Mechanisms of Hydraulic Fracture Offset Propagation in Layered Rocks: Insights from a Coupled Damage-hydro-mechanical Phase-Field Model

It has been observed that hydraulic fractures that interact with weak interlayers sometimes deflect and propagate laterally before re-initiating into adjacent layers. Despite being commonly noted in layered formations, this phenomenon, known as offset propagation, remains poorly understood. To bridge this gap, a novel fully coupled damage-hydro-mechanical Phase-Field Model is formulated. The model captures mixed-mode fracture propagation, damage-dependent poroelastic degradation, and anisotropic permeability evolution induced by fracturing. The model is implemented in the open-source finite element framework MOOSE, designed for large-scale multiphysics simulations. To enhance numerical stability and convergence, a multi-app strategy is employed: the hydraulic and mechanical fields are solved in the main application, while the damage field is updated in a sub-application. Both resolution schemes are staggered within each time step, until global convergence is reached. Benchmark tests confirm the model’s ability to reproduce analytical solutions of pressure- and flow-controlled fracture dynamics problems. Numerical simulations reveal that fracture propagation within weak interlayers is governed by shear-dominated failure, while re-initiation into the overlying layer may result from a local competition between fluid pressure evolution and in situ stress contrast near the fracture tip. A comprehensive parametric study identifies the key mechanical and hydraulic factors that control the magnitude of the offset distance and the occurrence of fracture re-initiation. Results provide new insights into the interactions between hydraulic fractures and weak or soft interlayers, and highlight the mechanical and hydraulic conditions that govern offset propagation in layered formations.