Drilling Optimisation

On a recent customer well, we encountered various challenges with drilling dynamic; slow hard drilling, with high levels of vibration. As a result of this, the downhole tools and top drive were taking a pounding; with higher WOBs and RPMs to combat the low ROPs and to mitigate stick slip, as well as varying drilling parameters continuously to feed the sweet spot between drillability and forward progress, to tool longevity and reliability. It was no use hammering the BHA to get improved drilling performance if it led to tools being rattled to death down hole.? Adding to the issues was the wellsite being a relatively remote location, with logistical constraints meaning that equipment resupply was difficult and with long lead times. With a view to increase on bottom drilling performance, as well as offer some protection to the downhole tools, drill string design analysis was carried out making use of the real time vibration data across various points of the BHA (i.e., multiple vibration sensors), and of the specific vibration mechanisms experienced by the drill string. Once we had a better understanding of how the downhole tools were interacting with the wellbore, various drilling technologies were evaluated to help address these challenges.

While some technologies were ruled out due to various physical or digital limitations (sadly no #AutonomousDrilling )?one technology which was evaluated and utilised successfully was the Tomax AS AST or “Anti Stick-Slip Tool”. In this weeks #TT (should that be triple T, Tomax Tool Tuesday?) we will take a closer look at this downhole BHA component which helped increase drilling performance while also reducing the drilling stresses on both the downhole BHA and surface system.

First up, what is an AST

The Anti Stick-Slip Tool (AST) functions by a mechanical damping system that absorbs and stores excess energy caused by uneven forces during drilling. The tool consists of a telescoping sub with internal splined helix and spring mechanism which will expand and contract based on the applied forces traveling through the length of the tool. When excessive forces are encountered while down hole and drilling, such as increased weight on bit (WOB) causing the drill bit cutters to hang up, or reactive torque causing the string to stall (e.g., stick slip), the AST compensates independently of the surface drilling parameters by instantly reacting and contracting. This momentary shortening of the sub length decreases the instantaneous WOB and hence reduces the drill bit cutter depth, resulting in a smoother and more consistent WOB and bit torque. Once the WOB and/or torque reduces below a set threshold, the internal spring mechanism will cause the sub to lengthen , reapplying the initial WOB to continue drilling. This continuous contraction and expansion acts as a buffer and compensator in the BHA, to provide a smoothed out and consistent WOB, leading to improved ROP, less saw tooth drilling, longer bit life, and (a cause close to my heart) less vibration that can lead to downhole tool damage or failures (#MWDOutOfSpec).

Principal

The Anti Stick-Slip Tool (AST) is designed to dynamically adjust the forces at the bit-rock interface, thereby stabilizing the drilling process. The AST works to mitigate vibrations and optimize the drilling process by addressing the stick-slip phenomenon. Stick slip is a common issue in drilling where the friction forces acting on the drill string can become greater than the force applied to rotate the string, causing it to momentarily stop or ‘stall’. On bottom stick slip occurs when the friction forces created by the drill bit interacting with the rock, influenced by ?various factors such as bit type and design (cutter shape, cutter angle, cutter size, number of cutters, depth of cut), the formation properties, mud properties etc,?increase to the point where it overcomes the force being applied to turn the drill bit, causing the bit and the string to stall. Once stalled, the rotation being applied at surface from the top drive can cause the drill string to torque up, with the energy being applied building up in the drill string until the force is enough to overcome the bit friction forces, whereby the string will release the stored energy, releasing the trapped torque in the string, and cause the bit to rapidly increase in RPM. This rapid stalling and massive RPM changes can lead to damage in not only the bit and the cutting surfaces of the cutters, but also the BHA such as MLWD tools (electronics don’t like high G vibrations surprisingly) and drill string (over torquing and fatiguing of connections for example).

