This paper examines mechanical and differential stuck pipe occurrences, giving all the details required to comprehend how the stuck pipe event is developed and, if occurred, how the issue needs to be resolved. In this study, recommendations and best practices from numerous stuck pipe incidents have been analyzed and provided as a reference to eliminate the stuck pipe events. According to industry literature, it is generally accepted that restoring a stuck pipe incident is challenging, and costly, and includes the high risk of losing the main wellbore and performing sidetrack. The most crucial factor in preventing the occurrence of stuck pipes is to think through potential scenarios and ensure that the step-by-step drilling techniques are prepared adequately to ensure that stuck occurrences are avoided or made manageable. The fundamental guideline is to make planning and response to stuck pipe incidents in the simplest and quickest way possible.
Keywords: stuck pipe, differential sticking, directional drilling, mechanical stuck, sidetrack, BHA .
A stuck pipe event which is the loss of mobility of a pipe string can occur for a variety of causes, including drill and casing sticking. This is one of the most frequent complications encountered while the construction of directional wells with a horizontal section of the wellbore [7].
Historically, pipe stuck has been divided into two main categories, including mechanical and differential. According to multiple resources, mechanical stuck includes mainly two distinct sub-categories: due to cuttings or loose rock formations and stuck in sections with the complex geometry of the wellbore, while the differential sticking typically happens due to the pressure difference between the well and drilled formation with a stationary drill string. Overall on a worldwide scale, differential sticking makes up 80 % of total pipe stuck events while the remaining 20 % is associated with mechanical and equipment or design-related stuck [1, 3, 10].
Pipe stuck typically occurs due to cuttings generated during drilling or collapsed formation rock. It often happens when the tool is pulled out of the hole, although a stationary tool string can potentially get stuck if the fluid circulation is interrupted for a long period of time. A similar event can possibly happen while running in the hole with the tool string. The most serious is getting stuck due to loose formation rocks that could potentially collapse and prevent the movement of the tool string. A string stuck in cuttings or loose rock is typically more challenging to release than a string stuck in a complicated hole geometry or due to differential pressure. In cases of tackling such stuck events, more frequently parts of the drilling string are lost in the hole and sidetracks need to be drilled accordingly. The predominant reason for getting stuck due to cutting or collapsed formations is poor hole cleaning and borehole instability [4, 7].
The stuck in the wells with the complicated hole geometry occurs mostly when the BHA design does not correspond to the wellbore shape and design. In such cases, BHA cannot pass easily through the complex well sections and if the tool string is forced to run through that sections by means of slacked-off weight — the stuck is possible. In other words, sticking in sections with complex hole geometry can potentially occur when the drill string is running in or pulling out the hole [6].
Differential sticking typically happens due to the pressure difference between the well and drilled formation when a stationary drill string is forced into the filter cake developed on the borehole across that permeable formation. It becomes impossible to move the drill string because of concentrated friction between the string and the formation. This kind of sticking happens more frequently in the wells that traverse depleted reservoirs. Additionally, differential sticking is mostly observed if the drill string is left static for an extended period of time [3].
As for all drilling complexities, the first step in tackling an issue is to correctly identify it. Therefore, defining the sticking mechanism is the first step in the process of addressing the sticking issue, and subsequently, taking the required actions to tackle the stuck event. It is crucial that the early steps of the response plan to stuck events are completed as fast and accurately as it’s technically possible. Regardless of the origin of the stuck, the stuck case and conditions usually get riskier and much more complex with time. Statistics show that within the initial 4 hours of the sticking, the stuck tool string may be got free in 50 % of stuck cases, but beyond that time period, the percentage lowers to 10 %.
The releasing of the stuck string is only the beginning of the problem's remedy. The investigation and assessment of the activities done are the subsequent steps in the process of addressing any stuck problem, allowing to draw conclusions and advance future jobs.
