A Study on the Influence of Core Stability on the Coring Process of Long-Barrel Coring Tools

In the process of long-barrel coring, the improper selection of operating parameters can easily cause blocked deformation, violent vibration of the core, core fracture, and impact crushing, which lead to a reduction in the stability of the core and core harvesting rates. Accurate knowledge of the influence of relevant factors on core stability is the key to improving core harvesting rates. Therefore, in this study, a numerical calculation model for tight and fractured cores in a barrel was constructed based on the Drucker–Prager criterion, using the finite element method. A numerical calculation model of a core broken into a barrel was constructed using the discrete element method. A study was conducted on the influence law of core stability under different core lengths, rotational speeds, weights on bit, and well inclination angles. The influence of each factor on core stability was analysed based on the vibration displacement and stress distribution characteristics of the core. The calculations show that increasing the weight on bit and reducing the rotation speed can effectively reduce the radial vibration displacement and local stress in tight and fractured cores, reduce the possibility of core fracture or breakage, and improve core stability. When the well inclination angle is large, it can easily cause core deformation and wall sliding, generating large contact stress and radial vibration displacement, significantly reducing the core stability. A broken core has the worst stability and is easily compacted in the core barrel, producing secondary crushing and plugging effects. Increasing the core barrel length resulted in a more unstable core. Compared with single-barrel coring, the distortion of the core column under double-barrel coring was more evident. In addition, the coring process, cuttings distribution, and drill bit hydraulic characteristics were studied based on the CFD-DPM method. The conclusions of this study are of great significance for optimising coring operation parameters to further improve core stability and coring harvest rate.