Horizontal & Multi-Fractured Wells
Tony Martin
Director, Offshore Stimulation
Baker Hughes
Royal School of Mines, Imperial College
30 April 2012
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Fracturing Basics
2
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Pressure
What is Pressure?
Pressure is Stored Energy
(per unit volume)
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Pressure, Rate, Proppant Concentration
Basic Concept
BHTP
STP
Rate
Prop Conc
Time
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BHTP = Bottom Hole Treating Pressure
STP = Surface Treating Pressure
Net Pressure
Net Pressure
pnet = BHTP - Dpnwf - pclosure
given that
BHTP = STP + HH - Dpf
BHTP
Dpnwf
pclosure
STP
HH
Dpf
5
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= Bottom Hole Treating Pressure
= pressure loss due to near wellbore friction
= closure pressure
= Surface Treating Pressure
= Hydrostatic Head
= pressure loss due to friction in the wellbore
Basic Fracture Characteristics
Length, xf
Width, w
Height, hf
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What Does Fracturing Do?
• High Permeability Formations
– Conductive path through skin damage
– Re-stressing of weak formations
– Reduction in turbulence in gas formations
– Increased effective wellbore radius
• Low Permeability Formations
– Increased inflow area/reservoir contact
– Change from radial flow to linear flow within reservoir
– Massive reduction in drawdown
– Increased drainage
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Permeability Drives Everything
High k
Medium k
Low k
In high permeability formations, fractures are designed to be
short and highly conductive
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Permeability Drives Everything
Very Low k
Ultra Low k
In low permeability formations, fractures are designed to
maximise reservoir contact
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Permeability Drives Everything
• Example inflow areas:– 100 m, OH vertical well, 8.5” diameter = 67.8 m2
– 3000 m, OH horizontal well, 6” diameter = 1,436 m2
– Single 50 m radial hydraulic fracture = 15,708 m2
• For ultra low permeability formations (e.g. shale gas)
planar fractures do not provide sufficient inflow area
– Hydraulic fractures designed to exploit natural fracture
networks
– Stimulated reservoir volume (SRV)
10
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The Importance of Fracture
Conductivity
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Fracture Conductivity, Cf
• Fracture Conductivity is a Measure of How Conductive the
Fracture is
• It is Analogous to the kh Derived by Well Testing
• Fracture Conductivity Defines How Much can be
Produced by the Fracture
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Fracture Conductivity, Cf
Proppant Fracturing:-
Cf =
Where wave
kp
wave kp
= average propped width
= proppant permeability
Remember that kp is Not Constant
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Dimensionless Fracture Conductivity, CfD
• Also called Relative Fracture Conductivity
– Previously known as FCD
• CfD is a Measure of How Conductive A Fracture is
Compared to the Formation
• In Order to get the Maximum Possible Production
Increase, the Optimum Value for CfD must be Obtained
15
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Dimensionless Fracture Conductivity, CfD
CfD =
Cf
xf k
Where xf
k
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=
wave kp
xf k
= fracture half length
= formation permeability
Dimensionless Fracture Conductivity, CfD
CfD =
The Ability of the Fracture to
Deliver Fluid/Gas to the Wellbore
The Ability of the Formation to
Deliver Fluid/Gas to the Fracture
In Order to Achieve the Maximum Possible
Production Increase, the Optimum Balance
Between Fracture and Formation Deliverability
Must be Found
17
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Fracturing Horizontal Wellbores
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Vertical, Deviated or Horizontal?
• Vertical Wells
– Cheap to Drill
– Easiest to Fracture
– Requires lots of wellbores and lots of locations
• Deviated Wells
– Significant Fracturing Problems
– Increased Costs
– Reduced number of locations
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Vertical, Deviated or Horizontal?
• Deviated Wells (continued)
– Usually very complex connection between fracture and
wellbore
• Affects both treatment placement and production
– Solution is to plan well correctly
• Azimuth of deviated section parallel to maximum horizontal
stress, or
• Drill S-shaped wells to penetrate reservoir with vertical wellbore
20
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Vertical, Deviated or Horizontal?
• Deviated Wells (continued)
Uncontrolled Wellbore
Azimuth
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Wellbore Azimuth Parallel
To Fracture Azimuth
S-Shaped Wellbore
Cased and Cemented or Open Hole?
• Open Hole Fracturing
– Easier Connection Between Fracture and Wellbore
– Cost Savings
• Liner, Cementing, Rig Time
– Specialised Systems Required to Isolate Individual Sections
to Control Fracture Initiation
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Cased and Cemented or Open Hole?
