Estimating methane emissions from natural gas extraction using tower-based atmospheric monitoring
Monitoring methane around the Marcellus Shale is necessary to understand the actual leakage of greenhouse gases from shale gas production. Leakage rates are not well known (Jiang et al. 2011), and this has large implications for the greenhouse impact of this energy source. If the total leakage rate is greater than 3.2% of production, then coal production is better in terms of greenhouse gas emissions in the short term (Alvarez et al. 2012).
2. Research questions and objectives
How do atmospheric mole fractions (concentrations) upwind of, and within an area of shale gas production vary with atmospheric conditions? Our ultimate goal was to infer methane emission rates from these data.
Two towers, one in rural New York as a background site and the other in the midst of natural gas extraction in northeastern Pennsylvania (Figure 1), were the main sources of data for this project. The background site was selected assuming there is no fracking at Site 1 and when the wind is aligned Site 1 does not receive methane signals from Site 2. A cavity ring down spectrometer collected measurements of H2O, CH4, and CO2 mole fractions at these tower sites. Surface weather stations were used for weather monitoring. The weather and mole fraction data provided the basis for our analyses of methane emissions. We compared the "background" conditions to mole fractions downwind or within the fracking region, and segregated these data according to wind direction and atmospheric stability conditions.
We will also estimated emissions from one strong CH4 plume using a Gaussian plume model. The plume model solved for the source strength (Panofsky and Dutton, 1984), applicable to the dispersion of nearby sources. The dispersion coefficients are functions of the distance downwind. Equation (1) was applied to the spike in methane on the afternoon of 23 June in PA (Figure 2) assuming we measured a plume emitted from a nearby, early production well.
The data in Figure 2 indicate that Pennsylvania location usually picked up a slightly higher methane signal than the New York tower.
Figure 1 (above): Locations of the wells in reference to the towers- red dots represent legally permitted wells as of January 2013.
Figure 2 (below): All day methane mole fraction data from June-July 2013
We focused in on the spike in the data on 23 June. This was done to obtain a sample of leakage rate data for a PA well within 2 miles of the tower collection site. The estimated leakage rate based on our plume calculation was 261,000 grams of methane per day. While a typical well in early production in the Marcellus area produces 59,956,000 grams of methane per day, the actual production on any particular day of the well or wells causing the measured peak is difficult to determine An early production rate of a typical Marcellus well was used because that is when the well is the most productive.
These results show evidence of regional methane emissions from gas production (Figure 2). We have also made a first attempt at estimating emissions as a fraction of gas production. This study shows the first continuous dataset of atmospheric methane concentrations from a tower-based network. The data from the two sites will be used to estimate regional emissions in future studies.
Acknowledgements. I would like to thank the National Science Foundation and this REU Program for supporting my research, Penn State Earth and Environmental Sciences Institute for support for our field measurements, and all Penn State faculty and staff who helped me complete my research goals