Quantitative Prediction of Rock Drillability in Various Drilling Directions in Shale Formation

Abstract

As a typical bedding formation, the rock mechanical properties of shale are strongly anisotropic. Quantitative evaluation of shale formation drillability in various directions is essential for drilling bit selection, drilling parameters optimization and directional well trajectory design. Based on drillability testing device and rock acoustic velocity measurement apparatus, we carried out drillability anisotropy and acoustic characteristic experiments in various drilling directions. The experimental results showed that 1) shale formations have strong drillability anisotropy and acoustic anisotropy characteristics; 2) the drillability at perpendicular to the bedding plane direction is better than that parallel to the bedding plane; 3) the acoustic travel time at perpendicular to the bedding plane direction is larger than that parallel to the bedding plane and the peak value appears at 60° to the bedding plane. Mathematical statistics methods have been applied to established a shale formation drillability forecasting model at various drilling directions, with an average relative error as low as 4.9%. This model provides an important approach to quantitative assess shale formation drillability in various drilling directions using logging data, and has vital importance for the efficient development of shale gas.