Developing reserves in the southern Negros, Philippines, requires the operator to drill through volcanic formations known for their abrasiveness and high unconfined compressive strength (UCS). Efficient penetration with conventional bits through the volcanic rock, which contains silicified andesitic tuff, breccia, quartz, chert, and pyrites, historically has been very challenging. Typically, when a bit encounters the hard formation, the strong impact exerted on the bit induces cutter damage, forcing a round trip of the BHA for a new bit.

PDC bits have many advantages over roller cone bits and have outperformed them in many applications. However, in geothermal applications, roller cone has been performing better than PDC bits. Traditionally, it was believed that it impossible to drill this type of formation using conventional PDC bits. In an offset well, the operator chose to use a tungsten carbide insert (TCI) bit in an attempt to reduce bit consumption and trip time and to increase rate of penetration (ROP). However, postrun analysis shows that the offset well requires at least three TCI bits with and average meterage of 200 m at an ROP of 3.5 m/h. The bits were pulled out with broken cutters and graded with 5-8-LT-TQ, 4-7-WT-PR, and 7-8-WT-TD.

To deliver a bit solution, a research initiative was launched to investigate new types of cutting elements. The project was successful and yielded an innovative conical diamond element (CDE). This element has twice the diamond thickness of conventional PDC cutters, resulting in higher impact strength and 25% more resistance toward abrasive wear. A new bit type was designed with the CDEs strategically placed across the bit face from gauge to the bit center utilizing a finite-element-analysis (FEA) -based modeling system. The placement of CDEs is mainly to support and protect the conventional PDC cutters from impact damage and to strengthen the overall cutting structure.

Before the job commenced, engineers used an FEA-based modeling system to predict the bit performance in a virtual environment. The analysis was completed by comparing simulation results between drilling dynamics of a conventional PDC bit, TCI bit, and the new CDE bit. The simulation output indicated that the CDE bit is more suitable to drill the hard compact carbonate formation compared with the conventional PDC bit and TCI bit. The CDE bit appeared to be more stable and yielded significantly lower vibration at the bit and along the BHA.

The 8½-in CDE bit was run and drilled the entire 8½-in hole section through the hard volcanic formation to TD at a significantly higher ROP compared with the offset well. Compared with the offset well that required three TCI bits to reach TD, the CDE bit delivered more drilled interval at a higher ROP while providing a smooth, high-quality wellbore, enabling casing to be set on the first attempt. Also, the dynamic response predicted by the modeling system matched the bit, BHA, and drillstring vibration profile recorded during the actual field run. Improvement in drilling performance for this particular run has saved the operator 10 days off their AFE.

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