Fracturing Review

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Fracturing (Review)
Department of Geology University of Texas at
Arlington
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Joints and shear Fractures

• There are 3 modes or end members of fracturing
• Mode I sides move perpendicular to fracture
• Mode II Sides move parallel to fracture but
  perpendicular to fracture edge
• Mode III Sides move parallel to fracture and
  parallel to fracture edge.
• Mode I are called joints. The others are called
  faults or shear fractures

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Relationship between stress and fracturing

• Review of Stress
• Stress is the force/area on all the surfaces
  passing through a point.
• Traction is the stress acting on a particular
  plane
• Normal and shear stress is the resolution of the
  traction perpendicular and parallel to the
  surface
• Principal stresses are the maximum and minimum
  tractions.

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Relationship between stress and fracturing

• Review The Mohr Circle
• Normal and shearing stress on the axes.
• The Principal stresses

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Relationship between stress and fracturing

• Review The Mohr Circle
• The normal and shearing stress acting on a
  particular plane
• The orientation of the normal to that plane

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Relationship between stress and fracturing

• Experimental Testing
• Three types of tests producing fractures
• (Middle test should show s3 in compression)
• Shear fractures (faults) are inclined to s1 and
  s3
• Joints parallel s1 and s2 and are perpendicular
  to s3

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Relationship between stress and fracturing
Mohr Circle for increasing Tension only
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Relationship between stress and fracturing
Mohr Circle for increasing Axial Extension with
compression
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Relationship between stress and fracturing
Mohr Circle for increasing Axial Compression with
no tension
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Relationship between stress and fracturing
As the tensile stress increases, fractures form
producing joints perpendicular to s3
How do you find the normal stress s and shearing
stress t on the joints when they fracture? That
stress is called the tensile strength, To
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Relationship between stress and fracturing
When there is both tension and compression, shear
fractures (faults) form.
How do you find the normal stress s and shearing
stress t on the faults when they fracture? That
stress is called the shearing strength, and it
varies with mean stress.
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Relationship between stress and fracturing
When the rock is everywhere under compression,
only faults form. How do you find the normal
stress (s) and shearing stress (t ) on the faults
when they fracture?
Again, that stress is called the shearing
strength, and it varies with mean stress.
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Relationship between stress and fracturing
Finding the failure envelop of rocks The stress
necessary to fracture a rock under a variety of
conditions is called the failure envelop. A
series of experimental tests are run until the
rock fails, the failure stresses are recorded and
plotted as a Mohr diagram noting the orientation
of the fractures.
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Relationship between stress and fracturing
Finding the failure envelop of rocks
For example, a tension test is run to find the
tensile strength of a rock.
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Relationship between stress and fracturing
Finding the failure envelop of rocks
Next, increase the mean stress and run test
again.
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Relationship between stress and fracturing
Finding the failure envelop of rocks
Finally, run a series of tests at increasing mean
stress.
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Relationship between stress and fracturing

• These tests define a failure envelop for a
  particular rock.
• All of the normal and shearing stresses inside
  the envelop are stable no fractures produced.
• All of the stresses on or outside the envelop
  will producing fracturing

