Page 100 - Reservoir Geomechanics

4 Rock failure in compression, tension

and shear

In this chapter I review a number of fundamental principles of rock failure in compres-
sion, tension and shear that provide a foundation for many of the topics addressed in
the chapters that follow. The first subject addressed below is the classical subject of
rock strength in compression. While much has been written about this, it is important
to review basic types of strength tests, the use of Mohr failure envelopes to represent
rock failure as a function of confining stress and the ranges of strength values found for
the rock types of interest here. I also discuss the relationship between rock strength and
effective stress as well as a number of the strength criteria that have been proposed over
the years to describe rock strength under different loading conditions. I briefly consider
rock strength anisotropy resulting from the presence of weak bedding planes in rock,
which can be an important factor when addressing problems of wellbore instability.
This is discussed in the context of two specific case studies in Chapter 10.
In this chapter I also discuss empirical techniques for estimating rock strength from
elastic moduli and porosity data obtained from geophysical logs. In practice, this is
often the only way to estimate strength in many situations due to the absence of core
for laboratory tests. This topic will be of appreciable interest in Chapter 10 when I
address issues related to wellbore stability during drilling. I also discuss a specialized
form of compressive failure – that associated with pore collapse, sometimes referred
to as shear-enhanced compaction or end-cap failure. This form of rock failure will
be revisited in Chapter 12 when compaction associated with depletion is addressed in
some detail. I then go on to discuss rock strength in tension. Because the tensile strength
of rock is quite low (and because tensile stress only acts as short-term transients in the
earth at depth), this subject is principally of interest in hydraulic fracturing in forma-
tions and drilling-induced tensile wellbore failures that form in the wellbore wall
(Chapter 6).
The final subjects I discuss in this chapter are related to shear sliding and the
frictional strength of faults. These topics are important in a number of ways. First, I show
how the shear strength of pre-existing faults constrains in situ stress magnitudes in the
crust. These constraints will be further refined by combining with direct measurements
of the least principal stress through some form of hydraulic fracturing in Chapter 6
and with observations of compressive and tensile wellbore failure in Chapters 7 and 8.
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