Utilising an AST can help ensure a more stable loading of PDC and RC bits, and allows the cutters to interact with the drilled formation with minimal waste of energy and lower the risk from vibrations. As the tool reacts almost instantaneously downhole, the mechanical response of the AST reduces losses to friction at the rock interface, leaving more energy available to cut rock faster and at the same time reduces cutter heat and, consequently, wear. This autonomous regulation of the drill bit interface helps prolong the bit cutting structure condition, which in turn helps improve section ROPs and run lengths, meaning less bit trips. ?Who doesn’t like the sound of #ReducedRisk of unplanned bit trips with the bit ‘dying on you’ and #Minimising Cost of picking up the back up bit #RunItYouBuyIt.

Vibration the tool killer

I’m sure you’ve all come across the poor MLWD hand presenting the out of spec form due to someone walking past their tools on deck and looking at them in a funny way. While an exaggeration, there’s a good reason why one of the biggest challenges facing the field crew is to avoid dangerous and damaging vibrations damaging or even destroying the down hole tools. A key reason for this is that most Directional Drilling and MLWD tools have electronics inside them and one thing electronics don’t like is excessive vibration. Electronics can be damaged by vibration due to several reasons; vibration causes mechanical stresses and strains that can lead to failure in electronic components and solder joints. Over time, even small amounts of vibration can cause vibration fatigue, particularly in delicate parts like solder balls and fillets on surface-mounted components. This fatigue results in cracks and eventual failure of the joints, compromising the integrity of the circuit board. Additionally, the physical bending and flexing of the printed circuit boards (“PCB’s”) due to vibration can damage the solder joints and component leads within the bending region. This kind of mechanical deformation can cause micro-cracks and lead to circuit or component failure. To mitigate these effects, several design strategies can be employed. Using secure mechanical fixation to enclosures and adding damping at fixation points either on the PCB boards or between the electronics packages and the drill collars themselves as a dampener, can help suppress vibrations. Proper selection of materials and insulation for wires and components can also reduce susceptibility to vibration damage. Routing and configuring wires to avoid chafing and ensuring appropriate tensioning can prevent additional mechanical stress on the components.

Where does the AST come in? By preventing bit generated SSLIP vibration occurring, and as a result reducing lateral and axial vibration. Vibration caused from whirl effects produced when severe stick-slip makes the angular velocity of the BHA continuously traverse the critical RPM regions for the BHA. Bad for MLWD electronics. It can also help with high frequency torsional vibrations an issues, as the tool helps keep the cutting interface in a condition that will keep the energy dissipated through such oscillations at a minimum while drilling (due to the internal absorber mechanism, there is a critical frequency range around 50 Hz which cannot be as effectively reduced by the tool itself, i.e. self-excited drill-collar harmonics).

Autonomous Activation

The AST operates autonomously, meaning it does not require manual adjustments or interventions from the surface or human input. The tool mechanically reacts to the torque and rotational speed of the drill bit and when encountering stick-slip, it autonomously adjusts the mechanism to stabilize the rotation and reduce vibrations. Similar attempts to monitor and react to downhole WOB and torque spikes at bit often require the use of a LWD ‘WOB/TOB’ tool, however even if this technology is available (not all #MLWD vendors have this as a service…you’ve been warned). WOB/TOB tools function most accurately when closer to the bit, meaning that the LWD sensors have to be pushed further back in the data, resulting in a compromised BHA design and potentially a discussion with the subsurface team about sensor priority.

In our case, the AST was run in a RSS BHA, above all the MLWD tools, meaning there was no compromise in the sensor offsets, and also removed the need to manually react to the WOB/TOB values. The tool can be positioned at various points within the bottom hole assembly (BHA), however its typically run above the MLWD electronics (otherwise you need a short hop/wired system to allow electrical signals to cross over the AST sub). The system can also be run with rotary, motor ,milling and coring BHAs, and in a wide range of well types, from vertical wells to highly deviated and horizontal wells, giving similar results.?

