Data from adjacent wells will be useful in predicting borehole stability problems that can occur in troublesome formations (eg unstable shales, highly permeable zones, lost circulation, overpressured zones)
Shale instability is one of the most common problems in drilling operations. This instability may be caused by either one or both of the following two mechanisms:
1. The pressure differential between the bottomhole pressure in the borehole and the pore pressures in the shales and/or,
2. Hydration of the clay within the shale by mud filtrate containing water.
The instability caused by the pressure differential between the borehole and the pore pressure can be overcome by increasing the mudweight. The hydration of the clays can only be overcome by using non water-based muds, or partially addressed by treating the mud with chemicals which will reduce the ability of the water in the mud to hydrate the clays in the formation. These muds are known as inhibited muds.