Formation mechanism of core discing during drilling under deep in-situ stress environment: Numerical simulation and laboratory testing

Core discing often occurs during drilling under deep in-situ stress environment. To determine its formation mechanism in sandstone under deep in-situ stress environment, PFC2D was used to study the crack distribution and energy evolution during drilling under different in-situ stress, and specific in-situ stress conditions prone to core discing were obtained. An independently developed testing system was utilized to verify the stress conditions required for core discing in laboratory settings, and to analyze the relationship between the failure and fracture surface morphology characteristics of the core and in-situ stress. The results show that the higher the in-situ stress, the more tensile cracks will be generated in the rock during drilling, especially at the hole wall and the root of the core. The cracks in the core develop from the outside surface inward. Higher in-situ stress levels also result in greater energy transformation, leading to fracture of the rock. The formation of core discing requires specific stress conditions. Core discing occurs at the root of the core when the radial stress (sigma(r)) is the maximum principal stress at a constant value of 45 MPa and the axial stress (sigma(a)) is either 25 MPa or 30 MPa. When the difference between sigma r and sigma a increases or the drilling depth increases, the disc thickness decreases, resulting in smoother fracture surfaces and smaller fractal dimensions and thus more pronounced core discing. This study can provide technical and data support for scientifically elucidating the formation mechanism of core discing under deep in-situ stress conditions.