Underbalanced Perforating

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Underbalanced Perforating
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Underbalanced Perforating

• Early tests by Exxon showed that flow patterns
  and perforation geometry prevent the cleaning out
  of an appreciable percentage of mud-or
  silt-plugged perforations by simple production
  from a well
• Published studies of the flow rate necessary to
  remove damage observed that serious perforation
  plugging occurred whenever the pressure was
  higher in the wellbore than in the formation

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Underbalanced Perforating

• Plugs that formed when perforating in heavy mud
  were almost impossible to remove by reversing
  pressure.
• "Permanent plugging of a high percentage of
  perforations may result from killing a well with
  mud or dirty fluid during well completion,
  servicing, or workover."
• "When perforating in mud with a pressure
  differential into the formation, perforations are
  filled with mud solids, charge debris, and
  formation particles." Not easily removed.

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Underbalanced Perforating

• Differential pressure required to initiate flow
  through each plugged perforation, varies.
• When a few perforations requiring low
  differential pressures open up, flow into these
  perforations makes it difficult to create the
  higher pressure drawdown needed to open
  additional perforations.
• Crushed and compacted rock around the
  perforation has essentially zero permeability and
  further reduces the probability of perforation
  cleanout.

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Underbalanced Perforating

• The post-shot flow into the wellbore (a function
  of the formation-wellbore pressure differential,
  the formation fluid viscosity and the formation
  permeability) helps remove the crushed formation
  from the perforation and provides improved flow
  channels.
• High post-shot formation to wellbore flow
  generally provides optimum perforation cleanup
  and minimum skin.

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Underbalanced Perforating

• Underbalance perforating followed by flow has
  been shown to be the best method for cleaning
  perforations and establishing high flow capacity
  from natural completions in moderate to high
  permeability core
• Even when compared to surging and washing,
  underbalance perforating followed by flow can be
  superior

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The Level of Underbalance

• Must balance perforation cleanup and well
  performance potential enhancement against the
  downside aspects of mechanical problems such as
  perforators or wireline sticking in the wellbore,
  near-wellbore rock formation disintegration,
  downhole tubulars and equipment damage, etc.

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The Level of Underbalance

• The pressure differentials necessary to achieve
  the flow rates required to remove perforation
  and/formation-skin damage are affected by
• Formation pressure
• Reservoir permeability
• Perhaps limited by formation integrity
• Usually range from approximately 500 psi to over
  5000 psi
• Have been established by trial and error in many
  fields

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Implementation

• Downhole and surface mechanical equipment to
  achieve desired underbalance and maintain the
  integrity of the well
• Control the well during deployment, perforating
  and retrieval,
• Deploying the perforating device(s) to the proper
  downhole position,
• Activating the perforating mechanisms,
• Monitoring the downhole perforating process,
• Retrieving the perforating system

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Precautions!

• It is more costly to employ than conventional
  perforating.
• Safety and well control is a primary issue.
• It requires the appropriate combination of
  reservoir pressure, reservoir fluid properties
  and formation permeability to achieve the
  required underbalance for effective application.
  
• Prospects must be thoroughly screened, and there
  are those that may not be, or are not,
  appropriate candidates.

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Jet PerforatingFormation Properties

• Compressive strength, effective stress and
  specific rock characteristics can have
  significant impact
• In unstressed rock, penetration decreases with
  increasing compressive strength
• Pore fluid compressibility affects performance.
• Increasing liquid saturation improves penetration
• Stress reduces penetration (other factors being
  kept constant)

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Jet PerforatingFormation Properties
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Design

• Perforator Type, Charge Strength and Gun
  Clearance
• Conveyance Logistics, Surface and Downhole
  Equipment/Apparatus
• Existing Wellbore Tubular/Cement-Sheath
  Limitations

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Design

• Formation Mechanical Rock Properties and
  Characteristics
• Reservoir Pressure and Fluid Flow
  Characteristics
• Perforating Downhole Environment
  Conditions/Limitations

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Design (Kings Method)
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Hsia and Behrmann
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Tariq
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Cleanup Criterion Based on Reynolds Number
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Design

• Of the factors influencing the determination of
  correct underbalance for cleanup the fluid
  properties, especially viscosity, are important
  but the key factor is the formation permeability

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Efficiency

• The major factors affecting the efficiency of a
  perforation include shot density (spf),
  penetration depth into the formation, angular
  phasing, and diameter
• Injectivity increases as shot density increases?
• Injectivity increases with increases in
  perforation penetration?
• The effect is greater at shallow depths.

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Efficiency

• Angular phasing other than 0 increases
  injectivity by reducing the interference with
  flow resulting from the presence of the wellbore.
• Perforation diameter plays a relatively minor
  role in determining injectivity?
• The strength, in-situ stress conditions and
  lithology can effect the penetration length, the
  extent and severity of the damage zone around the
  perforation, and the cleanup characteristics.

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Modular Gun System

• Deployment systems for multiple guns.
• Guns are loaded at the surface, deployed downhole
  individually, and stacked on each other at the
  perforating zone, with the lower-most gun module
  being supported by the gun hanger.

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Modular Gun System

• An entire interval can be perforated over- or
  underbalanced
• Gun sizes from 2 to 7-inch OD can be run for
  casing from 3 ½ to 8 5/8 inches,
• Zone can be perforated and tested with no
  downhole restrictions below or above the packer.