Chapter 5 Flashcards
is a strain relaxation method
where, in principle, a sample (core or block) is extracted from a rock unit, measured, and then released so that it can freely expand.
Overcoring
zones of failure of the wall of a
well that give the borehole an irregular and typically elongated shape
Borehole breakouts
is about how a rock responds to stress
below the limit where strain becomes permanent
Elasticity
means increasing the fluid pressure until
the rock fractures. The technique is frequently applied to
petroleum reservoirs to increase the near-well permeability
Hydraulic fracturing
give information about the Earth’s immediate response to stress
release along new or preexisting fractures.
Earthquake focal mechanism
the simplest general stress
model for the interior of the Earth. It is based on an idealized
situation where the rock has no shear strength (ss¼ 0).
lithostatic reference state
In a rock column where the rock is porous, the lithostatic stress is distributed over the grain contact area, and
this stress is called
effective stress
No real, solid rock experiences a perfectly lithostatic
reference state. Only magma and other fluids do, in which
case the term
hydrostatic pressure
forms when formation fluid in porous
formations is trapped between non-permeable layers.
Overpressure
Stress can be locked in and preserved after the external
force or stress field has been changed or removed, and is
then referred to
residual stress
The reference states of stress discussed above relate to natural
factors such as rock density, boundary conditions (uniaxial
versus plane strain), thermal effects and the physical properties of rock. Natural deviations from a reference state are
generally caused by
Tectonic Stress
is the gravitational pull exerted
by the sinking slab on the rest of the plate. It is largest for
old and cold, dense oceanic lithosphere, and negative if
light and buoyant continental crust is subducted
Slab Pull
simply the push from the topographically high oceanic ridge that marks divergent plate boundaries. This
ridge rises several kilometers from the ocean floor and
thus produces a significant lateral force.
Ridge Push
which is the frictional resistance
or shear force acting at the base of the lithosphere
basal drag
is expected to be
particularly common along convergent plate boundaries
and major active orogenic zones.
thrust-fault stress regime
are found along divergent plate boundaries, but are more pronounced in areas of active continental
rifting and extension.
normal fault regime stress
measures the deformation in the material in a direction perpendicular to the direction of the applied force.
Poisson’s Ratio
No real, solid rock experiences a perfectly lithostatic reference state. Only magma and other fluids do, in which case the term
hydrostatic pressure
reference state is based on the boundary condition that no elongation (positive or negative) occurs in the horizontal directions.
Uniaxial-strain reference state
Stress can be locked in and preserved after the external force or stress field has been changed or removed and is then referred
Residual stress
stress related to plate movements and plate tectonics. Locally, however, tectonic stresses may be influenced by such things as bending of layers, e.g. ahead of a propagating fault, fault interference and other local effects.
tectonic stress
is the gravitational pull exerted by the sinking slab on the rest of the plate. It is largest for old and cold, dense oceanic lithosphere, and negative if light and buoyant continental crust is subducted.
Slab pull
is simply the push from the topographically high oceanic ridge that marks divergent plate boundaries. This ridge rises several kilometers from the ocean floor and thus produces a significant lateral force.
Ridge push
which is the frictional resistance or shear force acting at the base of the lithosphere
Basal drag
the principal stresses are all equal. Hence, the lithostatic model itself provides no differential stress to the lithosphere, regardless of depth of burial.
lithostatic stress
