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由于地层的各向异性，在钻定向井时，沿钻头瞬时作用力方向的岩石可钻性是变化的。为了确定沿钻头瞬时作用力方向的岩石可钻性，作者将钻头的瞬时合力分解成钻压、变井斜力和变方位力３个分力。通过地层与钻头三维力的空间几何关系，推导出了任意井斜角和方位角时钻压、变井斜力和变方位力方向与边层内法线的夹角公式。在假设均匀层状地层内岩石可钻性级值呈椭球型分布的基础上，通过求解岩石可钻性级值椭球方程与三维力方向之一的直线方程所构成的方程组，得到了沿钻头瞬时三维力方向的岩石可钻性级值的计算表达式。结果可用来定量地分析地层因素对井眼轨迹的影响，对于深入研究钻头－地层相互作用关系具有重要意义。
Due to the anisotropy of the formation, the drillability of the rock along the instantaneous force of the drill bit varies when drilling the well. In order to determine the rock drillability in the direction of the instantaneous force of the drill bit, the author decomposed the instantaneous resultant force of the drill bit into three components: weight-on-bit, variable well inclination and variable azimuth. Based on the spatial relationship between the formation and the three - dimensional force of drill bit, the formulas of the angle between the direction of weight - shifting, variable well inclination and variable azimuth and the normal in the boundary layer are deduced. Based on the assumption that the values of drillability of rock formations in uniform stratigraphic layer are ellipsoidal, the equations formed by solving the linear equation of one of the directions of ellipsoid of rock drillability and the direction of three-dimensional force are obtained. Calculation expression of rock drillability grade along the instantaneous three-dimensional force of the drill bit. The results can be used to quantitatively analyze the influence of formation factors on the trajectory of the wellbore, which is of great significance for further studying the bit-formation interaction.