Our intense and escalating addiction to sweet, sweet hydrocarbons combined with our collective paranoia about dependency on foreign oil (damn you Canada!) has driven some pretty clever advances in the petroleum geosciences. Whether in terms of important scientific concepts (like chronostratigraphic approaches to understanding the rock record, or facies model approaches to understanding paleoenvironmental characteristics) or in terms of applied engineering (i.e., all the different ways we now have to acquire and understand subsurface data sets), the need to exploit more and more of the Earth's hydrocarbons have led us into heady intellectual territory.
Some of the schemes utilized to find and recover hydrocarbons from the subsurface are downright Strangelovian. While THE goofiest has to be the Wagnerian boondoggle of CO2 sequestration, a close second in terms of seeming craziness is fracking. Hydrofracking, or simply "fracking" if you're "hip" and "with-it", is the process by which humans enhance the connectivity of an existing oil or gas field. Briefly, let's say you've got an gas field sitting out in the middle of nowhere. You know there's hydrocarbons in them there interstitial pores, but your wells aren't producing at the rate or capacity that you had hoped. What you do then is what every red-blooded human has done since we knapped our first flint; you IMPROVE on nature! In this case, you pump a slurry of quartz and who-knows-what chemicals under high pressure into the reservoir in question. The high pressure fluids initiate and build-up a series of new fractures, while the quartz grains prop open the newly anthropogenic fractures, allowing your hydrocarbon fluids to flow more freely.
Fracking has been done for quite a while; commercial use of hydrofracture techniques began in 1949, although people were apparently sending water/nitroglycerine mixtures into wells to do the same thing as early as the 1860s. A pretty good historiography of hydrofracking techniques can be found here.
Despite this long-term history, Fracking has been in the news recently, on account of Oil Companies expanding interests in shale gas reservoirs. Shales, of course, are noted for their very poor porosity and permeability; fine-grained muds undergo some pretty impressive compaction in the subsurface, resulting in "tight" rock that really restricts the fluid flow paths. Since the universe is largely ruled by Irony, however, mudrocks and shales are also often extremely rich in organic material. Organics, when subjected to the right conditions, are what we pump out of the ground in the form of oil and gas. So, these mud-rich reservoirs are often very high in organic content, but actually REMOVING the fluids-of-interest from those sopping shales can be tricky! What nature fails to provide, human ingenuity must produce; thus, frack the shit out of some shales, and viola! You've got a productive field where once was only a tight, uneconomical subsurface lithosome.
However, some spoilsports don't seem to be willing to embrace the Inevitable March of Progress, and have voiced some concern regarding hydrofracking. One of the things that seems to have really caught a lot of people's attention is the supposed link between fracking and seismicity. People are worried that, by producing new fracture networks in the subsurface, hydrofracking might be causing earthquakes. The idea of human-induced seismicity is one of those sort of Doomsday type things that freak folks out, and there have been a number of reports and anecdotes from folks in and around fracking sites claiming that there has been a noticable increase is earthquakes. But is there any evidence for such a link?
A company in the UK, Cuadrilla Resources, recently published a report that does, for perhaps the first time, actually seem to support the idea that hydrofracking can be linked to seismic events. The company was in the Bowland Basin in Lancashire, trying to exploit a shale gas play through hydrofracking. Their press release can be found here, while the pertinent (and actually fairly interesting) technical report is available as well, titled "Geotechnical Study of Bowland Basin Seismicity".
Briefly, the company had been monitoring seismic activity during their fracking proceedures, generally through the use of down-hole geophones. In the past, these had recorded seismic events well below the "0" magnitude threshold that were apparently closely associated with active hydrofracking. However, there were some larger events, including a 2.3 Magnitude event and a 1.5 Magnitude event that the company believes are highly likely to have been caused by their fracking. Importantly, neither of these events had any sort of structural effect at the surface, and didn't actually cause any sort of damage.
What is interesting is the fact that the company's study has identified a few contingent geological factors that seem to have been part of the reason for the fracking-induced seismic activity. These include structurally complicated stratigraphies with locally variable dips as well as pre-existing faults in the succession, which interacted in a sort of "perfect storm" with the hydrofracking to induce slip and result in a series of small earthquakes, including the two "big" ones mentioned above (2.3 and 1.5). The companies conclusion is that, yes Virginia, fracking DID (most likely) cause these specific earthquakes, but a) they weren't that big and didn't do any damage to structures or the the surface and b) the seismic activity was a result of fracking AND the presence of pre-existing structure. The company points out that these local conditions imply that fracking induced seismicity can be the result of very specific conditions that have to be met. Regardless, the report advocates continued monitoring in this and other settings, which would allow us to better understand both the mechanisms and processes that produce fracking seismicity, as well as predict those conditions in other fields in the future.
It's a pretty interesting report, and freely available online, which is nice. And while it does show that fracking CAN cause seismicity, for the reasons stated above it also shows that this seismicity is not that big a deal and actually probably really rare. And, to be honest, I never really FELT like there was much danger of fracking being a big deal in terms of causing earthquakes or destroying towns. Still, it's nice to have some actually science on the subject.
Of course, the REAL worry regarding fracking doesn't have anything to with earthquakes, in my opinion. That's just the sort of sexy catastrophe-mongering peddled by the popular press, and, at the risk of sounding conspiratorial, just the sort of low-risk/high-publicity kind of thing big Industrial/Business concerns like people worrying about. As Pynchon wrote in Gravity's Rainbow, "if they can get you asking the wrong questions, they don't have to worry about the answers".
The Earthquake worry is the wrong question to ask about fracking; what should be eliciting some raised eyebrows is the fact that companies are pumping huge volumes of unknown, proprietary fluids into the ground, with the expressed purpose of making it easier for fluids of ALL kinds to move more easily in the subsurface. Don't worry about earthquakes destroying central Pennsylvania; worry about ground water contamination! And not just from the mystery fluids used in fracking. There's all sorts of unpleasant things in the brines that lurk in the subsurface. I'd like to see a little more transperancy, and a LOT more science, on the question of what impacts, if any, fracking is actually having on groundwater and the environment.
And, as pointed out by the Cuadrilla report, it's often a question of local contingencies that matter. What might be perfectly safe in upstate New York COULD be disastrous in North Dakota, or in Wyoming, or in Romania. The question of fracking needs some serious study, particularly in light of the complicated natural systems that we geologists know characterize the actual Earth.