A comparative experimental study of shale indentation fragmentation mechanism at the macroscale and mesoscale

Abstract

Low drilling rate is one of the key difficulties in shale formation, but the shale fracture mechanism during drilling operation is unclear. The shale dynamic fragmentation and crack from the multiscale perspective need to be studied. A multiscale shale indentation experiment was investigated. Comparative analyses of macroscale and mesoscale experimental results were provided. Macroscale shale fragmentation was established on the accumulation of mesoscale fracture. The accumulation of shale mesoscopic crack and breakage develop into macroscale breakage. The typical four stages of mesoscale shale fragmentation were overserved by the indentation test. With increasing loading rate of indenter, indentation displacement of Longmaxi shale decreases and the critical crack load of Longmaxi shale increases. The probability of two-three times macroscale crack for Longmaxi shale is greater than four times. The indentation displacement and critical load have a strong nonlinear relationship. The macroscale statistics of Longmaxi shale hardness and elastic modulus are close to the mesoscale statistics of Longmaxi shale. The elastic modulus of Longmaxi shale is proportional to the loading rate of indenter. The study results are benefit for the optimization of drill rate and drilling tools design.