Bottomhole Stress Factors Affecting Drilling Rate at Depth

Abstract

The mean formation stress near the bottom of a borehole is reduced by strain relaxation when a well is drilled. This causes a PV increase that can significantly reduce the local pore pressure of impermeable rocks, such as shales, but does not affect the pore pressure of permeable rocks. Since the penetration rate is strongly affected by the difference between the local pore pressure and the borehole fluid pressure, impermeable formations drill slower than adjacent permeable formations.