Physics of
Fracturing: All solid materials will fail if enough stress is applied.
Hydraulic fracturing (as well as pneumatic fracturing) methods use fluid
pressure to create the stress needed for a crack to nucleate in soil or
bedrock. Once the crack starts, lesser stress is required to propagate it
through the medium. Realistically, this implies that fluid fracturing methods
will create only one crack because the propagation dissipates the stress. In
contrast, explosives create shock waves of stress that can, effectively,
outpace the propagation so multiple cracks nucleate.
Fluid fracturing nucleates a single crack, and it can be
propagated by continuing to add (i.e. inject) fluid into it. The fracture can
continue to grow until it hits an obstruction or the injected fluid leaks out
of it into the surrounding permeable media as fast as the injection rate. Of
course, the law of diminishing economic returns can affect a feasible limit to
size.
Shape and
Orientation: The form of the fracture are
characteristics of interest and, practically, should be exploited to best
effect the purpose of fracturing. All induced fractures have the same
sheet-like shape, with apertures typically less than 1% of the extent.
Variation of injection rate and fluid viscosity as well as local geological /
geotechnical properties can impose some effect on this ratio as well as upon
the ratio of the two major dimensions of length and width.
In situ state of stress controls the ultimate orientation of
an induced fracture. Vertical in situ stress is typically the weight of the
overburden. Lateral in situ stress can result from a variety of geologic
processes. The fracture will propagate in the plane that is perpendicular to
the least principal in situ stress, i.e. it will overcome the least resistance.
Essentially the fracture plane will demonstrate whether it is easier to
“jack-up” the overburden or “push” the oceans apart.
Manipulating Stress:
That said, it is important to note that the fracture
nucleates in the plane defined by induced stress overcoming the strength of the
solid media. Since fluid fracturing relies upon pressure, the induce stress can
be manipulated by constructing the geometry of the surfaces upon which the
pressure is applied. So, we diligently create thin gaps of known orientation so
that the greatest stress can be developed at the perimeter of the shape. For
example, pressure applied in a disk-shaped cavity focuses stress in the hoop defined
by the cylindrical surface, and a fracture nucleated in the plane of this disk
will propagate some distance before being diverted to its ultimate direction by
the least in situ stress.
Conversely, application of fluid pressure to a long cylinder
results in a fracture that has a major dimension parallel to the axis of the
cylinder. This is why we avoid trying to create horizontal fractures from open
well. (It also the reason that a frozen pipe fails with a
split parallel to its axis.)
Propagation: Sometimes
it is said that fractures “follow the path of least resistance.” That is true
as long as resistances being considered are the matters of in situ stress as
discussed above. Notably, many readily observable characteristics of a
formation do not necessary align with the in situ stresses. These include
coloration, lamella, variations in stratigraphy,
existing fractures, etc. Consequently, a propagating fracture may cut across
features that might be perceived as able to control the fracture form. A most
prominent example was observed during the Stressoil
project when a excavation revealed that a fracture
created with white sand cut across a fracture created two days earlier with red
sand.
Hydraulic fractures grow in episodic bursts even when the
injection rate remains constant. In other words, the fracture expands as a
collection of lobes, each lobe created during a portion of the overall
injection event. In the end, the fracture comprises many lobes. Each lobe is
filled with whatever material is being injected while it is being created.
Since lobes created early are closest to the injection point, the last material
injected ends up the farthest from the injection well. This order contrasts
with the might be expected if the fracture grew uniformly with later material
displacing earlier.
Related Phenomena:
Fracture features can be created in a wide variety of media. Of course brittle
rocks fracture. Also, physics allows fractures to form in material that might
seem to be more compliant. For example, clay materials that traditionally are
conceived as plastic will exhibit Type I (tensile) fractures when subjected to
internal fluid pressure. Interestingly, failure
processes very similar to fracturing occur in non-cohesive materials such as
sand packs. Recent work at Georgia Tech shows that thin spaces of narrow
aperture, which are identical in form to fractures, can be propagated through
sand packs because the friction of rearranging the grains at the tip of the
space mimics the energy consumption of matrix failure that occurs during
creation of proper fractures. Accordingly, fracturing methods and models can be
applied to such formations.