Range A Pilot Study by Michael A. Levi by Gabrielle Petron et al. Accepted for publication in Journal of Geophysical Research Atmospheres

Transcription

1 Reply to Reply to Comment on Hydrocarbon emissions characterization in the Colorado Front Range A Pilot Study by Michael A. Levi by Gabrielle Petron et al. Accepted for publication in Journal of Geophysical Research Atmospheres Michael A. Levi 1 1 Council on Foreign Relations, New York, NY, USA Abstract The reply by Petron et al. [2012a] to my comment, Levi [2012], on their paper, Petron et al. [2012], defends their original conclusion: fugitive emissions in an area that they observed in 2008 were likely larger those indicated by previous estimates. The reply also claims to have now better characterized the uncertainties in their and my approaches. In particular, they question the representative nature of a data set that Petron et al. relied on, and that Levi retained, though they do not recalculate their estimates. Petron et al. also claim to show that the analysis in Levi [2012] is flawed because, they argue, it implies the physically unrealistic conclusion that fugitive emissions from the natural gas operations that Petron et al. studied may be negative. Petron et al. [2012a] are reasonable to be skeptical of the data set that they highlight. I show in this reply that if that data set is indeed not representative of condensate tank flashing in the area under study, it is impossible to produce the results in Petron et al. [2012] that the authors now defend. I also show that the other critique of Levi [2012] offered by Petron et al., i.e. that

2 Levi s analysis has physically unrealistic implications, relies on an incorrect interpretation of the formulas in that comment. Only one of two conclusions is possible: the results in Levi [2012] are correct, or the conclusions in both Levi [2012] and Petron et al. [2012] are unjustified. 1. Introduction and Questionable Assumptions Petron et al. [2012] and Levi [2012] both estimate fugitive methane emissions from natural gas operations by assuming that observed methane and light alkanes in the air sampled by Petron et al. [2012] came primarily from fugitive emissions at natural gas operations and flashing of condensate tanks. Both juxtapose the same atmospheric observations of the ratio of methane-topropane with the same range of possible profiles for flashing emissions, replying on profiles from 16 condensate tanks (all reported in [Petron et al. 2012]). Petron et al. [2012] and Levi [2012] also constrain the possible composition of fugitive natural gas emissions by using the same set of 77 profiles for Colorado natural gas wells (reported in [COGCC 2007]), though in different ways. Following Petron et al. [2012a], we use the term fugitive emissions rather than venting here, in order to be more general regarding possible emissions sources. We include in fugitive emissions all methane, propane, and n-butane emissions upstream of natural gas processing facilities, including from well venting, incomplete combustion of flares, compressor engines, and gathering systems. We assume here (as is done implicitly in Petron et al. [2012] and Petron et al. [2012a]) that these all exhibit the same ratios of methane-to-propane and methaneto-n-butane as raw gas, an assumption that we revisit below. (Our definition of fugitive

3 emissions does not include products of incomplete combustion, such as formaldahyde, which are not fugitives and are of no consequence to the analysis here.) Petron et al. [2012] discard data points directly downwind of a natural gas and propane processing plant that leads to strong propane enhancements, eliminating that as a possible source. Petron et al [2012] assume that the ratio of methane to propane in fugitive emissions is bounded by the mean (24.83) and median (15.43) of that ratio in the 77 wells. Levi [2012] notes that the results in Petron et al. [2012] are highly sensitive to this assumption and argues that there is no basis for it, given that no claim is made that the 77 wells are representative of either producing or fugitive-emissions-prone wells in the area under study. Levi [2012] notes that a ninety percent confidence interval for the ratio of methane-to-propane in the 77 well sample more appropriate given that nothing is known about whether the sample is represenatative contains wells with methane-to-propane ratios that range from 8.79 to It observes that, using the methodology in Petron et al. [2012], this implies a fugitive methane emissions rate that is bounded below by 66 Gg/yr well below the lower uncertainty bound presented in Petron et al. [2012] and defended in Petron et al. [2012a], but consistent with previous estimates and is not bounded above. Petron et al. [2012a] do not respond to this critique directly. They do, however, address it indirectly in the context of arguing that other results in Levi [2012] are implausible. They observe that in the WRAP Phase III inventory [Bar-Ilan et al 2012] of oil and gas activities in the area under study, 49% of the total VOC fugitive emissions were attributed to pneumatic devices and 32% to unpermitted fugitives emissions from various pieces of equipment at well pads. They then note that, in 2009, Colorado implemented regulations requiring low-bleed controllers on all valves in the area under examination, and infer that high-bleed valves were

