It's frustrating to be unable to reach the desired wellbore depth with a specific tool because it can't pass through a dogleg in the well bore. Ever experience a bad day in the field where the coiled tubing (CT) was accidentally parted or buckled as a result of not stopping the injector fast enough? Many of us have observed well intervention stacks swaying during land or offshore field operations. Have you ever pondered if the stack design had an adequate margin of safety under those conditions? While there's no uniform solution to these problems, proper pre-job design and use of advanced software tools can help avoid unpleasant and potentially unsafe outcomes during CT operations.
Using specialized software for pre-job modeling of downhole forces and remaining fatigue life helps users to optimize the life of their CT strings. Some report that they have safely reduced CT replacement costs by over 40%, with no field failures, by properly managing CT fatigue profiles. The functionality of CT modeling software has continued to mirror the growth of the CT industry. New developments include the ability to optimize CT string design, identify requirements for a downhole tractor, improve velocity-string design, and even design wellbore cleanouts.
However, performing safe, efficient, and cost-effective CT intervention operations under increasingly challenging conditions requires the use of more advanced tools and techniques. For example, many conventional rules-of-thumb and simple spreadsheet calculations are no longer valid for operations in highly deviated well bores or those with tortuous pathways.
Operations on floating structures (with independent wellhead and platform movement) can further complicate equipment selection. Fortunately, continued developments in well intervention modeling software for CT applications has provided new tools that can help reduce costs and provide for safer field operations.
Finite element analysis modeling
A fit-for-purpose, finite element analysis (FEA) software program, specifically developed for oilfield applications, has been incorporated into the CTES Cerberus for Coiled Tubing software, and can provide much-needed answers to challenging questions.
The most basic of these FEA modeling applications involves a bottomhole assembly (BHA) analysis for determining the feasibility of successful CT operations in deviated wells. This software analysis determines the maximum BHA size that will pass around a dogleg without tool bending. Alternatively, the software will calculate the maximum BHA length that will traverse a given wellbore geometry.
A more advanced FEA analysis can also be performed, which allows for normal bending of the BHA to be accounted for when determining its ability to pass through a dogleg. The wall contact forces caused by the bending of the BHA result in additional friction, which hinders tool movement through this curved region of the well bore. Again, software tools greatly simplify this type of analysis, and only require the user to input the physical geometry and material properties of the BHA. These inputs enable bending for a specific BHA to be calculated, and are used to model its ability to traverse sections of the well bore containing significant curvature or azimuth changes. The software also provides a graphical view of displacement and forces between the tool and well centerline as the BHA moves through the area of interest. This result can be used to determine the impact of adding centralizers, standoffs or rollers to the BHA, and whether these changes will provide for easier negotiation through the dogleg.
The Zeta Model, another unique, FEA-based modeling tool, has recently become commercially available. This tool is used in the job-design phase to model the intervention stack and determine the most cost-effective equipment and support system required to safely perform the field operation.
The industry no longer has to rely on static stack-analysis equations or guesswork to determine proper stack size, the need for a compliant stack section, the location of supports and guy-wire locations, or the need for a lift frame or compensation device. This approach performs a dynamic analysis of the stack, providing answers that more closely represent actual field operations. The software can also be used in conjunction with a specially instrumented lubricator section, providing measurements of real-time stack stress levels during the job.
Pre-job stack modeling determines deflection and stress at each point along the length of the entire intervention stack because of pressure, bending, external support placement, axial load and dynamic forces. A tall intervention stack may have relatively low combined stresses under static conditions, but these stress levels can increase dramatically as the stack begins to sway dynamically because of reel-back tension surge. Stack stress levels on floating structures could be further elevated as a result of wellhead movement and platform movement.
Intervention stack modeling results have produced insights that may be counterintuitive. For example, the worst-case loading scenario for bending stress may occur when the stack is under lower compressive loading, versus high compressive load. This is counter to the concept of a buckling load, where the stress increases as the load increases.
Without proper pre-job dynamic stack analysis, a lifting frame or compensated support structure may be used to apply tension to the stack. This approach is used to avoid potential stack buckling failures. However, the operation may not have been at risk with regard to a buckling failure. Instead of improving the safety margin of the operation, these additional supports may increase the risk of a stack bending failure while adding unnecessary cost and complexity.
Avoiding operational failures
A software-controlled emergency stop system integrated with existing CT data acquisition systems can avoid injector-control operational errors. This system is configured by the CT operator prior to field operations. It will automatically stop the injector (in approximately 1 second) and set the injector brake if certain critical parameters are exceeded. Injector shutdown as a result of emergency-stop system activation requires no action by the CT operator. As an added safety measure, the system does require the injector controls be placed in the "neutral" position prior to rearming the system following activation.
The emergency-stop system continuously monitors maximum and minimum weight values resulting from the amount of CT in the well. These values will require periodic readjustment by the CT operator during the operation, as the weight will vary with the total length of CT in the well. The software monitors both minimum (snub) and maximum CT weight and activates if the values measured during field operations exceed the settings entered in the software. Two auxiliary values, rate of weight change (RWC) and CT speed, are also used by the emergency stop software in order to provide proper injector control.
RWC is monitored over a 1-second interval and compared with the previous value. The emergency stop system activates if the measured RWC value exceeds the setting entered. Monitoring this value is critical where the minimum and maximum CT weight values are not updated on a timely basis as the amount of CT in the wellbore changes. Monitoring CT speed prevents unwanted activation of the system under certain conditions. For example, CT weight could increase rapidly as the tubing begins to move off bottom while being pulled out of a deviated well. This rapid weight increase could easily exceed the RWC setting, resulting in an erroneous injector shutdown by the emergency stop system. However, the software recognizes this situation as a result of monitoring CT speed, and ignores the rapid weight increase until such time as the CT reaches normal operating speed and the weight stabilizes.
It's never a "good day" in the field when the CT is accidentally parted or buckled as a result of not stopping the injector soon enough following a change in downhole conditions.
This injector emergency stop system provides a layer of defense that could easily prevent a simple distraction from becoming a very costly learning experience.

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