A new study revealed the process, which uses high-pressure liquid pumped deep underground to split shale rock and release gas, caused fractures running upwards and downwards through the ground of up to 588 metres from their source.
The research, published in the journal Marine and Petroleum Geology, found the chance of a fracture extending more than 600 metres upwards was exceptionally low, and the probability of fractures of more than 350 metres was 1%.
Researchers said the study showed it was “incredibly unlikely” that fracking at depths of 2km to 3km below the surface would lead to the contamination of shallow aquifers which lie above the gas resources.
Shale gas extraction has been controversial in the US because of claims that cancer-causing compounds used in the process have polluted water supplies, and that the flammable methane gas itself can pollute drinking water.
But Prof Richard Davies, of Durham University, said it was more likely any contamination came from drilling down through rock containing methane and where the cement or steel well casing failed, rather than the separate fracking procedure carried out kilometres down where shale gas forms.
Davies said: “What everyone’s interested in is: how far can fractures go upwards from that depth? Could they go far enough to intersect and contaminate aquifers with fracking fluids or create pathways for methane to contaminate aquifers.
“There’s a lot of debate over contamination of water supplies and that could be the case if they are less than 600 metres above shale gas fracking.”
He said that if the process was taking place 1km to 2km below aquifers it was very unlikely to be the source of pollution.
In most cases fracking occurs around 2km to 3km below the surface, where geological conditions are right for shale gas to form, but in one case in Wyoming it took place at about 600 metres down and there is now evidence of chemicals in the water supply.
Davies said there was “just reason to be cautious” and said regulators should set a distance limit, which should be well in excess of 600 metres when fracking in new areas where there was no existing data on possible fractures.
He said the UK’s only shale gas exploration scheme near Blackpool, carrying out fracking 3km down, would not affect water supplies in the area, which are around 300 metres below the surface.
The scheme was halted in 2011 after it caused two small earthquakes. But on 17 April, ministers were advised to allow fracking to be extended in Britain, despite the emergence of doubts over the safety of the wells that have already been drilled.
The researchers from Durham University, Cardiff University and the University of Tromso, Norway, looked at thousands of natural and induced rock fractures in the US, Europe and Africa, and found none of the artificially caused ones were more than 600 metres.
However, the largest measured fracture found naturally occurring in the world – created over millions of years – was 1.1km high, prompting the researchers to suggest the 600-metre limit should be an “absolute minimum guideline”.
Davies added: “Based on our observations, we believe that it may be prudent to adopt a minimum vertical separation distance for stimulated fracturing in shale reservoirs. Such a distance should be set by regulators; our study shows that for new exploration areas where there is no existing data, it should be significantly in excess of 0.6km.”