When and how EOP, Propellant or EUP could effectively improve the well’s perforation
Extreme Overbalance,Propellant
OR Extreme Underbalance
1865, Tin torpedos filled with gunpowder, later with nitroglycerin 1910, the single-knife casing ripper 1948, shaped charges 1970s, under-balance 1980s propellant 1993, extreme over-balance
The first 130 years of perforating
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Overbalanced CompletionBefore Flowing
Cement
Casing
Mud-damaged zone
Virgin formation
Charge debris
Crushed and compactedlow-permeability zone
Overbalanced CompletionAfter Flowing
Part of low-permeability zonestill exits
Perforation partially pluggedwith charge debris
Ideal Underbalanced Completion
Immediately After FlowingLow-permeability zone andcharge debris expelled bysurge of formation fluid
Perforation Clean Up Concepts
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• Overbalanced perforating creates a crushed zone with substantial flow restrictions
• Underbalanced perforating helps to remove debris and crushed formation fragments from the perforation tunnel
• Cleanup efficiency is a function of applied differential pressure and transient flow velocities in the rock
Perforation Clean Up Concepts
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• Why EOP The remaining reservoir pressure or underbalance is insufficient to
effectively clean the perforations The formation competence is questionable and the risk of sticking
perforating assemblies is greater, sufficient underbalance pressure is notpossible
• To address the perforation damage in these cases, extremeoverbalance perforating technique has been applied EOP is a near-wellbore stimulation technique EOP perforating also provides perforation breakdown in preparation for
other stimulation methods; and therefore, eliminates the need forconventional perforation breakdown methods
• The Extreme Overbalance Perforating technique was developedindependently by Oryx Energy and ARCO
Extreme Overbalance Perforations (EOP)
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EOP Technique• Pressuring the wellbore with compressible gases
(the gases have a high level of stored energy) above relatively small volumes of liquid
• Typically Nitrogen is pressured up to levels significantly higher than the formation break-down pressure
• The formation is instantaneously exposed by perforating the casing or shearing a plug placed in the tubing bottom
• Compressed N2 provides the energy to drive the wellbore fluid into the formation to create short fractures around the wellbore
• Proppant carriers have also been incorporated into the perforation assembly to introduce proppantsinto the flow path as the gun detonates.
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• Energy stored in tubular creates shock wave opposite
perforated zone.
• Energy impact and injection rates are significantly higher
than during hydraulic fracturing.
• Overbalance pressure needs to be above 1.4 psi/ft.
• Expansion of N2 creates short fractures.
• Fracture propagation and width are function of pressure
sustenance above fracture initiation pressure.
• EOP fracture initiation pressure is always higher.
EOP Fundamentals
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• Effectiveness of EOP is a function of: Type of fluid across target formation
Size of tubular (i.e., amount of finite energy available)
Length of perforated interval, size, and gun phasing
In-situ stress, σ, and permeability, k
Applied overbalance pressure gradient
Effectiveness of EOP
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• Opening existing (damaged or plugged) perforations
• Removal/bypass of skin damage and fines migration
• Stimulation of well where other treatments are impractical
• Pre-stimulation to permit reservoir evaluation tests
• Upfront hydraulic fracturing operation
• Stimulation of intervals with proximity to water/gas layers
EOP Candidate Selection
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• More than 500 jobs performed: 88% showed negative skin after EOP• Most treated reservoirs with k < 10md• Maximum treated interval length 300 ft• Success depends on overbalance gradient > 1.4 psi/ft• If reservoir does not respond to EOP, it will not respond to more
expensive treatments.• Reserves do not increase but are recovered in a shorter time• Clean fluids are a key to technique• 80% of fractured wells showed lower fracture pressures• Decline in use to less than 100 jobs a year (TCP operations are more
than 8,000 per year)
EOP Results Worldwide
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• In a real case, two jobs with EOP were performed• Neither of the jobs resulted in a commercial success:
In first case the perforation guns went off prematurely and forcedcompletion brine into the formation – the well never producedanything measureable.
In second case EOP surge did increase production from 1.5MMscfd to 2.4 MMscfd. Based on pressure transient test, skinreduced from 33 to 18. No long term test results available.
• The jobs proved that EOB with surface and downhole pressures of15,000 psi and 19,000psi were operationally possible.
• Cost for EOP much higher than conventional methods, economicjustification is still questionable????
Field Experience of EOP
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• Safety: High pressure gas at surface (well depth), well hardware andequipment ratings
• Logistics - Nitrogen and pumping units• Very high instantaneous flow rate….may exceed 100 bbl/min• Very high surface pressure…. has to be increased to more than 10,000
psi• Liquid cushion ….up to maximum of 1000 ft• Minimize fluid volume inside tubing, preferably 100% N2 to reduce
friction losses• Erosional effects are significantly higher• Large diameter perforations and perforation phasing are more
important than penetration…completion limitations for various gunoptions
• Pressure buildup before and after EOP• EOP creates multiple fractures near the wellbore…if no fracture is
created, perforations are plugged….completely
EOP Limitations and Issues
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• It is an Oxidizer and a Fuel
• It burns very quickly
• It generates gas
• Post-perforation Propellant Pulse System delivers the maximum energy produced to the formation to enhance near wellbore treatments
• the generated pressure pulse is powerful enough to break the formation
• Conveyable on wireline, tubing, or coiled tubing, it is run stand-alone or in combination with a perforating gun system for a one-step process
What is a Propellant?
