The recent rapid expansion of natural-gas developments and usage worldwide are bringing into focus the need to improve understanding and characterization of greenhouse-gas (GHG) emission sources, including methane (CH4), associated with petroleum and natural-gas systems. New production technologies and practices, including those involving hydraulic fracturing, necessitate a thorough review of existing quantification methods for fugitive CH4 emissions from venting, flaring, and equipment leaks associated with petroleum and natural-gas systems and operations.

In the past few years, widely divergent estimates have emerged regarding CH4 emissions from the US natural-gas-industry sector. Some discrepancies noted by industry surveys have led to a thorough review of newly available information and are leading to the improvement of estimation methods and emission factors associated with activities that comprise natural-gas systems. This has manifested itself in the engineering estimations that are used for compiling the national GHG Emissions Inventory and in the methods used by companies for reporting under the mandatory national GHG Reporting Program. Both the inventory and the reporting program are programs of the US Environmental Protection Agency (EPA).

This paper presents results of a comparative analysis of GHG-emissions data, including CH4, for key industry segments such as on-shore natural-gas production and natural-gas processing and their contribution to the Petroleum and Natural Gas Systems sector. The data analyzed will contrast the “top-down” assessments used in developing the GHG Emissions Inventory with the “bottom-up” estimation of actual emissions as reported under Subpart W of the GHG Reporting Program. The analysis will provide a comparison of the estimation methods and evaluation of the contribution of key sources with overall CH4 emissions.

The ultimate goal of this effort is to incorporate the new information that is becoming available into consistent methods that can be used both for national GHG inventory development and for corporate reporting. Harmonization of these methods is expected to contribute to informing the public debate on natural-gas use and its role in mitigating overall GHG emissions.

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