Natural gas produced in the Marcellus Shale gas basin in Pennsylvania and New York is not as big a contributor to climate change as coal, according to a study of the "life cycle" greenhouse gas emissions of natural gas by researchers in Pittsburgh.

Researchers at Carnegie Mellon University weighed into a growing body of analyses about the environmental impact of a natural gas boom stretching across the Northeast. The report refutes a recent study out of Cornell University that found that extracting gas from deep shale basins results in at least as big a greenhouse gas emissions footprint as that of coal.

Opponents of gas drilling leapt on the Cornell study as evidence that gas isn't a clean replacement for coal and has just as big an impact on global warming. Many independent analysts found that study had significant flaws.

Researchers in Carnegie Mellon's environmental engineering and business departments disagreed with Cornell in their study published earlier this month in the journal Environmental Research Letters.

"Marcellus shale gas adds only 3 percent more emissions to the average conventional gas, which is likely within the uncertainty bounds of the study," concludes the paper.

The report sticks with gas's role as a source of power generation. Gas, however, is also an ingredient at heavy industrial plants like chemical factories. Disregarding how gas is used, the industrial process of drilling for shale gas means an 11 percent increase in greenhouse gas emissions compared to average U.S. gas production.

"There is significant uncertainty in our Marcellus Shale GHG emission estimates due to eventual production volumes and variability in flaring, construction and transportation," it concludes.

Ongoing effort to measure 'footprint'
There's also not a lot of data on the start-to-finish emissions footprint for natural gas production. U.S. EPA has proposed a new rule that would require companies to collect those data, and a panel at the Department of Energy is also urging more data collection on air emissions.

The Pennsylvania chapter of the Sierra Club, which the paper gives credit to for funding, has not yet publicized the study and could not be reached immediately for comment. The Sierra Club has been a critic of the shale boom.

Gas produces about half of the carbon dioxide emissions of coal when power plants burn it to make electricity. But there is concern that drilling for shale gas requires so much equipment, water and power that the process could be just as environmentally harmful as mining and burning coal. Methane, the main ingredient of natural gas and a potent greenhouse gas, is also vented during the production process. That has raised questions about the overall impact of drilling for so much gas.

"The GHG emission estimates shown here for Marcellus gas are similar to current domestic gas," said the Carnegie Mellon study.

"Green completion and capturing the gas for market that would otherwise be flared or vented could reduce the emissions associated with completion," said the researcher team, "and thus would significantly reduce the largest source of emissions specific to Marcellus gas preproduction."

Well longevity remains among the unknowns
Still, according to the study, those emissions "are not substantial contributors to the lifecycle estimates, which are dominated by the combustion emissions of the gas."

In defining the life cycle of Marcellus gas, the researchers started with "preproduction." That includes all the preparation of a site before drilling a well, including building access roads. It also includes hydraulic fracturing, more commonly called "fracking." Producers use high-pressure injections of water, sand and chemicals to fracture shale rock about 8,000 feet below the surface. That releases the natural gas.

Once companies extract the resource, gas then requires processing before entering a compressor and getting piped to customers. The researchers assumed that greenhouse gas emissions for shale gas production, transmission, distribution and combustion are roughly the same as for conventional gas production.

The study assumes that 45 percent of water used in the process is reused at the site, and it assumes half is surface water and half is bought from a local water treatment plant.

"Marcellus shale gas production is in its infancy," it says. "Thus, industry practice is evolving and even single well longevity is unknown. Assumptions related to production rates and ultimate recovery have considerable uncertainty."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500