Wellbore Mechanics and Stability in Shale

Wellbore instability is the major cause of nonproductive time and increased well cost in oil and gas drilling. Most wellbore stability problems occur in shale where the poroelastic effective stress, together with chemical and electrokinetic potential gradients in the rock pore space, enhances the rock failure mechanisms. The described processes become more complex when the thermal gradients between the wellbore and subsurface induce thermal stresses within the rock. Furthermore, shale often exhibits variation in strength properties along and across the bedding planes. The porous structure of shale includes a system of multiple-porosity networks. The contrast between the mechanical properties and flow conductivity of these networks causes the dual-pore pressure and dual-effective stress behavior in shale. The described aspects of wellbore stability in shale are reviewed. The dual-porosity, dual-permeability poroelasticity, together with bedding plane strength properties, as well as chemical and thermal gradient effects are incorporated into the wellbore stability model through a bottom-up and step-by-step approach. A field case study is selected to illustrate these effects and their interplay. It is shown that the time-dependent margins of safe mud weight window of drilling may be fine-tuned when the contribution of each factor is superposed on the overall wellbore stress solution.