“We need to understand the cause and prevalence of these conditions to better understand the role [unconventional] natural gas can play in greenhouse gas emission mitigation,” Garvin Heath of the US National Renewable Energy Laboratory told environmentalresearchweb, “and to verify the estimates found through lifecycle assessment with a robust set of measurements at all scales – component to continental – and in different locations and times.”

What makes the difference, it seems, is the way the shale gas is produced. Shale wells with low lifetime production and high emissions from liquids unloading – a periodic clearing of built-up liquids and debris from the well – had the largest lifecycle greenhouse gas emissions, Heath and colleagues from the US National Renewable Energy Laboratory and Joint Institute for Strategic Energy Analysis found. The most emitting liquids unloading processes could produce was around 200 g of carbon dioxide equivalent per kWh of electricity generated, potentially producing 30% of shale gas’s entire lifecycle emissions.

The US Environmental Protection Agency’s most recent New Source Performance Standard and National Emissions Standards for Hazardous Air Pollutants rules do not address liquids unloading, the team notes in PNAS.

To investigate, the researchers harmonized results from nearly 100 lifecycle assessments, as well as carrying out a sensitivity analysis for well completion, well recompletion and liquids unloading. To create (“complete”) a shale gas well, liquids are forced underground at high pressure to fracture the rock and allow trapped gas to flow out, in a process known as hydraulic fracturing or fracking. This process may also be repeated later to extend the lifetime of the well, in what’s known as recompletion. Shale gas lifecycle emissions were most sensitive to assumptions about liquids unloading, the study showed.

Only the hydraulic fracturing process and subsequent flowback of the liquid to the surface are unique to shale gas, compared with conventional gas, the team stressed.

“We have clarified that the lifecycle greenhouse gas emission profile of natural gas somewhat depends on how it is produced, in addition to the thermal conversion efficiency of how it is used [to produce electricity],” said Heath. “We need to investigate this further to assure the climate mitigation potential of natural gas.”

To harmonize the lifecycle assessments, the team altered factors such as the unit of measurement of greenhouse gas, the assumed thermal efficiency of the power plant, the global warming potential of methane in line with the latest IPCC assessment, and more. The team did not harmonize methane leakage rates as the values reported covered a wide range, and new measurements of this parameter are underway.

“Harmonization is a meta-analytical technique that follows state of the science procedures for systematic review of prior work with an approach that ensures fair comparisons between two independently produced estimates of lifecycle greenhouse gas emissions by ‘harmonizing’ certain technical assumptions and methodological choices,” said Heath. “Transparency in our method and results is highlighted, with users able to adjust results to reflect their own choices on the harmonized factors. Harmonization is distinct from other meta-analytical techniques such as Monte Carlo, which is more statistically focused and requires assumptions to construct the input probability distribution functions on many parameters we don’t have much information about.”

The team’s harmonization process for a subset of the 100 lifecycle assessments resulted in median emissions values of 465 g CO2e/kWh for shale plus unconventional gas and 461 g CO2e/kWh for conventional natural gas.

Heath plans to continue his work on lifecycle assessments in order to help the interpretation of results for decision-making about energy choices. “In addition, I am helping to lead a new study that aims to reconcile the divergence in results between measurements of methane emissions and those derived from an approach analogous to lifecycle assessment – inventories, such as the US EPA’s national greenhouse gas inventory,” he said.

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