reservoir pressure or underbalance is insufficient to effectively clean the perforations as suggested by King et al. (1985) and others. In other cases, where 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, some (Handren et al. 1993, Pettijohn and Couet, 1994; Snider and Oriold, 1996) have suggested using extreme overbalanced (EOB) perforating, which is a near-wellbore stimulation technique. EOB perforating also provides perforation breakdown in preparation for other stimulation methods, and therefore, eliminates the need for conventional perforation breakdown methods.
The EOB technique involves pressuring the wellbore with compressible gases above relatively small volumes of liquid. The gases have a high level of stored
energy. Upon expansion at the instant of gun detonation, the gases are used to fracture the formation and divert fluids to all intervals. The high flow rate through relatively narrow fractures in the formation is believed to enhance near-well conductivity by extending the fractures past any drilling formation damage. Recently, Marathon Oil Company incorporated proppant carriers into the perforation assembly to introduce proppants into the flow path as the gun detonates. The
POWR*PERF SM process, patented by
Marathon Oil Company, further enhances productivity by scouring the perforations to leave some residual conductivity on the fracture plane. Most EOB perforating jobs are designed with a minimum pressure level of 1.4 psi/ft of true vertical depth. For optimum results, it is suggested to utilize the highest possible pressure level without compromising wellbore integrity or operation safety.
Typical Extreme Overbalanced (EOB) Perforating Assembly
Packer Bauxite VannGun® Assembly Wellhead Isolation Tool Nitrogen 300 ft of Fluid Radioactive Collar Tubing Pressure-Operated Venture Firing Head
Proppant Carrier H AL1 53 14
Along with standard EOB perforating with applied pressure from
compressible gases and proppant carriers, propellant-assisted perforating techniques are becoming more widely accepted. The StimGun™ assembly, patented by Marathon Oil Company, combines solid propellant technology with conventional perforating. The StimGun assembly may be utilized for either EOB or conventional
underbalanced perforating. The hardware utilized for either system remains the same aside from added protection by using centralizer rings to protect the brittle propellant material. The propellant sleeve in the StimGun assembly simply slides over the perforation scalloped carrier and is held in position on the gun with the centralizer rings. The propellant material is potassium perchlorate, an oxidizer that burns rapidly, creating carbon dioxide gas. As the shaped
charges detonate, the propellant is ignited by extreme heat from the gun system. As it burns, the propellant generates carbon dioxide gas at high peak pressures typically well above the formation fracture gradient. The StimGun assembly is an effective method for mild stimulation (fractures on order of 2 to 9 ft) for treating near- wellbore problems.
One of the benefits of licensing the StimGun assembly technology is the access gained to the proprietary design called the PulsFrac™* program. PulsFrac software package is utilized to safely design EOB perforating or propellant-assisted perforating jobs. The PulsFrac software output indicates anticipated peak pressure and the degree of fracturing that can be expected. PulsFrac software is a very useful tool for screening candidate wells for types of EOB perforating techniques and for identifying potential
operational issues. Centralizer RA Marker Safety Joint Retrievable Packer Fill Disk Firing Head Fast Gauge Recorder HAL1 59 77 StimGun™ Assembly
PerfPro
®Process
2-17
ShockProSM Shockload Evaluation Service
Engineer Perforated Completions to Evaluate the Mechanical Integrity of All System Components
Relying on old rules of thumb or utilizing standard mechanical configurations to cover all perforating cases can lead to catastrophic results. To help avoid such potential disasters, Halliburton utilizes its proprietary ShockPro™ software package* to evaluate the mechanical risk factors of all well components to ensure that all aspects of HSE and Service Quality are covered.
Advanced System for Analyzing Every Completion or Reservoir’s Unique Characteristics
Halliburton’s ShockPro service determines the dynamic pressure behavior during the perforation event in addition to the solid loading that is imparted to the tubulars, packers, and other completion hardware in the perforating assembly.
