Research progress of wellbore stability in hard brittle shale

Wellbore instability in hard brittle shale is a key problem restricting the effective development of deep resources and unconventional resources (e.g. shale gas). In this regard, this paper presents the basic analysis methods of wellbore stability under three scenarios (i.e. mechanics, stress-seepage coupling and seepage-stress-chemical coupling) and their application conditions, with consideration to the features, composition and structure of hard brittle shale. This paper also discusses four mechanical instability mechanisms of hard brittle shale, the driving action for drilling fluid invading into fractures, the strength weakening mechanism with and without active clay mineral, and the calculation model for wellbore stability with different forms of fractures. It is concluded that the key factors influencing wellbore stability analysis are the existence of active clay minerals in hard brittle shale matrix and fracture fillings and the fracture distribution laws. Microscopic description of the progressive wellbore instability process which is induced by propagation and connection of fractures in hard brittle shale under the action of multiple-field coupling will become a hotspot and difficulty in future research.