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Oil and Gas » Rogue Fractures
Rogue Fractures from Hydraulic Fracturing
Republished from a press release issued by Durham University, April 23, 2012
Editor's Comment:
Many objections to the use of hydraulic fracturing in the production of natural gas and oil focus on the possibility that artificial fractures might extend beyond the target rock unit and into shallow rock units where ground water contamination might occur. This study by Durham University suggests that a separation of at least 600 meters between the hydraulic fracturing level and the deepest sensitive rock unit will minimize the possibility of ground water contamination caused by the generation of rogue fractures. Contamination from well casing leakage or spills at the surface can still occur. |
Fracture Propagation Distance
The chances of rogue fractures due to shale gas fracking operations extending beyond 0.6 kilometres
from the injection source is a fraction of one percent, according to new research led by Durham University.
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.
The Fracking Process
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.
Shale Gas Development is Global
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.
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.
Fractures as Contaminant Pathways
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.
Minimum Vertical Separation Distance
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."
Professor Davies' team looked at published and unpublished datasets for both natural and stimulated fracture systems in
sediment of various ages, from eight different locations in the USA, Europe and Africa.
Professor Richard Davies said: "Sediments of different types and ages are potential future drilling targets and minimum
separation depths are an important step towards safer fracturing operations worldwide and tapping into what could be a valuable energy resource.
"We need to keep collecting new data to monitor how far fractures grow in different geological settings."
The team accepts that predicting the height and behaviour of fractures is difficult. They now hope that the oil and gas industry will continue to provide data from new sites across the globe as it becomes available to further refine the probability analysis.
Analysis of new sites should allow a safe separation distance between fracking operations and sensitive rock layers to be
further refined, the scientists say. In the meantime, the researchers hope that governments and shale gas drilling companies
will use the analysis when planning new operations.
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| Durhan University researchers have analyized data from thousands of shale gas fracking operations in the United States. They determined that rogue fractures are unlikely to extend more than 600 meters beyond an injection source. The illustration above demonstrates how a recommended separation distance could be used to create a buffer between an injection source and the deepest sensitive rock unit. Image © geology.com |
| Thousands of natural gas and oil wells are drilled in the United States each year and stimulated with hydraulic fracturing. Establishing recommended separation distances between the fracking process and the deepest sensitive rock unit would be a way to protect ground water quality. At the same time it would make potential resources off-limits for drilling. Fortunately, most of the shale gas plays in the United States are over 1000 meters deeper than sensitive rock units. |
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