Accuracy - Physics Based Numerical Modeling
Physics based numerical model accounts for fluid dynamics and dynamic failure of solids by accounting for the following forces:
• Pressure on surfaces • Drag
• Internal stress waves and reflections • Gravity
The time-marching finite differences technique is applied as the numerical method for both fluids and solids. The software is compiled on a personal computer and typically executes in times of several minutes to several hours, depending on complexity of job design. The following failure modes are accounted for in the numerical solution:
• Tubing burst / collapse • Packer axial load / differential • Tubing axial buckling or bending • Tubing compressive / tensile yield • Gun burst / collapse
• Gun compressive / tensile yield • Casing burst
• Sump packer / bridge plug axial load • Wireline tensile yield / pull-out
Buckling / Collapse of Tubing Joint Below Retrievable Packer During Perforating Event *Software programs used under license from John F. Schatz Research and Consulting, Inc.
ShockPro™ Software Graphic Display with Error Flags for Tubing Yield and Buckling Failure
This information is utilized to determine the peak pressure applied to a packer, for instance the maximum tension or compression on a joint of pipe or the differential pressure applied to the packer. Once dynamic failure criteria have been established, ShockPro software can be utilized to examine whether or not potential problems may occur with a given perforating assembly.
Steps can then be taken to correct unusually high peak loads to manage job risk factors. The physics based model has been validated special high speed recorders that sense pressure, temperature, and acceleration at sampling frequency on the order of 115,000 samples per second.
PerfPro
®Process
2-19
SurgePro
SMService
Halliburton’s SurgePro™ perforating-design software program* is robust and can be used for a large variety of dynamic wellbore calculations. The sub-models contained in the program are physics-driven and rely on measurable or estimated actual input parameters, no curve fitting or back of the envelop calculation.
As a result, the SurgePro program is ideal for predicting: • Wellbore, perforation, and gun pressurizations • Wave propagation—fluid injection/production • Perforation behavior—perforation damage • Completion integrity—burst/collapse and packer
differential
Accuracy—Physics Based Solution with Documented Validation
The SurgePro program is based on a proprietary analysis developed from:
• API Section IV perforation flow laboratory studies • Time marching finite difference modeling
• High-speed pressure measurements • Empirical field data
Mass, momentum, and energy are conserved for each time step. The solution is derived by using energy release
equations for the gun, simultaneous coupled finite-difference solutions of the Navier-Stokes equations for wellbore, perforation and fracture flow, and solid rock mechanics for perforation breakdown.
Capability to Model a Wide Range of Wellbore Conditions
To fully represent dynamic wellbore behavior, the SurgePro program takes into account a wide variety of factors: • Thermodynamic mixing and multiple compressible fluid
types/phases
• Various energy sources, including perforating gun ignition, and residual energy deposition (gun, well, and perforation tunnel)
• Valves, pumping, and orifices
• Multiple diameter effects in the well including: - Surface pressurization, pumping, and flow back
of fluids
- Flow into and breakdown of perforation tunnels - Subsequent transient return flow from perforations
A typical screen capture from SurgePro™ software simulation; understanding and prediction of dynamic pressure behavior becomes paramount when conventional underbalance techniques are not an option.
H
AL15
56
Dynamic underbalance is created with the application of a special fast opening surge vent assembly. Note the gauge reading atmospheric pressure in the chamber prior to the perforating event following a sustained minimum surge pressure across the perforated interval of ± 1,000 psi for 0.5 seconds.
This minimum surge pressure across the formation results in a dynamic underbalance 3,200 psi that can potentially improve well productivity. The high speed gauge readings are in good agreement with the theoretical prediction from the physics based model. Hundreds of high-speed pressure records have been collected under varying well conditions to validate the modeling results generated.
Identical sandstone targets perforated with the same 39 gram shaped charge at the same reservoir pressure and effective stress condition. The picture on left is perforated in a balanced condition and the picture on the right is perforated ideally with 3,000 psi underbalance pressure. The difference in productivity or core flow efficiency in this case is on the order of 82% by not completely cleaning up the perforation tunnel with proper underbalance pressure or differential surge flow. In cases where conventional underbalance perforating is not applicable, it may be possible to apply the SurgePro service to create a localized dynamic underbalance pressure to overcome the perforation damage or skin factor associated with balanced or overbalanced perforating techniques while still maintaining well control.
*Software programs used under license from John F. Schatz Research and Consulting, Inc.
HAL15568
Actual High Speed Field Pressure Measurement
HAL1 55 70 Balanced Underbalanced H AL1 55 69