Page 176 - Applied Petroleum Geomechanics

170 Applied Petroleum Geomechanics

where C I is the weight fraction of illite clay content and C I is between
0 and 1.
Therefore, the coefficient of friction of the fault is lithology-dependent,
which needs to be considered when one applies Eqs. (5.11) and (5.12) to
estimate in situ stresses.
Field measurements of in situ stresses show that the horizontal stresses
are highly dependent on lithologies (e.g., Warpinski and Teufel, 1989;
Wileveau et al., 2007; Gunzburger and Cornet, 2007; Gunzburger and
Magnenet, 2014). For instances, coal seams and shales have much higher
minimum horizontal stresses than the adjacent sandstones, as shown in
Fig. 5.4. Measured data from West Texas (Miller et al., 1994) also show that
shale formations have much larger minimum horizontal stresses than those
in adjacent sandstones, as shown in Fig. 5.5. The minimum horizontal stress
results analyzed from leak-off test data in the North Sea (Breckels and van
Eekelen, 1982) have the same conclusion.
Fig. 5.6 shows the comparison of two stress polygons: one in a sandstone
with m f ¼ 0.6 (the case shown in Fig. 5.3) and the other in a shale with a

Figure 5.4 The minimum horizontal stress profile (s h ) measured from mini-frac tests in
Colorado. (Modified from Warpinski and Teufel, 1989.)