Hydraulic fracturing, commonly known as ‘fracking’, is a process used to extract what are known as unconventional gas deposits such as shale gas or coal seam gas. Unconventional gas is more difficult to extract than conventional natural gas. The process used is a long bore hole drilled vertically down to varying distances depending on the area, but it could be vertical for a mile and then it is drilled horizontally into the shale gas area for another mile or so.
Millions of litres of fracking fluid, that is water mixed with sand and toxic chemicals, including known carcinogens, are forced into the well under immense pressure, to fracture the rock. This releases the gas. It can also cause uncontrolled cracking in the rock layers. It can allow previously undisturbed, naturally occuring natural gas, heavy metals and radioactive materials in the rock to leak into and contaminate the water table.
The toxic fracking fluid can also migrate into the water table when it is forced down underground. Not all of the fracking fluid returns to the surface once it has done its job. The ‘produced water’, or waste fracking fluid that does return to the surface, carrying extra toxins picked up underground must be disposed of carefully.
Problems occur when:
- the cement linings of the boreholes somehow fail, cracking or crumbling, allowing the fracking fluid, gas and other substances to escape
- produced water / waste fracking fluid is not handled properly or disposed of safely, and leaks from its storage pond into the surrounding landscape
- earthquakes / tremors are triggered by the fracturing process creating new cracks and other uncontrollable damage
Have a look at how wikipedia explains fracking at http://en.wikipedia.org/wiki/Hydraulic_fracturing.
A fantastic description of what fracking is, how it works, and what it would mean for us is included in this powerpoint presentation by Dr Aedin McLoughlin Shale Gas Extraction: The Future for Northern Ireland