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Use of Geothermal Resources: Environmental Considerations 237

seismiciTy associaTed wiTh hiGh-pressure injecTion of fluid To
enhance reservoir permeabiliTy
In order to improve recovery of oil and gas deposits, the oil and gas industry developed a technique
of fracturing rock formations at depth using high-pressure fluid. The technique is called hydrof-
racturing and can result in a significant increase in formation permeability. This process is also
called reservoir stimulation or reservoir enhancement. The geothermal industry has adopted the
approach as a means of improving permeability near production wells, and as a means for develop-
ing a permeability pathway between an injection well and a producing well. Hydrofracturing is a
crucial element in the development of Enhanced Geothermal Systems (EGS, which is discussed in
some detail in Chapter 14).
Hydrofracturing is accomplished by using a pump capable of injecting fluids at pressures between
2 and 20 MPa (300 to 3000 pounds per square inch) into a well that is lined with high strength steel
pipe. In the part of the subsurface where increased permeability is desired, the steel pipe is perfo-
rated, allowing the high pressure fluid to enter the enclosing rock. Depending upon the rock prop-
erties and fracture sets, the rock will either form new fractures or preexisting fractures will open.
The pressure will be maintained for hours to days, depending on the distance over which the new
permeability is sought.
Research has shown that the primary effect of this process is to cause preexisting fractures to
fail by shearing (Kraft et al. 2009; Tester et al. 2006). Fluid injection reduces the normal stress that
effectively increases the ratio of shear stress, τ, to normal stress, σ . At sufficiently high pressures,
n
the resulting τ to σ ratio exceeds the fracture frictional strength μ and the fracture slips or ruptures.
f
n
The size of the microseismic events suggest the rupture area is commonly on the order of a hundred
2
m to a few thousands of m .
2
Induced fracturing produces swarms of many small seismic events. From an operational per-
spective, these events are useful for monitoring the location and progress of the hydrofracturing.
The use of high sensitivity seismometers on the surface and installed in boreholes provides the
ability to map the location, orientation, and extent of the fracturing.
Figure 12.6 shows the concentration and distribution of seismic events for a hydrofracturing effort
undertaken at the European EGS site at Soultz-sous-Forêts in eastern France in 2000–2004 (Baria et
al. 2006). The shaded region encompasses tens of thousands of seismic event. The strongest events
had magnitudes of 2.6 and 2.9. Thousands of events were recorded but were not sensed as ground
motion by the local inhabitants. The vast majority of the seismicity had magnitudes less than 2.0.
Although a broad range of injection rates, injection volumes, and pressures were used for this effort,
the seismic moment magnitudes were small, suggesting the overall risk of large events was small.
However, 150 km south of this region, near the city of Basel, a similar EGS effort resulted in a
magnitude 3.4 seismic event. Although no structural damage was reported, the unexpected shock
resulted in concern regarding seismic risk. This event occurred several hours after injection had
been stopped. Similar post shut-in seismic events have been observed elsewhere including at the
site in Soultz-sous-Forêts.
Such seismic events associated with fluid injection are not a new phenomenon. Nicholson and
Wesson (1990) documented that injection of fluids can induce earthquakes in a variety of settings.
The largest induced earthquake was a magnitude 5.5 event in 1967 associated with waste disposal
at the Rocky Mountain Arsenal in Colorado. Bachmann, Wössner, and Wiemer (2009) have shown
that the seismicity associated with fluid injection, specifically that at the Basel site, exhibits charac-
teristics similar to aftershock sequences observed for natural earthquakes, which suggests that the
physical processes are similar.
Mitigation of seismic risk associated with fluid injection for hydrofracturing purposes can be
accomplished by coordinating site studies, as described above in the section on injecting cool water,
with several operational approaches. These aspects are briefly described by Baria et al. (2006). They
include detailed monitoring of the microseismic response to changes in injection rate, volume, and