gas industry greenhouse

EXHIBIT 4-6: Uncertainty Example for Statistical Sampling, continued
o
The monthly CO 2 emissions factor is then calculated by applying the following equation:
6
lb CTotal 44lb CO
2/lbmole CO
2
lbTotal lbmoleTotal
10 scf tonne CO2
× × × × ×
lb 12lb C/lbmole C lbmole 379.3 scf MMscf 2204.62 lb CO
Total Total 2
=55.8856 tonnes CO2/MMscf for January.
o
The uncertainty for this quantity is calculated by applying Equation 4-3 for the relative uncertainties
of MW Total and wt% C Total
2 2
⎛UX
⎞ ⎛UY
⎞
Urel ( ) = ⎜ ⎟ + ⎜ ⎟ = 4.13 + 5.59
⎝ X ⎠ ⎝ Y ⎠
2 2
=6.95%
o
The same methods are applied to each of the monthly samples. Table 4-12 summarizes the resulting
emission factors (tonnes CO 2 /MMscf) and uncertainties.
Table 4-12. Measured Monthly Emission Factors
Tonnes
CO 2 /MMscf
U(abs), Tonnes
CO 2 /MMscf U(rel), %
January 55.8856 3.8840 6.95
February 55.7698 3.9276 7.04
March 55.9697 3.9414 7.04
April 56.1644 3.9304 7.00
May 54.7437 4.1879 7.65
June 55.2278 4.1441 7.50
July 54.7069 4.1606 7.61
August 55.4646 4.1751 7.53
September 54.6648 4.1705 7.63
October 54.6332 4.1647 7.62
November 54.4964 4.1527 7.62
December 54.8858 4.0798 7.43
4.5 Aggregating Uncertainty
Figure 4-5 provides the decision tree for aggregating uncertainties. This diagram steps through the
combination of activity data and emission factors to result in the final emission estimate. It also addresses
the aggregation of emissions from multiple sources, applying the GWP to convert non-CO 2 emissions to a
CO 2 equivalent basis, and the final summation of CO 2 equivalent emissions.
An important point to note is that, although the GWP values reported by IPCC have corresponding
uncertainties, the GWP values are treated as constants in compiling a facility- or entity-wide GHG
inventory. Multiplying by a constant does not change the relative uncertainty.
Pilot Version, September 2009 4-29