== Measuring methane emissions ==
Natural gas makes up about a quarter of the energy used in the United States.<ref
name="r1">Ritter, K., Shires, T.M., Lev-On, M. 2015. Methane Emissions From Natural-Gas Systems: A Comparative Assessment for Select Industry Segments. SPE Economics & Management '''7'''(01): 22 - 28. SPE-168379-PA. http://dx.doi.org/10.2118/168379-PA. </ref> New production methods, including those related to hydraulic fracturing, are expanding access to natural gas and have increased natural gas based power, which is credited with decreasing greenhouse gas emissions in the country. Recent estimates for methane emissions from natural gas industries in the United States have been widely divergent. 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).
The EPA reports that nearly 29% of all methane emissions in 2012 came from oil and gas industry activity<ref
name="r2"> Rassenfoss, S. Pressure to Reduce Methane Emissions Highlights the Need for Better Monitoring. J Pet Tech '''67''' (3): 46-52. http://www.spe.org/jpt/article/8437-pressure-to-reduce-methane-emissions-highlights-the-need-for-better-monitoring/. </ref>. It is the largest source, followed by enteric fermentation, at 25%, which is the digestive process used by grazing animals, such as cattle, producing methane as a gassy byproduct. Those animals also produce manure, which is responsible for an added 9% of methane emitted. Other estimates suggest far higher industry emission levels. That matters to the oil and gas business because those numbers will guide future emissions rule making.
Using direct measures of emissions from devices on 377 wells operated by 10 companies cooperating on the survey, Allen gathered data that reshaped the EPA’s pie chart of E&P emissions of methane. The study concluded that emissions during flowback have sharply dropped from 26% in the EPA’s Greenhouse Gas Inventory to 1%. That decline, plus larger-than-expected emissions by gas-powered pneumatic control devices, pushed its share of the natural gas emissions from the EPA estimate of 26% to 42%, making it the largest single category.