One of the main challenges drilling within the deep gas field "X" in Oman is the very hard and abrasive Al Khalata sandstone. The 8 3/8″ vertical section drilling performance is restricted due to abrasive wear and toughness of the polycrystalline diamond compact (PDC) cutter technology utilized, which limits both penetration and durability within the hard sandstone. Drill bit providers have typically tried heavy set drill bits to combat the early wear or damage to the cutting structure to gain performance improvements.
After an in-depth study of the application, engineers worked closely with a customer to fully understanding the true failure mechanisms through each lithology. Rock strength was analyzed along with a careful review of the current bit technology utilized. The team hypothesized that although the Al Khalata sandstone presented a cutting structure durability threat, the primary mode of PDC failure was thermal degradation and that simply trying heavier set drill bit designs to overcome this failure was not the solution.
Instead, the goal was to use a lighter set design to increase the penetration rate with improved point loading while fully engineering the PDC cutter to provide increased toughness and, more importantly, overcome the thermal degradation.
Al Khalata formation tests in Field "X" were completed after detailed laboratory testing that included cutter-rock interaction modelling to investigate the improved thermally stable PDC cutter. The initial field test utilized two 8 3/8 in. lighter set six-bladed designs equipped with the new thermally stable PDC cutter technology. Both designs successfully drilled the entire interval through the Al Khalata, Hasirah, Saih Nihyda, Ghudun, Lower Ghudun and Al Bashair formations. The result was a breakthrough in performance, setting a new interval field record including the longest 8 3/8 in. interval drilled in the Al Khalata with a PDC bit.
The results of this research led to new cutter and bit technologies which have proven that the combination of improved thermal resistance and more efficient cutter rock interaction will provide higher rate of penetration (ROP) for the section and demonstrate superior durability while drilling through the hardest stringers to complete the section. By focusing on the true failure mechanisms of the operation, the team was able to prove that improvements in cutter technology can yield breakthrough performance and in this case allow for a lighter set six-bladed design. The improved bit designs have repeatedly drilled further and faster than any previous attempts, resulting in significant cost savings for the operator.