Abstract：Abstract：The evaluation of drillability anisotropy of the formation is very important for well trajectory control. The velocities of ultrasonic wave and rock drillability are measured in perpendicular and parallel directions of the bedding plane of core samples which were cut from the approach hole of Chinese continental scientific drilling. Rock drillability index and P-wave velocity anisotropy coefficient are calculated from the testing data；and the results are discussed. Correlation of drillability anisotropy index of the roller cone bit and PDC bit，the difference between drillability of the roller cone bit in perpendicular and horizontal directions of the bedding plane with the P-wave velocity anisotropy coefficient are studied respectively by means of mathematical statistics. A mathematical model is established for predicting drillability anisotropy index by using P-wave velocity anisotropy coefficient. Thus，drillability anisotropy of a formation can be evaluated conveniently from seismic or logging data by using this model. The method is used to evaluate drillability anisotropy of a formation of Well AN2004 in Henan Oilfield in China. The result shows that the formation drillability anisotropy index predicted by using this method can properly reflect the deviation characteristics of the formation being evaluated.
