Drilling with a bottomhole pressure less than the formation pore pressure (Underbalanced Drilling, UBD) usually increase the risk of borehole instability due to yielding or failure of the rock adjacent to the borehole. But evaluation criterion of this failure mechanism is complex and very often diagnosis did not fit with field operational practices. It is believed that, shear and radial tensile failures with negligible chemical instability are the common mechanisms to cause mechanical instability in UBD in shales. In a mechanical borehole instability perspective, UBD causes lots of potential challenges and formation uncertainities due to facts like:
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