Pennsylvania's status as "a major energy producing state," and the possible impacts the state may see from climate change have not changed significantly over the last four years, according to a report commissioned by the Pennsylvania Department of Environmental Protection (DEP).
But researchers cautioned that while the natural gas industry had the potential to bring substantial change to the energy sector, there remains uncertainty over how natural gas could replace other fuels and disagreement over the best method for calculating greenhouse gas (GHG) emissions over the life cycle of wells in the Marcellus Shale. They said both issues require further study.
The 156-page report released Monday was the DEP's second on climate change since passage of the Pennsylvania Climate Change Act in 2008. The first report was issued in 2009.
"Natural gas from the Marcellus Shale has represented the largest energy growth area for Pennsylvania since 2009," said researchers from Pennsylvania State University, who conducted the study for the DEP. "Gas production in the Commonwealth has increased from under 1 trillion Btu/d (1 Bcf/d) to more than 3 trillion Btu/d (3 Bcf/d) in the past two years...it is largely self-sufficient in natural gas."
The manner in which natural gas could replace other fuels still needs to be assessed, the researchers said.
"The introduction of natural gas into energy utilization and delivery systems...is more complex than just making calculations based on replacing one Btu of another fuel with a Btu of natural gas. For example, while it is true that burning a Btu of natural gas in a power plant releases less CO2 [carbon dioxide] than burning a Btu of coal or fuel oil in a power plant, the Btu-to-Btu comparison can be misleading.
"The GHG impacts of additional investments in natural gas-fired power generation will depend on the efficiency with which natural gas is utilized in the plant, the costs of utilizing the natural gas plant versus other technologies, and the location of the natural gas plant...A more valid comparison in this case would incorporate these system effects to compare the impacts of additional gas-fired power generation on a kWh-basis, not a Btu-basis."
On the issue of GHG calculations for the life cycle of a well, the researchers touched on a controversial, and discounted, report by Cornell University ecologist Robert Howarth (see Shale Daily, April 13, 2011).
"Life cycle comparisons of GHG emissions from the natural gas sector are subject to uncertainties due primarily to lack of data, but also due to other modeling assumptions. Differences in GHG implications of Marcellus Shale development largely come down to assumptions made over vented and fugitive CH4 [methane] emissions and the relevant time frame for life cycle analysis.
"Howarth [assumed] high levels of vented CH4 and fugitive emissions, [but] these estimates are viewed as unrealistically aggressive in other studies," the researchers said, citing reports by Carnegie Mellon University, the Department of Energy's National Energy Technology Laboratory, and other academics at Cornell (see Shale Daily, Dec. 1, 2011; Aug. 19, 2011).
"Direct measurement of CH4 venting and fugitive emissions is rare and expensive," the researchers said, adding that a 2012 study conducted by Nature reporter Jeff Tollefson found methane released from a gas field in Colorado "[is] more in line with Howarth than with other studies.
"While the [Tollefson] study represents only a single data point, it is suggestive of the high degree of uncertainty that exists in current estimates of direct methane releases from natural gas drilling."