It's always critical to have a specific appropriate drillbit design for an application. However, limited data is challenging in exploration wells, especially in geothermal applications. It is already well known that geothermal wells consist of various types of igneous and volcanic rock with high ranges of hardness and abrasiveness. Thus, proper bit selection analysis must be carefully performed in order to balance the bit aggressiveness with bit durability.
Statistical analysis from many geothermal wells in Indonesia was used for preliminary bit selection. Advanced finite element analysis (FEA) drilling dynamic modeling was then performed to simulate several bit options responses to some rock models. In the past, a TCI bit was believed to be the best option for drilling hard and abrasive geothermal wells due to its crushing action. However, TCI bits are limited by the maximum revolution the bit can achieve before bearing seal failure. PDC bits with conical diamond element (CDE) were also included in the simulation. Based on the simulation with CDE bits for three sections, the 17 1/2-in, 12 1/4-in, and 9 7/8 in bits generated better stability and higher ROP compared to TCI bits. Bits with CDE provide significant improvement of wear and impact resistance. The point loading of this cutter is more concentrated on the formation and enables the bit to remove rock in a more efficient way. The simulation also provided a stable drilling parameter roadmap for applying CDE bits in drilling operations.
On the first exploration well in this field, a CDE bit with seven blades and 16-mm cutters was used to drill the 9 7/8-in section with a rotary BHA. PDC cutters on the primary position provide aggressiveness to the bit, while CDE on the backup position protect the bit from heavy impact and abrasion. The bit drilled 872 m with 15 – 30 m/h instant ROP on tuff and breccia formation, resulting in 12.1 m/h average ROP. By using the CDE bit in this section, it is estimated to save at least one TCI bit trip, equivalent to about 120 hours. On the second well, a CDE bit was run in the 17 1/2-in section and it drilled 547 m with 7.5 m/h ROP, while previous TCI bits could only drill 224 m with 4.8 m/hr ROP. The next 12 ¼-in section was also drilled with CDE bit for 1,060 m with 13.1 m/h ROP, eliminating at least two bit runs with significantly higher ROP.
Changing the TCI bit to a PDC with conical diamond elements is a proven method drilling optimization in these exploration wells, saving about 6 – 9 drilling days per well.