The
chances of rogue fractures due to shale gas fracking operations
extending beyond 0.6 km from the injection source is a fraction of 1%, 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 m was found to be 1%.
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.
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 U.S.,
Europe and Africa. Of the thousands artificially induced, none were
found to exceed 600 m, with the vast majority being much less than 250 m
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.”
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.
Source: Durham University
