A multiscale poroelastic damage model for fracturing in permeable rocks

A new poroelastic damage model is developed in the paper to describe the macroscopic failure of rock materials due to microcrack nucleation and propagation based on a multiscale framework. The model is deduced from locally periodic microstructure with dynamically evolved microcracks in heterogeneous rock body. The homogenization method based on asymptotic expansions gives rise to the damage evolution law coupled with the poroelastic fracture system, which includes the fracture opening induced permeability change. The obtained model takes into account the complex coupling between fluid pressure-deformation and hydro-mechanical (HM) properties at the microscale, leading to the nonlinear anisotropic mechanical behavior, degradation of both elastic stiffness and poroelastic properties at the macroscale, which is fundamental to describe the complex fracturing behavior influenced by microcrack distribution. The homogenized coefficients are illustrated in detail for a given set of initial material parameters, with dependence on the normalized damage variable. Results of numerical simulations well reproduce specific experimental observations where fracturing in heterogeneous rocks is shown to be a multiscale phenomenon that initiates from the microcrack nucleation and propagation, while the fracture propagation direction is shown to be influenced by both external loadings and microcrack distribution. The easy implementation in finite element framework and revealed micro-mechanism for macroscopic failure under strict mathematical formulations make the wide application of model in rock mechanics problems.