Bit induced stick-slip occurs when the torque load from the bit – formation interaction is erratic, either because the weight transfer is poor or because the formation is not easily drillable. Stick-slip is the root cause of many costly and time consuming problems in drilling operations; vibration related equipment failures and slow ROP.
The Anti Stick-slip Tool (AST) was developed to mitigate stick-slip through stabilizing the bit torque; the depth-of- cut is continuously adjusted to maintain constant torque on the bit. Unlike traditional methods of mitigating stick- slip, the AST provides dynamic depth-of-cut regulation, thus can be run in combination with aggressive bits and achieve high ROP while stick-slip and vibration are maintained at low levels.
The field results from the past six years show that the technology works as intended and that many of the problems cascading from erratic bit torque are mitigated. The technology is available from four locations world- wide; Norway, the UK, USA and Brazil.
MITIGATING STICK-SLIP, TRADITIONAL METHODS
Traditional methods of mitigating stick-slip are based on adjusting drilling parameters from surface and modifying the bit with static features to limit the depth- of-cut and reducing the cutter attack angle. These methods are capable of reducing stick-slip, however, at the expense of ROP as the efficiency of the bit is reduced. The costs associated with vibration related failures, and consequential round-trips, are typically higher than the savings that can be achieved from high ROP. The drillers have had to make a choice between low risk and high ROP.
Using traditional methods of mitigating stick-slip, the driller has to choose between high ROP and low risk from vibration. The low risk option is often preferred as the costs associated with vibration related failures outweigh the savings from high ROP.
MITIGATING STICK-SLIP WITH THE ANTI STICK-SLIP TOOL
The Tomax Anti Stick-slip Tool (AST) stabilizes drilling torque originating from the bit through a direct mechanical process where excess torque causes the tool to contract along internal helical splines. This contraction will effectively decrease the depth-of-cut and prevent further accumulation of torque. An internal spring is energized in the process and will cause the tool to extend, thereby increasing depth-of-cut when drilling torque decreases. This process ensures continuous efficient drilling is maintained and destructive stick-slip is limited.
The AST is a stick-slip mitigation tool that unlike static bit features provides dynamic depth-of-cut regulation, thus can be run with aggressive bits and provide both high ROP and low levels of stick-slip.
The Anti Stick-slip Tool (AST) from Tomax mitigates stick-slip without sacrificing ROP.
EXTENDING BIT LIFE
Two major contributors to PDC bit damage are high temperature and impact. The toughness of PDC material is significantly reduced at temperatures exceeding 350°C. The Anti Stick-slip Tool has a positive effect on both these factors; the cutter temperature will be lower when drilling with an efficient cutting structure due to the lower WOB requirement, resulting in less heat from friction, and dynamic depth-of-cut regulation will reduce impact damage.
The net result is fewer trips for damaged bits and more footage per run.
HIGH VALUE ADDED FOR ALL TYPES OF DRILLING AND MILLING
The Anti Stick-slip Tool has been utilized in coiled tubing, snubbing and rotary-drilling operations, in applications like milling, coring, drilling with mud motor and rotary drilling. The run record includes over 750 runs and 500 000 meters drilled. The contributors to the value from running the AST are plentiful and will be discussed in more detail below.
In rotary drilling the AST will stabilize torsional forces created by the drilling action and prevents these forces from being amplified by the drill string, causing severe torque variations and associated vibration. The technology has been used successfully with the main Rotary Steerable Systems on the market. In direct comparisons run to run, leading offshore operators have calculated improvements in penetration rates ranging from 15 to 300%, while keeping drill string vibration below critical levels.
The main challenge in motor drilling is to control the torque on bit, through controlling the weight on bit, in order to avoid motor stall-outs and to control the motor tool-face setting. Issues like weight transfer, wellbore friction, heave, often make this challenge very difficult. The AST tool will stabilize torque variations from the bit and the motor will be subjected to a stable load, thus reducing motor stalls and improving tool-face control. Another effect by using an AST tool is that the motor can be run at a higher load, as the margin against stall-outs can be reduced. More power can be delivered to the bit, resulting in higher rates of penetration.
Drilling and under-reaming simultaneously is cost effective, however, the operation is complex as there are two cutting elements working at the same time and the weight distribution between the two elements cannot be fully controlled. The operation is often associated with severe vibrations, occasionally so severe that they result in parting of the string.
The Anti-Stick-slip Tool has been used extensively in such operations and has proven its ability to effectively stabilize the torque from both the bit and the under- reamer.
EASY TO USE
The AST tools are designed such that they cover a wide operating range, hence no field specific adjustments are required. A User Manual will be produced prior to the job, confirming that the tool placement and drilling parameters are within the operating window of the tool, and providing information on handling and running the tool. There is no requirement for a field engineer. To learn more about the Anti Stick-slip Tool, view technical data, news, contact details and datasheets please visit www.tomax.no.
The AST tool dynamically adjusts depth-of-cut to maintain a constant torque on bit.