PETE 406 - Underbalanced Drilling, UBD

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PETE 406 - Underbalanced Drilling, UBD

• Lesson 9
• Benefits of Underbalanced Drilling
• UDM - Chapter 3

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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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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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Chip hold down effect
As drilling fluid enters the fracture, the
pressure differential across the rock fragment
decreases, releasing the chip
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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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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 rocks
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.
Bit tooth
Crack in the formation
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Field example switching from air to mud
Switch to mud
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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
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The volume of rock excavated per revolution is
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The mechanical specific energy is give by
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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

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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.

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Reduced Differential Sticking

• Fs AcDPms144 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

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

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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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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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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 9