• Cased Hole Fracturing
– Increased Cost
• Liner, Cementing, Rig Time
– Requires Complex Completion Systems
– Precise Control of Fracturing Process
– Traditionally, Most Horizontal Wells that are Planned to be
Fractured are Cased and Cemented
• New Technology is Changing This
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Horizontal Wellbores
sh,min
sh,max
Longitudinal Fracs
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sh,max
sh,min
Transverse Fracs
Longitudinal or Transverse?
• Longitudinal
– Longitudinal fracs are easiest to pump and have the
simplest connection to the wellbore
– Post-fracture production is not “choked” at the contact
between fracture and wellbore
– Easiest to predict post-fracture production
– Wellbore must be drilled within +/- 15 ° of maximum
horizontal stress azimuth.
• Anything else behaves like a transverse fracture
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Longitudinal Fractures
A
Approximately Equivalent
Post-Frac Behaviour when
A≈B
B
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Longitudinal Fractures
• Designing Longitudinal Fractures
– Start with “equivalent” single fracture on vertical wellbore
– Use Unified Frac Design to design geometry of single
fracture
– Place multiple fractures along horizontal wellbore
• Sufficient number to provide complete coverage
• Maintain UFD length to width ratio
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Longitudinal Fractures
• Unified Frac Design*:
– Proppant number, Np
Np =
2 kfwave
rek√p
=
xek
square
drainage
area = xe2
radial
drainage
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2 kfwave
*
Economides et al, 2001
Longitudinal Fractures
• Unified Frac Design:
– Optimum dimensionless fracture conductivity, CfD,opt
CfD,opt = 1.6
For Np < 0.1
-0.583 + 1.48 ln Np
CfD,opt = 1.6 + e
CfD,opt = Np
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1 + 0.142 ln Np
For 0.1 < Np < 10
For Np > 10
Longitudinal Fractures
• Unified Frac Design:
– Optimum length, xf,opt, and width, wopt
wopt
xf,opt
= CfD,opt
k
kf
• Adjust Np for Dietz* shape factor (CA):
Np,e = Np
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CA
30.88
*
Dietz, 1965
Longitudinal Fractures
• Calculate maximum dimensionless productivity
index, JD,max:
1
JD,max =
For Np,e ≤ 0.1
0.99 – 0.5 ln Np,e
0.423 – 0.311Np,e – 0.089Np,e2
JD,max =
6
p
-e
1 + 0.667Np,e + 0.015Np,e2
For Np,e > 0.1
Economides & Martin, 2007
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Transverse Fractures
Angle of Fracture from Wellbore
15°
15°
15°
15°
LONGITUDINAL
LONGITUDINAL
TRANSVERSE
TRANSVERSE
Most Wellbores, Drilled Without Knowledge of (or Planning
for) Fracture Azimuth, will Produce Transverse Fracs
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Transverse Fractures
• Transverse fractures have a very poor connection to the
wellbore.
– This makes frac jobs hard to pump due to tortuosity
– This chokes production and dramatically reduces fracture
effectiveness
– Open hole fractures have a much cleaner connection
between the fracture and the wellbore than cased and
perforated fractures
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Transverse Fractures
ye
xe
Np =
34
Ix2
kf wave xe
xf k ye
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where Ix = 2
xf
xe
Drainage Area
Productivity per Frac
Transverse Fractures
No of Fractures
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Transverse Fractures
• How Many Fractures?
