Page 69 - Geothermal Energy Systems Exploration, Development, and Utilization

2.3 Relevance of the Stress Field for EGS 45
Australian Stress Map; Hillis and Reynolds, 2000); some do not require information
from existing wells (1 and 2), while the others generally rely on information derived
from drilling (3 and 4) and borehole engineering (5 and 6) activities in the
surrounding region:
1) the determination of focal mechanism solutions from earthquakes of suffi-
ciently high magnitudes occurring in the region, from which the principal
stress orientations can be inferred;
2) geological observations such as recent fault slip and volcanic alignments can
also serve as first-order stress indicators;
3) failure along the borehole walls (borehole breakouts), which occur in the
direction of the minimum horizontal stress (S h )or
4) drilling-induced tensile fractures that form parallel to the maximum horizontal
stress direction (S H );
5) hydraulic fracturing that can be induced by well and/or reservoir engineering;
and
6) overcoring.
In the early stage of field development, before drilling or with no available
stress magnitude data, stress models can be developed assuming that in situ
stress magnitudes in the crust will not exceed the condition of frictional slid-
ing on well-oriented faults. Commonly, geometrical constraints such as fault
throw and fault intersections in mapped 3D fault patterns, for example from
seismic surveys, indicate a limited variation of stress regimes, ranging from
normal faulting (S V > S Hmax > S hmin ) to transtensional (S V = S Hmax > S hmin )to
strike slip (S Hmax > S V > S hmin ) or reverse faulting (S Hmax > S hmin > S V ), as shown
in Figure 2.2a, where S Hmax and S hmin are the maximum and the minimum
horizontal stresses, while S V is the vertical stress.
Stress values for any given stress regime can be predicted using Equation (2.1)
and assuming Andersonian fault theory (Anderson, 1951) and the Mohr–Coulomb
criterion. Applying the known stresses S V (vertical stress) and S hmin (minimum
horizontal stress) and Equation (2.1), the value for S Hmax (maximum horizontal
stress) in the reservoir can be constrained. The frictional equilibrium applicable
for a geothermal reservoir is (after Jaeger, Cook, and Zimmerman, 2007)
(σ 1 − P p )
σ 1eff
=
σ 3eff (σ 3 − P p )
2 1/2
2
= (µ + 1) + µ (2.1)
Parameters used in this equation include a frictional coefficient µ,ranging from
0.6 to 1.0 for most rock types, as suggested by Byerlee (1978) on the basis of
experimental data, and pore pressure P p ; σ 1 and σ 3 are the maximum and the
minimum principal stresses, respectively (see also Peˇ ska and Zoback, 1995; Moeck
et al., 2009).
The in situ stress tensor in a reservoir can be derived only from failure along the
borehole wall, that is from borehole breakouts and tensile fractures. The opening
angle of borehole breakouts can be used to determine the maximum horizontal