PETE 689 Underbalanced Drilling, UBD

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PETE 689 - Underbalanced Drilling, UBD

• Lesson 3
• Benefits of Underbalanced Drilling
• Read UDM - Chapter 3

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Benefits of Underbalanced Drilling

• Increased penetration rate.
• Increased bit life.
• 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 treatments.

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Increased Penetration Rate

• In permeable rocks, a positive differential
  pressure 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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Chip Hold Down Effect
Bit tooth.
As drilling fluid enters the fracture, the
pressure differential across the rock fragment
decreases, releasing the chip.
Crack in the formation.
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Effect of Pressure Differential
Micro-bit test

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

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Indiana Limestone Confining Pressure 6000
psi Bit weight 1000 lbm 50 rpm
10
8
Drilling Rate (ft/hr)
6
4
2
0
0 1000 2000 3000
4000 5000
Overbalanced Differential Pressure (psl)
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Gas Drilling Vs. Mud Drilling
Drilling Days
0 20 40 60 80
100 120
0
Drilled With Mud Drilled
With Gas
1000
2000
3000
Mud
4000
Depth (feet)
5000
6000
7000
8000
Gas
9000
10000
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Penetration Rate As A Function Of The
Differential Pressure Across The Workfront
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Penetration Rate in Impermeable Rocks
Bit tooth
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.
Crack in the formation.
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Borehole pressure 440 psi
150
X Pore Pressure 87 psi O Pore Pressure 508
psi
125
100
75
Rate of Penetration (ft/hr)
50
25
0
0 5000 10000 15000 20000
25000 30000 35000 40000 45000
50000
Downhole Weight on Bit (lbf)
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Borehole pressure 1.450 psi
150
X Pore Pressure 580 psi O Pore Pressure 870
psi Pore Pressure 116 psi
125
100
Rate of Penetration (ft/hr)
75
50
25
0
0 5000 10000 15000 20000
25000 30000 35000 40000 45000
50000
Downhole Weight on Bit (lbf)
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Borehole pressure 4.800 psi
150
X Pore Pressure 2320 psi Pore Pressure 4930
psi
125
100
Rate of Penetration (ft/hr)
75
50
25
0
0 5000 10000 15000 20000
25000 30000 35000 40000 45000
50000
Downhole Weight on Bit (lbf)
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Normalized Drilling Strength

WOB x RPM
ROP Pb
WOB x RPM
ROP Po DSn Normalized Drilling
Strength Index. WOB Weight on Bit (lbf). RPM
Rotary speed (rpm). ROP Rate of penetration
(ft/hr). P Pressure (psia). Subscript
b Indicates borehole conditions. Subscript
o Indicates atmospheric conditions.
DSn
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Influence Of BHP On Normalized Drilling Strength
In Hard Shales
Normalized Rock Drilling Strength, DSn
A value of 5 means that the penetration rate at
an specific BHP will be 1/5 of the penetration
rate at atmospheric pressure.
0 500 1000 1500 2000
2500 3000 3500 4000 4500
5000
Bottomhole Pressure (psi)
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Normalized Shale Strength Example

• A well drilled with an unweighted
• (8.5 ppg) mud at a depth of 6000.
• BHP 2900 psi.
• Reducing the effective MW to 7 ppg reduces BHP to
  2400 psi.
• Decreases the drilling strength, i.e., increase
  the penetration rate by less than 15.

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Normalized Shale Strength Example

• To double the penetration rate the BHP would have
  to be dropped to
• 1500 psi.
• A BHP of 100 psi might be expected if drilling
  with air and would increase the penetration rate
  approximately 5 times.
• Note This assumes equal WOB and RPM.

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Normalized Shale Strength Example
10
9
8
7
6
5
Normalized Rock Drilling Strength, DSn
4
3
2
1
0
0 500 1000 1500 2000
2500 3000 3500 4000
4500 5000
Bottomhole Pressure (psi)
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Field Example Switching From Air To Mud
3000
Well 1 Well 2
Well 3
4000
DRY AIR
5000
SWITCH TO MUD
Depth (feet)
6000
7000
8000
9000
5 10 15
20 25
30
Days
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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.
• The number of bits required to drill an interval
  will be inversely proportional to the footage
  drilled by each bit.

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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
WOB x ROP 60 x RPM
W 2 pt
Where W work done by the bit (ft/ lbf/
revolution) t torque (ft- lbf) WOB weight on
bit (lbf) ROP rate of penetration (ft/hr) RPM
revolutions per minute
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The Volume Of Rock Excavated Per Revolution Is
V volume of rock excavated per revolution (ft
3) db bit diameter (feet)
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The Mechanical Specific Energy Is Give By
480 t x RPM 4WOB d b2 x ROP pd b2


MSE
MSE mechanical specific energy (psi)
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What Does This Mean?
480 t x RPM 4WOB d b2 x ROP pd b2


MSE

• Bit torque is not a function of borehole
  pressures.
• Penetration rates generally increase with
  decreasing borehole pressures.
• MSE are therefore, usually lower at lower
  borehole pressures.

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What Does This Mean?

• Therefore, cutting structure wear rates (in terms
  of distance drilled) should be inversely related
  to the MSE.
• If the bit has to do less work to remove a given
  volume of rock, its cutting elements should wear
  less.
• A bit should be able to drill more footage, when
  drilling underbalanced.

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Reduced Differential Sticking

• Fs Ac DPms 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 between the
  string and the mud cake.

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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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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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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.

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

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Formation Damage Mechanisms During Drilling
(Overbalanced)

• 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 downhole 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.
• Drill string running too fast after a bit is
  changed.
• Equipment malfunctions or procedural errors.

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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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Spontaneous Imbibition
800
Countercurrent Imbibition is Possible for Initial
Wetting Phase Saurations Between 20 and 47 for
the Underbalance Pressure Shown in this Example
(200 psi).
Zone of Potential Spontaneous Imbibition
700
600
500
Capillary Pressure (psi)
400
300
Example Underbalance Pressure
200
100
Sa c 47 (Equilibrium)
Sa i 20
S a irr 40
0
0 20
40 60
80 100
Wetting Phase, a, Saturation ()
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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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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.

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Environmental Benefits

• Closed loop systems produce less wasted drilling
  fluids.

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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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End of Lesson 3
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