Bit - Tooth Penetration Under Simulated Borehole Conditions

Abstract

A study of bit-tooth penetration, or crater formation, under simulated borehole conditions has been made. Pressure conditions existing when drilling with air, water and mud have been simulated for depths of 0 to 20,000 ft.These crater tests showed that a threshold bit-tooth force must be exceeded before a crater is formed. This threshold force increased with both tooth dullness and differential pressure between the borehole and formation fluids.At low differential pressures, the craters formed in a brittle manner and the cuttings were easily removed. At high differential pressures, the cuttings were firmly held in the craters and the craters were formed by a pseudoplastic mechanism.With constant force of 6,500 lb applied to the bit teeth, an increase in differential pressure (simulated mud drilling) from 0 to 5,000 psi reduced the crater volumes by 90 per cent. A comparable increase in hydrostatic fluid pressure (simulated water drilling) produced only a 50 per cent decrease in volume while changes in overburden pressure (simulated air drilling) had no detectable effect on crater volume.Crater tests in unconsolidated. sand subjected to differential pressure showed that high friction was present in the sand at high pressures. Similar friction between the cuttings in craters produces the transition from brittle to pseudoplastic craters.