Page 75 - Geothermal Energy Systems Exploration, Development, and Utilization

2.3 Relevance of the Stress Field for EGS 51
stress distribution along fault planes, and by solving Equation (2.5) for nor-
mal stress distribution along fault planes plotting the results in equal area
stereonets (Morris, Ferrill, and Henderson, 1996; Ferril and Morris, 2003). As
such, this slip and dilation tendency analysis is a technique that permits the
rapid assessment of stress states and related potential fault activity through easy
visualization.
Faults with a high slip tendency are critically stressed faults with a high amount
of shear stress. They have a high reactivation potential as shear fractures and are
therefore prone to seismicity during stimulation of critically stressed reservoirs.
Faults with a high dilatational tendency bear low shear stresses and low normal
stresses (Moeck et al., 2009). During the operations at the Groß Sch¨ onebeck field,
a massive water stimulation lasting six days induced surprisingly low seismicity
of magnitudes −1to −2 as described by Moeck et al. (2009). The slip tendency
analysis, however, revealed a low slip tendency of optimally oriented faults resulting
from high rock strength and therefore a high frictional resistance of any faults.
Thus, slip is very unlikely to occur under initial reservoir conditions and a
significantly higher pore-fluid pressure of 20 MPa is needed to increase the slip
tendency. Increasing the pore pressure means a reduction of the normal stress
acting on a fault plane. An increasing ratio of shear to normal stress is effectively
an increase in slip tendency. The visualization of slip tendency is given in the
lower hemisphere projection and shows all faults prone to high slip. Figure 2.4
shows the distribution of faults with highest slip tendency in the red areas for
the volcanic succession of the Groß Sch¨ onebeck reservoir. However, the slip
tendency is below the value of 0.8 which is the limit of frictional resistance
(Byerlee, 1978). During stimulation and consequent pore fluid increase, a fracture
plane was generated as evidenced by microseismic events in the area of high
slip tendency (Figure 2.5). The concert of both results indicates that the slip
tendency analysis, originally developed for earthquake assessment, is an appropriate
method to investigate, characterize, and understand fault behavior of engineered
reservoirs.
The compilation of all available data from the surface and/or subsurface into
one integrated a priori 3D geological model will facilitate a comprehensive inter-
pretation. Depending on the geological setting and on available data, conventional
geological maps can be used for 3D geological modeling (Moeck et al., 2007). A
priori 3D geological models are the portal to further modeling, including flow
simulation as part of reservoir engineering or stress modeling, to understand the
stress state and fault behavior under initial and changing stress conditions (e.g.,
during stimulation).
The ultimate purpose of geological exploration studies on geothermal fields
is the comprehensive characterization of geological controls on the geothermal
systems. A broad understanding of a geological system, including a quantitative
structural geological site characterization, does not only delineates favorable areas
for future geophysical exploration and drilling but also facilitates all levels of field
development and utilization. Exploration geology grounded in field-based and/or