As opposed to running a WOB/TOB MLWD tool and reacting to real time data from surface, the AST will, similar to autonomous drilling systems, automatically compensate for drilling parameter fluctuations, regardless of human input, to smooth out drilling parameters and help lower vibration to reduce tool damages. Some models of the AST can also be equipped with sensors that measure the forces through the AST while? in hole, which can be reviewed once the tool is at surface to understand how much ?work the AST was doing, and potentially help optimise drilling parameters and bit choice further on subsequent runs or future wells.

Key benefits:

• Drilling performance optimisation: The continuous adjustments made by the AST lead to a more stable drilling process, by maintaining ?an optimal WOB and minimizing vibrations. This reduces wear on the drill bit and improves on bottom time, leading to more efficient drilling operations. The result is improved ROP and longer bit runs.
• Reliability improvements via vibration mitigation: The tool effectively minimizes high-frequency oscillations of both torsional and lateral vibrations, which can lead to uneven cutting and wear on the drill bit, MLWD tools and the drill string. By reducing these vibrations, this allows smoother and more controlled drilling parameters and also reduces harmful vibrations which can damage string components, potentially leading to premature bit trips or tool failures.
• Autonomous operation: AST’s operate autonomously while in hole to regulate the drilling conditions in real-time, without the need for electronics or human intervention. This can make for an effective solution to soft torque, soft speeds or WOB/TOB tools for optimizing drilling performance and reducing the risks associated with stick-slip vibrations.
• Flexibility and Versatility: AST’s are designed to work in various positions within the BHA and are designed to be compatible with other drilling tools and systems. They can be used alongside rotary steerable systems and performance motor-powered drills, and are effective in vertical and deviated wells. This flexibility allows it to be used in different drilling scenarios, from standard wells to complex, high-curvature wells.
• Pressure and Temperature Handling: The tool is built to handle high-pressure and high-temperature environments, making it suitable for challenging drilling conditions such as deep wells and HTHP operations. It maintains its performance even under these extreme conditions, ensuring consistent drilling efficiency.

AST Reliability

The AST is a mechanical tool with an excellent reliability record. ?The latest XC version has an MTBF figure of over 100,000 hours for the past 12months. ASTs have been run in over ?8,500 wells ?with over 11,000 runs, with the only two mechanical failures since 2008 having had root causes outside the tool design.

Operator Challenges:

• Unconventional Wells: The AST, particularly the C-Series, is designed for wells with significant curvatures and long lateral reaches. This series is optimized for performance motor-powered, rotary steerable systems, making it suitable for extreme drilling suites.
• High Temperature and High Pressure (HTHP) Operations: The 6 ?” X-Series AST can operate in temperatures up to 425°F (220°C), making it viable for HTHP environments. This capability helps in managing vibrations and stick-slip at deep depths, which is crucial for operations involving PDC type drill-bits and mud motors in hot, compacted rock.
• Performance Drilling: The 5 ?” X-Series AST is suitable for high load performance drilling, with improved capabilities in combination with various drill-bit sizes and under-reamers. It is designed for extensive operational capability in terms of drilling parameters and well curvature, enhancing efficiency in unconventional wells.
• Flexibility: The AST is used across different well configurations, including vertical, deviated, and multi-lateral producers. Its flexible design and extensive service record in coring, milling, and under-reamer operations make it adaptable to diverse drilling needs.

