Photoelastic observation of toughness-dominant hydraulic fracture propagation across an orthogonal discontinuity in soft, viscoelastic layered formations

Abstract

Hydraulic fracture (HF) unavoidably encounters natural pre-existing discontinuities in geologic formations, which complicates the propagation and containment behavior across a discontinuity. This study explored the HF propagation across an orthogonal discontinuity in layered formations by exploiting the photoelastic, transparent, soft (or deformable), and viscoelastic characteristics of gelatin. First, photoelastic HF experiments in homogeneous gelatin media were carried out while monitoring the fluid pressure and stress intensity factor (SIF). The SIF was observed to stay constant during steady-state HF propagation. Second, photoelastic HF experiments in layered gelatin media were conducted, in which biwing-type fractures encountered the bounding layers with different levels of stiffness. Two contrasting containment behaviors - crossing or dilation/arrested - were observed. The fracture crossed the discontinuity when it encountered the bounding layer with a lower toughness. By contrast, the fracture was arrested by the bounding layer and/or dilated the discontinuity, propagating along the discontinuity when the bounding layer had a greater toughness. In addition, competition between debonding of the layer interface and creating a fresh fracture in the bounding layer was found to play a significant role in fracture containment behavior. This study provides unique experimental data with photoelastic images that are comparable to analytical or numerical models. Furthermore, the results contribute to a better understanding of HF propagation and containment behaviors across a discontinuity in viscoelastic media.