Drillability characteristics of formation and evaluation of bit selection for the Zhongtaishan structural strata in Sichuan Basin, China

Abstract Selecting a suitable bit for a specific formation is very important in increasing the rate of penetration (ROP) and reducing the drilling cost during the drilling process. Drillability and rock mechanical parameters are frequently used to evaluate the resistance of strata to drilling engineering. Based on the drilling well history data for the Zhongtaishan structural strata in the Sichuan Basin, it can be observed that the slow ROP and high drilling cost have become critical factors in restricting the efficient development of tight gas in this tectonic region. Furthermore, a new model for the rock drillability index was established based on the experimental rock drillability data under simulated bottom-hole conditions. Part of the drillability test data was used to verify the accuracy of the model, which can accurately describe the effect of wellbore pressure on drillability. Well history and logging data were also used to calculate the rock mechanics parameters and rock drillability index. In the Xujiahe Formation, characterized by high uniaxial compressive strength (UCS) and drillability index, the Poisson’s ratio of the formation rock was approximately 0.21–0.24, the elastic modulus was 80 GPa–110 GPa, the average value of the internal friction angle increased from 40° to 45°, and cohesive increased from 18 MPa to 36 MPa. The formation drillability index without wellbore pressure ranged from 5–7, whereas that under wellbore pressure conditions ranged from 8–9 with a high Cerchar abrasion index (CAI). Mechanical specific energy (MSE) models were used to optimize the bit quantitatively, and the results demonstrated that the SH633, HJ537GK, I437, and KPMD1642DRT combined drill bits were more suitable for the Da’anzhai, Zhenzhuchong member of Ziliujing Formation, and the 6th, 3rd, and 2nd variables of the Xujiahe Formation respectively.