Standard drillability test is carried out at atmosphere pressure without considering the effect of confining pressures. The test results mainly reflect the intrinsic characters of rock. Downhole rock is under different stresses and these stresses influence the rock drillability a lot. Based on analysis of rock drillability test

the invasion depth of tooth is an important influential factor on rock drillability. A model for calculation invasion depth is presented considering both rock features and confining pressures

based on Mohr-Coulomb strength criterion. Through comparing rock drillability tested with invasion depth calculation

a close relationship can be built between invasion depth and drillability. Since the invasion depth formula includes the influences of rock features and confining pressures

the rock drillabilty under real drilling condition can be predicted by calculating invasion depth.