Energy Technologies: How They Work and Their Induced Seismicity Potential - Induced Seismicity Potential in Energy Technologies - page 86

1. Many factors are important in the relationship between human activity and in-

duced seismicity: the depth, rate, and net volume of injected or extracted fluids,
bottom-hole pressure, permeability of the relevant geologic layers, locations and
properties of faults, and crustal stress conditions. These factors, some of which are
interdependent, are also described in Chapter 2. For an induced seismic event to
occur, at least two criteria have to be satisfied: (1) the pore pressure change in the
reservoir has to exceed a certain critical threshold and (2) a certain net volume
of fluid has to be injected (or extracted) to achieve a particular magnitude. The
available data suggest, but do not prove, that the net volume of fluid may serve
as a proxy for these factors, which indicates what set of conditions will generate
small and large earthquakes. Particularly because the other data—bottom-hole
pressure, permeability of the relevant geological layers, crustal stress factors, high-
resolution well data (full waveform dipole and resistivity and waveform borehole
imaging logs), seismic reflection images (two- and three-dimensional surface seis-
mic techniques, 3D vertical seismic profiles or cross well seismic data) to reveal the
subsurface structure such as the location, orientation, and properties of faults in
the area—are not generally available, total volume can be a tool to draw inferences
about various technologies. However, a pure causal relationship between the larg-
est induced magnitudes and fluid volume should not be assumed. Important also,
exceptions occur in those cases where fluids are injected into sites such as depleted
oil, gas, or geothermal reservoirs, or at sites where the volume of extracted fluids
essentially equals or exceeds the volume injected. In those cases pore pressures may
not reach the original levels, or in some cases may not increase at all due to the
relative volumes of injection and extraction. These data (specifically for oil and gas
withdrawal and geothermal energy) are included in Figure 3.16, but it is noted that
these specific data points do not necessarily represent the total (net) fluid (injected
and withdrawn) that may be related to the maximum magnitude event.

2. The volumes indicated in Figure 3.16 include both volumes for individual wells

in single projects and volumes for fields. The data cannot be used to predict earth-
quake magnitudes for an entire region or industry, but rather only to infer what
magnitudes might be possible for individual wells or fields.

3. The data in Figure 3.16 are maximum magnitudes associated with fluid injection

or extraction and support the requirement, outlined in Chapter 2 and elsewhere in
this chapter, that a certain net volume of fluid has to be injected to cause a seismic
event of a certain magnitude (or in a similar sense for net fluid withdrawal). The
graph does not represent causality, but a condition for an induced seismic event of
a certain magnitude to occur. Importantly, the correlation in the figure does not
predict what earthquake magnitude will be induced by a specific project, but it
reports instead the observed limits (to date) of what earthquake magnitudes have