4 likely used across the oil and gas field during the period that Petron et al. [2012] studied (in 2008). From this, they conclude that unpermitted fugitive emissions were also likely ubiquitous in the Denver-Julesburg Basin in There are two main problems with this reasoning. The fact that regulations were imposed in 2009 does not imply that emissions were uniformly distributed ( ubiquitous ) among different types of wells prior to that year. (Regulations often impose uniformity on previously uneven practices.) More important, as pointed out in Levi [2012] but not addressed in Petron et al. [2012a], Petron et al. [2012] show that assuming that the WRAP Phase III figures are correct implies that fugitive emissions in the Denver-Julesberg basin are far lower than the alarmingly high ones that Petron et al. [2012] estimate using their top-down methods. Indeed a central conclusion in Petron et al. [2012] is that bottom-up estimates of methane emissions based on the WRAP Phase III figures are far too low (which suggests that something is missing from them). The implication is that either the WRAP Phase III figures are incorrect, in which case they should not be used to defend the conclusions in Petron et al. [2012], or they are are correct, in which case they contradict the conclusions in Petron et al. [2012] that are defended in Petron et al. [2012a]. Neither possibility reinforces the conclusions in Petron et al. [2012]. 2. Summary of Critique of Levi [2012] Unlike Petron et al. [2012], Levi [2012] makes no assumptions about the ratio of methane to propane in fugitive natural gas emissions. Instead, it uses observations of the atmospheric propane-to-n-butane ratio reported in Petron et al. [2012], together with a 95-percent confidence interval for the propane-to-n-butane ratio in fugitive natural gas emissions for the 77 wells also

5 used by Petron et al. [2012]. This allows it to constrain estimates of fugitive methane emissions. Its results are consistent with previous bottom-up studies but not with the top-down estimates in Petron et al. [2012]. Petron et al. [2012a] do not directly challenge any of the physical reasoning behind the analysis in Levi [2012]. Instead they claim to show that the approach taken in Levi [2012] implies physically implausible results, and hence is problematic. They then infer that the supposedly implausible results may be partly explained by the fact that the full range of possible flashing emission profiles and the total VOC flashing source are likely not captured by the set of 16 flashing emission estimates used by Petron et al. [2012] and Levi [2012]. Some skepticism regarding the representative nature of the flashing profiles is reasonable (though Petron et al. [2012a] do not justify the conclusion that they are likely wrong). We show below, however, that their critique of the remainder of the methodology in Levi [2012] is unsound. Moreover, applying the critique of the 16 flashing emission profiles in Petron et al. [2012a] to the methodology in Petron et al. [2012] severely undermines their own original results. 3. Physical Realism of Levi [2012] Levi [2012] derives formulas for fugitive emissions of methane, propane, and n-butane given atmospheric methane-to-propane and methane-to-n-butane ratios; assumptions about the relationship between propane and n-butane concentrations in fugitive-emissions-prone wells derived from the dataset of 77 wells; and a range of possible emissions from condensate tank flashing (which are constrained to be linear combinations of the profiles considered by Petron et al [2012]). Petron et al. [2012a] do not directly challenge the physical logic of any of the

6 analytical steps in Levi [2012], nor do they challenge the atmospheric concentration ratios used in Levi [2012], or the assumptions that Levi [2012] makes about gas that enters the atmosphere as fugitive emissions. They do note in passing that Levi [2012] does not discuss the possibility that there is a third major source of n-butane emissions, other than fugitive emissions and flashing, that is co-located with, and proportional to, existing sources of propane emissions. For there to be a third source matching this description, there would need to be a common driver of all fugitive and flashing emissions that also drove significant n-butane emissions from this third source. This same source could not generate significant propane or methane emissions; otherwise, it would invalidate the critical assumption in Petron et al. [2012] that there are only two main types of sources of such emissions in the area under study. Petron et al. [2012a] offer no suggestions for what this highly unusual source might be. That said, it is possible in principle that incomplete combustion of raw gas in flares and compressors could result in emissions that have a different ratios of methane-to-propane and methane-to-n-butane from those characteristic of either raw gas venting or condensate tank flashing. If such fractionation is occurring strongly and at a significant scale, then the two-source assumption used in Petron et al. [2012] and retained in Levi [2012] and Petron et al. [2012a] is incorrect. This could, in principle, fatally undermine the results in both papers. Petron et al. [2012a], in any case, focus instead on the use the 16 condensate tank flashing profiles, first used in Petron et al. [2012], in Levi [2012]. They write that Levi s calculation implies that only one condensate tank flashing profile (tank #14) out of the set of 16 in the CDPHE dataset results in non-negative propane and butane fugitive fluxes. For the other 15 modeled flashing composition profiles, Levi s calculations require that fugitive emissions must remove propane and n-butane to match the atmospheric ratios.

