ATMPETE 406 UBDATM

ATM ATM

PETE 406 - Underbalanced Drilling, UBD

Lesson 9

Benefits of Underbalanced Drilling

UDM - Chapter 3

Harold Vance Department of Petroleum Engineering

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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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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

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Increased Penetration Rate

• In permeable rocks, 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

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Chip hold down effectBit tooth

Crack in the formation

As drilling fluid enters the fracture, the pressure differential across the rock fragment decreases, releasing the chip

Harold Vance Department of Petroleum Engineering

ATMPETE 406 UBDATM

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Effect of Pressure Differential

• In permeable rocks penetration rate is a function of the differential pressure not the absolute pressure

Micro-bit test

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ATMPETE 406 UBDATM

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Gas drilling vs. 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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Penetration rate in impermeable rocksBit tooth

Crack in the formation

In impermeable rock, the instantaneous initial pressure in the crack itself is close to zero, i.e. the penetration rate is now a function of absolute wellbore pressure.

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Field example switching from air to mud

Switch to mud

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ATMPETE 406 UBDATM

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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, is defined 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)-(ft torque τ

602

whereRPM

ROPWOBW

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The volume of rock excavated per revolution is:

(feet)diameter bit d240

b

2

RPM

ROPdV b

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ATMPETE 406 UBDATM

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The mechanical specific energy is give by:

22

4480

bb d

WOB

ROPd

RPMMSE

Harold Vance Department of Petroleum Engineering

ATMPETE 406 UBDATM

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What does this mean?

1. Bit torque is not a function of borehole pressures.

2. Penetration rates generally increase with decreasing borehole pressures.

3. MSE are therefore, usually lower at lower borehole pressures

22

4480

bb d

WOB

ROPd

RPMMSE

Harold Vance Department of Petroleum Engineering

ATMPETE 406 UBDATM

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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 MSE

5. 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.

Harold Vance Department of Petroleum Engineering

ATMPETE 406 UBDATM

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Reduced Differential Sticking

• Fs = AcPs*144 sq.in./sq.ft.

• Fs = force required to free pipe (lbf)

• Ac= contact area (sq. ft)

P= pressure differential across the mud cake (psid)

s = coefficient of friction

Harold Vance Department of Petroleum Engineering

ATMPETE 406 UBDATM

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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

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ATMPETE 406 UBDATM

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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.

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ATMPETE 406 UBDATM

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Improved Formation Evaluation

• Production rates while drilling UB can be measured with no filtrate invasion occurring

• No filtrate invasion can mean more accurate LWD measurements.

ATMPETE 406 UBDATM

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Reduces formation damage

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Formation damage mechanisms during drilling (overbalanced)

• Scales, sludges or emulsions due to interaction between filtrates and pore fluids

• Interaction between aqueous mud filtrate and clay particles in the formation

• Solids invasion

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Formation damage mechanisms during drilling (overbalanced):

• Phase trapping or blocking

• Adsorption of drilling fluid additives, leading to permeability reductions or changes in wettability

• Migration of fines

• Generation of pore-blocking organic byproducts from bacteria entering the formation from the drilling fluid

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ATMPETE 406 UBDATM

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Formation damage mechanisms during drilling (underbalanced):

• Temporary overbalance

• Spontaneous imbibition

• Gravity-induced invasion

• Wellbore glazing

• Post-drilling damage

• Mechanical degradation

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Temporary overbalance

• Can be intentional to:– kill well for trips, – transmit MWD surveys, – log the well, – completion and WO operations

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Temporary overbalance

• Can be unintentional:– Slug flow or liquid holdup causing fluctuations

in annular pressure– High fluid pressures across the face of diamond

and TSP bits– Near wellbore production reduces the formation

pressure near the face of the wellbore

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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

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Spontaneous Imbibition

• Due to capillary effects - even if drilling underbalanced

• The underbalance pressure necessary to prevent water from being drawn from an aqueous drilling fluid into the formation will depend on the initial formation water saturation and the pore sizes

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Gravity-induced invasion

• Can occur during UBD in the formation produces from natural fractures or vugs

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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

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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

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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 and available flow channels leading to

• reduced permeability

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ATMPETE 406 UBDATM

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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 completed underbalanced, wells from depleated reservoirs will not need swabbing.

Harold Vance Department of Petroleum Engineering

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Environmental Benefits

• Closed loop systems produce less wasted drilling fluids

Harold Vance Department of Petroleum Engineering

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

Harold Vance Department of Petroleum Engineering

ATMPETE 406 UBDATM

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