1. IPS –
2015 International Perforating Symposium Europe The Renaissance Hotel, Amsterdam 19th - 21st May 2015
An Integrated Testing and Modeling Approach to Understand the Competing Effects of Static vs. Dynamic Underbalance
Graham Fraser, Tullow Oil
Noma Osarumwense, Baker Hughes
Rajani Satti, Baker Hughes

2. Outline Introduction - Background - Objectives
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Introduction
- Background
- Objectives
- Deepwater Well Description
- Design Philosophy
Perforation Flow Laboratory
- Description
- Test Configuration/Matrix
Experimental Results
Computational Modeling
Conclusions
Experimental Results:
Charge Performance Study
Clean Up Study under different wellbore conditions
Computational Modeling: To better understand the effects of static vs. dynamic underbalance

3. Background Focus of this study is to exploit laboratory-based testing
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Focus of this study is to exploit laboratory-based testing
as well as reliable modeling to design and optimize
perforated completion strategies.
The objectives of this study are therefore, three-fold:
Utilize the flow laboratory to provide a better understanding of shaped charge perforating/performance
Effect of underbalance/overbalance
Zinc vs steel case
Understand the effects of Static and Dynamic Underbalance on Perforation Clean up
Gain insight into how numerical modeling can be used to complement experiments.
Understand the effects of Static and Dynamic Underbalance on Perforation Clean up: Perforation Clean up as indicated by the magnitude of Dynamic Underbalance generated.
Testing to Investigate the effects of drilling mud invasion on flow performance was also performed but not discussed here (see SPE MS for details).

4. Well Description Offshore West Africa 1000 m – 1980 m water depth
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Offshore West Africa
1000 m – 1980 m water depth
Sandstone reservoir
Natural completion
Typical Well Data
Reservoir depth: 3300 – 3600 mTVD
Reservoir pressure: 6300 – 6412 psi
Formation fluid : Oil ( 33o – 35o API)
Permeability : 200 – 600 md
Porosity : 14 – 21%
Temp: 200 – 230°F
Bore Hole Size: inch.
Casing Size : 9-5/8 inch.

5. Design Philosophy
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The process begins with the examination of the reservoir and well properties, completion objective, and basic economics that allows the initial selection of broad options.
Preliminary downhole wellbore dynamics models (field and lab scale) are run using charge performance estimates from downhole gun performance modeling programs or API 19B Section II data if available, for initial completion evaluation to set realistic conditions for necessary testing, and to bring the calibration insight from the lab into the field-scale model to optimize the design for each specific well.
Depending on the test objective (flow and/or charge performance), proper lab tests are designed and executed to more accurately understand the performance of the various options.
Based on the lab results, more accurate productivity and inflow analysis can be performed.
Once the downhole model is validated with Gauge Data, model can be used to predict other scenario (s) such that experiments are only performed where there is significant difference in well or formation parameters.
After the testing is complete, analytical tools are again used to apply the results for strategy selection.
Integrated Engineering Workflow that combines experimental and numerical methods to develop perforating solutions and optimize pay zone productivity

6. Perforation Flow Laboratory
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API Recommended Practice 19B Section-II and IV testing.
Provides the capabilities to:
Study and qualify the performance of different perforating systems in formation rock at reservoir conditions
Study the influence of various factors on well productivity
Integrate this knowledge to select the optimal perforating system and clean up strategy for improved productivity.

7. Perforation Flow Measurements
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Pre-shot porosity
UCS from Scratch testing
Pre/post perforation permeability
Perforation tunnel diameter
Perforation tunnel depth
Detailed tunnel characterization using advanced CT scanning methods
Dynamic pressure data
Core flow efficiency and productivity ratio
Evaluate perforation cleanup mechanisms
Optimization of underbalance methods

8. Test Configuration Three Charge Types Rock Type: Buff Berea
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Three Charge Types
39 gm steel case DP, HMX
39 gm zinc case DP, HMX
39 gm steel case Ultra DP, HMX
Rock Type: Buff Berea
Permeability: ~ 200 mD
UCS: ~ 4000 psi
Average Porosity: 22%
Confining pressure : 9,300 psi
Pore pressure: 6,000 psi

9. Test Matrix Effect of underbalance conditions
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Effect of underbalance conditions
Overbalanced (250 psi) with minimum Dynamic Underbalance
Static Underbalance (500 psi) with Dynamic Underbalance
Higher Static Underbalance (2,000 psi) with Standard Gun Volume
Higher Static Underbalance (2,000 psi) with Dynamic Underbalance
Performance comparison of steel vs. zinc case charges
For the purpose of this presentation, we are looking at performance comparison of Steel vs. Zinc based on charge performance and Dynamic Underbalance generated. We have done additional work to look at compatibility with completions fluids but this will be discussed in a separate paper/presentation.