7 This is incorrect. No tank profile results in anything; the relationships in question are only arithmetic. To be certain, if one assumes that all flashing emissions come from a single tank profile, then for 15 of the 16 profiles, the implied fugitive emissions of propane and n-butane are indeed negative. But there is no reason to believe that flashing emissions are due to a single tank profile. The correct implication to take from the exercise in Petron et al. [2012a] is that, as shown in Levi [2012], it is not plausible that all flashing emissions come from the same profile. Indeed it would be quite surprising if they did. Petron et al. [2012a] continue by arguing that Levi s minimum fugitive methane emission estimates rely on a combination of the tank #14 positive estimate and a negative contribution from the estimate associated with tank #8. Given that we are dealing with source processes [leaks] that must be zero or positive [emitting to the atmosphere, not removing mass from the atmosphere], any single fugitive emission has to be positive to be considered part of the solution. Based on this, Petron et al [2012a] reject the remaining analysis in Levi [2012]. Their reasoning is incorrect: it attributes physical causation ( removing mass from the atmosphere ) to an arithmetic manipulation. Given a set of constraints on atmospheric concentrations of methane, propane, and n-butane, and a set of possible values of emissions from certain sources that contribute to these, Levi [2012] estimates emissions from other (unknown) sources by requiring that the combination add up to the observed concentrations. There is nothing unusual or unphysical about the fact that higher emissions from some sources will imply lower emissions from others, given constraints on their total. These lower emissions are the inappropriately labeled negative or removed emissions that Petron et al. [2012a] have identified. Petron et al. [2012a] insist this is unacceptable, and thus conclude that there is something fundamentally flawed in the approach in Levi [2012]. There is no reason to do so.

8 4. Condensate Tank Flashing Profiles Petron et al. [2012a] do, however, reasonably point out that the results in Levi [2012] indicate that fugitive emissions are biased toward dry gas wells. In part for that reason (though in part because they are skeptical of the negative propane and butane fugitive emissions) they reasonalbly suggest that the 16 flashing profiles used in Petron et al. [2012] and retained in Levi [2012] may be unreliable. (This is in part why Levi [2012] noted that statistically meaningful samples of flashing emission profiles from condensate tanks would be valuable.) The implication is that the results in Levi [2012], which rely on the flashing profiles in question, are likely incorrect. This critique is inconsistent with the defense of the ultimate results in Petron et al. [2012] by Petron et al. [2012a]. In Petron et al. [2012], the formula for fugitive methane emissions contains a factor equal to the difference between the mass of propane from flashing emissions (scaled by the atmospheric ratio of methane to propane) and the mass of methane from flashing emissions (equation 3). But Petron et al [2012a], after calling the tank flashing profiles relied upon in Petron et al [2012] into question, suggest no new constraints on tank flashing emissions. If the flashing profiles used in Petron et al [2012] are unreliable, there is now no way to calculate that paper s estimates of fugitive methane emissions, which depend vitally on the flashing profiles. One can only conclude that either the flashing profiles are reasonably representative in which case Petron et al [2012a] have presented no reason to question the results in Levi [2012] or the flashing profiles are unrepresentative, in which case neither Petron et al. [2012] nor Levi

9 [2012] have any basis to report reliable estimates of fugitive methane emissions. In either case, the results reported in Petron et al. [2012] are without foundation. Since the flashing profiles of condensate tanks in the area under study have likely changed since Petron et al. [2012] collected their data in 2008 (Petron et al [2012a]), this part of the debate is unlikely to be resolved definitively. Debate and observations should focus on rigorously understanding what is happening today through multiple observational and analytical methods. Several data collection efforts that could enable this are currently underway [EPA, 2012]. References Bar-Ilan, A., and R. Morris (2012), Final Emissions Technical Memorandum, No. 4a (Colorado Basins), 33p, available at Colorado Oil and Gas Conservation Commission (COGCC) (2007), Greater Wattenberg area baseline study, report available in the Library section at EPA (2012), Summary of Stakeholder Workshop on the U.S. GHG Inventory for Natural Gas Systems Held September 13-14, 2012 in Washington, DC, available at orkshop_ng_in_the_ghg_inventory_report.pdf Levi, M. A. (2012), Comment on Hydrocarbon Emissions Characterization in the

10 Colorado Front Range A Pilot Study, J. Geophys. Res., 117, D21203, doi: /2012jd Pétron, G., et al. (2012), Hydrocarbon Emissions Characterization in the Colorado Front Range A Pilot Study, J. Geophys. Res., 117, D04304, doi: /2011jd Petron, G., G. J. Frost, M. K. Trainer, B. R. Miller, E. Dlugockencky, and P. P. Tans (2012a), Reply to comment on "Hydrocarbon emissions characterization in the Colorado front range - A Pilot Study" by Michael A. Levi, J. Geophys. Res., doi: /2012jd018487, in press. Pétron, G., et al. (2012), Hydrocarbon Emissions Characterization in the Colorado Front Range A Pilot Study, J. Geophys. Res., 117, D04304, doi: /2011jd