Fracking requires a minimum distance from sensitive rock strata

Posted: 25 April 2012

The chances of rogue fractures due to shale gas fracking operations carried out more than 0.6 kilometres vertically from the injection source are shown to be exceedingly low - a fraction of one percent, according to new research led by Durham University. 

Fracking operations in the USA are growing in number and many countries across the world are looking at shale gas as a potential energy resource. The process of fracking has come under increasing scrutiny. A recent test well in the UK near Blackpool, Lancashire, was stopped after some minor earthquakes were felt at the surface. The UK government is allowing the test fracking to resume but critics have also warned of other possible side-effects including the contamination of groundwater.

The analysis is based on data from thousands of fracking operations in the USA and natural rock fractures in Europe and Africa. It is believed to be the first analysis of its type and could be used across the world as a starting point for setting a minimum distance between the depth of fracking and shallower aquifers used for drinking water.

The new study, published in the journal Marine and Petroleum Geology, shows the probabilities of ‘rogue’ fractures, induced in fracking operations for shale gas extraction, extending beyond 0.6 kilometres from the injection source is exceptionally low. The probability of fractures extending beyond 350 metres was found to be one per cent.

Fracking diagram

During fracking operations, fractures are created by drilling and injecting fluid into the rock strata underground to increase oil and gas production from fine-grained, low permeability rocks such as shale. These stimulated fractures can significantly increase the rate of production of oil and gas from such rocks.

Researchers from Durham University, Cardiff University and the University of Tromsø looked at thousands of natural and induced fractures from the US, Europe and Africa. Of the thousands artificially induced, none were found to exceed 600 metres, with the vast majority being much less than 250 metres in vertical extent.

Fracture heights are important as fractures have been cited as possible underground pathways for deep sources of methane to contaminate drinking water. But the likelihood of contamination of drinking water in aquifers due to fractures when there is a separation of more than a kilometre is negligible, the scientists say.

Professor Richard Davies, Director of Durham Energy Institute, Durham University, said: “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 0.6 km.

“Shale gas exploration is increasing across the world and sediments of different ages are now potential drilling targets. Constraining the maximum vertical extent of hydraulic fractures is important for the safe exploitation of unconventional hydrocarbons such as shale gas and oil, and the data from the USA helps us to understand how fracturing works in practice. Minimum vertical separation distances for fracturing operations would help prevent unintentional penetration of shallow rock strata.”