When and how EOP, Propellant or EUP could effectively improve the wells perforation
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
Overbalanced CompletionBefore Flowing
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
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
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)
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.
Energy stored in tubular creates shock wave opposite
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.
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, , and permeability, k
Applied overbalance pressure gradient
Effectiveness of EOP
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
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
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
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 penetrationcompletion limitations for various gunoptions
Pressure buildup before and after EOP EOP creates multiple fractures near the wellboreif no fracture is
created, perforations are plugged.completely
EOP Limitations and Issues
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?
Invented late 1970s (DynaFrac by Chuck Godfrey of
Extensively studied by Sandia (US DOE) Predicted fractures of 100s feet
Extensively evaluated by Mobil and other majors
1980s and discarded as an acceptable stimulation
History of Propellants
Expanding bubble increases localized pressure This in turn fractures the rock
What Propellant Does??
Energy application rates and resulting fracture patterns for various fracture stimulation technologies
Fracture Patterns for Various Techniques
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
At its peak 1,000 jobs per year comparedto more than 40,000 hydraulic stimulationjobs per year
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
Risk of completion damage such as unseating packers or splitting casing
Will destroy hydraulic cement bond
Safety No liquid to surface
Well hardware and equipment ratings
Collapsing guns with propellant sleeves
Formation damage If no fracture initiated, perforations are plugged
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
Less than optimum underbalance results in variable perforation
damage skin and variable flow rate/perforation (SPE 22809 & SPE
Under-balance Perforation; Some Fundamentals
EUP jobs were performed employing modular gun system. This
technique takes the underbalance perforating to the extreme.