Effect of Rotary Speed and Bit Weight on Penetration Rate of a Diamond Microbit

Abstract

Presented are results of experiments on the penetration of a 1.25-in. OD diamond microbit into Georgia granite and Beekmantown dolomite. A study of the effect of bit weight and rotary speed on the penetration rate was carried out, at simulated borehole conditions, while following the effect of wear on the performance of the diamond bit. A microbit drilling appartus was used, which enabled the control of rotary speed, weight on bit, circulation rate, hydrostatic pressure and overburden and formation fluid pressures. A 500 psi hydrostatic pressure and an atmospheric formation pressure were held constant since the only pressure that substantially affects drilling rate is the differential between the hydrostatic and formation pressures. Since it has been found that "confining pressure has no systematic influence on drilling rate" it was controlled at 500 psi in order to decrease equipment problems. Quarried samples of Georgia granite and Beekmantown dolomite with respective grain densities of 2.64 and 2.72 gm/cc and compressive strengths of 17,000 and 24,200 Ib/ sq in. were drilled. The very low permeabilities of the samples enabled the use of water as a drilling fluid. A circulation rate of 7.3 gal/min was maintained throughout the tests. A 1.25-in., surface-set, spiral-type diamond drill bit was used. The bit consists of 14.75 carats of 219 diamonds, giving an average diamond size of 15 diamonds per carat.