Wellbores drilled for hydrocarbon exploration and development purposes usually penetrate at least one shale formation. Because shales are mechanically weak, they commonly fail under the influence of the relatively high stresses acting around the wellbore. Often, failure involves the development of an annular zone of yielded (weakened) rock around the wellbore. If this zone becomes large, the wellbore might collapse [i.e., become unstable). In order to combat wellbore instability, it is common to use high-densitv drilling muds that impart a support pressure on the wellbore wall. Although this may be effective initially, the high fluid pressures in the wellbore will eventually penetrate the surrounding fonnation, altering the state of effective stress and causing the yielded zone to grow.

Mathematical models that predict the initial and final extent of the yielded zone around a wellbore in an elastic-brittle-plastic material have been derived previously. This research involves the extension of such models, so that they can predict the evolution of the yielded zone with time as it grows in response to transient pore pressures. The first model developed is applicable in situations where the fluid transport properties of the shale are unchanged as it yields. The second model developed is applicable in situations where the shale penneabilily is significantly increased as it yields. For the latter model, an existing numerical technique is modified to solve the coupled diffusion equation in a domain with a moving boundary. Both models are also capable of including the effects of time-dependent strength parameters (cohesion) on the extent of the yielded zone.

In order to demonstrate the usefulness of these models in practice, a series of sensitivity analyses were performed using mechanical parameters for Blackstone Formation shale, a troublesome rock unit in wells drilled in the Foothills of the ( ‘anadian Rockies. An essential - and previously unknown - parameter for these analyses is the permeability of this rock, which was measured using a pulse-deeay penneability testing apparatus designed and built as part of this research.