Modeling and validation of percussive drilling performance in a simulated visco-elasto-plastic rock medium

Sazidy, Mahmud Sharif (2011) Modeling and validation of percussive drilling performance in a simulated visco-elasto-plastic rock medium. Masters thesis, Memorial University of Newfoundland.

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Abstract

Optimal force profiles are essential for extracting maximum performance from a percussion drilling system. In this investigation, a visco-elasto-plastic model of rock is simulated using the Bond Graph modeling technique to study the effect of different percussive force profiles on rock failure and to generate optimal force profiles. Physical parameters of the model are estimated from rock material properties like compressive strength, density, elastic modulus and Poisson's ratio using Hsieh's equations. The model predicts penetration due to crushing when applied force is greater than a threshold force of the rock medium. However, this model does not account for penetration due to rotary drilling bit shear or fluid flow. The simulated rock model is tested for three different strength rock formations. -- A Specific Energy Index (SEI) and a Performance Index (PI) are employed to evaluate percussive force profiles. SEI reflects the effects of rate of penetration (ROP) and average hammer power where as PI considers rate of penetration, bit force, and input power. SEI is a limited metric because it recommends low frequencies and low rate of penetration. The Performance Index (PI) seems to strike a better compromise between ROP and power, and has the additional potential benefit of accounting for bit wear. -- To validate the simulated model, rock physical parameters are tuned numerically by introducing a stiffness correction factor (Ks), a damping correction factor (Kb), and considering two different impact test scenarios. Published experimental results from the TerraTek Single Cutter Impact Tester [22, 43-45] is used to verify four different rocks, and to study the effect of Bottomhole Mud Pressure (BHP) on penetration and damping correction factors. Another published experimental data from Drilling Research, Inc (DRI) implemented drop tests [48] on Indiana limestone is used to verify the model as well as to observe the change in damping correction factor with four different drop heights. Overall model validation results are in good agreement with the experimental data. However, further investigation is required to resolve many important issues and to characterize percussive drilling system. -- The present visco-elasto-plastic rock model can be studied under both percussion and vibrational loading but here only percussive force profiles are analyzed. Follow up experimental work is ongoing, focused on characterization of different bit-rock type with the help of the developed rock model by measuring actual hammer force profiles and bit wear due to impact.

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