Page 18 - Reservoir Geomechanics

1 The tectonic stress field

My goals in writing this book are to establish basic principles, introduce practical
experimental techniques and present illustrative examples of how the development of
a comprehensive geomechanical model of a reservoir (and overlaying formations) pro-
vides a basis for addressing a wide range of problems that are encountered during
the life-cycle of a hydrocarbon reservoir. These include questions that arise (i) during
the exploration and assessment phase of reservoir development such as the prediction
of pore pressure, hydrocarbon column heights and fault seal (or leakage) potential;
(ii) during the development phase where engineers seek to optimize wellbore stability
through determination of optimal well trajectories, casing set points and mud weights
and geologists attempt to predict permeability anisotropy in fractured reservoirs;
(iii) throughout the production phase of the reservoir that requires selection of optimal
completion methodologies, the prediction of changes in reservoir performance dur-
ing depletion and assessment of techniques, such as repeated hydraulic fracturing, to
optimize total recovery; and (iv) during the secondary and tertiary recovery phases
of reservoir development by optimizing processes such as water flooding and steam
injection. Chapters 1–5 address basic principles related to the components of a com-
prehensive geomechanical model: the state of stress and pore pressure at depth, the
constitutive laws that commonly describe rock deformation and fractures and faults in
the formations of interest. Chapters 6–9 address wellbore failure and techniques for
using observations of failure to constrain stress orientation and magnitude in wells of
any orientation. Chapters 10–12 address case studies that apply the principles of the
previous chapters to problems of wellbore stability, flow associated with fractures and
faults and the effects of depletion on a reservoir and the surrounding formations.

Why stress is important

The key component of a comprehensive geomechanical model is knowledge of the
current state of stress. Wellbore failure occurs because the stress concentrated around
the circumference of a well exceeds the strength of a rock (Chapters 6 and 10). A fault
will slip when the ratio of shear to effective normal stress resolved on the fault exceeds
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