by Shannon Julius
The long term environmental concerns having to do with shale gas development are primarily greenhouse gas (GHG) emissions and freshwater consumption, as other forms of environmental degradation can be remediated over time. Ian Laurenzi and Jersey Gilbert (2013) of the ExxonMobile Research and Engineering Company performed a life cycle assessment (LCA) of both GHG emissions and freshwater consumption of Marcellus shale gas. The life cycle begins with well drilling and ends with burning the fuel for power generation. Using their elaborated system boundaries, the researchers found that a Marcellus shale gas well 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) over the course of its lifetime. The biggest contributor to both GHG emissions and freshwater consumption is power generation. The result of this study are highly dependent on the variables chosen to represent the shale gas well life cycle, especially the expected ultimate recovery of natural gas. Despite the potential for variability of results, 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 other studies fall within the 10%–90% range of 450–567 CO2eq/MWh. 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 what is required for an average coal life cycle.
Laurenzi and Jersey used a “from well to wire” approach to study the carbon and water footprints of Marcellus gas. In this study, the shale gas life cycle was defined to include drilling, well completion, wastewater disposal, transportation of gas from well via gathering pipelines, treatment and processing, transmission, and power generation. It considered water consumed for hydraulic fracturing and evaporative cooling at the power plant. Excluded from the study were 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 purpose for this unit is to make the “global warming potential” for all greenhouse gases comparable. This study used 100-year global warming potential values of 25 kg CO2eq/kg for methane (CH4) and 298 kg CO2eq/kg for nitrous oxide (N2O). 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). These units mean that the study did not report GHG emissions and power consumption as a gross total amount but as a ratio of resources inputted to power outputted. 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 were 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 (CCGT) power plant operating at 50.2% efficiency.
The researchers’ calculations revealed that the total life cycle GHG emissions of a Marcellus shale gas well are 466 kg CO2eq/MWh. Almost 78% of emissions occur at the power plant, where the shale gas is burned to create electricity. The second most significant source of GHG emissions are the gas engines that drive the gathering system compressors, which are part of the network that transports gas from various wells to a central location. Hydraulic fracturing activities are only responsible for 1.17% of the life cycle GHG emissions. As hydraulic fracturing is the major difference between shale gas and conventional gas life cycles, only 1.17% of GHG emissions are specific to Marcellus shale gas production and processing, making the difference between Marcellus shale gas and conventional gas emissions 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 gal/MWh, with 93.3% of that total occurring at the power plant, where water is used for cooling. Hydraulic fracturing requires 6.2% of the total (13.7 gal/MWh), representing the majority of the water consumed before the power plant. That figure takes into account the water used in the life cycles of gasoline or diesel that are used to power the fracturing process or vehicular transportation. The final 0.5% of water is consumed during drilling, casing manufacture, and road transportation for well maintenance.
Researchers ran a simulation that varied different parameters of the LCA for GHG emissions and measured how much the final results changed. This simulation determined that life cycle GHG emissions are most dependent on the expected ultimate recovery of natural gas from a well. This reflects the fact that the initial investment of resources for well drilling and completion will yield more power over the course of the well’s lifetime if more natural gas is recovered from the well. Other important parameters are pipeline length, gas composition, water scarcity in the region, and other infrastructural elements.
The efficiency of the power plant is a factor that could greatly change the final result, as it is dependent on power output. This assessment assumed an efficiency of 50.2%, which is relatively consistent with the 80% of US power plants that operate within the range of 42–48% efficiency. An additional life cycle assessment using the efficiencies of the currently operating US CCGT fleet resulted in a distribution that was wider with a higher amount of GHG emissions. Even so, comparing the results of this higher, more realistic life cycle distribution to the GHG emissions of an average coal life cycle (calculated by a different research group) shows that the carbon footprint of Marcellus shale gas is approximately 53% that of coal. The highest possible level of Marcellus shale gas emissions from this higher life cycle distribution is lower than the lowest possible GHG emission level of coal. Using the same LCA for coal, researchers determined that the freshwater consumption of shale gas is approximately 50% that of coal.
Laurenzi, I., Jersey, G., 2013. Life cycle greenhouse gas emissions and freshwater consumption of Marcellus shale gas. Environmental Science and Technology 47, 4896–4903. http://bit.ly/1sMLdtS