Lesson 9 Benefits of UBD

8/13/2019 Lesson 9 Benefits of UBD

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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



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



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



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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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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Gas drillingvs. mud

drilling Mud

Gas


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Penetration rate as a function of the

differential pressure across the workfront

For permeable rocks


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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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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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The work done by the bit is:

lb)-(fttorque

60

2

where

RPM

ROPWOBW


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The volume of rock excavated

per revolution is:

(feet)diameterbitd

240

b

2

RPM

ROPd

V b


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The mechanical specific energy

is give by:

22

4480

bb d

WOB

ROPd

RPMMSE


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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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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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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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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



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



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



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




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




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



Temporary overbalance

Can be intentional to:

kill well for trips,

transmit MWD surveys,

log the well,

completion and WO operations

ATMPETE 406 UBDATM


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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

ATMPETE 406 UBDATM


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Can be unintentional:

Varying pore pressure along the wellbore

Excessive surge pressures

Equipment malfunctions or procedural errors

ATMPETE 406 UBDATM


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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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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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Post-drilling damage

Due to:

Killing the well for completion

Cementing

Mobilization of fines during production

Liquid coning in gas reservoir

ATMPETE 406 UBDATM


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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


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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Environmental Benefits

Closed loop systems produce less wasted

drilling fluids

ATMPETE 406 UBDATM


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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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Lesson 9 Benefits of UBD

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