Mechanisms of borehole instability in heavily fractured rock media

Whereas borehole stability in continuous media has been extensively studied, little attention has been paid to what happens in the case of fractured formations. This paper presents some field evidence gathered during several attempts to drill at great depth through heavily fractured-layers of basalt and tuff. Discrete element modelling is then used to model the field conditions and to identify the mechanisms behind the instabilities experienced. It is shown that these instabilities probably resulted from the opening of joints communicating with the borehole and the subsequent invasion of a limited region of the fracture network by the drilling fluid. The consequent loosening of the corresponding blocks made it possible for the drilling fluid circulating in the well to erode them from the wall. It is shown and corroborated by the field data, that a mud density increase can play a major destabilizing role insofar as it contributes to the opening of radial cracks and hence favours the invasion of the network of open fractures by the drilling fluid.