Fixed cutter bits – generally termed PDC bits for their poly-crystalline diamond cutters – have been achieving high rates of penetration (ROP) for decades, helping operators reach target zones as quickly as possible. However, fast drilling creates several challenges, especially in larger hole diameters that are usually drilled with water-based mud. The cuttings must be cleared from the bit face rapidly or the ROP suffers due to bit balling. The bit blades must resist solids accretion, and the cutters must remain sharp over long intervals. The bit design must match the formations to be drilled.
To address these needs, operators typically choose between tungsten carbide body bits, known for longevity, and steel body bits, known for speed but more susceptible to erosion. Another factor is formation type: drilling in shales and carbonates is less abrasive than sandstone or hard rock. In these more forgiving conditions, a properly designed steel body bit can change the equation, allowing operators to benefit from both speed and longevity.
Bit Cleaning Facilitated
In the Anaima field in Oman, the carbonate and shale formations are not particularly abrasive, but frequent bit balling was slowing the ROP to an average of 65–70 ft/hr (21m/hr). The fluid flow across the face of the bit could not prevent cuttings from lodging between the blades. The operator had been using mostly tungsten carbide bits. A switch to steel body fixed cutter bits showed some gains as the ROP rose to an average of 100–105 ft/hr (32 m/hr).
The lessons learned from this improvement were applied to a new fixed cutter steel body design. The fine tuning paid off; while drilling the 12¼” interval, the operator set a field record of 5,360 ft in 23 hours at an ROP of 233 ft/hr (71m/hr). When the operator reached total depth (TD) for the interval, the new SteelForce™ bit was pulled and graded 1-1 – equivalent to nearly new condition.
This success was accomplished primarily by resolving the bit cleaning issue. The steel body bit has several characteristics that make this possible. Steel is more ductile, or flexible, than tungsten carbide, so the blades of the steel bit can extend further from the body, creating improved flow paths for the fluid emerging from the bit nozzles. The high blade standoff design of the steel compared to standard matrix bit facilitates cleaning for a faster ROP, and the strong blade design combats drillstring vibration. Tungsten carbide, which is much more brittle than steel, lacks the ductility necessary for extended blade construction.
Another success factor is the use of micro nozzles, allowing for an increased number of smaller nozzles to replace the standard number of conventional ones. While the diameter of the nozzle orifice typically remains consistent with established drilling practice (i.e., up to 16⁄32” opening), the smaller nozzle casing takes up less space. More nozzles can be installed on the bit, increasing the cleaning efficiency without reducing the total flow area needed to optimise bit hydraulics.
To combat erosion and potentially abrasive formations, the new steel body fixed cutter bits are coated in a special hardfacing that strengthens the outside surface with carbide pellets, helping to extend the life of the bit so that it can compete favourably with tungsten carbide bits. In addition, each bit has an antiballing coating that changes the electrical potential of the steel to a strong negative. This repels the negative ions in the drilling fluid so that a lubricant barrier of water forms on the surface, preventing the bit from balling.
The cutters, like the rest of the bit, are designed to extend bit life without sacrificing ROP. Traditionally, cutters provide abrasion and impact resistance, but thermal degradation can cause damage through the differing expansion coefficient of the materials in the cutter. The ability of a cutter to manage the frictional heat generated during the drilling process allows it to stay sharper longer and wear at a slower rate.
Each bit is designed specifically for the application. The combination of these optimised features means that the steel body fixed cutter bit will drill farther faster, and reduce the cost per foot drilled.