Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Andrew K. Wojtanowicz

Abstract

A new mechanistic drilling model for PDC bits was derived from the balance of forces acting at the PDC cutter. The model combined the torque and the drilling rate equations, cutter's geometry and rock properties. A new understanding of frictionally generated heat between rock and PDC cutter is introduced. The numerical analysis revealed that neglecting the heat generated at the cutting surface area results in underestimation of the actual wearflat temperature by 10% to 530%, depending upon bit dull and downhole hydraulics. The example bit performance comparison made by calculating the MBP curves showed 18% reduction of drilling rate when the new and more rigorous temperature limitation is used. A new PDC bit wear model was derived and used for bit performance prediction. The model relates bit life with temperature, weight on bit, rotary speed, and cutter geometry. The predictions showed that the effect of the friction dominates bit life, and this effect is greater than the effect of convective cooling. A new laboratory instrument was constructed and succesfully used to measure friction forces between sliding surface of a PDC cutter and the rock surface. Results showed that friction coefficient did not change considerably within the range of tested rock and fluid types. The concept of maximum bit performance (MBP) curve was introduced. The curves represented the maximum values of average drilling rates for various pre-assumed footage values. A new method for preparing a multi-bit drilling program, the dynamic drilling strategy, was developed. The dynamic drilling strategy provided the best combination of PDC bit runs to achieve the minimum drilling cost for a long borehole interval. The method was numerically compared to the conventional drilling optimization and to the field practices. Considerable savings of 25% and 60% were estimated, respectively. Based on the drilling model, a new method was developed for the insitu measurements of the PDC bit condition and lithology change detection. The technique was verified by comparing the predicted and measured PDC bit wear and by showing the correlation between rapid formation changes and discontinuities in the diagnostic plots.

Pages

373

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