Time-dependent borehole stability in hard-brittle shale

Rock damage appears in brittle shale even prior to peak stress (i.e., before failure) due to the occurrence of microcracks in these rocks. In this work, a coupled hydromechanical model was built by incorporating the mechanical and fluid seepage induced stresses around a wellbore during drilling. The borehole instability mechanism of hard-brittle shale was studied. The results show that even if a well is simply drilled into a hard-brittle shale formation, the formation around the borehole can be subjected to rock damage. The maximum failure ratio of the formation around the borehole increases with drilling time. A lower drilling fluid density corresponds to a faster increase in the failure ratio of the borehole with time and a shorter period of borehole collapse. When the initial drilling fluid density is too low, serious rock damage occurs in the formation around the borehole. Even though a high-density drilling fluid is used after drilling, long-term borehole stability is difficult to maintain. While drilling in hard-brittle shale, drilling fluid with a proper density should be used rather than increasing the density of the drilling fluid only after borehole collapse occurs, which is more favorable for maintaining long-term borehole stability.