American Oil and Gas Reporter - February 2015 - 142

including several with magnitudes greater
than 5.0.
Because operations such as wastewater
disposal continue over months and years,
one study contends that dispersed fluids
have the potential to interact with fault
systems tens of kilometers from injection
points.
Earthquake Basics
DOE research points to subsurface
stresses and fluid pressures as major
causes of induced seismic activity, with
fluids playing a dominant role in controlling pressures-defined as displacements
across a fracture, joint or fracture zone-
acting on faults. Pore pressure, contained
in the rocks' fractures and pores-acts
against the weight of the rock and the
tectonic forces holding the rocks together.
If pore pressures are low, especially compared with the forces holding the rock
together, the DOE study says any imbalances in natural in situ stresses will cause
occasional earthquakes. If pore pressures
increase, it takes less of an imbalance of
those stresses to lead to an earthquake.
The agency says injecting fluids into the
subsurface is one way of increasing pore
pressures.
Faults in the earth's crust are common,
but they can sustain high stresses without
slipping because the weight of the overlying rock maintains the two sides of the
fault together, with friction providing
even more resistance to any slippage, geologic studies say. One way that balance
is upset is by injected water reducing
frictional forces.
According to plate tectonic theory,
the movement of large blocks of the
earth's crust is another source of seismic
events, typically experienced near plate
boundaries. Faults range in size from a
few meters in length to hundreds of kilometers long, and the size of an earthquake
depends on how much slip occurs across
the fault, how much stress there was on
the fault before the slip, how fast it fails,
and over how large an area the slip occurs.
The DOE research says most large
earthquakes start more than five kilometers
below the surface, at depths where there
is enough stored energy to provide the
large amounts of force needed to move
the volumes of earth required. It adds
this implies that if seismicity is induced
at shallow depths of less than five kilometers, those seismic events may be numerous, but are unlikely to include large
events.
According to the National Research
Council, geological processes-such as
deposition or erosion of surface rocks-
affect the balance of opposing stresses

and serve to trigger most earthquakes
that occur away from plate boundaries.
Changes in the strain weaken the stability
of faults confined by stresses, creating
the possibility for a fault to slip.
Induced seismic events can occur when
increasing pore pressures caused by fluid
or gas injection reduce the effective stress
acting perpendicular to pre-existing faults
and thus decrease the effective clamping
force resisting shear movement, the council
says. The reduced stress can allow the
elastic energy already stored in the rock
formation to be released as an earthquake.
One line of research suggests deep fluid
injections can trigger seismic activity
only if the fluids reach and relieve the
friction on a suitably oriented adjacent
fault that is subject to regional tectonic
stresses.
Injecting wastewater into a formation
does not mean that an earthquake will
occur or that it will happen immediately,
the council asserts, saying in most cases
a significant amount of fluid must be injected over a prolonged period before a
properly oriented fault is activated. It
says as fluids are injected into porous
rocks, the pore pressure would have to
increase to the point that it caused the
rock volume to expand, destabilizing a
fault. Similarly, it says a pore pressure
decrease when fluid is extracted also can
affect fault stability.
According to one study of eight earthquakes, maximum fluid injection rates
near the faults exceeded 150,000 barrels
of water a month. The study says that
while this suggests seismic incidents
are more likely to be triggered if injection
reaches a critical rate, that rate depends
on local subsurface conditions, and thus
is likely to vary. Within the Barnett
Shale region, the study says there are
more than 100 disposal wells with injection rates higher than that threshold
that experienced no identifiable earthquakes during the study period.
The study emphasizes that water injection associated with hydraulic fracturing
is unlikely to have triggered any seismic
events because pressurization during fracturing affects only limited volumes of
rock, typically several hundred meters in
extent, and usually lasts only a few hours.
While small earthquakes have been reported in conjunction with fracturing operations-including a magnitude 2.3 event
near Blackpool, England, in 2011, a series
of events ranging from magnitude 2.2 to
3.8 in the Horn River Basin of British
Columbia, Canada, between 2009 and
2011, and smaller incidents in Oklahoma
and Ohio-the study argues these are extremely rare events.

142 THE AMERICAN OIL & GAS REPORTER

Basement Rocks
McGarr suggests that wastewater injection in aquifers near the base of sedimentary columns may enhance the likelihood of induced seismic episodes large
enough to be felt; even more so if injection
is directly into the basement.
"The likelihood of inducing felt earthquakes is higher in cases in which injection
is into the basement," he says. "Realization
of this goes back to the 1960s and the
Rocky Mountain Arsenal wells near Denver, where the intention was to drill into
the basement. That has been a very educational case history."
It is difficult to detect faults in basement
rock using seismic techniques because
of the thousands of meters of sediment
overlaying them, McGarr goes on. Most
times they are found only after they are
imaged by precisely located induced
earthquakes. "Sometimes the basement
topography can provide a clue that there
is some faulting going on, but there is
more than one explanation for basement
topography, so even that is difficult," he
adds.
Lawrence Berkeley's Majer says the
type of fault is a major factor in determining how difficult it is to find a basement
fault. Strike-slip faults, where one side
slips relative to the other, induce no
vertical movement on either side of the
fault, which he says makes them difficult
to detect. A thrust fault, with one side
moving up relative to the other side, produces offsets in the overlaying sediments
and are relatively easy to detect with surface seismic reflection and similar techniques, he adds.
McGarr says the proposition that it is
more beneficial to have a series of small
"stress-relieving" events rather than one
larger event is incorrect.
"In almost all cases, it is a few of the
very largest earthquakes that account for
all of the deformation," he says. "The
occurrence of a lot of smaller earthquakes
does not mean that stresses are being relieved to any extent. In fact, if you have
100 magnitude 2.0 earthquakes, there are
likely to be 10 magnitude 3.0 earthquakes
and one magnitude 4.0 earthquake, and
so on up the magnitude scale. If anything,
the occurrence of many smaller earthquakes would have the opposite interpretation. Charles Richter pointed that
out in the 1950s."
Colorado Quakes
The Rocky Mountain Arsenal was a
20,000-acre federal chemical weapons
manufacturing and disposal plant in operation from World War II through 1995.
Research into the linkage of fluid injections
to seismic events began in Colorado after

American Oil and Gas Reporter - February 2015

Table of Contents for the Digital Edition of American Oil and Gas Reporter - February 2015