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• Invented late 1970’s (DynaFrac by Chuck Godfrey of
Physics International)
• Extensively studied by Sandia (US DOE) Predicted fractures of 100’s feet
• Extensively evaluated by Mobil and other majors
1980’s and discarded as an acceptable stimulation
technique
History of Propellants
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• Expanding bubble increases localized pressure• This in turn fractures the rock
What Propellant Does??
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• Energy application rates and resulting fracture patterns for various fracture –stimulation technologies
Fracture Patterns for Various Techniques
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• Performs a mini fracture on the formation by: Using a high energy pressure wave to drive a fluid piston into the
formation to initiate a fracture Short non-propped, bi-wing fracture created
• Potential short-term increase in production rates• Results can determine whether conventional frac job is needed or not• Fractures tend to stay in zone (zonal isolation)• Fracture past wellbore/formation damage• Fracture past perforation damage• Potential communication with natural fractures
Propellant Uses
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• At its peak 1,000 jobs per year comparedto more than 40,000 hydraulic stimulationjobs per year
Propellant Results
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• Pressures at least 1.4 psi/ft or .6 plus frac gradient• Intervals up to 300 feet (< 50 feet most common)• Fluids in wellbore can vary• Completion brine
Acid - mini acid wash Resin - for sand control (more failures than successes) Minimum liquid column required for propellants to prevent tool
movement and to initiate propellant NO liquid to surface for propellants otherwise wellhead will be blown
off
• Risk of completion damage such as unseating packers or splitting casing
• Will destroy hydraulic cement bond
Propellant Operations
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• Safety
No liquid to surface
Well hardware and equipment ratings
Splitting casing
Packer movement
Collapsing guns with propellant sleeves
• Formation damage
If no fracture initiated, perforations are plugged
Propellant Limitations
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• Perforation cleanup occurs in about the first 10 msec from generation
of perforation tunnel (SPE 30081).
• Optimum underbalance to achieve clean perforations is a function of
permeability, porosity, reservoir strength and type /size of charge
(SPE 30081).
• Less than optimum underbalance results in variable perforation
damage skin and variable flow rate/perforation (SPE 22809 & SPE
28554)
Under-balance Perforation; Some Fundamentals
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• EUP jobs were performed employing modular gun system. This
technique takes the underbalance perforating to the extreme.
• The wells perforated with virtually no hydrostatic pressure (about 50
psi) at the perforated interval
• In addition well is open at the choke to the pit
• No problem of sand influx reported
• Very encouraging 5-fold increase in MCFD/md-ft was reported (in low
producers)
Field Experience with EUP
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• More than 800 EOB Jobs have been reported, most of these in US andCanada
• While there have been notable success, EOB is not a globalreplacement for underbalance perforating
• It is rather a complementary process for specific applications: Lack of engineering design tools can inhibit treatment success Uncertain reliability, results Operationally complexity and cost
• Convergence with propellant technology may provoke key to success: Same principle, superior dynamics as EOB Operationally simple and low in cost
• 2,000-3,000 psi dynamic underbalance pressure possible with PUREand Combo PURE gun system
• Field experience with EUP has been very encouraging
Conclusion
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• Extreme overbalance pressure perforations are not feasible,
and hence not recommended at current state of its technology
• For Post Perforating Propellant Pulse Stimulation right
candidate selection is crucial
• PURE perforation system (or Equivalent technology) for Wells
having higher bottom hole pressure i.e. 1,000 – 1,400 psi
• Extreme under balance perforation technique to be tried on
the candidates, where creating underbalance is virtually
impossible
Take Away
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• Poveda et al. (2013) “Case History_Combining Extreme Overbalance and Dynamic Underbalance Perforating Techniques in Ecuador” SPE-166420-MS
• Wang et al. (2003) “Successful Application of Combining Extreme Overbalance Perforating and Alcoholic Retarded Acid Technique in Abnormal High Pressure Gas Reservoir ” SPE-82272-MS
• Ghalambor et al. (1998) “Performance Evaluation of Extreme Overbalanced Perforating” SPE-39459-MS
• Azari et al. (1997) “Well Testing and Evaluation of Tubing-Conveyed Extreme Overbalanced Perforating” SPE-37326-MS
• L.A Behrmann (1996) “Underbalance Criteria for Minimum Perforation Damage” SPE-30081-PA
• Mason et al. (1994) “Block Tests Model the Near-Wellbore in a Perforated Sandstone” SPE-28554-MS
• Behrmann et al. (1991) “Measurement of Additional Skin Resulting From Perforation Damage” SPE-22809-MS
Further Reading…
Need More…
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