Extreme Overbalance, Propellant OR Extreme Underbalance When and how EOP, Propellant or EUP could effectively improve the well s perforation
The first 130 years of perforating 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
Perforation Clean Up Concepts Cement Mud-damaged zone Casing Virgin formation Charge debris Crushed and compacted low-permeability zone Part of low-permeability zone still exits Perforation partially plugged with charge debris Low-permeability zone and charge debris expelled by surge of formation fluid Overbalanced Completion Before Flowing Overbalanced Completion After Flowing Ideal Underbalanced Completion Immediately After Flowing
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
Extreme Overbalance Perforations (EOP) 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 not possible To address the perforation damage in these cases, extreme overbalance 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 for conventional perforation breakdown methods The Extreme Overbalance Perforating technique was developed independently by Oryx Energy and ARCO
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 proppants into the flow path as the gun detonates.
EOP Fundamentals 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.
Effectiveness of EOP 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
EOP Candidate Selection 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 Results Worldwide 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)
Field Experience of EOP 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 forced completion brine into the formation the well never produced anything measureable. In second case EOP surge did increase production from 1.5 MMscfd to 2.4 MMscfd. Based on pressure transient test, skin reduced from 33 to 18. No long term test results available. The jobs proved that EOB with surface and downhole pressures of 15,000 psi and 19,000psi were operationally possible. Cost for EOP much higher than conventional methods, economic justification is still questionable????
EOP Limitations and Issues Safety: High pressure gas at surface (well depth), well hardware and equipment 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 gun options Pressure buildup before and after EOP EOP creates multiple fractures near the wellbore if no fracture is created, perforations are plugged.completely
Propellant
What is a Propellant? 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
History of Propellants 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
What Propellant Does?? Expanding bubble increases localized pressure This in turn fractures the rock
Propellant Pulse
Fracture Patterns for Various Techniques Energy application rates and resulting fracture patterns for various fracture stimulation technologies
Propellant Uses 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 Results At its peak 1,000 jobs per year compared to more than 40,000 hydraulic stimulation jobs per year
Propellant Operations 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 Limitations 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
Extreme Under-balance
Under-balance Perforation; Some Fundamentals 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)
Field Experience with EUP 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)
Conclusion More than 800 EOB Jobs have been reported, most of these in US and Canada While there have been notable success, EOB is not a global replacement 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 PURE and Combo PURE gun system Field experience with EUP has been very encouraging
Take Away 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
Further Reading 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
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