Generally, the induced fractures are stress-related
petal coring-induced fractures, which initiate from near the
core boundary and propagate downwards from a dip of 30°
to 75°. Some of these induced fractures form at the same
depth on opposite sides of the cores and several of them propagate
downward into the core but usually stop at about 1/3 to 1/4
of the core diameter
A schematic illustrastion of bit-induced stress
array below core, modified from Jeager & Cook,
1979 (After Lorentz and Finley, 1979)
It is
commonly thought that the petal fractures are tensile
fractures that form beneath the core bit and propagate
ahead of the core bit. The fractures form along the principal
stress trajectories beneath the core bit, as illustrated
in the figure. Petal fractures follow optimum lines of
stress towards the bit centreline (see traces of lines
a, b, c) and possibly outwards as well (lines d, e). The
strike of the petal fractures is usually parallel
to the maximum in-situ horizontal stress(and is perpen-dicular to the plane shown in the
figure).
In a deviated well, the orientation of the induced fracture
is only relevant when measured in the drilling position.
These stress-related fractures are developed during or
after coring.
The development of induced fractures
is under the control of pre-existing mechanical anisotropy
(natural fractures and bedding planes). Their orientation
is influenced by the in-situ stress.
These fractures were generally observed in the fine-grained
and competent rocks. The planes presented moderate dip
values, generally around 55°.