Key considerations

• Optimal placement in BHA? The standard placement of the AST tool is on top of the non-magnetic portion of the BHA. This is the same for rotary steerable systems, downhole motors and rotary assemblies. BHA configuration examples for RSS/Under Reamer BHA RSS/LWD BHA and motor BHA. For hard rock and high friction formations, a roller reamer above the AST is recommended.
• Will the AST perform better closer to the bit? The effect of the AST is virtually the same anywhere in the BHA. The force and displacements required for the AST to affect the cutters move at the speed of sound in steel. At this speed, a difference of 120’ or 40m has little impact. The advantage of moving the AST 120’ or 40m closer to the bit is 1.2% and 2.4% improvements in response at 120 and 240 RPM respectively. Placing the AST closer to the bit in a low angle well also puts increased weight on the tool. This potentially increases the normal internal forces and friction losses. This must be weighed against the advantage mentioned above.
• Is there a limitation on dropping balls through the AST? None other than the given ID. 1/16” (1 ? mm) clearance is recommended. Consideration should be given to the drift if running ball drop or dart activated reamers or circulation subs.
• Heat Tolerance? The AST is designed to function in both standard temperature and HPHT environments. Standard tool spec is configured to withstand up to 200 °C (390F) and for HPHT applications the 6 ?” tools are capable of operating at temperatures up to 425°F (220°C).
• Can the AST have an effect on directional drilling? The AST tool is generally placed in a position where it does not affect directional performance. In cases where stick-slip is a problem, the effect of the AST will be to improve the orientation ability of 3D systems. When used with a motor, the AST will make tool-face orientation more effective. In fact, it becomes so effective that care needs to be taken not to “over-steer”. In some challenging environments, such as in offshore drilling, the AST will make it easier to go on bottom without disturbing the MWD survey transmission.
• Why is it recommended to space out the AST when drilling with an under-reamer? The telescopic portion of the AST will otherwise come close to the under-reamer and potential side-forces will create friction that makes the AST less effective. This is also the reason why the AST should NOT be positioned right on top of a stab or roller-reamer. Minimum spacing is about 10’ (3m).
• Does the AST have a job-specific setup? There is no upper RPM limit. When drilling vertical wells, the bit RPM should be 120 or higher to ensure the contraction of the AST is faster than the elongation of the string when reacting to torque spikes. As the friction above the AST increases with increased deviation, the requirement for a minimum RPM goes away.
• Is there an RPM limit on the AST? No, as long as the tool is right for the main hole parameters (see AST comparison), the long stroke length accommodates a wide variety of parameters.

Any others worth a look?

Its important to note that depending on the well challenges that there are several tools from different manufacturers that perform similar functions to the TOMAX AST in mitigating vibration during drilling operations. Each of these tools employs different technologies and designs to address the common issue of vibration in drilling operations, enhancing performance and equipment longevity.

RIPstick by Downhole Well Solutions: This tool autonomously mitigates downhole torque and weight on bit (WOB) spikes through a fully mechanical design, which helps in reducing stall and stick-slip issues during drilling.

SoftSpeed? II by NOV: This tool uses automated vibration dampening to address torsional vibration and reduce stick-slip oscillations. It features an auto-tuning speed controller for optimal dampening, thus improving drilling efficiency and reducing equipment wear.

HI TOOL? by Expro: Designed to mitigate stick-slip and lateral vibrations, this tool improves the rate of penetration (ROP) by 35% for operators. It is strategically placed in the bottom hole assembly (BHA) to replace an integral blade stabilizer, thus optimizing drilling performance without disrupting the existing setup.

Steady Scout? and Steady Torque? by Scout Downhole: These solutions are specifically designed to manage stick-slip in downhole drilling environments, protecting drill string components from damage due to this phenomenon.

Conclusion

Hard drilling, slow ROP, bit trips and down hole vibration issues are not a new phenomenon in the drilling world. Most wells will face one or more of these during its operational phase, and the most challenging wells often face all of them in the same section! Any performance advantages that can be gained from incorporating ASTs should be at least considered during the planning stages of well design. While there is normally a financial cost and increased risk to add more complexity to any BHA, evaluating the risks versus rewards can have significant advantages in the right situations or drilling environments. In areas with multiple offset wells, the use of ASTs allows for a more direct comparison of performance; did we see higher ROPs and lower vibrations compared to when run without an AST in the string? For areas with smaller offsets, or true wild cat exploration drilling, it is worth considering the use of vibration mitigation tools as a low cost drilling optimisation and BHA longevity policy.

And as a minimum, it might even result in avoiding practicing your signature on an out of spec form!

#ToolTuesday #TOMAX #ROP #Vibration #StickSlip #PleaseSignMyOutOfSpecForm #BitTrip #HassleFreeOperations #DD #MLWD #MLWDAtoZ