Page 19 - Reservoir Geomechanics

4 Reservoir geomechanics

its frictional strength (Chapters 4, 11 and 12). Depletion causes changes in the stress
state of the reservoir that can be beneficial, or detrimental, to production in a number of
ways (Chapter 12). As emphasized throughout this book, determination of the state of
stress at depth in oil and gas fields is a tractable problem that can be addressed with data
that are routinely obtained (or are straightforwardly obtainable) when wells are drilled.
In this chapter, I start with the basic definition of a stress tensor and the physical
meaning of principal stresses. These concepts are important to establish a common
vocabulary among readers with diverse backgrounds and are essential for understanding
how stress fields change around wellbores (Chapters 6 and 8) and in the vicinity of
complex structures such as salt domes (as discussed at the end of the chapter). I also
introduce a number of fundamental principles about the tectonic stress field at a regional
scale in this chapter. These principles are revisited at scales ranging from individual
wellbores to lithospheric plates in Chapter 9. While many of these principles were
established with data from regions not associated with oil and gas development, they
have proven to have broad relevance to problems encountered in the petroleum industry.
Forexample, issues related to global and regional stress patterns are quite useful when
working in areas with little pre-existing well control or when attempting to extrapolate
knowledge of stress orientation and relative stress magnitudes from one area to another.

Stress in the earth’s crust

Compressive stress exists everywhere at depth in the earth. Stress magnitudes depend
on depth, pore pressure and active geologic processes that act at a variety of different
spatial and temporal scales. There are three fundamental characteristics about the stress
field that are of first-order importance throughout this book:
Knowledge of stress at depth is of fundamental importance for addressing a wide

range of practical problems in geomechanics within oil, gas and geothermal reservoirs
and in the overlaying formations.
The in situ stress field at depth is remarkably coherent over a variety of scales. These

scales become self-evident as data from various sources are analyzed and synthesized.
It is relatively straightforward to measure, estimate or constrain stress magnitudes

at depth using techniques that are practical to implement in oil, gas and geothermal
reservoirs. Hence, the state of stress is directly determinable using techniques that
will be discussed in the chapters that follow.
In short, the in situ stress field in practice is determinable, comprehensible and needed
to address a wide range of problems in reservoir geomechanics.
In this chapter I review a number of key points about the state of stress in the
upper part of the earth’s crust. First, we establish the mathematical terminology that
will be used throughout this book and some of the fundamental physical concepts and
definitions that make it possible to address many practical problems in subsequent