addressing uncertainty in oil and natural gas industry greenhouse

DOCUMENT AT A GLANCE
This document is designed to provide a summary of technical considerations that are important for
understanding and calculating GHG emission inventory uncertainty. The document will provide needed
technical background and specific calculation methods to determine uncertainties with targeted
measurements and emission factors and determine how to aggregate these individual terms to derive
uncertainty ranges (at a pre-designated probability level) for entire GHG inventories, at any given level.
These emission inventories of typical oil and natural gas (O&G) operations are quite complex; they are
based on a combination of measured and estimated emissions data, according to local requirements and
available information. The overall range of uncertainty associated with an entity GHG inventory is
determined primarily by the uncertainty associated with the largest (“key”) sources of emissions. In turn,
the confidence interval associated with each individual source depends on the availability of sufficient
data to estimate emissions, or on the quality of that data, in order to properly account for emission
variability.
Uncertainty analysis is a potential tool to not only assess confidence intervals, but more importantly, to
allow the targeting of specific areas for enhanced data collection. Such an analysis will enable a user to
rank order the importance of different emission sources in terms of their overall contribution to the
emissions inventory and its overall uncertainty range.
This document is a companion to the API Compendium of Greenhouse Gas Emission Methodologies for
the Oil & Gas Industry (2009 API Compendium). It provides a range of background information on
industry practices and specific calculation methods that would enable inventory developers to quantify
and better understand the uncertainty associated with the resultant GHG emissions.
Section 1.0 introduces some basic concepts and terms that provide a foundation for understanding GHG
emissions inventory uncertainty. This terminology is used throughout the document. This section covers:
the importance of reliable GHG accounting; a terminology overview; definition of error types; and a
description of the determination of uncertainty ranges (also known as confidence intervals).
Section 2.0 discusses the major sources of uncertainty in GHG inventories. It moves from general
concepts to issues that are germane to GHG inventories in the O&G industry. It also describes factors
that could introduce errors into the emission measurements process and contribute to the range of
uncertainties of estimated emissions. It introduces the categories of emission estimation approaches and
their uncertainty implications, and concludes with a short description of emission inventory steps and data
aggregation.
Section 3.0 provides an overview of measurement practices, focusing on gas flow measurements and the
determinations of carbon content and heating values of combusted fuels. The section recognizes industry
recommended practices and standards that have traditionally applied to “custody transfer”. This section
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