The new hybrid drilling technology which combines the two primary rock failure modes of crushing and shearing was effectively deployed in the challenging drilling environment on the west coast of Ireland. It improved drilling efficiency by reducing the mechanical specific energy (MSE) and mitigating stick-slip and excessive torsional oscillations.
The 12¼-in. section was drilled through a relatively homogenous cretaceous limestone sequence with a small percentage of chert bands. The limestone became progressively harder with depth but remained drillable using the PDC bit. The initial bit, an eight-bladed PDC design, was considered the best choice to drill through the limestone sequence. The initial rate of penetration (ROP) through the top of the limestone was 20 to 30m/hr. After 30 to 40m of drilling, the ROP decreased rapidly to 0 to 2 m/hr over a 10m interval. The bit was pulled. When on surface, the dull condition of the bit confirmed the presence of interbedded chert.
When the hybrid drill bit was run into the hole it drilled an average of 5 to 7m/hr with an instantaneous ROP of 22 to 24m/hr. Isolated chert bands occasionally reduced the ROP to a 3 to 4m/hr range. However, the bit returned to the average ROP upon exiting the chert. The operator, therefore, selected a second identical bit to complete the section. Both hybrid bits endured similar wear on the PDC and TCI cutting elements without any evidence of bearing or seal damage.
The hybrid cutting action generated smoother rotation and lower vibrations throughout the drilling system which improved ROP and extended bit life. The bit thus provided a viable solution to drilling firm limestone with interbedded chert stringers.