Types and Causes of Induced Seismicity - Induced Seismicity Potential in Energy Technologies - page 34

SUMMARY
Both the conditions that lead to the initiation of a seismic event and the factors that
affect the magnitude of the resulting event are well understood. The conditions of initiation
are embodied in the Coulomb criterion (involving a comparison of the shear stress on the
fault to the fault frictional strength), while the magnitude of the seismic event is related to
the area of the fault undergoing slip. Inducing a seismic event requires a triggering event
that will either increase the shear stress or reduce the normal effective stress on the fault and/
or reduce the fault frictional resistance, for example, an increase of the pore pressure that
reduces the frictional strength to a level at which it is overcome by the driving shear stress.
However, to cause a significant event requires activating slip over a large enough area; for
example, a seismic event of M 4 involves a fault area of about 1.4 km 2 (~0.5 square miles)
and a slip of about 1 m (~39 inches).
Unfortunately, despite our understanding of the factors affecting the initiation and the
magnitude of a seismic event, the values of the process parameters (such as the injection
rate or the volume of fluids injected) that will trigger the seismic event and what magnitude
the event will be are generally not possible to quantify. The inability to make these kinds
of predictions is due to several factors: (1) fragmentary knowledge of the state of stress in
the Earth; (2) lack of knowledge about the faults themselves, including their existence (if
they have not yet been mapped) and their orientations and physical properties; and (3) dif-
ficulty in collecting the basic data (hydraulic and mechanical parameters, geometry of the
geological structure, such as the reservoir) that are required to calculate the pore pressure
and stress change induced by the fluid injection or withdrawal.
Nonetheless, the insights into the mechanisms causing seismic events allow us to make
some broad conclusions. In processes involving fluid injection, the pore pressure increase is
the dominant factor to be considered, as stress change can often be ignored. Any increase
of the pore pressure above historical undisturbed values may bring the system closer to
critical conditions. The probability of triggering a significant seismic event increases with
the volume of fluid injected: the larger the volume injected, the more likely a larger fault
will be intersected. However, injection of fluid in depleted reservoirs (such as in secondary
recovery stimulation—waterflooding) is unlikely to create an earthquake, irrespective of the
volume of fluid injected, if the pore pressure remains below preproduction values.
The transient region of high pore pressure that surrounds a newly created hydraulic
fracture is not expected to be large enough for a significant seismic event to be triggered,
except in rare cases where the new hydraulic fracture intersects or is very near an existing
fault. Even in such cases, the magnitude of the event is expected to be small because a large
fault area will not be affected.
The fluid injected in crystalline basement rocks is essentially transmitted by a network
of interconnected fractures and joints. Because of the high transmissivity and low storativity