| How to Create Hydraulic Fractures |
Certainly any of a variety of methods can be
used to create horizontal hydraulic fractures, and FRx has experience in
most. Furthermore, we are willing to
fine-tune methods to most effectively satisfy project and site
constraints. The site-specific factors
that weigh in selection of methods include target depths for the fractures,
remedial processes to be used, surface access, subsurface obstructions such as
utilities or existing wells, schedule, budget, etc. Since no two projects are identical, a
description of fracturing methods for a hypothetical site may not be
representative.
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The following steps describe what is perhaps the simplest method for creating hydraulic fractures. This is especially effecting in cohesive soils at depths as great as 75 feet. Click on the thumb nail sketches for larger schematics.
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| (1) Push casing equipped with a drive point to
desired depth. Either hammering or
direct push can provide the requisite force.
If convenient, a pilot hole may be drilled to a depth a few feet above
the fracture target zone. Also, a small
diameter soil core may be taken through the target zone prior to installation
of the casing.
<-- Click the icon to the left
The casing should be
inserted at least 1 ft into native soil, so that stress induced by lateral
displacement of soil can effect a sufficient seal for subsequent fracturing
pressures. The drive point can be
designed to include a sump or rat-hole, which permits subsequent installation
of a pump below the level of the fracture. (Sump not shown in this sketch)
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| (2) Expose a short cylindrical wall of soil by either (a) displacing the drive point downward by a few
inchesof (b) retracting the casing a few inches.
<-- Click the icon to the left.
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| (3) Cut a fracture initiation notch by directing a
high-energy jet against the cylindrical wall that is exposed above the drive
point. Cuttings from the jet fill and
overflow the casing and are collected at the surface for proper disposal. By rotating the jet in the horizontal plane, the largest possible notch can be created.
<-- Click the icon to the left.
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| (4) Nucleate the fracture by applying pressure
to the well and notch. The disc shape of the notch focuses stress along its
perimeter, and the fracture nucleates along that edge.
<-- Click the icon to the left.
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| (5) Propagate the fracture to desired size by
injection of slurry of granular material in a suitable carrier fluid. Rheological properties of the carrier fluid
allow it to suspend large concentrations of solid particles yet permit the
slurry to flow when pumped. Fluid and solids are mixed prior to pressurization.
Generally positive displacement pumps are best suited to granular slurries, so
propagation usually is conducted at constant volumetric rate.
<-- Click the icon to the left.
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(6) Monitor process and
geophysical parameters to facilitate interpretation of resultant fracture
form. Common measurements (referenced by
lowercase letters) include:
(a)
Pressure provides real-time information about fracture orientation because
horizontal and vertical fractures generate distinctly different signatures in
pressure logs. (b) Upward displacement of the ground surface (also known as
heave or uplift) observed by use of surveying equipment. The observation station is placed well
outside the expected radius of the fracture and remains stationary. (c) Angular deformation of the ground surface as
measured by tiltmeters. Small changes in surface orientation, which may be
critical under some structures or equipment, can occur without significant
uplift, so tiltmeters can provide assurance of adequate fracture control. (d)
Measurement of electromagnetic potential or current. When the injection well and fracture fluid
are excited electromagnetically (not indicated in the schematic), the resultant
field can be measured by an appropriate sensor.
An inversion algorithm
that is calibrated for the properties of the soil matrix and fracture material
can be run on a suitable computer to obtain a real-time description of fracture
form.
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