Methane is a remarkable molecule that is the center of attention in the conflict on fracking (hydraulic stimulation of gas and petroleum production). Methane is a greenhouse gas with heat trapping efficiency that is about 25 times that of carbon dioxide.
Thus, small leaks of methane would adversely impact global warming. Plus leaks of methane represent an economic loss and a potential safety hazard. Recall the San Bruno Fire in 2010.
A webinar lecture by Prof. Mark Zondlo of Princeton University (Princeton, NJ) discussed various estimates of leakage associated with the production of natural gas. Usually, natural gas from petroleum-bearing formations contains higher hydrocarbon homologs e.g., “condensates,” including ethane and propane. In contrast, methane from coal deposits is usually free of ethane and condensates other than water. This helps differentiate between possible sources. Next, surface sources include radioactive 14C from cosmic rays interacting with nitrogen in the atmosphere plus residue from the nuclear weapons programs.
Methane remains in the atmosphere for a decade or longer, with a 1/e folding time of 13 years. Methane degrades in the upper atmosphere by oxidation with water.
Prof. Zondlo pointed out that recent estimates of leakage associated with the production and distribution of natural gas vary over a range of 12%–0.3%. Important variables include location and analytics and well history. Others have reported that more gas escapes during drilling and rework than during normal production. At the high end, the financial loss would be sufficient to justify fixing the problem. Even at 1% ($0.03/MBTU) this should be attractive.
The wide range in leakage depends on the analysis method selected by the author. However, it also probably reflects the difficulty in measuring methane production and distribution. Prof. Zondlo used a variety of sensors to study the sources, including wellheads and compressors. Remotely piloted drones were used for aerial surveys at low altitude. When the air was very still, one could occasionally correlate the gas with expected sources, but when the air was moving, the methane plumes were narrow and very active vertically, producing narrow currents that were difficult to correlate with surface features.
Long-range optical transits were also investigated with new lasers, such as quantum discharge lasers, that operate at wavelengths where methane has strong absorbance (~3.7 μm). Lasers and reflectors facilitate monitoring atmospheric methane over a pathlength of several kilometers. This seems to be suitable for macroscopic events, but not very useful for helping to locate and remediate leaks.
It seems that controlling methane emission will need to be very localized with sensors mounted on pipes, valves, tanks, and compressors. Indeed, Prof. Zondlo solicited others to work on developing field-deployable methane sensors. Potential point sources will need to be identified and evaluated. If the risk is not negligible, monitoring is called for.
Recognizing that once the gas is above ground at the wellhead, it is vulnerable to leaks in collection piping, compression, and transmission to the eventual consumer. The longer the trip, the greater the risk of loss. For this reason, it seems that natural gas should be converted to more benign energy forms such as electricity close to the wellhead. Gas turbine generators seem to be the most obvious choice. Exhaust gas should be recovered and reinjected into the producing formation.
Robert L. Stevenson, Ph.D., is Editor, American Laboratory; e-mail: [email protected].