Numerical study of the influence of fluid viscosity on wellbore spalling in drained fractured rock

The objective of this work is to model the influence of shear stresses induced by viscous fluid flow on wellbore spalling. We simulated a drop of stress and pore pressure at the wall of a meter-scale borehole with a plane strain Finite Element model. The rock mass was modeled as a jointed continuum. Block sliding was predicted from the tangential displacements in the joint after the shear failure criterion was reached. Simulations show that: (1) Higher far field stresses induce more normal stress in the joints, which prevents the occurrence of shear plastic strains in the joints and reduces block sliding at the wall; (2) Shear stresses and consequent shear plastic strains that are induced by viscous fluid flow in the joints are higher for higher fluid viscosities, and decrease over time as the blocks on each side of the joint slide on each other; (3) In joints that are in contact with the borehole, a change of one order of magnitude in the fluid viscosity results in a change in joint shear stress by a factor of 2. Results suggest that if drainage had been simulated over a longer period of time or for a smaller borehole diameter, the failure criterion would have been reached on a larger zone around the borehole, which could have a critical impact on the risk of borehole spalling. The numerical approach proposed in this work is expected to be useful to recommend wellbore operation modes so as to avoid excessive spalling and clogging.