Page 278 - Geothermal Energy Renewable Energy and The Environment

The Geothermal Energy Future: Possibilities and Issues 269

TechnoloGical requiremenTs
As discussed in detail in Chapter 9, the principle means whereby heat is extracted from the subsur-
face and used to generate electricity is by bringing to the surface water that has equilibrated with a
geothermal reservoir. However, in most regions in the subsurface where temperatures are in excess
of 150°C there is insufficient porosity to accommodate significant volumes of fluid. To overcome
this problem, the strategy has been developed to enhance (hence enhanced geothermal systems) the
porosity and permeability of a volume of rock by hydrofracturing (discussed in Chapter 12). Once a
volume of rock has been hydrofractured, or stimulated, other wells can be drilled into the stimulated
zone of fractured rock (Figure 14.7).
The volume of fractured rock that can be generated using hydrofracturing techniques is about
3
2 km (see results presented in Baria et al. 2006). Experience has shown that hydrofracturing com-
monly reactivates one or more preexisting fracture sets. Since the preexisting fracture sets formed
in response to stress fields that had specific orientations for the maximum, minimum, and interme-
diate principle stresses, the enhanced permeability that results from the stimulation process will
generally not be distributed in random orientations. Instead, it is likely that a preferred orientation
will exist, resulting in preferential flow directions for the fluid that will be injected into the stimu-
lated zone. Monitoring microseismic activity allows the shape and location of the stimulated zone
to be mapped. Using that information, production wells are drilled with specific target depths and
locations, in order to intersect the most likely regions of enhanced permeability.
Enhancing the permeability of a rock volume is a process that perturbs the existing steady state
condition of the rock mass. Without some means to prop open the fractures that dilated during the
hydrofracturing process, it would be anticipated that the fractures would close back up once the high
pressure used with the hydrofracturing fluids was reduced. However, as noted in Chapter 12, the
hydrofracturing process encourages rock failure to occur by shear along preexisting fractures. Since
fracture surfaces tend to be irregular and rough, shearing of one fracture surface past another surface

Production Injection Production
well well well

100° C

7 km 150° C

200° C

Stimulated
zone

FIGUre 14.7 Schematic diagram of an EGS system. The injection well is initially used to hydrofracture a
zone in the rock that is at the target temperature which, in this case, is 200°C at a depth of about 6.5 km. That
same well is then used to pump fluid into the stimulated zone. Production wells that have been drilled into the
stimulated zone then recover the heated fluid and transfer it to electrical generating facilities.