– Dependent upon xf, k, kf, xe, and wave
– Complex iterative process
– Useful to fix a value of xf based on height growth
•
•
•
•
•
•
Zone height, water or gas contacts
Find Np and CfD,opt for fixed proppant volume
Calculate JD per frac for optimum geometry
Calculate total JD against number of fracs
NPV analysis to get optimum number of fracs
Repeat for different proppant volumes, to get plot of optimum
NPV against proppant volume per frac, for various numbers of
fracs
• Repeat process for different values of xf
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Transverse Fractures
• Gas Wells – Important
– Near well bore choking effect
• Caused by the very limited area of contact between fracture
and wellbore
• Can seriously affect productivity in medium and high
permeability gas wells
sc =
h
kh
ln
2rw
kfw
JDTH =
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p
2
1
(1/JDV) + sc
Economides & Martin, 2007, 2010
Transverse Fractures
• Gas Wells – Important
– Turbulent flow effects are also significant
kf,g =
kf
1 + NRe
• The combined effect of choking and turbulence can
reduce the flow by 80 to 90% in high permeability gas
formations
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Economides & Martin, 2007, 2010
Transverse Fractures
• Consider which type of completion is best for your
gas well
Permeability Range,
md
>5
0.5 to 5
0.1 to 0.5
< 0.1
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Best Technical
Solution
Comments
Horizontal Wellbore,
Longitudinal Fractures
In all cases
Horizontal Wellbore,
Longitudinal Fracture
OR
Vertical Well with Fracture
Dependent upon relative
costs of vertical and
horizontal wells
Horizontal Wellbore,
Transverse Fractures
Above 0.5 md, the choked
connection means that
transverse fractures are
relatively inefficient
Horizontal Wellbore,
Transverse Fractures
OR
Vertical Well with Fracture
Dependent upon relative
costs of vertical and
horizontal wells
Economides & Martin, 2007, 2010
Fracturing Multiple Intervals
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Completion Options
• Open Hole
– Sliding side doors separated by open hole packers
• Cased Hole
– Sliding side door systems
• Liner-conveyed
• Completion-conveyed
– “Plug and Perf” systems
• Various different systems available
– Coiled tubing-based systems
• Fracturing through CT
• Annular
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Open Hole Systems
• Multizone open hole
completion systems use
a series of sliding side
doors, separated by open
hole packers
• SSDs are initially closed
and are opened by a ball
landing on a seat
• Seats have progressively
larger diameters moving
upwards
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Open Hole Systems
• Up to 40 zones per completion
• 3 different types of packer available
– Inflatable, swellable, squeeze
• Typically run as a liner
– Liner hanger set conventionally
– First ball sets the packers and opens the lowest interval
• Swellables have to be left 24 to 48 hours
– Subsequent balls open successive intervals and close off
the previous interval
• All zones flowed back together after fracturing
operations have finished
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Open Hole Systems
• Applications
– Horizontal or vertical wellbores
– Cased or open hole
– Acid or proppant stimulation treatments
• Advantages
– One-trip installations
– Reduction in completion time
• Disadvantages
– Control of fracture initiation
– Fluid recovery
– Lack of flexibility
– Ball recovery
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Cased Hole Systems
• In general, cased hole systems offer greater
flexibility and better control of fracture initiation
– Most systems allow perforations to be designed zone
by zone
– The point of fracture initiation is tightly controlled
• However, in general cased hole systems are more
expensive and require significantly more rig time
– In addition to the time and expense of cementing a
horizontal liner in place
– In spite of this, there are still more cased and
cemented horizontal multizone wells being
completed than open hole wells
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Cased Hole Systems
• Casing-Conveyed SSDs
– SSD run on casing or liner and cemented into place
– SSDs can be opened in several different ways
• Coiled tubing, with a packer positioned below the SSD to
provide isolation
• Balls, similar to open hole systems
• Darts or “frac bombs”
– Fluid pressure is used to break cement behind SSD
• Acid soluble cement systems are also used
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Cased Hole Systems
• Completion-conveyed SSDs
– A series of SSDs separated by squeeze packers are RIH on
a tubing string.
• Liner is perforated prior to completion running
• SSDs manipulated by coiled tubing between zones
– Technically the best system for zonal isolation, controlling
fracture initiation and post-treatment fluid recovery
• Very heavy on rig time
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Cased Hole Systems
• “Plug and Perf” systems
– Perforate, stimulate, isolate
– Move from the bottom of the well to the top
•
•
•
•
Perforate the lowest interval
Perform the treatment
Recover the frac fluid, if desired
Isolate the interval
– Wireline/CT conveyed plugs
– Sand plugs
• Repeat as often as required
• Go back in with CT and remove isolation systems
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Cased Hole Systems
• Coiled Tubing Methods
– Fracturing through CT
• All intervals perforated before frac operations
• Straddle packer placed on the end of the CT
• Treatments pumped down CT into perforations
– Treating pressure “energises” packer elements
– Circulating and reversing possible
• Multiple zones treated consecutively using a single CT run
• Much greater pressure can be placed on the CT than is normal
– Static vs dynamic
• Large diameter CT required
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Cased Hole Systems
• Coiled Tubing Methods
– Annular CT Fracturing
• No pre-perforating
• Perforations either cut using jetting tool or shot
via selective perforating guns on the CT
• Zonal isolation
– Packer placed below jetting tool or perforation
guns
– Sand plugs pumped down the CT/completion
annulus
• Treatment is pumped down the CT/completion
annulus.
• CT string used to monitor BH pressure
• Multiple zones treated consecutively using a
single CT run
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Summary
• Transverse or Longitudinal?
– Formation stresses
– Wellbore azimuth
– Gas?
• How many fracs?
• Cased or Open Hole?
– Fluid recovery
– Rig time
– Operational flexibility
• Would a Vertical Well be Better?
52
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Horizontal & Multi-Fractured Wells
Thank you.
Any Questions?
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