Here’s a slightly edited description of fracking in the US (posted as a comment by Steve (North Shore) on Kiwiblog):
There are several different types of fracking jobs. Slickwater, CO2, Nitrogen, and Gel fracks are the most common. Slickwater is the basic frac job – a stage usually takes about 150,000 to 200,000 gallons of water, and around 150,0000 lbs of sand. (of course, this can vary due to the depth of the gas formation – very deep wells will use more water).
A well has multiple stages – 12, 15, 20 – depends on the formation they want to get the gas from. Around here, we’re usually pumping at 6000psi to get the water and sand into the formation, and pumping the stage takes around 2 hours.
The chemicals used in slickwater, co2, and nitrogen jobs are very safe to work with. Co2 and Nitrogen jobs are a little bit more dangerous than slickwater fracs due to the explosive expansion capabilities of liquid co2 and nitrogen – but dangerous to the frac hands, not the public.
Gel jobs are a bit of a different bird – the chemicals are a bit nastier. We’re not using a lot of chemicals on the job, tho – think in terms of 1/4 or 1/2 gallon of chemical per 1000 gallons of water pumped. Think of it like this – no one will survive drinking a gallon of bleach. Dump a gallon of bleach into a swimming pool, and you can drink it all day long…
The only chance for gas and gas byproducts to migrate is if the well and casing are compromised. The formations we are pumping into are a mile or two under ground, and theres an impermeable barrier of bedrock between the formation and any water table. If the well and casing get ruptured, tho, gas can seep up the line and contaminate water tables, so the company will pull the string and re-run the line and casing.
Failures in the casing/well are pretty rare, but all wells are monitored for just this eventuality. In my 5 years of working, I’ve only seen one well in which we couldn’t frac because the pressure readings indicated a problem with the casing.
Some of the weird Greenies here think fracking caused the earthquakes in Christchurch.
No problem – and they’re talking about injection wells. After a frack, they flow back the well. Basically, after the frack, the pressure of the gas in the formation pushes the water that they fracked with back out of the well and into tanks or storage ponds.
Some of the water is “pressed”, or filtered and used again. If you need to get rid of a ridiculous amount of water, you drill an injection well and pump it down a mile or two underground.
There is a veeery tenous connection between injection wells and mild earthquakes.