Drillability prediction in some metamorphic rocks using composite penetration rate index(CPRI)–An approach

Assessment of drillability of rocks is vital in the selection,operation,and performance evaluation of cutting tools used in various excavation machinery deployed in mining and tunneling.The commonly used rock drillability prediction methods,namely,drilling rate index(DRI)and Cerchar hardness index(CHI)have limitations in predicting the penetration rate due to differential wear of the cutting tool in rocks with varied hardness and abrasivity.Since cutting tools get blunt differently in different rocks,the stress beneath the tip of the bit decreases until it reaches a threshold value beyond which the penetration rate becomes constant.In this research,a new composite penetration rate index(CPRI)is suggested based on the investigations on four metamorphic rocks viz.quartzite,gneiss,schist and phyllite with varied hardness-abrasivity values.The penetration-time behavior was classified into active,moderate,passive,and dormant phases based on the reduction in penetration rate at different stages of drilling.A comparison of predicted penetration rate values using DRI and CPRI with actual penetration rate values clearly establishes the supremacy of CPRI.Micro-structure and hardness-based index was also developed and correlated with CPRI.The new indices can help predict cutting tool penetration and its consumption more accurately.

A NEW METHOD FOR CLASSIFICATION OF ROCK DRILLABILITY

A New method of rock drillability classification for impregnated diamond drilling is recommended. The essence of the method is comparing the area of the slots cut respectively on a standard synthetic rock sample and the rock sample being classified by one diamond saw to determine the rock drillability in diamond core drilling. This method has the advantages of good in simulation and stable in comparison standard.

Rock fragmentation mechanisms and an experimental study of drilling tools during high-frequency harmonic vibration

Resonance drilling is a new technology, still at the laboratory stage. It has great potential to improve rock fragmentation efficiency. We analyzed the amplitude-frequency characteristics of steady- state mechanical vibration excited by harmonic vibration in rocks and an apparatus was built to achieve high fi＇equency vibration of rock. The influence of rock drillability, rotary speed, excitation frequency, and other parameters on the rate of penetration （ROP） in resonance drilling was analyzed. The results show that the rock drillability decreased with an increase in excitation frequency. When drilling with a large size drill bit, the ROP increased with excitation frequency. The ROP reached a maximum value at the resonant frequency of the rock. Tile ROP of the bit increased linearly with rotary speed when no vibration was applied on the rock and increased approximately exponentially when harmonic vibration was applied. In addition, the resonant frequency of the rock was changing during the process of rock fi＇agmentation, so in order to achieve tile desired resonance of the rock, it is necessary to detemaine an appropriate hamlonic vibration excitation frequency.

Anti-drilling ability of Ziliujing conglomerate formation in Western Sichuan Basin of China

The conglomerate rock is usually featured by strong heterogeneity,high abrasiveness,and poor drillability due to its complex composition and texture,which brought a huge challenge for drilling efficiency.In order to guide the drill bit selection and high-efficiency drilling,the physical,mechanical,and drillability characteristics were investigated for conglomerate rock that collected from the lower Jurassic Ziliujing formation in the Western Sichuan Basin of China.The mineral composition,SEM micro-structure,P-and S-wave velocities,uniaxial and triaxial compressive testing,drillability,abrasiveness were systematically tested and analyzed.The mechanical properties and anti-drilling ability of Ziliujing formation were proposed for a typical deep well of S-07,and the distribution characteristics were analyzed.The results indicated that the Ziliujing rock is rich-in quartz and clay minerals,due to the co-existing of strong quartz gravel and weak argillaceous cement,the Ziliujing rock shows strong heterogeneity.The relationships are roughly linear among UCS,drillability,and grinding weight loss with P-wave velocity.The Young's modulus,UCS,internal friction angle,drillability,and abrasiveness meet the Weibull distribution pattern,while only the Poisson's ratio meets the Kernel Smooth distribution pattern.Logging interpretation results reval that the Ziliujing formation has the Young's modulus of 38.61±17.08 GPa,the Poisson's ratio of 0.327±0.006,the internal friction angle of 49.21±11.00°,the drillability of 8.04±1.54,and the abrasiveness grade of 4.32±1.94.The mechanical properties and anti-drilling ability of logging interpretation are in good agreement with the experimental data.The Ziliujing formation is a kind of hard rock with strong heterogeneity,high strength,poor drillability,and medium abrasiveness.Based on the characteristics of Ziliujing formations,the SV516TAUL PDC bit with non-planar cutters was selected for the field application due to the good abrasion resistance,impact resistance,self-sharpening and thermal stability of the non-planar cutters.The field application shows that the average ROP of the new type drill bit in Ziliujing formation is 2.93 m/h,and the average footage is 225.9 m.Comparing with the traditional PDC bit,the ROP of the new drill bit with non-planar cutters has increased by 67.4%,and the footage has increased by 92.1%.The results of this paper can be utilized to guide the drill bit selection and high-efficiency drilling in conglomerate formation.