20th September 2013.
A study published in PNAS has reported new measurements of methane emissions associated with shale gas production sites. The research included 150 production sites with nearly 500 wells produced through fracking, as well as measurements of 27 “well completion flowbacks”. After a well is drilled it has to be cleaned of sand and liquids that had been injected into it during its creation, and during this cleaning (the “flowback”) methane can be emitted. An average value of 1.7 Mg of methane was reported for the flowbacks, with a range of approximately 0.01 – 16 Mg. The large range is partially due to the duration of flowbacks, some of which lasted for two weeks and featured large amounts of gas flaring. This figure is substantially lower than the average value of 81 Mg per flowback cited by a recent EPA inventory, and the authors of the PNAS paper suggest that this discrepancy is due to efficient capture and control of potential methane emissions.
As well as flowbacks, the study also examined “unloadings”; during the lifetime of a well, water and liquid hydrocarbons accumulate and periodically need removing. For the 9 unloadings measured, the average release of methane was 1.1 Mg (range 0.02 – 3.7 Mg), although the frequency of unloading events varied between wells. Taking this into account, the average emission per well per year was calculated as 5.8 Mg. Whilst acknowledging the limited sample size and large uncertainty, the authors state that current EPA estimates overpredict measured unloading emissions.
Finally, the paper looked at 150 well sites that were under routine operation. Methane emissions from pneumatic chemical injection pumps averaged 3.7 g CH4 per minute, slightly lower than EPA estimates. However, intermittent pneumatic devices averaged 5.9 g CH4 per minute, whilst the average for low bleed pneumatic devices was 1.7 g CH4 per minute; both higher than EPA estimates. Equipment leaks averaged 1.2 g CH4 per minute which is similar to the EPA inventory.
If these figures are scaled up the cited figure is that these sources emit 957 Gg CH4 per year, compared with EPA’s estimate of 1211-1250 Gg CH4 per year. The authors do cite the caveat that their estimated uncertainty in this calculation is 200 Gg. Adding this to other associated sources gives a total of 2300 Gg of methane from US natural gas production. To conclude, it seems that methane emissions from natural gas production may be decreasing, partly due to federal regulations and EPA standards.
Reference: Allen et al, 2013. Measurements of methane emissions at natural gas production sites in the United States. PNAS, 10.1073/pnas.1304880110.
Photo: By Joshua Doubek (Own work) [CC-BY-SA-3.0] via Wikimedia Commons.