The drill bit industry is now focusing on the study and measurement of downhole vibration, the primary cause of drilling inefficiency. More than 10 years ago ReedHycalog began developing tools to model, predict and measure lateral and torsional vibration. These are used in development, testing and field performance validation of not only the bit but also the entire drilling system.
Overcoming vibration
This effort first started with the development of finite element mathematical drilling simulation software. The model allows the user to attach a drill bit to a drill string and simulate the motion of both at prescribed parameters.
Calculations are based on forces measured using a single-cutter test rig with a variety of
Figure 1. The BlackBox recorder is a memory tool with 200 hours of life downhole. It records three types of downhole vibration data. (All images courtesy of ReedHycalog) |
A proprietary software tool, VibraSCOPE, was also developed to model lateral, axial and torsional modes of the drill string and bottomhole assembly (BHA). This tool takes a model of the drill string, BHA and well trajectory to calculate critical RPM for a complete interval or well prior to spud. By knowing the RPM ranges that would excite harmful vibration, they can be avoided wherever possible during drilling.
The company’s Pressurized Drilling Laboratory can test bits up to 17 1/2 in., at downhole pressures. It is used to verify performance, in terms of mechanical efficiency and stability, of new roller cone and fixed cutter drill bits before they are run in the field.
To validate laboratory predictions and field performance, the Drilling Research Tool (DRT) was developed. This sophisticated device measures motion in all 6 degrees of freedom and records them for post-well analysis. Even when its dimensions and complexity became an issue for many customers, it was clear that there was a need to quantify vibration in order to define drilling problems more accurately, enabling the development of targeted solutions. It showed that looking at surface drilling logs and dull bit condition was clearly not enough.
Measuring drilling dynamics
The company built on the knowledge acquired with the DRT and developed a compact version known as the BlackBox drilling dynamics recorder. Like the DRT, the BlackBox recorder is a memory tool with 200 hours of life downhole. It records three types of downhole vibration data: maximum lateral accelerations, RMS accelerations and torsional vibration/ stick-slip indicator. Downhole RPM can be derived from the torsional vibration indicator.
This versatile tool can be run with any type of bit, fixed cutter or roller cone, regardless of
Figure 2. Derived downhole RPM from the BlackBox recorder. |
In a recent application in western United States, BlackBox services were successfully used to isolate a single BHA component as the origin of severe stick-slip. This assembly used was a powered RSS BHA system composed of the drill bit, RS unit, BlackBox carrier sub, downhole motor and a stabilizer. The BHA was tripped due to poor directional performance as a result of high stick-slip affecting toolface response from the RSS.
When the next BHA was picked up, the system drilled to total depth (TD) without any stick-slip issues. However, the original bit did not show any signs of mechanical damage associated with stick-slip problems. The graph (Figure 2) shows the derived downhole RPM from the BlackBox.
Observe the high level of RPM oscillation traditionally seen during stick-slip. However, it does not show a traditional “stick” phase where the rpm drops as low as zero before ramping up to extreme levels. Closer investigation confirmed that the rpm level never dropped below 95 rpm (red line) which was exactly the calculated output of the motor.
By running the BlackBox below the motor, it was possible to determine that the source of stick-slip was located somewhere above the motor. The motor did not stall and operated at a constant 95 rpm. It was the surface rpm that came to a complete stop and then violently spiked. During this time, the bit underwent rpm fluctuations as a result of excessive torque levels, but no mechanical damage to the bit occurred.
Upon careful examination of the run reports, it was found that stick slip first commenced after drilling 50 ft (15.2 m) of a sandstone formation, which coincided with the placement of the uppermost stabilizer entering the formation. RPM was reduced to prevent abrasive wear of the bit in the sandstone, which enhanced the conditions for stick-slip to develop. This is the point at which steering performance deteriorated.
The subsequent run reached TD by dropping the stabilizer from the BHA, and showed enhanced directional performance with no stick-slip events. It would have been impossible to make this correction if the BlackBox recorder had not been run below the motor and the drilling engineer had only relied on surface logs and dull bit condition analysis.
Closing the loop
Closing the loop from prediction to field performance leads to further development of the mathematical model to predict each of the four fundamental drilling indices:
• Rate of penetration (ROP), or how fast the bit will drill for a given weight on bit (WOB);
• Durability – How resistant the bit is to abrasive wear;
• Stability – How resistant the bit is to lateral vibration; and
• Steerability – How the bit responds to side forces and therefore how steerable it is on a given drive type.
There are interdependencies and tradeoffs between the indices. For instance, bits with high durability will have low ROP. Working with the customer to establish the relative importance of each index in their application allows optimal bit selection and drilling performance.
However there are other, more subtle design features that can lead to significant changes in bit performance. This is where the use of more sophisticated indices that look beyond the number of blades, the cutter size and bit profile is a very powerful way to truly obtain a bit solution for a particular application.
As well as solutions to predict, verify and validate performance, the company has developed the V-Stab vibration dampening tool. This string tool not only continuously disrupts drillstring resonance, it also dampens string vibration. This dramatically extends downhole tool and bit life and is proven to enhance ROP resulting in overall reduced drilling costs.
In a recent deepwater well in the Gulf of Mexico, the V-Stab tool was run with a Bi-Center bit and subsequently with concentric hole openers. One concentric hole opener run did not utilize the V-Stab tool. Downhole vibrations were monitored on all three runs. Torsional vibration, lateral vibration and peak shock levels were all reduced by over 50% as compared to the run without the V-Stab tool, enhancing ROP, footage and ultimately reducing the associated drilling costs.
By using predictive technologies, laboratory verification, downhole validation, and unique tools, the company has been able to provide drilling vibration solutions by taking into account the complete drilling system and moving beyond the bit.
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