You probably already know what fracking is, but just in case there are a few folks who don’t, I’ll start with a brief explanation.
Traditionally, “natural gas” is extracted from wells similar to oil wells, releasing gas that was trapped in porous rock beneath a cap of impermeable rock. You drill a hole through the impermeable rock, insert a pipe, and the gas flows (fairly) freely through the porous rock, and up your pipe.
Fracking is a process for extracting gas, mostly methane, that’s trapped within non-porous rocks. To do this, you have to fracture the rock to make it somewhat porous, hence the word “fracking”. To fracture the rock, you pump water under extremely high pressure into holes drilled in the rock. These holes typically go down thousands of metres, cunningly turn horizontal in a long, smooth curve, and then go kilometres horizontally in the target rock – lined with metal pipes all the way, but with perforations in the part of the pipe that’s in the target rock. You don’t use pure water; it contains chemicals and a proppant – sand or other particles to prop the crack open after you remove the hydraulic pressure (which you have to do to let the gas out!)
For far more detail, see en.wikipedia.org/wiki/Hydraulic_fracturing.
There are many objections to fracking, each dealt with in following sections:
There are other objections, but these are the main ones – with the possible (not certain) exception of earthquakes, they are all very important issues. Contamination of water supplies is not a trivial matter, and nor is adding yet more greenhouse gas to the atmosphere!
The gas produced is basically the same as any other natural gas (mostly methane). Of all fossil fuels, methane produces the least carbon dioxide in proportion to the energy produced when it burns, but using more fossil fuel is still adding additional carbon dioxide to the atmosphere, and it’s time we were moving away from fossil fuels as fast as we can, not developing new ones.
Ideally, all the methane produced by fracking would be burned to carbon dioxide (and water vapour) when it’s used, but inevitably a significant amount of methane will leak to atmosphere. Methane is a much more potent greenhouse gas than carbon dioxide, and although it does slowly oxidize to carbon dioxide in the air, it persists for many years in the atmosphere.
Fracking certainly causes earthquakes. How significant these are is an open question; to date, all the many known cases have been quite minor. The earthquakes occur either due to the weakening of the rock by fracturing, or due to fracking water leaking into and lubricating pre-existing faults. Each earthquake relieves stress in the Earth’s crust in the location where the earthquake occurs – but relieving stress in one location increases stresses in surrounding areas. Increased stress may result in larger earthquakes which can be difficult to prove to have been indirectly caused by the fracking. Things are further complicated by the fact that all these earthquakes, or similar ones, would almost certainly have occurred eventually anyway, as the crust moves. The initial earthquakes directly caused by the fracking would indeed be smaller (and more frequent) than they would otherwise have been; the secondary ones may occur sooner (possibly decades or centuries sooner) than they would have, but whether they’ll be weaker or stronger is hard (maybe impossible) to predict.
The biggest risk here is of failure of holding ponds for the fracking water. Fracking water is pumped down the wells, but then much of it returns when the pressure is removed to allow the gas to escape.
Fracking water contains high concentrations of toxic chemicals, some of them artificial chemicals put into the water to assist in the fracking process, and some of them leached out of the fractured rocks. The latter include radioactive isotopes from the decay of uranium in the rocks, which escape from the mineral grains the original uranium was trapped in, because one of the isotopes in the decay chain is Radon, an inert gas which diffuses out of the mineral grains into the interfaces between grains, where its subsequent decay products then remain. This means that fracking water comes into contact with these isotopes preferentially, as the cracks produced during fracking follow the interfaces between grains rather than breaking grains.
This radioactive contamination is not trivial, but nor is it extremely alarming. Compared to concentrations of such materials in the vicinity of uranium mines, it’s very minor; even more so compared to the effects of accidents at nuclear reactors or reprocessing facilities, or leakages of nuclear waste from waste repositories.
The biggest risk here is from seepage from the surface after the failure of holding ponds, but there is also some risk from fracking water escaping from depth, generally by following up the well hole, around the outside of the well pipe, rather than through the solid rock.
The main risk here is of fracking water escaping from the target layer up around the outside of the well pipe, but there is also some risk that the fractured region may extend beyong the target layer, resulting in fracking water escaping into neighbouring regions of porous rock connected with deep aquifers.
Like many such organizations, these companies think they’re above the law. See A Dam Big Problem.