Page 31 - Reservoir Geomechanics

16 Reservoir geomechanics

Indicators of contemporary stress orientation and relative magnitude

Zoback and Zoback (1980) showed that a variety of different types of stress-related
data could be used to produce comprehensive maps of stress orientation and relative
magnitude at regional scales. A stress measurement quality criterion for different types
of stress indicators was later proposed by Zoback and Zoback (1989, 1991) which is
discussed in detail in Chapter 6.A key decision that Mary Lou Zoback and I made
in these initial compilations was to consider only stress data from depths greater than
several hundred meters. This was to avoid a myriad of non-tectonic, surface-related
sources of stress (due, for example, to topography, thermal effects and weathering) from
having a large effect where tectonic stresses are small (see Zoback and Zoback 1991).
The success of our initial stress mapping efforts demonstrated that with careful attention
to data quality, coherent stress patterns over large regions of the earth can be mapped
with reliability and interpreted with respect to large scale geological processes. The
Zoback and Zoback criterion was subsequently utilized in the International Lithosphere
Program’s World Stress Map Project, a large collaborative effort of data compilation
and analyses by scientists from 30 different countries led by Mary Lou Zoback (Zoback
1992). Today, the World Stress Map (WSM) database has almost 10,000 entries and
is maintained at the Heidelberg Academy of Sciences and the Geophysical Institute of
Karlsruhe University, Germany (http://www-wsm.physik.uni-karlsruhe.de/).
The following provides a brief description of stress indicators described in the stress
compilationspresentedthroughoutthisbook.AsindicatedinTable1.2,thesetechniques
are discussed in detail in subsequent chapters.

Wellbore stress measurements

The most classic stress measurement technique used in wellbores at depth is the
hydraulic fracturing technique (Haimson and Fairhurst 1970). When a well or bore-
hole is drilled, the stresses that were previously supported by the exhumed material
are transferred to the region surrounding the well. The resultant stress concentration is
well understood in terms of elastic theory, and amplifies the stress difference between
far-field principal stresses by a factor of 4 (see Chapter 6). Under ideal circumstances,
recording the trace of a hydraulic fracture on a wellbore wall can be used to determine
stress orientation. However, such measurements are usually limited to hard rock sites
and relatively shallow depths (<3 km) where open-hole hydraulic fracturing is possi-
ble. In most oil and gas wells, hydraulic fracturing cannot be used to determine stress
orientation because the wells must be cased in order to carry out hydraulic fracturing
without endangering the downhole equipment and wellbore. As discussed in Chapter 6,
hydraulic fracturing enables the least principal stress magnitude to be determined with
some accuracy (Zoback and Haimson 1982).