New Conical Diamond Element Bit Technology Coupled with FEA-Based Modeling for Bit Selection Demonstrates High Durability and Improved ROP in Compact Carbonate Formation in Deep Water

Abstract

Developing reserves offshore East Malaysia, especially in deep water, often requires the operator to drill -through formations that consists of compact carbonate with high unconfined compressive strength (UCS). Efficiently penetrating these carbonate formations with conventional PDC bits has been challenging; typically, when a bit encounters this formation, a high vibration level is induced that leads to impact cutter damage, forcing a round trip for a new bit and causing nonproductive time (NPT) and additional cost for a new bit. Consequently, the cost impact is more severe to the operator, especially in deep water environments. Traditionally, an operator has used four- and five-bladed PDC bits to drill this formation because this configuration had set a benchmark in this field compared with roller cone bits. However, a recent engineering analysis on a recently drilled offset well showed that a minimum of two PDC bits are required to reach the section total depth (TD). On the first run, the bit was pulled out of hole (POOH) due to slow ROP 6.2 m/h) and received a poor dull grade (6-7-HC-ROP). Although the second PDC bit produced slightly higher ROP compared with the first bit, it was POOH with broken cutters. A research initiative was launched to investigate new types of cutting elements. The project was successful and yielded an innovative conical-shaped polycrystalline diamond element (CDE). This element has twice the diamond thickness of conventional PDC cutters, resulting in higher impact strength and more resistance toward abrasive wear by approximately 25%. A new bit type was designed with the CDEs strategically placed across the bit face from gauge to the bit center utilizing 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. The 8½-in CDE bit was run and drilled the entire 8½-in hole section through the hard carbonate formation to TD at a significantly higher ROP compared with the offset well. Although the CDE bit was POOH due to downhole tool failure and graded 1-2-BT-DTF, the same bit was rerun and successfully completed the hole section in good condition and dull graded as 1-3-BT-TD. Compared with the offset well that required two conventional PDCs 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 drill string vibration profile recorded during the actual field run. Improvement in drilling performance for this run has saved the operator 15 hours for the same drilling interval.