10. Results Effect of Dynamic Underbalance IPS – 15 - 10
CT scan imaging is used to visualize the perforation tunnel in its as-shot or undisturbed state before the cores are split. CT scanning provides tunnel shape, tunnel length/diameters, clean and debris zones, etc. Picture shown in this slide is the CT Scan images for Test 2045 (500psi Static Underbalance with Dynamic Underbalance). Images are consistent with the Split cores as shown.
CT scan, and split core images
CT Scanner Lab

11. Results Effect of Underbalance Conditions IPS – 15 - 10 Test ID
Simulated Gun System/Charge
Test Description
Gun Body Volume
(cc)
Core Penetration Length (in.)
Productivity Ratio
Magnitude of Dynamic Underbalance Achieved
(psi)
2044
7-in. X 6 spf X 60 deg
39 gm steel case Ultra DP, HMX
Minimum Dynamic Underbalance with 250psi Static Overbalance
100
17.38
0.83
1,857
2045
500psi Static Underbalance with Dynamic Underbalance
615
16.00
1.14
3,355
2036
2000psi Static Underbalance with Standard gun volume
585
20.69*
1.06
2,298
2050
2000psi Static Underbalance with Dynamic Underbalance
19.63*
1.03
2,431
Productivity Ratio (PR) = (Pre- Perf Permeability)/(Post - Perf Permeability)
* Penetration Length includes the narrow extended tip (see CT Scan image in next slide)

12. CT Scan and Split core images
Results
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CT Image – 2044
CT Image – 2045
CT Image – 2036
CT Image
Core Penetration (in.)
250psi OB
100cc GV
500psi UB,
615 cc GV
2000psi UB,
585cc GV
2000psi UB,
615cc GV
1. Compared to the minimum dynamic underbalance case, the 2,000psi static underbalance and 500psi static underbalance cases indicate a uniformly larger perforation tunnel along its length.
2. The entry hole diameter (as measured from the CT scan) for the 2,000psi and 500psi static underbalance cases is larger than that of the minimum underbalance case.
3. The case with 2000 psi Static Underbalance indicates the highest penetration (20.69 inches for Test 2036 and inches for Test 2050). Penetration depth includes the narrow tip as seen in the CT Scan image. It is believe that this portion will contribute to flow as it was easily removed with light scrubbing after core was split. However, further investigation and experimentation is required to quantify the consistent presence and effect of the tip region.
4. The test with 500 psi Static Underbalance yielded the Highest Dynamic Underbalance compared to the other conditions.
5. Higher flow Efficiency in the Static Underbalance cases compared to the Overbalance with minimum dynamic underbalance case.
DUB Achieved (psi)
CT Scan and Split core images
250psi OB
100cc GV
500psi UB,
615cc GV
2000psi UB,
585cc GV
2000psi UB,
615cc GV

13. Results
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The 2,000psi and 500psi static underbalance cases indicate higher flow efficiency, uniformly larger perforation tunnel along its length, and a larger entry hole diameter in the core.
The case with 2,000psi static underbalance yielded the highest penetration length*
Dynamic pressure data shows that static underbalance affects the magnitude of dynamic underbalance achieved.
Penetration depth includes the narrow tip as seen in the CT Scan image. It is believed that this portion will contribute to flow as it was easily removed with light scrubbing after core was split. However, further investigation and experimentation is required to quantify the consistent presence and effect of the tip region.
* Penetration Length includes the narrow extended tip (see CT Scan image)

14. Core Penetration Length Magnitude of Underbalance Achieved (psi)
Results
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2. Performance of Shaped Charges (steel vs. zinc case)
Test ID
Simulated Gun
System/Charge
Core Penetration Length
(in.)
Productivity
Ratio
Magnitude of Underbalance Achieved (psi)
2038
7-in. X 12 SPF X 135 deg 39 gm zinc case DP, HMX
11.25
1.00
2,982
2039
8.13
0.97
2,749
2035
7-in. X 12 SPF X 135 deg 39 gm steel case DP, HMX
13.31
1.07
3,570
2048
12.25
1.16
3,550
Productivity Ratio (PR) = (Pre- Perf Permeability)/(Post - Perf Permeability)

15. Results
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Core Penetration (in.)
Split Core - Zinc case charges
Zinc
Split Core - Steel case charges
Depth of penetration and magnitude of dynamic underbalance is higher with the steel charges.
Dynamic Underbalance is achieved with zinc case charges.
Steel case charges indicates a higher Penetration depth for both test shots.
The productivity ratio is higher with steel case charges, which is also supported by the magnitude of dynamic underbalance achieved (steel ~ 3,500psi and zinc ~ 2,800psi).
DUB Achieved (psi)
Zinc
Zinc
Steel
Steel

16. Wellbore Dynamics Modeling
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Industry leading software platform for computational modeling of transient, downhole perforating events.
Scientific code capable of simulating short-time dynamic events in the coupled wellbore-perforation-fracture-reservoir system.
Application space of perforation / stimulation jobs
Flexible input for tooling and conveyance
The Simulator used is a software tool that assist in calculations of dynamic mixed-phase compressible flow in a wellbore, perforations, fractures, and porous rock formation as created by events like burn of a perforating gun, propellant, and the expansion of a overpressured gas or liquid.
This Physics-based predictive modeling software determines the wellbore dynamics and predicts the changes in pressures that occur in the duration of a few hundred milliseconds.
It also allows us to predict and confirm the underbalance needed to reach optimum levels of clean up in the perforation tunnels.

