Science Magazine last week had an excellent review of fracking and water well contamination. It presented the technology in a fair manner although I think it was a little spotty on exactly how a gas well is dug and what can go wrong during this step that leads to contamination of aquifers. After all, the natural gas is located far beneath aquifers. How can it end up in someone's water well thousands of feet higher up?
This picture from the article shows a typical well, but I don't think it is really obvious how the well ended up looking like that, what with all these different cement annuli, the decreasing width of the well with depth, etc.
This is a 3-stage well. The first stage is the uppermost, widest part. The well is started with a large diameter drill (let's call it 12 inches just to have a number to work with) that drills down a certain distance. The drill is removed and pipe of a slightly smaller diameter (call it 11 inches) is placed in the hole. Wet cement is then shoved down the inside of the pipe and forced with a piston-like device to the very bottom of the well and then up into the annulus between the rock and the pipe. That's the first stage. The second stage is quite similar, but would start with a smaller drill, say 9 inches in this case. The drilling continues for another span, the drill is removed and then like before, pipe of a smaller diameter (call it 7 1/2 inches) is put in the well and similarly cemented in place. And so on for the third stage.
The process is far more complicated than this in practice as the gas in the ground is under pressure and wants to exit the ground before anyone is ready for it (a la the Deepwater Horizon disaster) and this is where drilling muds and cement density and other factors come in. That is far too involved of a subject for today, but know that these are issues that need to be addressed by the drilling engineers.
The key issues in avoiding water contamination is ensuring that the cement is properly set. If not, the gas then has an easy access route up to the aquifer along the annulus outside of the pipe. The inset in the picture above shows 5 different ways in which the cementing can fail. With fracking, there are additional concerns because different chemicals are driven into the rock formations to fracture (hence the term "fracking") the rock and increase the gas flow. And just like the gases, these chemicals can also end up in aquifers when the well is improperly cemented.
I hope that clarifies for one and all how well construction occurs and that you can see the importance of doing it properly. A properly constructed well should not cause any issues to aquifers or the general environment that people encounter, while a poorly constructed one can be an environmental nightmare. This is no different than any other technology - mining, pulp and paper, chemical/auto/semiconductor manufacturing, construction...Any industry can be a boon or a bane to the environment. We have to decide how we want to proceed, but understanding the basics of the technology (something very few people do when it comes to fracking) is essential so that the decisions are not based on just fear and other emotions.