Investigating Hole Opener Stinger Length on Drilling Performance and Founder Point in Soft and Hard Rock Formations

Abstract Drilling efficiency is crucial in reducing operational time and costs, primarily in high-cost environments where even minor improvements can lead to significant savings. Drilling bit configuration is one of the factors that improves drilling efficiency. Pilot holes in particular, have shown enhanced performance by improving the rate of penetration (ROP). However, previous studies have primarily focused on ROP improvements but have often neglected the impact on Mechanical Specific Energy (MSE), and have largely been limited to soft rock formations. This research aims to fill these gaps by investigating the influence of pilot hole length on drilling performance under different hole cleaning efficiencies for sandstone and gabbro as representative of soft and hard rock formations respectively. Experiments were conducted using a Large Drilling Simulator (LDS) with a hole opener configured with a 2-cutter Polycrystalline Diamond Compact (PDC) stinger bit and a 4-cutter PDC hole opener (HO) bit with 3 stinger length configurations: Full Face (FF), 1 cm pilot hole (1-PH), 2 cm pilot hole (2-PH), and 3 cm pilot hole (3-PH). Drilling performance was evaluated by varying Weight on Bit (WOB) from 3 kN to 15 kN and flow rates of 6 L/min (representing poor hole cleaning) and 30 L/min (representing efficient hole cleaning). The study focused on the impact of stinger length (and corresponding pilot hole length) on Rate of Penetration (ROP), Mechanical Specific Energy (MSE), and founder point. The results show that drilling with a stinger (as compared to full-faced drilling) improves ROP, reduces MSE, and delays reaching the founder point, highlighting the enhancement of the overall drilling performance. Varying stinger lengths impacted relative improvements in performance likely due to challenges clearing cuttings from deeper pilot holes.