Analysis of Sand-Control-Installation Failures Provides Insights, Paths Forward
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Sand-control-installation failures range from minor issues that can be remedied easily to catastrophic events that put the entire well and investment at risk. The complete paper introduces several case studies of such failures and details the investigative process and techniques used to identify root causes. A commercial software tool incorporating data-analysis techniques and a methodical evaluation work flow were used to understand the cause of the failures and determine how they can be prevented in future operations. Examples include events such as screen and washpipe damage, bridging, hole collapse, and packer-seal failure.
Potential modes of failure traditionally have been investigated with material-balance calculations and surface-pressure measurements. However, this simple approach is unable to diagnose clearly the specific failure mode and often is insufficiently precise to be conclusive, especially for high-value wells. While surface data provide an overview of the operation, the data are influenced by many factors involving the entire system, making it difficult to differentiate multiple events and locate them in the flow path. Mass-balance calculation is also a rudimentary analysis with many inherent assumptions. Often, estimations of perforation or openhole volumes are inaccurate, location of voids cannot be identified in the event of an incomplete pack, and gravel volume and mass measurements do not help in understanding packing mechanisms. Attempts have been made to develop logging techniques to obtain a clearer picture of the outcome of a gravel pack. While this effort has met with some success in cased holes, it has proved to be difficult to apply in open holes. Regardless, identification of the presence of voids in a gravel pack does not provide a clear explanation of why they formed and how they may be prevented in the future.
Operators need to understand failures in detail: where open holes have collapsed, how viscous carrier fluids have performed in the well, where shunt failures have occurred, where alpha waves have stalled, which perforations have packed and to what extent, and where and when screens and washpipes have been damaged. As illustrated by the case studies in the complete paper, detailed analysis of downhole pressure and temperature gauges can provide answers to these questions. The complete paper presents seven examples from openhole completions, but the concepts and principles covered are equally applicable to cased-hole completions. For reasons of space, only one of these case studies is discussed in this synopsis. The case studies address the following failures:
- Stuck string during run in hole
- Bridging during viscous gravel packing
- Bridging during water packing
- Gravel pack carrier-fluid failure
- Shunt failure
- Screen and washpipe erosion damage
- Openhole packer-seal failure
Principles of Analysis
While the trends in the measured downhole pressures can provide insights, section friction pressure analysis provides much more detail. This method relies on the fact that, in a circulating system, the pressure recorded by any downhole gauge is the sum of the hydrostatic and friction pressures. Removing the hydrostatic from a gauge pressure enables the calculation of the friction-pressure component attributed to the flow path downstream of that gauge. Taking the difference between the friction pressure at subsequent gauges isolates the friction pressure for the portion of the flow path between them, the response of which is independent of events in the remainder of the system. Therefore, the friction trends observed can be attributed to events occurring between the two gauges, enabling events to be located and tracked accurately through the system.
Downhole temperature measurements can also be used to determine fluid arrival and displacement through the system. Fluids with different thermal properties conduct heat differently during displacement, creating different temperature signatures as they pass the downhole gauges in the system.
Post-Job Evaluation Work Flow
A detailed post-job evaluation work flow coupled with an appropriate software modeling tool can be used to compare measured data with simulated data and validate the placement model. After the treatment is completed, surface and downhole data must be obtained from the gauges, then processed and prepared for analysis. First the surface and downhole data files are imported into a software tool and time-synchronized. Data offsets and noise are corrected, and the data is parsed into appropriate data ranges that describe the job. The initial conditions for each range must be defined or hydrostatic corrections made, depending on the evaluation mode chosen. Once these steps are complete, analysis calculations can be performed to generate the friction and temperature curves that are used to identify the events that have occurred during the job.
Failure Investigation and Lessons Learned
Understanding sand-control failures is the first and most critical step to avoiding them in the future. The authors demonstrate how the work flow presented in a previous paper can be used to understand catastrophic events, including shunt failure, screen and washpipe erosion, openhole collapse, annular bridging, and openhole packer failure.
The case histories use the standardized gauge locations and section layouts described in Fig. 1 of the complete paper. Washpipe-gauge numbering starts from the first gauge below the service tool and commences down the well. Similarly, section numbering starts from the work-string gauge above the service tool and works toward the bottom of the well. The charts display all data in dimensionless form. This hides the magnitude of the data, but the scale and trends are fully representative of actual events.
Case History: Shunt Failure
Shunt failure can result in a leak that bypasses the shunt packing tube and nozzles, causing incomplete wellbore packing and possible screen erosion or failure. It can be caused by failure of the jumper tube seals (blue circles in Fig. 1) or by erosion. Erosion has been observed at various locations in testing and in screen retrieved from wells. The two main locations where it may occur are at the junction between the transport and packing tubes and the entrance to the nozzles (red circles in Fig. 1).
Shunt-tube failure occurs after shunt activation and can be identified by a reduction in the observed shunt friction at a constant rate. After shunt activation, all the flow should exit the shunts below the annular bridge through a limited number of nozzles. Erosion of either the nozzles themselves or the shunt tubes above them results in flowpath-enlargement reduced friction. The magnitude of the pressure drop depends on the number of nozzles taking fluid and the extent of the erosion.
This type of failure can be prevented by minimizing shunt erosion, which can be accomplished by reducing the rate as much as practically possible (generally less than 1.5 bbl/min per shunt tube). Some of the newer high-pressure shunt systems have been designed with erosion tolerance in mind and would be a good choice for any long-shunted interval in which the potential of a significant volume of slurry being pumped through tubes exists, or in deepwater or high-value completions in which the cost of failure is significant. Where shunt failure does occur, remediation depends on the severity of the failure. If the failure leads only to an incomplete pack, the unpacked section of the well can be isolated and the remainder of the interval produced normally. However, if failure leads to more-significant issues, remediation becomes much more difficult.
Software for Continuous-Improvement Strategy
The analysis of the various case studies was performed with a commercially available software tool that, according to the authors, has the following capabilities:
- Allows the model to be easily validated and calibrated, providing more-accurate predictions for better planning of future operations
- Enables any discrepancies between the model and reality to be identified quickly and located in the system, helping to pinpoint the potential cause
- Provides a means of investigating root causes by applying each to the model and determining if the results match the observed trends
- Facilitates the implementation of new technology or processes to determine the benefit they may have in resolving identified issues
The complete paper presented several case histories that summarize the importance of understanding the root cause of sand-control-installation failures. The case studies included the following:
- One case in which the string became stuck during run in hole, and data analysis determined it was the result of shale collapse, a cause confirmed by the model
- Two cases of bridging during gravel-pack treatments in which data analysis narrowed the potential causes and the model was used to identify the effect of each incident individually and collectively
- One case of viscous carrier-fluid failure wherein data clearly showed alpha wave deposition when it was not expected
- Multiple hardware issues from which data analysis identified shunt failure
- Screen and washpipe erosion and a partially set packer confirmed by the model
A commercial software tool incorporating data-analysis techniques and a methodical evaluation work flow was used to understand the cause of the failures and to determine how they may be prevented in future operations.
Analysis of Sand-Control-Installation Failures Provides Insights, Paths Forward
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