Temperature-dependent abrasivity of Bukit Timah granite and implications for drill bit wear in thermo-mechanical drillin
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RESEARCH PAPER
Temperature-dependent abrasivity of Bukit Timah granite and implications for drill bit wear in thermo-mechanical drilling Yinlin Ji1 • Lu Wang1 • Yanlong Zheng2 • Wei Wu1 Received: 5 February 2020 / Accepted: 11 August 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Thermo-mechanical drilling combines the advantages of flame thermal treatment and rotary head drilling to achieve a high penetration rate in granitic rocks and a low wear rate of drill bits. This drilling strategy has been recognized as a potential technique to develop deep geothermal energy, but further improvement is still needed to optimize the drilling efficiency, such as the reduction in drill bit wear in thermally treated rocks. We use the Cerchar abrasivity test to investigate the abrasivity characteristics of Bukit Timah granite after the thermal treatment at different high temperatures. The Cerchar abrasivity index (CAI) decreases from 3.5 at 25 °C to 3.0 at 400 °C, remains constant between 400 and 600 °C and subsequently increases to 4.1 at 800 °C. Based on the meso-scale and micro-scale observations using polarizing microscope, 3D scanner and scanning electron microscope, the reduction in CAI value at lower temperatures is due to the abrasive wear of stylus tip, while the mechanical interaction between the stylus cone and abrasive minerals causes the increase in CAI value at higher temperatures. The Cerchar abrasivity test results indicate that overheating rocks may result in the exposure of drill bits to more abrasive minerals and enhance drill bit wear. Keywords Cerchar abrasivity index Drill bit wear Microstructure evolution Thermal treatment
1 Introduction Thermo-mechanical drilling combines flame thermal treatment and rotary head drilling to enhance the drilling efficiency in granitic rocks. This drilling strategy has been known as a promising technique to develop deep geothermal energy, which is currently impeded by high drilling cost [19]. A high heat flux induces the spallation of rocks and improves the rate of penetration [7]. After the removal of produced fragments using circulating water, the drill bits cut the weakened rocks [18] and the wear rate of drill bits can be reduced in the mechanical drilling [25]. The low thermal conductivity of rocks leads to large temperature gradient and crack accumulation near the surfaces [8]. The tensile stress in compressive regions below the surfaces & Wei Wu [email protected] 1
School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore
2
School of Civil Engineering, Southeast University, Nanjing 211189, China
promotes crack generation [11, 22]. Previous studies also indicate that the mechanism of drill bit wear in heat-treated rocks could be different from that in untreated rocks, which may be attributed to the density and distribution of induced cracks. Understanding the interaction between the drill bits and heat-treated rocks could further impro
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