Case studies and recommendation
Some stuck pipe events from real case studies across the globe have been analyzed with the BHA, details of drilling parameters, field and well parameters, and recommendations are provided as a reference to prevent stuck pipe incidents.
If it is determined that cleaning the hole might create complexities, the string must be maintained in motion whenever possible at all times [2, 9]. The loss zone must be identified as soon as practical and with the greatest degree of accuracy to allow for effective mitigation.
Longer periods of circulation may be established since, in the event that the flow rate is decreased owing to the loss of circulation, the hole might get dirtier. Hole cleaning might be aided by raising the low shear rate viscosity and yield point. Drilling fluid characteristics should be continuously controlled, and API fluid loss must be maintained as low as possible to avoid creating excessive filter cake over permeable layers [2, 4, 5].
Based on the case studies reviewed, the below-mentioned best practices for preventing stuck pipe incidents while drilling can be proposed: [7, 8, 11]:
— To guarantee effective hole cleaning, the pumping of high-viscosity pills at certain defined intervals should be considered (e.g., every 15 meters or every two hours). Once the pills have been introduced into the mud system, good rheology will ultimately develop. To ensure efficient hole-cleaning, the drilling fluid's rheology may be enhanced by the use of the polymer [4].
— Never keep the workstring in stationary while drilling an open hole; if the drilling is stopped for any reason (taking GeoTap points, etc.), always check that the string is reciprocating, rotating, and circulation is in process.
— To eliminate sloughing and avoid tight spots during reaming, back-reaming, and wiper trips, using two roller reamers in BHA at the proper spacing is beneficial.
— Once each stand has been drilled, the newly drilled section must be sufficiently reamed after sufficient circulation or bottoms-up.
Additionally, it is crucial that the early steps of the response plan for stuck events are completed as fast and accurately as it’s technically possible, for the cases if the stuck event has been already developed. The initial steps for releasing from mechanical and differential stuck are different and require a good evaluation of situations prior to implementation [4, 6].
The initial steps to release the string from the stuck point can be taken once the stuck mechanism has been figured out. In case of stuck events due to cuttings accumulated around BHA or collapsed formation rock, the following steps should be considered as a part of the response plan:
— Apply torque and perform jarring down or up depending on the stuck scenario. The max torque needs to be applied to rotate the string in the opposite direction of its movement prior to sticking. The goal is to release the string from the stuck point and establish the circulation to carry cuttings and plug material up to the surface. However, it’s not recommended to apply over-pull and rotate at the same time as it could lead to damage to the string (twisted, parted pipe events, etc.)
— In case the circulation can be established, the circulation or BU at the maximum acceptable flow rate needs to be performed to clean the well from cuttings or collapsed rock particles.
— Once the stuck event is tackled, it’s necessary to work out the stuck depth interval and run to the bottom for performing further BUs to clean the well prior to casing or liner run.
In case of differential sticking happened, the following set of steps should be considered as a part of the response plan:
— Apply the maximum possible torque to the string to transfer it to the sticking point
— Commence with circulation at the utmost achievable flow rate and try to keep the applied torque
— Do abruptly unload the string to create the axial load by maintaining the torque applied to the sticking interval of the string
— If the string design includes jars, cock the jar and apply jarring down. It is necessary to reduce the pump rate to the possible minimum value prior to jarring operation to not deteriorate the impact force.
Conclusion
Each stuck pipe incident or event must be analyzed in detail, and the root cause must be brought to the discussion table by drilling engineering experts. The well should be handled appropriately, and any warning signs it may be giving should be promptly and practically comprehended. Unless the incident was unavoidable owing to a problem with the well design or another technical cause, the driller's response will be crucial in determining whether to deal with the stuck pipe event or not. With consistent efforts, such as using the actual stuck-pipe incidents as lessons learned, best practices, and recommendations, stuck-pipe occurrences may be avoided. Also, if the stuck-pipe event has occurred, the set of steps with the initial response plan should be in place to follow for addressing both mechanical and differential stuck scenarios.
References:
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