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ATMPETE 406 UBDATM
ATM ATM
PETE 406 - UnderbalancedDrilling, UBD
Lesson 9
Benefits of Underbalanced Drilling
UDM - Chapter 3
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Benefits of Underbalanced
Drilling
Increased Penetration Rate
Increased Bit Life
Reduced Differential Sticking
Minimize Lost Circulation
Improved Formation Evaluation Reduced Formation Damage
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Benefits of Underbalanced
Drilling Reduced Probability of Differential Sticking
Earlier Production
Environmental Benefits
Improved Safety
Increased Well Productivity Less Need for Stimulation
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Increased Penetration Rate
In permeablerocks, a positive differential
will decrease penetration because
increases the effective confining stress which
increases the rocks shear strength
Therefore increasing shear stress (by drilling UB)
increases penetration rate
and increases the chip hold down effect
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Chip hold down effectBit tooth
Crack in theformation
As drilling fluid
enters the fracture,
the pressuredifferential across
the rock fragment
decreases, releasing
the chip
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Effect of Pressure Differential
In permeable
rocks
penetration rateis a function of
the differential
pressure not theabsolute
pressure
Micro-bit test
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Gas drillingvs. mud
drilling Mud
Gas
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Penetration rate as a function of the
differential pressure across the workfront
For permeable rocks
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Increased Bit Life???
Increased vibration with air drilling may
actually decrease bearing life
Bit may drill fewer rotating hours but drill
more footage - fewer bits
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Effect of UBD on cutting
structure of roller cone bits
Mechanical Specific Energy, MSE, isdefined as the mechanical work that must be
done to excavate a unit volume of rock
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
The work done by the bit is:
lb)-(fttorque
60
2
where
RPM
ROPWOBW
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
The volume of rock excavated
per revolution is:
(feet)diameterbitd
240
b
2
RPM
ROPd
V b
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
The mechanical specific energy
is give by:
22
4480
bb d
WOB
ROPd
RPMMSE
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
What does this mean?
1. Bit torque is not a function of borehole
pressures.
2. Penetration rates generally increase withdecreasing borehole pressures.
3. MSE are therefore, usually lower at lower
borehole pressures
22
4480
bb d
WOB
ROPd
RPMMSE
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
What does this mean?
4. Therefore, cutting structure wear rates (in
terms of distance drilled) should be
inversely related to the MSE5. If the bit has to do less work to remove a
given volume of rock, its cutting elements
should wear less.6. A bit should be able to drill more footage,
when drilling underbalanced.
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Reduced Differential Sticking
Fs= AcDPms*144 sq.in./sq.ft.
Fs= force required to free pipe (lbf)
Ac= contact area (sq. ft)
DP= pressure differential across the mud
cake (psid)
ms= coefficient of friction
TPETE 406 UBDT
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Example
Contact area is 30 feet long and 0.25 ft wide
Pressure differential is 300 psid
The coefficient of friction is 0.3
The force to free the pipe (in excess of
string weight) is
30 x 0.25 x 300 x 0.3 x 144 = 97,200 lbf
Note equation 3.5 in text is incorrect
TPETE 406 UBDT
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Minimized Lost Circulation
If the pressure in the wellbore is less than
the formation pressure in the entire open
hole section, lost circulation will not occur.
TPETE 406 UBDT
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Improved Formation Evaluation
Production rates while drilling UB can be
measured with no filtrate invasion occurring
No filtrate invasion can mean more accurateLWD measurements.
TPETE 406 UBDT
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ATMPETE 406 UBDATM
ATM ATM
Reduces formation damage
TPETE 406 UBDT
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Formation damage mechanisms
during drilling (overbalanced) Scales, sludges or emulsions due to
interaction between filtrates and pore fluids
Interaction between aqueous mud filtrateand clay particles in the formation
Solids invasion
TPETE 406 UBDT
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Formation damage mechanisms
during drilling (overbalanced): Phase trapping or blocking
Adsorption of drilling fluid additives,
leading to permeability reductions orchanges in wettability
Migration of fines
Generation of pore-blocking organic
byproducts from bacteria entering the
formation from the drilling fluid
TPETE 406 UBDT
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Formation damage mechanisms
during drilling (underbalanced): Temporary overbalance
Spontaneous imbibition
Gravity-induced invasion
Wellbore glazing
Post-drilling damage
Mechanical degradation
TPETE 406 UBDT
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Temporary overbalance
Can be intentional to:
kill well for trips,
transmit MWD surveys,
log the well,
completion and WO operations
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Temporary overbalance
Can be unintentional:
Slug flow or liquid holdup causing fluctuations
in annular pressureHigh fluid pressures across the face of diamond
and TSP bits
Near wellbore production reduces the formationpressure near the face of the wellbore
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Temporary overbalance
Can be unintentional:
Varying pore pressure along the wellbore
Excessive surge pressures
Equipment malfunctions or procedural errors
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Spontaneous Imbibition
Due to capillary effects - even if drilling
underbalanced
The underbalance pressure necessary toprevent water from being drawn from an
aqueous drilling fluid into the formation
will depend on the initial formation watersaturation and the pore sizes
ATMPETE 406 UBDATM
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department of
Petroleum Engineering
Gravity-induced invasion
Can occur during UBD in the formation
produces from natural fractures or vugs
ATMPETE 406 UBDATM
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department ofPetroleum Engineering
Wellbore glazing
UBD can result in high wellbore
temperatures due to the friction between the
rotating drillstring and the borehole wall. This can cause a thin low permeability
glazed zone
ATMPETE 406 UBDATM
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department ofPetroleum Engineering
Post-drilling damage
Due to:
Killing the well for completion
Cementing
Mobilization of fines during production
Liquid coning in gas reservoir
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department ofPetroleum Engineering
Mechanical degradation
Rock around the wellbore experiences a
concentration of in-situ stresses due to
drilling the well. As the wellbore pressure is lowered, the
effective stresses increase,
resulting in a decrease in porosity andavailable flow channels leading to
reduced permeability
ATMPETE 406 UBDATM
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department ofPetroleum Engineering
Earlier Production
With the necessary equipment on location
during UBD operations, produced fluids can
go to sales. Open-hole completions are sometimes
performed.
If the well is drilled and completedunderbalanced, wells from depleated
reservoirs will not need swabbing.
ATMPETE 406 UBDATM
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department ofPetroleum Engineering
Environmental Benefits
Closed loop systems produce less wasted
drilling fluids
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ATMPETE 406 UBDATM
ATM ATMHarold Vance Department ofPetroleum Engineering
Less Need for Stimulation
If the formation is not damaged during
drilling and completion, stimulation to
remove the damage will not be needed
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