A new life cycle assessment for UK shale gas has been released by researchers from the University of Manchester. This study considers a range of potential environmental impacts. Most significantly, it compares shale gas extraction against a host of other energy technologies, including conventional gas, coal, nuclear power and renewables.
This sort of comparison is very important, because when it comes to energy sources, we have to choose how our energy mix should be balanced. All sources of energy have impacts, and we can't say no to all of them.
Equally significant is the fact that the study doesn't just consider the global warming impact of the various technologies (the global warming potential, or GWP), but a whole host of environmental factors , including the use of abiotic resources (rare earth elements etc), acidification, eutrophication, freshwater, marine, terrestrial and human toxicity, and ozone depletion.
Before I get into the details of the study, the first figure I'll borrow is the one that compares the global warming potential (GWP), according to the various previous studies:
I do so to re-iterate the point, made many times by many different people, that the Howarth study famous for claiming that shale gas is worse than coal, is a real outlier. There is clear consensus that shale gas is better than coal with respect to global warming.
The following bar charts show the environmental impacts of each technology in each category, and I discuss the results in greater detail below.
The consensus that shale gas is significantly better than coal is again reached in the new study, which finds that shale gas has a similar GWP to conventional gas, and in fact a lower footprint in comparison to LNG, which has to be liquified, transported across oceans and re-gasified. By assuming the worst case for all possible variables, the researchers are just able to make shale gas as bad as coal, and 2.5 times worse than North Sea gas, but they concede that this extreme end-member "is probably not realistic" (it requires an average EUR of 0.1bcf per well, and no attempts to mitigate gas venting during completion).
In the best possible case (high average EUR and no gas vented during completion), shale gas global warming potential (GWP) is only 2.8% higher than North Sea gas, but 15-19% lower than imported LNG. Compared to coal, the central shale gas case has a GWP 51-58% lower, a significant advantage. Of course, compared to non-fossil fuel sources of energy (nuclear, solar, wind), all of the above are still fossil fuels and so have far higher GWP.
An important issue facing some energy technologies is the rate at which they use up natural resources such as metals, and in particular rare earth elements which are vital in many modern technologies. The abiotic depletion potential of elements (ADP-E) for shale gas is 18% lower than wind energy, and 94% lower than solar energy. This demonstrates that while renewable energy is often considered clean, it does still have an environmental footprint. Because of the increased footprint of shale gas with respect to conventional gas, shale gas ADP-E is 81% higher than North Sea gas, but on a similar level to coal.
The acidification potential describes the quantity of SO2 released by an energy technology. This study finds that the AP for shale gas is between 4 - 7 times worse than North Sea gas. However, the error bars on this factor are huge, ranging from double that of any other technology to less than coal, and interestingly, 60% lower than solar power.
The main sources of SO2 are in diesel engines used to power the drill and fracking pumps, and in gas sweetening, where H2S present naturally in the gas is removed before it is sold. The study assumes an H2S level equal to that of conventional gas, which at face value is a reasonable assumption. However, I've not heard that so-called "sour" gas has been an issue in many shale plays, where the gas is generally found to be very "sweet" (low in H2S). If this is the case in the UK (I'm not aware of any analysis of the gas produced by Cuadrilla after their stimulations in 2011) then the AP would be towards the lower bound, making it a lower emitter of SO2 than solar power.
The report considers toxicity in marine, freshwater, terrestrial and humans separately. However, the results paint a very similar story for these different factors.
For freshwater toxicity potential, the report finds that shale gas is comparable to natural gas, and is an order of magnitude better than nuclear, offshore wind and solar.
The human toxicity potential for shale gas comes in with a lower impact than nuclear (which is 5 times worse), solar (6 times worse) and coal (10 times worse)!
The marine toxicity potential of shale gas comes in between 1.6-7.8 times lower than nuclear, offshore wind and solar, and a whopping 45 times lower than coal.
The only toxicity potential in which shale gas comes in worse than other sources is terrestrial toxicity, where the impact is 13-26 times worse than conventional gas, and between 2-4.4 times worse than coal, nuclear, wind and solar.
It is worth noting that the major source of this toxicity potential for shale gas is how drilling waste is disposed of. In the central scenarios, 60% of the waste is disposed of by landfarming, a process whereby waste is ploughed into the soil, allowing soil microbes to degrade any harmful contaminants. Alternatively, drilling waste can be treated and sent to landfill, which substantially reduces the toxicity potential. If instead 100% of the drilling waste is sent to landfill, the terrestrial toxicity potential can be reduced to an amount that is an order of magnitude lower than solar, wind and coal.
For me these toxicity potentials are a surprising result. We hear much about the potential pollution, and impacts on human health and the environment, engendered by an increase in shale gas production in the UK. However, when a full life-cycle analysis of impacts is performed, shale gas comes in lower for a range of toxicity potentials than a range of alternative energy sources - sometimes by an order of magnitude or more! Although the results are less surprising when you learn about the extraction of rare earth elements in places like China.
Shale gas is found to have a similar impact on ozone depletion as conventional gas. However, both shale gas conventional gas have a high impact compared to other sources, 25 times higher than wind and nuclear. This is because of fire-retartant gases such as halon used in pipelines. That said, shale gas still has a similar ozone depletion potential as solar (the high values for solar are due to the manufacture of tetrafluoroethylene used in the panels).
Photochemical ozone creation:
This seems to be the only factor in which shale gas appears to perform badly, with a POCP factor worse than solar, wind and nuclear by between 3, 26 and 45 in the central case, and 3.3 and 5.6 times even in the best case. The main source of POCP is in gas sweetening, and as I comment above, if UK shale gas has a low H2S content then these effects will be mitigated somewhat.
The advantage of a LCA paper like this is that you can look to see what aspects of the life cycle of shale gas production have the greatest impact, and therefore what should be the key steps taken to minimise impact.
With respect to GWP, the advantage of shale gas with respect to both coal and LNG requires low rates of methane venting during completion. It is therefore important that measures are put in place to ensure that methane is either captured and flared during the flowback processes.
This venting is also important with respect to the photochemical ozone creation factor (POCP) - unburned butane, ethane and other VOCs can contribute to smog. By minimising venting, the POCP factor can be reduced.
Similarly, with respect to toxicity factors, the disposal of drilling waste is the key factor. Landfarming of waste appears to increase toxicity factors substantially. The larger the portion of waste treated and taken to landfill, the lower the toxicity factors.
Therefore if we are to produce shale gas in the UK, green completions (where the amount of gas vented is minimised) should be used, and appropriate treatment pathways for drilling waste are identified.
Finally, I note that all of the above assumptions are determined to a certain extent by the Estimated Ultimate Recovery (EUR). The more gas you get per well, the lower the average impact is (as you have more gas for your buck). This study used a central case EUR of 1bcf. Over the last 5 years, as technology has moved forward we are seeing ever-increasing EURs. Of course, the better your EUR, the better off you are financially. By using better technology, shale gas extraction is better both economically and in terms of environmental impact.