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Relationship between stress and fracturing
When the Mohr circle becomes tangent to envelop,
then the s and t at that point causes a fracture.
No fractures are produced by any other
combination of s and t on the circle.
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Relationship between stress and fracturing
What is the orientation of the fracture produced
by the state of stress shown in the diagram? What
is s and t on that fracture surface?
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Relationship between stress and fracturing
The Coulomb fracture theory. The normal stress
across a potential fracture surface tends to
inhibit shear displacement. The shear stress
parallel to the surface tends to generate
faulting. Faulting occurs when the ratio is a
critical value (t-to)/s constant
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Relationship between stress and fracturing
The Coulomb fracture theory. tc to s Tan(F)
The equation is a straight line on the Mohr
diagram to is the intercept and called the
cohesion F Is the slope of the line and is called
the angle of internal friction.
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Relationship between stress and fracturing
The Coulomb fracture theory. tc to s Tan(F)
Meaning of equation For any value of normal
stress s, the value of the shearing stress on
that surface must be tc if a fracture forms.
Otherwise no fracture will occur.
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Relationship between stress and fracturing
The Coulomb fracture theory. tc to s Tan(F)
Note that the Mohr-Coulomb theory only applies to
the compressive stress region. Tensile stress
failure is different.
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Relationship between stress and fracturing
The fracture law in the tensile region is call
the Griffith law
tc (4tos 4to2)1/2 to is the tensile
strength s is the normal tensile stress acting on
a surface tc the critical shear stress on that
surface that will produce a fracture
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Relationship between stress and fracturing
The fracture law in the high pressure region is
called von Mises, and is transitional between
brittle and ductile failure
The diagram shows all three failure laws and the
pressure regions they apply to.
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Relationship between stress and fracturing
Each material (rock) has a different fracture
envelop. The graph shows data for several common
rock types determined experimentally.
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Relationship between stress and fracturing
Friction and preexisting fractures
If fractures are already present in a rock when a
new stress is applied, then those old fractures
may be reactivated. Whether old fractures are
reactivated or new ones produced depends on
friction and their orientation relative to the
compressive stress.
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Relationship between stress and fracturing
Friction and preexisting fractures
Friction depends on roughness of the fracture
surfaces and whether fault gouge is
present. Movement on a preexisting fracture is
determined by the friction law which is similar
to the Mohr-Coulomb fracture law tc to s
Tan(F) (Mohr-Coulomb Law) tc s mf (Byerlee
Friction Law) mf coefficient of sliding
friction TanFf There is no cohesion (to)
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Relationship between stress and fracturing
Friction and preexisting fractures
Experimental tests show that most rocks have a
friction coefficient that varies slightly from
low pressure to high pressure. The coefficient
does not depend on rock type
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Relationship between stress and fracturing
Friction and preexisting fractures
If the friction envelop is plotted on the Mohr
Diagram together with Mohr-Coulomb envelop, you
can see that pre-existing fractures are weaker
and will shear before intact rock fractures
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Relationship between stress and fracturing
Friction and preexisting fractures
Note that only preexisting fractures of a
particular orientation will shear (shown in the
upper right). All other orientations will not
shear.
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Relationship between stress and fracturing
Friction and preexisting fractures
If preexisting fractures of the right orientation
shear, then stress is relieved and will not build
up high enough to form new fractures.
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Relationship between stress and fracturing
Friction and preexisting fractures
As the stress increases, the orientations of
preexisting fractures that can shear expands. If
preexisting fractures of the proper orientation
arent present, then new fractures will form in
the intact rock.
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Pore Pressure
In a rock with pore space, The external stress
(s1, s3) is supported by the minerals. Fluids in
the pore space can have a pressure that is
independent of the external stress supported by
the minerals.
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Pore Pressure
The pressure in the pore fluids acts outward, and
acts like a tensile stress pushing the grains
apart.
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Pore Pressure
Normally, the pore pressure is just the
hydrostatic pressure of the overlying fluids if
the pores are connected to the surface Pp rw g
h Normally, the vertical stress is also the
weight of the overlying rock sv rr g h Because
the density of rock is greater than that of
fluids, sv gt Pp
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Pore Pressure
Because the pore pressure acts outward, it
offsets the external stress reducing any
compressive stress. So, in the presence of pore
fluids, the total stress, called effective
stress (s), is given by s s Pp The Mohr
equations become s 1/2(s1 s2) 1/2(s1
s2)Cos(2q) t 1/2(s1 s2) Sin(2q)
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Pore Pressure
Increasing pore pressure has the same effect as
reducing the mean stress and driving the Mohr
circle to lower pressures. Without the fluid
pressure, the stress may not be great enough to
cause fractures. But higher pore pressures can
reduce the mean stress enough to intersect the
fracture envelop.
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Pore Pressure
Remember, mean stress is related to depth of
burial, so increasing pore pressure is like
bringing the rock toward the surface. Normally,
joints (extension fractures) can occur only near
the surface. But high pore pressures can produce
joints at depth.
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Pore Pressure
The petroleum industry artificially fractures
reservoirs to stimulate production by pumping
fluids down a bore hole, increasing pore pressure
in the reservoir and generating fractures.
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Pore Pressure
Fractures can grow episodically by fluctuating
pore pressures.
As pore pressure increases, mean effective stress
lowers until a fracture occurs. The pore fluid
fills the fracture so pore pressure lowers and
stress increases. This can cycle several times
extending the fracture.
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Pore Pressure
Fractures can grow episodically by fluctuating
pore pressures.
Multiple arrest lines on joints probably are
produced by pore pressure cycling.
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Pore Pressure
Veins in fractures may also be associated with
high pore pressure. As fracturing occurs, fluid
pressure drops precipitating vein minerals.
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Pore Pressure
Fibrous vein filling can indicate the direction
of fracture opening and show episodic opening
events by fibers with different orientations.