Page 296 - Reservoir Geomechanics

274 Reservoir geomechanics

a. b.
620

0
Normal Faulting
622
200

400
624
600
Depth (meters) 800 626

1000
S
v
1200
PORE
628
1400 PRESSURE
NORMAL
FAULTING
m = 0.6
1600
0 10 20 30
Stress (MPa) 630
N E S W N
Figure 9.4. (a) Least principal stress measurements in the Yucca Mountain area of the Nevada Test
Site (hole USW-G1) indicate a normal faulting stress regime with least principal stress directions
consistent with frictional faulting theory for a coefficient of friction of ∼0.6 (after Zoback and
Healy 1984). Note the extremely low water table. (b) Drilling-induced hydraulic fractures imaged
with a borehole televiewer explain the total loss of circulation during drilling (after Stock, Healy
et al. 1985).

graben of the North Sea. One of the first places where frictional faulting theory was
demonstrated to be clearly applicable to faulted crust in situ was the Yucca Mountain
area of the Nevada Test Site. This site is located in the Basin and Range province of the
western U.S., a region of high heat flow and active extensional tectonics. As illustrated
in Figure 9.4a, the magnitudes of the least principal stress, S hmin , obtained at various
depths from mini-frac tests are consistent with the magnitudes predicted using Coulomb
faulting theory for a coefficient of friction of ∼0.6 (Zoback and Healy 1984). In other
words, at the depths at which the measurements were made (∼600–1300 m), the mea-
sured magnitude of S hmin wasexactly that predicted by equation (4.45) for a coefficient
of friction of 0.6 and appropriate values of S v and P p (dashed line in Figure 9.4a).
In addition, the direction of least principal stress in the region (WNW–ESE) is