GeoNerd Digest 7th Edition: Optimizing Drilling Strategies for Deep Wells in Complex Formations

Welcome to the seventh edition of GeoNerd Digest! In this edition, we explore a detailed technical paper presented at the 6th International Colloquium on Nonlinear Dynamics and Control of Deep Drilling Systems. Titled "Optimizing Drilling Strategies for Deep Wells in Complex Interbedded Formations", this study by Adriano Passos, Flank Lima, and colleagues from Petrobras focuses on the significant challenges faced when drilling through deep, complex interbedded formations, particularly in large hole diameters.

In July this year I was presenting our paper at this event and it was during the time when we also announced collaboration of GA Drilling with Petrobras. Therefore, I decided to choose this paper prepared by our friends from Petrobras for 7th GeoNerd Digest. It may seem that this paper has no relation with geothermal energy. However, it highlights the challenges which can be solved by similar technologies we develop for cost efficient geothermal drilling.

This article discusses various aspects of Bottom Hole Assembly (BHA) design, drill string failure, the influence of rig equipment, and vibration control systems, all of which are crucial to improving the safety and performance of deep-well drilling in these challenging conditions. Full article can be found in the Proceedings on this link.

The Challenge of Drilling in Interbedded Formations

Deep well drilling in large hole sections, particularly in complex interbedded formations like shale, mudstone, limestone, and sandstone, presents serious mechanical and operational challenges. These formations can induce significant stress on the drill string, increasing the risk of tool failure, bit wear, and column breakage. The geological makeup can vary drastically between formations, creating zones of unpredictable stress and leading to tool failures and poor performance.

In addition, the drilling of pre-salt wells in such environments is especially difficult due to the large diameters of drill bits and the well sections required to handle the casings and completion systems for deep formations.

The paper highlights several key challenges and solutions that are particularly important for drilling deep wells in these environments.

1. Bottom Hole Assembly (BHA) Strategy and Drill String Stress

The selection of the BHA, drill bit, and drill string is critical in determining the performance and stability of the drilling process. Historically, the use of 5-inch drill pipe with lower torque capacity was a limiting factor. With advancements, larger and more robust drill pipes, such as 5 7/8-inch drill strings with XT57 connections, have been introduced, offering much higher torque limits of up to 56,000 ft-lbs.

While this advancement improved drilling performance, it also led to increased vibration and stress in the drilling column, especially when drilling large-diameter sections. High-torque BHAs can increase penetration rates, but they also increase the risks of vibration-induced failures and column break events.

2. Simulation and Operational Limits

To mitigate these challenges, the study emphasizes the use of advanced simulation tools to assess operational limits and column stresses. These tools help to model the forces acting on the drill string, determine safe operational thresholds, and identify potential weak points in the BHA and drill string design.

However, current simulation models still have limitations. They are primarily static, meaning they do not accurately capture the dynamic conditions of actual drilling operations. As the well progresses, the mechanical properties of the wellbore can change drastically due to formation washouts, bit wear, and swelling in reactive formations, further complicating the drilling process. This discrepancy between static models and real-time drilling conditions remains a significant challenge.

The study proposes an improved real-time monitoring system that would provide a more accurate picture of downhole conditions, allowing for immediate adjustments to drilling parameters and better failure prevention.

3. Vibration Control and Mitigation Strategies

One of the central focuses of the paper is on the high levels of vibration encountered during drilling. The paper highlights the detrimental effects of stick-slip, backward whirl, and rotational variations, which can all cause significant damage to both the drill string and BHA tools.

SoftSpeed? and SoftTorque? systems, which are designed to mitigate stick-slip and torsional vibrations, play a crucial role in managing these challenges. These systems work by smoothing out the rotational torque and reducing vibration, allowing for more stable and efficient drilling. Figure 8 in the paper demonstrates how the use of SoftTorque technology stabilizes torque and rotation parameters, leading to a marked improvement in performance and a reduction in tool failures.

4. Rig Equipment: Heave Compensators and Top Drives

The paper also emphasizes the importance of rig equipment in improving the efficiency and safety of drilling operations. In particular, heave compensators and top-drive control systems help maintain consistent Weight on Bit (WOB) and rotational speed, which are critical for drilling in large-diameter sections of interbedded formations.

The use of Auto-Drill systems, which automatically adjust the drilling parameters based on the real-time condition of the formation, has been found to significantly reduce stress on the drill string and improve operational stability. Figure 6 of the study shows the clear benefits of using an active compensation system in combination with Auto-Drill for maintaining stable WOB, even in challenging conditions.

Case Studies: Lessons from the Field

The paper provides two detailed case studies, which illustrate the real-world impact of these technological advancements on deep-well drilling operations:

Case Study 1: Disconnection in Well A

• This case study describes a major event where the drill string experienced a disconnection during the surface phase of drilling. Despite drilling in soft formations, the well encountered severe stick-slip vibrations (up to 400 rpm variations), leading to multiple disconnection points.
• The use of SoftSpeed? technology helped reduce the severity of these vibrations, but it highlighted the need for better control systems and improved real-time monitoring to detect and respond to such issues more proactively.

Case Study 2: Column Break in Well B

• In this instance, the drill string experienced a break due to fatigue failure during the tripping operation. The break occurred at a stabilizer and later at the Drilling Jar, where high levels of bending stress and vibration were observed. This failure underscores the importance of optimizing BHA design to mitigate stress in challenging formations.

Conclusion and Recommendations

The study concludes with several important recommendations for improving drilling strategies in deep, interbedded formations. These include:

• Utilizing advanced vibration mitigation systems (SoftTorque and SoftSpeed) to reduce the risk of drill string and BHA failures.
• Incorporating real-time monitoring to capture dynamic downhole conditions and adjust drilling parameters on the fly.
• Enhancing simulation models to better account for the real mechanical conditions encountered during drilling.
• Improving tool maintenance and inspection to track fatigue cycles and prevent unexpected tool failures.
• Optimizing BHA design based on real-time data to minimize vibration and stress, and to enhance overall drilling performance.

These advancements will help reduce drilling time, lower costs, and improve safety when drilling large-diameter wells in complex interbedded formations.

Let’s Discuss

1. How can real-time monitoring systems be enhanced to better capture dynamic downhole conditions?
2. What role do vibration mitigation systems like SoftSpeed and SoftTorque play in improving deep well drilling performance?
3. What are the best strategies for optimizing BHA design in challenging interbedded formations?

Let’s engage in these critical questions as we continue exploring the future of deep-well drilling technology!

Stay tuned for the next edition of GeoNerd Digest, where we’ll continue to cover the latest innovations and challenges in the geoscience field.

Copyright Notice:

This summary is based on the paper “Optimizing Drilling Strategies for Deep Wells in Complex Interbedded Formations: Addressing Drill String Stress, BHA Tool Failure, and Performance Enhancement” by Adriano Passos, Flank Lima, Guilherme Britto, José Brígido, Márcio Francisco, Reinaldo Tomita, presented at the 6th International Colloquium on Nonlinear Dynamics and Control of Deep Drilling Systems, Rio de Janeiro, Brazil, 2024. All figures and tables from the paper are used under fair use for review purposes only.