Study on the dynamic mechanism of wellbore instability in deep fractured formations

Due to the presence of highly discrete rock blocks and extensively developed natural fractures in fractured formations, frequent losses of drilling fluid and wellbore instability incidents occur during the drilling process. Meanwhile, for some extremely fractured formations, even after increasing the density of the drilling fluid, the wellbore instability situation shows no obvious improvement, and even the wellbore collapse becomes more severe. Regarding the highly discrete rock blocks in fractured formations, current research mainly focuses on using the discrete element method (DEM) to reveal the mechanism of wellbore instability in fractured formations. However, the DEM emphasizes the representation of the contact behavior of rock blocks and has difficulty effectively representing the multi-field coupling behavior during the actual drilling process in fractured formations. Therefore, in this paper, a combined modeling method of discrete element method (DEM) + finite element method (FEM) is adopted, comprehensively considering the dual-medium seepage effect of fractures and matrix pores, to conduct numerical simulation studies on the wellbore instability after drilling formations with different degrees of fragmentation. The research indicates that as the degree of fragmentation of formation increases, the situation of wellbore instability becomes more severe. Meanwhile, when the degree of fragmentation of formation is low and the occurrence of natural fractures is within the risk range of wellbore instability, due to the large size of discrete rock blocks in the formation, the wellbore instability mainly stems from the secondary fracturing within the discrete rock blocks; while when the degree of fragmentation of formation is high, the initiation of fractures around the wellbore mainly depends on the opening of natural fractures, thereby causing a large amount of shedding of discrete rock blocks and subsequently leading to wellbore instability. Increasing the bottomhole pressure can effectively inhibit the secondary fracturing of discrete rock blocks. For formations with a low degree of fragmentation, the wellbore instability is mainly caused by the secondary fracturing of discrete rock blocks. Thus, increasing the bottomhole pressure can effectively reduce the risk of wellbore instability; while for highly fractured formations, the wellbore instability is mainly caused by the shedding of discrete rock blocks triggered by the opening of natural fractures in the formation. Therefore, increasing the bottomhole pressure cannot effectively reduce the risk of wellbore instability. In terms of drilling fluid loss, the opening of natural fractures is more conducive to the expansion of the range of drilling fluid loss. Therefore, for highly fractured formations, increasing the bottomhole pressure not only fails to effectively reduce the risk of wellbore instability but also exacerbates the situation of drilling fluid loss. By analyzing the influence of the ratio of wellbore size to the size of fractured blocks on wellbore instability, it is concluded that for fractured formations, when the wellbore trajectory cannot avoid it, a small wellbore size should be adopted as much as possible.