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94 Applied Petroleum Geomechanics

Figure 3.7 Variations of peak strength (s 1, peak ) with s 2 in Coconino and Bentheim
sandstones tested under the common loading path for all constant s 3 levels: solid dots
are the experimental data (Ma et al., 2017).

3.1.4 Polyaxial compression test
In the polyaxial compression test (also called true triaxial compression test),
three independent and mutually perpendicular uniform loads apply to the
faces of a rectangular prismatic specimen. That is, the maximum, inter-
mediate, and least compressive principal stresses are applied separately.
Earlier polyaxial cells built by Mogi (1971) and Spetzler et al. (1981) were
mainly suited for testing deformational and strength characteristics of weak
to medium strength rocks. The University of Wisconsin designed and
fabricated a true triaxial testing system suitable for testing both weak and
competent rocks (Haimson and Chang, 2000). The polyaxial compression
tests can simulate the in situ loading scenarios in actual field condition. The
results from the polyaxial compression tests experimentally demonstrate that
rock strength is a function of the major principal stress (s 1 ) and the minor
principal stress (s 3 ) as well as the intermediate stress (s 2 ). Fig. 3.7 presents
the true triaxial test results in two porous sandstones, Coconino and
Bentheim (Ma et al., 2017). It reveals the effect of s 2 and s 3 on rock
failures, and the rock peak strength increases as s 2 and s 3 increase.

3.2 Rock strengths from petrophysical and well log data

Compressive and tensile strengths are key inputs for geomechanical ana-
lyses, particularly the UCS. Rock strength parameters can be obtained from