17. Wellbore Dynamics Model: Physics
Simulator
Wellbore Flow Model
Perforation Flow & Cleanup
Reservoir Fluid Flow
Solid Object Models
Fracture Generation & Propagation
It utilizes a combination of complex and coupled physical models including wellbore flow, perforation flow, reservoir dynamics, tool models and fracture physics to model and simulate the dynamic perforating event.
All models are based on physics, although some approximations are used to avoid the extreme complexities of modeling this process on the micro-scale, but the overall philosophy of adhering to mathematical modeling using only reasonable physics-based phenomena is strictly adhered to throughout the code. The underlying finite-difference numerical scheme employs adaptive gridding and anti-diffusion flux for speed and accuracy.
Incorporates complex full-physics models to model full-scale dynamic perforating events

18. Transient Pressure Comparisons
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Recorded data from High Speed Gauge
Recorded data from High Speed Gauge
Calculated data
from Model
Calculated data
from Model
Recorded data from High Speed Gauge
This shows an overlay of the actual recorded data from High Speed Gauges ran in the experiment with predicted results from the model;
Minimum Dynamic Underbalance with 250psi Static Overbalance
500psi Static Underbalance with Dynamic Underbalance
2000psi Static Underbalance with Standard gun volume
Calculated data
from Model

19. Effect of Static UB on Dynamic UB
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Static Underbalance (psi)
Wellbore Pressure (psi)
Magnitude of Dynamic Underbalance Achieved (psi)
6000
4034
200
5800
3878
300
5700
3800
400
5600
3721
500
5500
3643
600
5400
3564
700
5300
3486
800
5200
3408
900
5100
3329
1000
5000
3251
2000
4000
2467
A parametric study using the validated model was then performed to study the effect of Static Underbalance on the magnitude of Dynamic Underbalance generated.
The Table in the slide is a summary of the results with the model from Test Observation was same in all cases.

20. Effect of Static UB on Dynamic UB
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Based on initial results, we believe that the decrease in dynamic underbalance as static underbalance is increased is primarily due to the competing effect of static pressure differential (from wellbore to gun body) and dynamic pressure differential (from formation to gun body). Further investigation and modeling is underway. IPS 15-5 (Fast-physics computational model to predict complex transient dynamics of API section IV lab tests) will provide an overview of our plans.

21. Field Run Comparison (Initial Overlay: Modeled vs. Actual)
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Typical Well Data
Well Type: 2 - Zone Gas Injector
Water Depth:
Reservoir Depth: 3856 m MD
Reservoir Pressure: psi
Permeability : md – md
Porosity : 17 – 20%
Temp: 211 – 213°F
Bore Hole Size: inch.
Casing Size : 9-5/8 inch.
Well Condition: Balanced
Gun System: 7 inch x 39gm x 135/45
deg x steel case DP, HMX
Recorded data from High Speed Gauge
Calculated data
from Model
Notes:
Formation Fluid is Gas
Initial Overlay of Modeled vs. actual is shown (History Match and Investigation into the difference in the response is in progress).
Actual Dynamic Underbalance Achieved = 4,180 psi, Calculated Dynamic Achieved from Model = 4,383 psi.
Response is similar in terms of well behavior/profile.
Difference could be as associated to changes/differences in the downhole environment – Reflections (peak pressure), Heterogeneity of the formation, Changes in fluid properties at downhole conditions etc.

22. Conclusions
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Perforation tunnel characteristics, cleanup and flow performance were better with the static underbalance conditions compared to the overbalance with minimum dynamic underbalance condition as seen in the experimental results.
The magnitude of dynamic underbalance achieved decreases with increase in static underbalance (as seen from both experiments/modeling). Further modeling efforts are underway to understand this complex phenomenon.
Dynamic Underbalance is indeed achieved with zinc case charges. However, penetration depth and the magnitude of dynamic underbalance achieved is higher with the steel case charges.
Again, we have performed preliminary studies into the compatibility of zinc charges with completions fluids. This will be discussed in a separate paper/presentation.
IPS 15-5 (Fast-physics computational model to predict complex transient dynamics of API section IV lab tests) will provide an overview of the plans to investigate further.

23. Acknowledgements / Thank You
Slide 23
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Acknowledgements / Thank You
Management of Tullow Oil and Baker Hughes for supporting this study.
Committee of the 2015 IPS Europe for giving us the opportunity to present this work.

24. Slide 24
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Questions ?