Page 72 - Geothermal Energy Systems Exploration, Development, and Utilization

48 2 Exploration Methods
high differential stresses (Ferrill and Morris, 2003). In a fractured rock, shear
reactivation of preexisting faults normally occurs at lower pore-fluid pressure than
tensile fracturing. However, new tensile fractures can form and serve as conduits
for fluid flow (Sibson, 1996, 1998) if
• rocks are intact and do not contain favorably oriented, cohesionless faults
• existing faults are not favorably oriented for shear reactivation;
• existing favorably oriented faults have become cemented and regained cohesive
strength.
The last point deserves attention, as it shows that the determination of fracture
orientation in space may not suffice to evaluate its potential as a fluid conduit
because of mineralization due to continuous circulation of hydrothermal fluids
(Morrow, Moore, and Lockner, 2001). In addition, Sibson (1998) pointed out that
faults may have regained cohesive strength and thus behave like an intact rock
rather than a cohesionless fault. Sufficient geochemical characterization of the
reservoir, including both rocks and fluids, is essential to address this specific case
adequately. In contrast, fractures that are not favorably oriented within the in situ
stress field may well serve as fluid conduits if they are propped open by grains that
prevent crack closure despite the stress orientation (Hillis, 1998).
A graphic evaluation of the orientation of fractures with respect to the in situ
stress field, the fault rock strength, and the corresponding likelihood of the fracture
to be critically stressed and hydraulically conductive is the fracture susceptibility
diagram (Mildren, Hillis and Kaldi, 2002; Hillis and Nelson, 2005). It is constructed
as a stereoplot (Figure 2.4), which is color coded by the amount in pore pressure
P p that leads to failure of a fracture for a given failure envelope in the Mohr circle
diagram.
Knowledge of fracture orientation with respect to the stress tensor is important
for well planning if deviated wells are considered in a certain direction, relative
to one of the principal stress axes, to cross natural tensile fractures, or to enable
multiple hydraulic fractures, which are both in the plane of the maximum and
intermediate stress axes (Figure 2.2a and b). The orientation of an induced tensile
fracture at the wellbore wall can be predicted, given knowledge of the in situ stress
tensor and wellbore trajectory (Peˇ ska and Zoback, 1995). A well path optimized for
fracture stimulation within the stress field is, however, not necessarily the safest
in terms of borehole stability; thus, changes in stress concentrations around the
borehole and mechanical behavior of rocks should be considered before reservoir
access (Figure 2.2b).
Fracture stimulation is used to enhance reservoir performance, particularly
in low-permeability reservoirs. It is achieved by artificially increasing pore-fluid
pressure (Chapter 4). This kind of human intervention can cause a modification of in
situ stress conditions that can be significant enough to change fault behavior. Also,
production and injection through wells, both important elements in sustainable
reservoir management, change the stress field by modification of the pore pressure,
which isalsoreferred toas formation pressure. Injection causes an increase in
formation pressure, which in turn causes a decrease of normal stresses acting