reservoir geomecanics

7 Determination of S 3 from mini-fracs and
extended leak-off tests and constraining the
magnitude of S Hmax from wellbore failures
in vertical wells
As mentioned at the outset of this book, arriving at practical solutions to many problems
in geomechanics requires knowledge of the magnitude and orientation of all three
principal stresses. This is well illustrated by the range of geomechanical topics and
case studies presented in Chapters 10–12. The first subject discussed in this chapter
is the magnitude of the least principal stress, S 3 ,asobtained by hydraulic fracturing,
specifically mini-frac (or micro-frac) tests done specifically for the purpose of measuring
stress. As discussed at length below, because hydraulic fracturing frequently occurs
during leak-off tests (LOT’s) and especially extended leak-off tests (XLOT’s), these
tests also can be used to determine S 3 .Innormal and strike-slip faulting environments,S 3
is equivalent to S hmin .Inreverse faulting environments, S 3 is equivalent to S v . Methods
for determination of S hmin from Poisson’s ratio (obtained from P- and S-wave sonic
logs) are based on questionable physical and geologic assumptions. These methods
will be discussed briefly in Chapter 9. Suffice it to say at this point that direct measurement
of the least principal stress through some form of hydraulic fracturing is the only
reliable method known that is practical to use in wells and boreholes at any appreciable
depth.
One can determine the magnitude of the least principal stress from a micro-frac, a
very small-scale hydraulic fracture induced only to measure stress at a particular depth,
usually at a specific depth through perforations in cemented casing. One could also
determine the least principal stress from a mini-frac, orthe relatively small-scale frac
made at the beginning of a larger hydrofrac operation intended to stimulate production
in a low-permeability formation. It can also be determined at the beginning of fracpack
operations, where a hydrofrac is made through some sort of gravel-pack screen
set in a well in weak sands, principally to spread out the depletion cone around the
well to reduce the likelihood of sand production. One can also determine the least
principal stress from a leak-off test: after the casing has been cemented in place at a
given depth and the well is drilled a short distance (usually 10–20 ft) the open section
of the well is pressurized to the point that a hydraulic fracture is created, and the
magnitude of the least principal stress can be determined. When leak-off tests are
carried out fully (as described below), they are referred to as extended leak-off tests.
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