Page 205 - Geothermal Energy Systems Exploration, Development, and Utilization

4.5 Definition and Description of Methods (Theoretical) 181
and hence, a high shear stress, should be best candidates for hydraulic fracturing
in low permeable rock.
Knowledge about the stress regime is of great importance to understand or
even to predict the hydraulic fracturing process (Cornet, B´ erard, and Bourouis,
2007; Evans et al., 2005b). Borehole breakouts, borehole fractures, microseismic
events, and stimulation pressures have been evaluated to confine the orientation
and amplitude of the principal stress components.
One method to reduce the risk of creating shortcuts is the isolation of intervals
in the borehole and the successive stimulation of these intervals. Such a strategy is
also favorable to reduce the risk of creating larger seismic events.
Cases of induced seismicity have been reported from hydraulic stimulation
programs in geothermal wells, but not all geological formations are prone to these
events. Induced seismic events, which could be felt at the surface, have been
reported from hard rock environments. Since the permeability in these formations
is a fracture- permeability, the pressures generated to frac the formation can only
diffuse through the fracture and fault network, which will lead to a reduction in
effective stress. In sedimentary environments, due to their matrix porosity and
permeability, elevated pressures will not focus on fracture and fault pathways, but
diffuse through the porous matrix. A potentially considerable sedimentary coverage
of a hydraulically stimulated hard rock formation will also damp induced seismic
events.
Controlling the reservoir growth while stimulating or circulating is an important
issue for all projects in low permeable rock (Baria et al., 2006). Microseismic
monitoring gives 3-D time-resolved pictures of event location and magnitude from
which the fractured rock volume can be inferred. This method has evolved to the key
technique to map the reservoir in HDR projects (Niitsuma, 2004; Wallroth, Jupe,
and Jones, 1996; Dorbath et al., 2009). In current projects, (Soultz; Cooper Basin,
Australia) the microseismic event distribution serves for the determination of the
target area for new wells. More recently, microseismic monitoring has become
important to detect and to control larger seismic events, which might occur during
stimulation in geological active areas (Bommer et al., 2006).

4.5.1.2 Waterfrac Treatments
Waterfrac treatments are applied in low permeable or impermeable rocks with
high amounts of water to produce long fractures with low width compared to the
following treatments. In general, waterfrac treatments produce long fractures in
the range of a few 100 m with low apertures of approximately 1 mm and hence low
conductivity. The success of the treatment depends on the self propping of the rock
and on the potential of shear displacement (Figure 4.4).
The flow rate during waterfrac treatments can be constant during the whole
treatment or vary in a cyclic manner with several high flow rates followed by low
stages. Simulations have shown that the impact of high flow rates for the fracture
performance is better, even if the intervals are limited in time, compared to a
constant flow rate.