PDC drill bit design techniques and product features improve RSS drilling efficiency

Rotary steerable systems are regularly used as effective and efficient drilling solutions for offshore wells. And over the years the number of and types of RSS resources has expanded. But the challenge for drilling engineers remains the same: “how do we improve ROP and borehole quality and keep our costs down?”

Varel International believes that they can help engineers meet their drilling objectives – particularly when a rotary steerable system is utilized. The company has a unique design philosophy for drill bits when it comes to rotary steerable systems. Varel believes drilling optimization can only be achieved when the drill bit is designed to match the RSS, directional goals of the well, and formations to be drilled.

“Tailoring the drill bit to the rotary steerable system is a very effective drilling solution for operators,” said Bruno Cuillier, Eastern Hemisphere PDC Product Manager for Varel International. “By considering the RSS and the bit as a ‘drilling system,’ we can help customers enhance their drilling efficiency.”

Image shows the design criteria used by Varel International when designing bits for directional applications.

Pairing the Drill Bit to the Drilling System
Engineers in Varel’s R&D group have used field run data and laboratory tests to develop tools that aid in designing the most effective bit for the drilling system. The design tools take into account the type of system employed – point-the-bit or push-the-bit – as well as the power generated and drilling characteristics delivered by each drilling system. Varel’s Navigator bits, the company’s PDC bits used for directional drilling, are custom designed to the operator’s specific directional profile and formation requirements. To do this, Varel engineers consider the operator’s parameters for dogleg severity, borehole quality and ROP through the formations to be encountered. The bit’s ability to achieve a given wellbore objective relies upon two key design factors: steerability and directional behavior.

Varel invested time and resources to prove this theory. Today Varel’s customers are benefiting from this work. Dr. Alfazazi Dourfaye, R&D Manager for Varel explains why the company made this research a priority. “Some RSS tools are more aggressive than others and we have to take this into account when designing the bit. The same bit designed for a Baker Hughes Inteq drilling system should not be used on a Schlumberger PowerDrive system. Each drilling system comes with it own unique characteristics and capabilities; therefore, the bit should be specifically designed to complement the individual system.”

Gage Influences Steerability and Walk
The bit’s ability to achieve a given wellbore objective relies on two important design factors: steerability and directional behavior – and gage design influences both of these factors. So, Varel engineers pay particularly close attention when designing the gage.

“The gage is what gives the bit its lateral aggressivity and steerability,” said Cuillier. “It is imperative to design the gage to complement the drilling system. To give an example: we don’t want to design an extremely gage aggressive bit for a drilling system that has a propensity to ‘walk.’ This pairing would make the bit too aggressive, making it too difficult to control the bit, which will result in low drilling performance and the operator missing the target. So, for optimum drilling efficiency, it is important to pair the drill bit to the drilling system.”

For example, on a point-the-bit system, gage length affects and influences the bit’s performance when it is in “hold mode” and when it is in “directional mode.” Varel engineers design all Navigator directional PDC bits to drill effectively in both modes. The gage is designed so that it is has less torque and minimizes the potential for stick slip issues when in “directional mode”. When the bit is in hold mode, the gage design complements the RSS tool resulting in better well bore quality and higher ROP.

Alternatively, in push-the-bit systems, the gage is designed differently to optimize the characteristics of the RSS tools. In these circumstances, due to the nature of the RSS tools, Varel’s active gage design is used to improve drilling efficiency and wellbore quality. When building angle the active gage design engages the formation to ensure the bit meets the dogleg severity requirements.

To further enhance the directional capabilities of the Navigator bits additional matrix material in the shape of an o-ring is placed directly behind specific cutters. This additional material helps manage the torque generated by modern RSS tools while maintaining orientation for increased ROP and reduced sliding time.

When drilling a directional well, negative occurences like hole wall sloughing, keyseating, or severe dog legs can occur. Varel’s updrill feature, a standard feature on Navigator products, allows hole wall cleaning as you ream the well bore helping eliminate such problems.

Field Proven Results
The development of specific tools to design bits for use in directional work has provided drilling optimization and continued success for Varel’s customers.

One example is the success in the Eastern Hemisphere with the 8-1/2” VRT619G. The bit was specifically designed to run on PowerDrive Vortex from Schlumberger. The bit entered the well at 5215’ and 0° incline and drilled 3178’ in 59 hours while building angle to 86° inclination. The results were a 21% ROP improvement over the best previous run completed in the same field. For this well, the bit went in with 27° inclination already built.

On another project, the same operator experienced an increase in ROP by 44% and achieved the best overall drilling performance for a particular field when Varel’s 8-1/2” VRT619G was used. Again, the bit was designed for Schlumberger’s PowerDrive Vortex RSS tool. The Varel bit entered the well at 5,524’ with a 20° angle already built and drilled to finish the section at 9145’ and 86° inclination. This run beat the best offset, which stood since June 2006, by 20.5 hours drilling time.

Contact Sherri Scott at sscott@varelintl.com.