Page 94 - Applied Petroleum Geomechanics

86 Applied Petroleum Geomechanics

compression tests. Empirical equations and correlations between rock strengths and
petrophysical and geophysical properties are presented for different lithologies. New
correlations for deepwater reservoirs and shale oil and shale gas formations are pro-
posed. These correlations enable one to obtain continuous rock strengths along the
depth interval of interest from well logging and seismic data (e.g., sonic velocity, transit
time, and porosity). Rock failure criteria are examined to reveal rock failure mecha-
nisms, including linear and nonlinear MohreCoulomb, DruckerePrager, true triaxial,
CameClay, and modified Griffith failure criteria. Their suitability and applicability for
geomechanical modeling are also discussed. The anisotropy of rock strengths and its
impact on rock failures are also investigated.

Keywords: Anisotropy; Failure criterion; Laboratory test; Rock strength; Strength
correlation; Well log.

3.1 Laboratory tests for rock strengths

Laboratory tests usually consist of simple experiments appropriate to the nature
of the rock in which important quantities, often stress and strain, are deter-
mined (Jaeger and Cook, 1979). Different types of laboratory tests can be used
to obtain rock strength, Young’s modulus, and other mechanical properties.
Fig. 3.1 shows some typical laboratory tests, as introduced in the following:
(1) Uniaxial compression test (Fig. 3.1A): the cylindrical rock specimen
is only compressed in the axial direction by the axial load or stress
of s 1 ;
(A) (B) (C)

(D) (E) (F)

Figure 3.1 Schematic diagram showing typical laboratory tests to obtain rock
mechanical properties. (A) uniaxial compression; (B) uniaxial strain; (C) tensile;
(D) hydrostatic; (E) triaxial compression; (F) polyaxial compression.