133
the strength of the rocks, the fault will remain “locked” and immobilized. The size
of the eventual earthquake that does occur when the fault finally breaks depends in
a large part on just how much stress had built up across it. Different rock types have
different strengths, and the induced earthquakes caused by wastewater injection
seem to occur primarily on faults in relatively strong rocks like sandstone disposal
formations or the granitic basement rocks below them. Shale as a rock type is generally too weak to build up much stress across faults. Although hydraulic fracturing
fluids can enter and pressurize pre-existing faults in shale, there is usually a limited
amount of stress to relieve if the fault slips. The large, induced earthquakes from
wastewater injection down UIC wells that are felt at the surface and cause damage
are rare in fracked shale.
Rare is not the same as absent, however. In the United Kingdom, multiple earthquakes in 2011 were linked to hydraulic fracturing operations in the Bowland Shale
near the town of Blackpool. The largest of these had a magnitude of 2.3 and was felt
locally (Clarke et al. 2014). Operations at the well site, known as Preese Hall, were
suspended immediately after the seismic events, and the well was plugged and
abandoned in 2013. The Bowland Shale is actually an organic-rich, shaly limestone,
and the limestone component may have given the formation higher rock strength
compared to clay-rich or even silica-rich shale. Greater rock strength would have
allowed more stress to build up across a fault, and when frack fluids increased the
pore pressure inside this fault it slipped, causing the earthquake.
As a result of these events, the U.K. has implemented the strictest regulations in
the world for induced seismicity from hydraulic fracturing operations, requiring
activity to cease for at least 18 hours if an induced earthquake as low as 0.5 magnitude is measured (far below anything that might be “felt”). Seven years after the
Preese Hall earthquakes, fracking operations resumed under these regulations at a
nearby site called Preston New Road, still targeting the Bowland Shale. Small earthquakes in October 2018 again raised public concerns. In August 2019, hydraulic
fracturing operations on a well at the Preston New Road site generated 128 earthquakes, including a magnitude 2.9 event that was widely felt across the region.
Operations at Preston New Road were suspended by the U.K. Oil and Gas
Commission until reviews could be completed on the cause of the earthquakes and
the implementation of the induced seismicity regulations. More information can be
found on websites of the British Geological Survey and U.K. government. (https://
earthquakes.bgs.ac.uk/research/BlackpoolEarthquakes.html)
The British Geological Survey, now considered the world’s foremost authority
on fracking-induced seismicity, has identified two types of induced seismic events.
“Fracked” events are caused by the brittle failure of the rock as the injection of fluid
creates new fractures in a rock mass that was previously intact. These quakes are
constrained by the energy of the injection process and are usually quite small, sometimes referred to as microseismic events. The second type of seismicity is known as
“triggered” events. These occur when the presence of fluid and the perturbation of
pre-existing faults cause them to fail. The size of these triggered events depends on
the amount of stored-up elastic strain energy present in the rocks.
7.3 Induced Earthquakes
the strength of the rocks, the fault will remain “locked” and immobilized. The size
of the eventual earthquake that does occur when the fault finally breaks depends in
a large part on just how much stress had built up across it. Different rock types have
different strengths, and the induced earthquakes caused by wastewater injection
seem to occur primarily on faults in relatively strong rocks like sandstone disposal
formations or the granitic basement rocks below them. Shale as a rock type is generally too weak to build up much stress across faults. Although hydraulic fracturing
fluids can enter and pressurize pre-existing faults in shale, there is usually a limited
amount of stress to relieve if the fault slips. The large, induced earthquakes from
wastewater injection down UIC wells that are felt at the surface and cause damage
are rare in fracked shale.
Rare is not the same as absent, however. In the United Kingdom, multiple earthquakes in 2011 were linked to hydraulic fracturing operations in the Bowland Shale
near the town of Blackpool. The largest of these had a magnitude of 2.3 and was felt
locally (Clarke et al. 2014). Operations at the well site, known as Preese Hall, were
suspended immediately after the seismic events, and the well was plugged and
abandoned in 2013. The Bowland Shale is actually an organic-rich, shaly limestone,
and the limestone component may have given the formation higher rock strength
compared to clay-rich or even silica-rich shale. Greater rock strength would have
allowed more stress to build up across a fault, and when frack fluids increased the
pore pressure inside this fault it slipped, causing the earthquake.
As a result of these events, the U.K. has implemented the strictest regulations in
the world for induced seismicity from hydraulic fracturing operations, requiring
activity to cease for at least 18 hours if an induced earthquake as low as 0.5 magnitude is measured (far below anything that might be “felt”). Seven years after the
Preese Hall earthquakes, fracking operations resumed under these regulations at a
nearby site called Preston New Road, still targeting the Bowland Shale. Small earthquakes in October 2018 again raised public concerns. In August 2019, hydraulic
fracturing operations on a well at the Preston New Road site generated 128 earthquakes, including a magnitude 2.9 event that was widely felt across the region.
Operations at Preston New Road were suspended by the U.K. Oil and Gas
Commission until reviews could be completed on the cause of the earthquakes and
the implementation of the induced seismicity regulations. More information can be
found on websites of the British Geological Survey and U.K. government. (https://
earthquakes.bgs.ac.uk/research/BlackpoolEarthquakes.html)
The British Geological Survey, now considered the world’s foremost authority
on fracking-induced seismicity, has identified two types of induced seismic events.
“Fracked” events are caused by the brittle failure of the rock as the injection of fluid
creates new fractures in a rock mass that was previously intact. These quakes are
constrained by the energy of the injection process and are usually quite small, sometimes referred to as microseismic events. The second type of seismicity is known as
“triggered” events. These occur when the presence of fluid and the perturbation of
pre-existing faults cause them to fail. The size of these triggered events depends on
the amount of stored-up elastic strain energy present in the rocks.
7.3 Induced Earthquakes