The increase in shale gas production in the United States has led to an interest in the environmental impacts of this unconventional and largely unstudied source of natural gas. Ian Laurenzi and Jersey Gilbert of the ExxonMobile Research and Engineering Company present a life cycle assessment (LCA) of both greenhouse gas (GHG) emissions and freshwater consumption of Marcellus shale gas. This assessment includes processes from drilling the gas well to power generation. Using their elaborated system boundaries, the authors found that a Marcellus shale gas life cycle releases 466 kg of carbon equivalent units per megawatt hour of power produced (kg CO2eq/MWh) and consumes 224 gallons of freshwater per megawatt hour of power produced (gal/MWh). The biggest contributor to both GHG emissions and freshwater consumption is the power plant. The results are similar to previous LCAs of conventional and shale gas and are far lower than results from LCAs of coal. Even considering factors that can increase total results, this study shows that average GHG emissions from shale gas are 53% lower and freshwater consumption is 50% lower than required for an average coal life cycle. —Shannon Julius
Laurenzi, Ian J., and Gilbert R. Jersey. “Life Cycle Greenhouse Gas Emissions and Freshwater Consumption of Marcellus Shale Gas.” Environmental science & technology 47.9 (2013): 4896-4903.
Laurenzi and Jersey use a “from well to wire” approach to study the carbon and water footprints of Marcellus gas. In this study, the shale gas life cycle is defined to include drilling, well completion, wastewater disposal, transportation of gas from well via gathering pipelines, treatment and processing, transmission, and power generation. It also includes consideration of water consumed for hydraulic fracturing and evaporative cooling at the power plant. Excluded from the study are gas distribution networks, which deliver gas for purposes other than electricity. The GHG emissions are expressed in units of CO2 equivalents based on an IPCC specification. The idea behind this unit is to make the “global warming potential” for all green house gases comparable. This study used 100-year global warming potential values of 25 kg CO2eq/kg CH4 (methane) and 298 kg CO2eq/kg N2O (nitrous oxide). The functional units for the whole study were kg CO2eq/MWh (amount of gas released per unit of power produced) and gal/MWh (gallons of water consumed per unit of power produced). The authors used data from over 200 Marcellus shale wells in West Virginia and Pennsylvania. Their information largely came from XTO Energy, a subsidiary of ExxonMobil, and where data was not available they used established standards from different regulatory agencies or publicly available data. Modeling of the power generation stage used a combined cycle gas turbine power plant operating at 50.2% efficiency.
The authors’ calculations revealed that the total life cycle GHG emissions of Marcellus shale gasses are 466 kg carbon equivalent units per MWh of power produced. The majority (almost 78%) of emissions occur at the power plant. The second most significant source of GHG emissions are the gas engines that drive the gathering system compressors, which are part of the system that transports gas from the well to a central location. Hydraulic fracturing activities are only responsible for 1.2% of the lifecycle GHG emissions. Only 1.17% of total GHG emissions are specific to Marcellus shale gas production and processing, making the difference between Marcellus shale gas and conventional gas statistically insignificant. Some other sources of emissions are: transmission compressors, transmission losses, processing plant compressors, processing losses, pneumatic devices and chemical injection pumps, and road transportation for well maintenance.
The total life cycle water consumption is 224 gallons of freshwater per MWh of power produced, with 93.3% of that total occurring at the power plant. Of the remaining water consumed, 91% (13.7 gal/MWh) goes towards hydraulic fracturing operations. That figure includes water used in the life cycles of gasoline or diesel used to power the fracturing process or for transportation. Water is also consumed during drilling, casing manufacture, and road transportation for well maintenance.
The results of this assessment are dependent on the particular boundaries chosen to represent the life cycle of shale gas. The most important parameter is the expected ultimate recovery (EUR) of natural gas from a well, since there will be more greenhouse gases released per unit of power produced if more wells are needed to yield the same amount of natural gas. The GHG emissions associated with life stages besides drilling and completion are independent of EUR. Still, there is a strong inverse relationship between EUR and total lifecycle GHG emissions. Other important parameters are pipeline length, gas composition, water scarcity in the region, and other infrastructural elements.
Another factor that could greatly change the final result is the efficiency of the power plant. This assessment used an efficiency of 50.2%, which is relatively consistent with the 80% of U.S. power plants that operate within the range of 42-48% efficiency. When the authors present a separate GHG distribution using data from the less efficient, currently operating U.S. power plants, they get a distribution that is wider with a higher average of GHG emissions. Even so, the highest possible level of Marcellus shale gas emissions from this higher life cycle distribution is lower than the lowest possible GHG emission for an average coal life cycle.
Despite the potential for variability of results due to the previously stated factors, the result of 466 CO2eq/MWh is consistent with other published life cycle assessments for conventional and shale gas, and almost all of the 14 studies fall within the 10%-90% range of 450-567 CO2eq/MWh.