Borehole Stability Analysis in Fractured Porous Media Associated with Elastoplastic Damage Response

Abstract

A dual-porosity finite-element model is presented for stability analysis of boreholes drilled in fractured porous media with the consideration of elastoplastic damage response. Governing differential equations adopted in this study can account for the elastoplastic deformation and continuum damage. The approximate solution for the governing differential equations was obtained numerically by applying FEM. Performance of the numerical model was evaluated through several published numerical/analytical examples. Simulation results demonstrate that the FEM model is robust and is capable of capturing essential issues for borehole stability analysis, such as responses of effective stress, damage evolution, fracture propagation, and stability analysis for boreholes under different conditions. Numerical results indicate that borehole failure starts with the initiation of microcracks (damage occurs), progresses with further extension of fractures (damage zone), and is accompanied by redistribution of fluid pressures in dual porosity and variation of stress around boreholes. Also, a more deformable geoformation may be characterized by more damage developing in the vicinity of the wellbore wall, indicating that elastic analysis gives an underestimation in terms of effective stress and an overestimation in terms of fluid pressure responses. In the case of borehole stability under hydraulic injection, this study produced a smeared damage zone (equivalent to fracture zone) instead of a single fracture along a specific direction, which manifests as a shorter damage zone.