FORMATION DAMAGE ISSUES IMPACTING THE PRODUCTIVITY OF TIGHT GAS PRODUCING FORMATIONS

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FORMATION DAMAGE ISSUES IMPACTING THE
PRODUCTIVITY OF TIGHT GAS PRODUCING FORMATIONS

• Brant Bennion
• Hycal Energy Research Laboratories Ltd.
• Canadian Well Logging Society
• June 9, 2004

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Presentation Summary

• What is formation Damage
• Definition of a tight gas reservoir
• Conditions generally required for economic tight
  gas reservoir production
• Common formation damage types occurring in tight
  gas reservoirs
• Reducing formation damage in tight gas reservoirs

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What is Formation Damage?

• Any process causing a reduction in the inherent
  natural permeability of an oil or gas producing
  formation

In Many Cases the Exact Cause of the Damage is
Difficult to Define - Another Good Definition of
Formation Damage is
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The Impairment of the Unseen, by the Inevitable,
Resulting in an Unknown Reduction in the
Unquantifiable!
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What is a Tight Gas Reservoir?

• Somewhat arbitrary classification
• Often defined as a gas bearing sandstone or
  carbonate matrix (which may or may not contain
  natural fractures) which exhibits an in-situ
  permeability to gas of less than 0.10 mD
• Many ultra tight gas reservoirs may have
  in-situ permeability down to 0.001 mD

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What Controls the Ability to Economically Produce
Tight Gas Reserves?

• Effective permeability
• Initial saturation conditions
• Size of effective sand face drainage area
  accessed by the completion
• Reservoir pressure
• Degree of liquid dropout from gas (rich vs. dry
  gas)

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Capillary Equilibrium in Gas Reservoirs High
Perm
Relative Permeability
Capillary Pressure - Psi
Krw
Krg
Water Saturation
Water Saturation
FWC
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Capillary Equilibrium in Gas Reservoirs LOW Perm
Relative Permeability
Capillary Pressure - Psi
Krg
Krw
Water Saturation
Water Saturation
FWC
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Generally if a Tight Gas Matrix is in Equilibrium
With a Free Water Contact, Unless Very Large
Vertical Relief is Present, Equilibrium Water
Saturation Reduces Reserves and Permeability to
Gas Below the Economic Limit for Production
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Non - Capillary Equilibrium in Gas Reservoirs
LOW Perm
Relative Permeability
Capillary Pressure - Psi
Krg
Krw
Water Saturation
Water Saturation
NO FWC
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For Significant Reserves and Mobile Gas
Production in Very Low Perm Gas Reservoirs, a
CAPILLARY SUBNORMAL Water Saturation Condition
Usually Must Exist
Water Gauge
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Subnormally Water Saturated Tight Gas Reservoirs
What Are They

• A gas reservoir in which the initial water
  saturation is less than that which would be
  achieved on a conventional drainage capillary
  pressure curve at the effective capillary
  gradient of the reservoir

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Postulated Mechanism For Establishment of the Low
Swi Condition
Pore System is isolated from Dynamic capillary
contact With active recharge water Source
(faulting, etc)
Long Term Migration of Gas Slightly out of
equilibrium with Reservoir results in
Desiccation Of water saturation to subnormal value
Low Perm matrix Initially 100 Saturated
with Water
Gas Migration commences Pore system displaced
to Capillary equilibrium swirr
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Postulated Mechanism For Establishment of the Low
Swi Condition
Results in unique combination Of low perm and low
Swi
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Subnormal Saturation Conditions

• Generally a pre-requisite for an economic gas
  reservoir in ultra tight rock (lt0.1 mD)
• Increases reserves and gas permeability
• Increases apparent salinity and suppresses Rw
  (proven by case studies)
• Swi often difficult to precisely measure using
  conventional logging
• Direct measurement via traced coring program
  common method used

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Common Subnormally Saturated Formations in
Western Canada

• Deep basin area
• Paddy
• Cadomin
• Cadotte
• Jean Marie
• Montney
• Rock Creek
• Ostracod
• Gething
• Bluesky
• Halfway
• Doig
• Cardium
• Viking

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Subnormal Initial Water Saturation Gas Reservoirs

• USA
• Powder River Basin
• Green River Basin
• DJ Basin
• Permian Basin
• Also documented in South America, Europe, Asia,
  Africa and Australia

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Dominant Formation Damage Mechanisms in Tight Gas

