ABSTRACT:
Drilling rates decrease continuously with depth in fluid-filled boreholes because of the confining effect of borehole pressure. Impulsive depressurization of the borehole can overcome the confining pressure and in-duce effective tensile stress at the cutting face. Pore pressure diffusion limits the magnitude of the effective tensile stress pulse. Suction pressure pulses can be generated during rotary drilling by a flow cycling valve that interrupts the flow of drilling mud downstream of the bit. A series of small-scale pressure drilling experiments were carried out using a prototype flow cycling valve capable of generating intense suction pres-sure pulses. Tests were carried out in shale and sandstone with a drag bit and in granite with an indentation bit. Suction pressure pulses with amplitude comparable to the rock tensile strength, cause a large increase in drilling rates in shale; and significant increases in drilling rates in sandstone and granite. The results reflect a combination of reduced pressure and hydraulic impulsive loading effects.
