In fractured formations, increasing mud weight can also cause a wellbore to collapse

Lee et al. (2012) developed an anisotropic wellbore stability model accounting for two types of shear failures: one within intact rocks and the other along weak planes. Their model assumes fluid pressure in weak planes remains at formation pressure even after the wellbore is drilled, and they found that increasing mud weight can help prevent rock collapse along weak planes. However, their findings contradict our drilling experiences in fractured formations, where increasing mud weight beyond a certain point can actually exacerbate wellbore collapse. The cavings have a platy/tabular shape, indicating they have fallen from natural fractures. Most incidents were accompanied by mud losses, and image log data reveal that the formation contains natural fractures with large openings and high dip angles. This evidence indicates that mud invasion into the weak plane is likely to occur, which can cause the fluid pressure therein to deviate from the formation pressure. Consequently, the assumption made by Lee et al. (2012) does not hold. Driven by the observations, we modified the strength criterion in Lee et al. (2012) by incorporating a parameter y to consider the effect of pressurizing a weak plane due to mud invasion. Our results suggest that the concept of lower bound mud weight for preventing wellbore collapse is not applicable when weak planes are permeable. Instead, there exists an optimal value that minimizes the shear failure: reducing mud weight below this optimum can reduce mechanical support for the rock and thus lead to collapse, while increasing mud weight can also be problematic because it promotes pressure diffusion into the weak plane, reducing effective normal stress, and ultimately causing rock slippage failure. These findings match closely with our own and a series of global onsite observations, based on which we propose practical strategies to optimize mud weight. In addition to shear failure, we also evaluated tensile failure to further illustrate the risks associated with increasing mud weight in fractured formations.