Hydraulic fracturing
is a method used to create fractures that extend from a
borehole into rock formations, which are typically maintained
by a proppant. The method is informally called fracing
or hydrofrac. The technique is used to increase or
restore the rate which fluids, such as oil, gas or water, can
be produced from the formation. By creating or restoring
fractures, the surface area of the formation exposed to the
borehole is increased and the fracture provides a conductive
path connecting the reservoir to the well, which effectively
increases the rate that fluids can be produced from the
reservoir formations.
The main
industrial use of hydraulic fracturing is in stimulating
production from oil and gas wells. Hydraulic fracturing is
also applied to stimulating groundwater wells, preconditioning
rock for caving or inducing rock to cave in mining, as a means
of enhancing waste remediation processes (usually hydrocarbon
waste or spills), to dispose of waste by injection into
suitable deep rock formations, and as a method to measure the
stress in the earth. Volcanic dikes and sills are examples of
natural hydraulic fractures. Hydraulic fracturing incorporates
results from the disciplines of fracture mechanics, fluid
mechanics, slid mechanics, and porous medium flow.
When
applied to stimulation of water injection wells, or oil/gas
wells, the objective of hydraulic fracturing is to increase
the amount of exposure a well has to the surrounding formation
and to provide a conductive channel through which the fluid
can flow easily to the well. A hydraulic fracture is formed by
pumping a fracturing fluid into the well bore at a rate
sufficient to increase the pressure downhole to a value in
excess of the fracture gradient of the formation rock. The
pressure then causes the formation to crack which allows the
fracturing fluid to enter and extend the crack further into
the formation. In order to keep this fracture open after the
injection stops, a solid proppant is added to the fracture
fluid. The proppant, which is commonly a sieved round sand, is
carried into the fracture. This sand is chosen to be higher in
permeability than the surrounding formation and the propped
hydraulic fracture then becomes a high permeability conduit
through which the formation fluids can be produced back to the
well. The fracture fluid can be any number of fluids, ranging
from water to gels, foams, nitrogen, carbon dioxide or even
air in some cases. Various types of proppant are used,
including sand, resin-coated sand, and man-made ceramics
depending on the type of permeability or grain strength
needed.
