STUDY AND PRACTICE OF THE BOTTOM-HOLE UNLOADING EFFECT ON ROCK DRILLABILITY

Rock drillability of rock samples was studied by means of experiment under zero pore pressure and simulated down hole conditions. The results of drillability grade shows a bow-shaped curve the unloading force rising as increasing the well depth. The magnitude of drillability grade experence four stages which are increasing lightly, keeping constant, decrease slightly and decreasing rapidly until the samples are unstable at the bottom of hole finally. This is a response of the bottom-hole unloading effect on rock drillability which is a new concept of deep well drillability. The significant benefit have been gotten by using this ideal mentioned above in bit selecting in Tarim oil field.