• Unless natural microfractures are present, almost
  all tight reservoirs must be fracture stimulated
  to obtain economic production rates
• In the case where fracture stimulation is
  required, classic formation damage associated
  with drilling is not normally problematic due to
  the radius of penetration of the fracture
  treatment

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Exceptions Tight Matrix With Enhanced Natural
Permeability Conduits

• Natural fractures
• Interconnected vugular porosity
• Possible deep invasion of whole drilling fluids
• Possible application of UBD

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Hydraulically Fractured Tight Gas Systems

• High fracture conductivity essential (proppant
  crushing, embedment, residual gel/frac fluid
  entrainment are issues)
• Water or hydrocarbon based phase trapping a major
  source of matrix damage in the near frac face area

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Highway Analogy
Optimum Situation Matrix Deliverability Balanced
Fracture Transmissibility
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Highway Analogy
Productivity Controlled by Fracture Perm,
Not Matrix Ability to Deliver Gas
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Highway Analogy
Very Low Perm Matrix, Limited Frac
Penetration, High Formation Damage Productivity
limited by Frac Face Deliverability, Not Fracture
Transmissivity
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Water Based Phase Trapping
Original Subnormal Swi Condition
Capillary Pressure
Water Saturation
Relative Perm
Water Saturation
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Water Based Phase Trapping
Water Based Fluid Invasion
Capillary Pressure
Water Saturation
Relative Perm
Water Saturation
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Water Based Phase Trapping
Subsequent Return Flow
Capillary Pressure
Water Saturation
Relative Perm
Water Saturation
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Reducing Water Based Phase Trap Potential

• Avoid use of water based fluids (OB, pure gas,
  etc)
• Use surface tension reducing agents to reduce
  capillary pressure and trapping potential (mutual
  solvents, alcohols, etc)
• Low fluid loss systems with rapid recovery times
  to minimize imbibition
• May also be an issue in some underbalanced
  operations

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Using Hydrocarbon Based Fluids in Reservoir Prone
to Water Trapping

• May still be the preferred method as relative
  volume of non wetting phase hydrocarbon which is
  trapped is often much less than water
• Resulting damage is far less than if water based
  fluid had been used in the same situation in many
  cases

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Hydrocarbon vs. Water Based Fluids in Low Perm,
Low Swi Gas Reservoirs
Water Based Fluid Swirr 55
Relative permeability
Krg 0.10
Total fluid saturation
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Hydrocarbon vs. Water Based Fluids in Low Perm,
Low Swi Gas Reservoirs
Oil Based Fluid Sor 15
Relative permeability
Krg 0.40
Total fluid saturation
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Common Stimulation Treatments for Water Blocks

• Dry gas injection (natural gas, CO2)
• Mutual Solvent Injection (methanol, CO2)
• Extended shut in time
• Formation heat treatment
• Direct penetration (fracturing/refracturing)
• High drawdowns normally not effective

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DJ Basin ColoradoReservoir Parameters

• Very fine grained sandstone
• Depth 2400 m
• BHP 20 MPa
• kh 1 4 mD-ft (0.3 1.2 mD-m)
• Typical treatment
• 550,000 lbs (250 tonnes) in X-linked water
• Post-frac production
• 50 mcf/day 500 mcf/day

Slide Courtesy of Calfrac
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DJ Basin J Sand
Two Production Cycles
Slide Courtesy of Calfrac
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Common Stimulation Treatments for Hydrocarbon
Blocks

• High pressure lean gas injection (natural gas,
  nitrogen)
• Lower pressure rich gas injection (CO2, ethane,
  propane, butane)
• Mutual solvent (heavy alcohol) treatments

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Diagnosis of Problems and Evaluation of Most
Effective Prevention or Stimulation Treatments

• A variety of lab/core evaluation techniques exist
  to evaluate
• Water and hydrocarbon phase trap potential
• Interplay of reservoir pressure, invasion and
  drawdown effects
• Evaluation of optimum stimulation methods for
  existing damaged wells
• Evaluation of optimum drilling and completion
  methods in naturally fractured formations

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Lab Regain Perm Test Equipment
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Conclusions
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Conclusions

• Tight gas reservoirs have a huge future potential
  for production
• Generally to be economic tight gas reservoirs are
  normally in a subnormal water saturation
  condition
• Fluid trapping tends to be a dominant damage
  mechanism for tight gas reservoirs
• Techniques exist to evaluate and minimize phase
  trapping problems and to stimulate existing
  damaged wells

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Thank You for Your Attention
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