Introduction to Fracturing
Duke Gray
August 26th, 2010
Objectives
• Overview of fracturing
• Identify and explain porosity and permeability and how
they relate to fracturing
• Identify the four principal parameters of fracturing
design
• Explain effective wellbore radius (rw’)
• Introduce general hydraulic fracturing materials
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Fracturing Overview
• Hydraulic fracturing involves the injection of fluids into the
formation at a rate and pressure above the fracture
pressure of the reservoir in order to create a fracture
within the rock itself. We then pack this space with
proppant which:
– Improves productivity
– Interconnects formation permeability
– Improves EUR (Estimated Ultimate Recovery)
• In the case of the Barnett Shale, the goal is to connect
the naturally fissured (fractured) portions with the
wellbore to bypass damage due to drilling and cementing
and to speed recovery of the hydrocarbons.
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Propped Frac & Acid Frac
open fracture
during job
(frac width = wf)
sand used to
prop the
frac open
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1/2"
fracture tends to close
once the pressure has been
released
acid etched frac
walls
P.E. Fitzgerald, “The Value of Repeated Acid Treatments”. Illinois State Geological
Survey, Urbana, 1934.
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Reservoir Rocks
• Reservoir rocks need two properties to be successful:
– Pore spaces able to retain hydrocarbon.
– Permeability which allows the fluid to move.
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Permeability and Rocks
• In formations with large grains,
the permeability is high and the
flow rate larger.
• In a rock with small grains the
permeability is less and the flow
lower.
• Grain size has a large effect on
permeability.
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k
k
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Fissures (Fractures): Secondary Porosity
• Fissures are caused when a rigid rock is strained beyond its
elastic limit.
• The forces causing it to break are in a constant direction, hence all
the fissures are also aligned.
• Fissures are an important source of permeability in low porosity
reservoirs (such as the Barnett).
Fissures
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Hydraulic Fracturing
• Increase well productivity by creating a highly conductive path
compared to the reservoir permeability.
Formation perm. = ke
Damage
Frac perm. = kf
lxf = Fracture half length
}
lxf
wf
• The fracture will extend through the damaged near wellbore area.
• The fracture size is limited to two criteria :
– Drainage Radius
– Cost
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Four Principal Parameters
• Hydraulic fracturing does not change the permeability of the given
formation, but rather creates a permeable channel for reservoir fluids
to contact the wellbore.
• The primary purpose of hydraulic fracturing is to increase the effective
wellbore area by creating a fracture of given geometry, whose
conductivity is greater than the formation
– Dimensionless Conductivity
kf wf
Fcd 
k elxf
Where;
Fracture conductivity
------------------------------- kf = fracture permeability
Formation conductivity w = fracture width
f
ke = formation permeability
Fcd of 10-30 considered optimal
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lxf = fracture half length
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Effective Wellbore Radius (rw’)
• By relating dimensionless fracture conductivity (FCD) to effective
wellbore radius (rw’) and fracture ½ length (lxf, or xf) a correlation can
be distinguished.
– As shown here, the
Cinco-Ley Relation for rw’,
as the ratio of rw’/xf increases,
so does the dimensionless
fracture conductivity.
rw’ = 0.5*lxf
In other words, the bit would
be as wide as ½ the half length
of the fracture (effectively)!
Fcd 10-30
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Effective Wellbore Radius (rw’)
• The expression (rw’) is to equate the in-flow areas of wellbore and
propped fracture as illustrated. This relation shows for an infinite
fracture, the effective wellbore radius (rw’) is approximately ½ fracture
penetration or ½ length (lxf).
rw’ = 0.5*xf
rw’ = 0.5*800 ft
rw’ = 400 ft
For
a half=
length
800 feet,
width
800 of
ft!!!
800 ft.
it would equate to a 800 foot
wide drill bit!
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Hydraulic Fracturing Materials
• Base Fluid Systems
• Chemical Additives
• Proppants
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Base Fluid Systems
• Slickwater Applications
• Energized Fluid Applications
–
Low Friction
–
Carbon Dioxide
–
Low Viscosity (<5cp)
–
Nitrogen
–
Low Residue, less damaging
–
Water Sensitive Formations
–
Low Proppant Transport capabilities
–
Depleted Under pressured wells
–
Low Permeable Gas Formations
–
High Proppant Transport capabilities
• Linear Gel Applications
–
Mild Friction Pressures
–
Adjustable Viscosity (10<x<60cp)
• Gelled Oil Fluids
–
High Residue, more damaging
• Acidizing Services
• Crosslinked Applications
–
High Friction
–
High Viscosity (>100cp)
–
Excellent Proppant Transport
capabilities
–
High Residue, more damaging
–
Expensive
–
Complex Chemical Systems
• pH and Temperature dependent
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Chemical Additives
• Gelling Agents
• Oxygen Scavengers
• Friction Reducers
• Surfactants
• Crosslinker Control
• Recovery Agents
• pH Adjusting Agents
• Foaming Agents
• Clay Control
• Acids
• Breakers
• Anti-Sludge Agents
• Scale Inhibitors
• Emulsifiers
• Corrosion Inhibitors
• Fluid Loss Agents
• Bactericide
• Resin Activator
– Multiple varieties of all additives differing in concentrations,
ionic charges, limitations, and compatibilities
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Proppant Permeability
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Proppants
• Frac Sand (<6,000psi)
– Jordan
– Ottawa
– Brady
• Resin-Coated Frac Sand (<8,000psi)
– Super LC® (Santrol)
• Cureable
– AcFrac Black® (Borden)
• Precured
• Intermediate Strength Ceramics (<10,000psi)
– Econoprop® (Carbo Ceramics)
– Nap-Lite® (Norton-Alcoa)
• High Strength Ceramics (<15,000psi)
– Carboprop® (Carbo Ceramics)
– Sinterball® (Sintex)
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Questions?
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