Page 157 - Reservoir Geomechanics

5 Faults and fractures at depth

In this chapter I consider a number of topics related to faults and fractures in rock.
Faults and fractures exist in essentially all rocks at depth and can have a profound
effect on fluid transport, mechanical properties and wellbore stability. As discussed in
Chapter 4 (and later demonstrated through a number of case studies), frictional slip
along pre-existing fractures and faults limits in situ stress magnitudes in a predictable –
and useful – way.
To begin this chapter, I distinguish between opening mode (Mode I) fractures and
faults, and briefly discuss the importance of faults in influencing fluid flow in low per-
meability rock. The influence of faults on permeability is discussed at length in Chapter
11,as well as the sealing (or leakage) potential of reservoir-bounding faults. The man-
ner in which slip along weak bedding planes can affect wellbore stability is discussed
in Chapter 10.I briefly discuss wellbore imaging devices as it is now routine to use such
devices to map fractures and faults in reservoirs, and then discuss common techniques
for representing fracture orientation data, including stereonets and three-dimensional
Mohr diagrams, when the state of stress is known. Faulting in three dimensions is
revisited in Chapter 11.I conclude this chapter by briefly discussing earthquake focal
mechanisms and their use in determining approximate stress orientations and relative
stress magnitudes.
There are a number of books and collections of scientific papers on the subject of
fractures and faults in rock. Of particular note are the compilations by Long et al.
(1996); Jones, Fisher et al.(1998) Hoak, Klawitter et al.(1997) and the collec-
tions of papers on the mechanical involvement of fluids in faulting (Hickman, Sibson
et al. 1995; Haneberg, Mozley et al. 1999;Faybishenko, Witherspoon et al. 2000;
Vigneresse 2001; Jones, Fisher et al. 1998;Davies and Handschy 2003). Hence, the
purpose of this chapter is not to provide a comprehensive review of this subject. Rather,
my goal is to cover a number of basic principles about the nature of fractures and
faults at depth and provide a basis for concepts discussed in subsequent chapters. For
reasons that will soon be apparent, we will be principally concerned with faults; planar
discontinuities associated with shear deformation. As the topic of this book is geome-
chanics, I consider only mechanical discontinuities at depth and not those associated
with chemical processes – dissolution features, stylolites, etc. – which are encountered
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