Abstract
Abstract
Jar twist-off is a well-known service quality issue for some applications, especially in relatively large borehole sizes, such as 22 in. and 16 in. These twist-offs cause costly nonproductive time (NPT) due to fishing operations and sidetracks, which might eventually cause well abandonment. The jar is one of the weakest points in the drill string due to moving parts, which include complex shapes, resulting in stress concentration.
This condition is especially challenging in some applications with multiple formation layers, which tend to increase vibrations during every layer transition. Other factors such as total mud losses and hole wash-out make drilling dynamics even more challenging.
Optimizing weight on bit (WOB), bit revolutions per minute (RPM), and flow rate using real-time vibration monitoring tools and indicators, such as mechanical specific energy (MSE), can be effective in minimizing vibrations and reducing stress at the jar. However, this approach often has the negative effect of limiting rate of penetration (ROP), and therefore increases drilling costs. This ROP reduction and drilling cost increase is because drilling parameters can be optimized either to maximize ROP or to minimize vibrations. In most situations, the selected parameters will cause a compromise between ROP and vibrations. Therefore, there is a need to have another method to reduce the stress, apart from drilling parameters.
This paper presents results of a study focusing on placing the jar in the bottomhole assembly (BHA) to reduce the bending moments and bending stresses on the jar connections. As a result, the risk of twist-offs during drilling are minimized, while maintaining the effectiveness of the jar should a stuck event occur. This paper briefly presents the study, the resulting recommendations, and a proposed change in standard BHA configurations to reduce service quality compromising incidents and productive time lost from jar twist-offs.
The first step in the study is to calibrate a transient dynamic, finite element analysis (FEA) model based on field data to ensure accurate outputs. Then, multiple BHA configuration guidelines are established, based on previous lessons learned. These configurations are simulated using a calibrated FEA model to quantify the different types and magnitudes of stresses the jar experiences. These outputs are then used as inputs for a FEA-based fatigue simulator to analyze all jar connections.
The results from the simulations are evaluated to confirm that the stress levels are reduced compared to the baseline configuration, resulting in reduced risk of failures and prolonged tool life. The output of the study is a proposal to change the placement of the jar from between drill collars to between heavyweight drillpipe (HWDP).
The connection between the jar and the HWDP would be accomplished by two saver subs, which will be fitted to the top and the bottom of the jar. These subs are specifically designed to ensure a smooth transition between the jar and the HWDP.
Analysis confirms that the bending moment at the jar is reduced by 40% because of this new hardware positioning. Furthermore, FEA-based fatigue analysis confirmed that all proposed connections in their new positions would have higher fatigue limits compared with those in a conventional BHA. Since its introduction, this new BHA configuration has been run 124 times, with no twist-off incidents to date.