Microstructure-based modelling of hydraulic fracturing in silicified metamorphic rock using the cohesive element method

Abstract

Hydraulic fracturing offers new opportunities for unconventional mining such as in-situ recovery (ISR) in impermeable ore deposits. Implementing the hydraulic fracturing design requires a robust modelling technique to accommodate the complex condition underground. Most numerical studies assume homogeneous material properties. In reality, the orebody is strongly heterogeneous and irregular in shape. To better understand the effect of heterogeneity on the propagation of fluid-driven fractures within an irregular ore-preserved metamorphic rock under in-situ stress, we extend the prevailing modelling efforts and propose a novel 2D finite element modelling framework, namely: (i) modified Voronoi tessellation for explicitly considering the microstructure of the rock in the model, in which the heterogeneous microstructure is reconstructed by digital image processing (DIP) of computed tomography (CT) images; (ii) Gaussian random distribution for assigning the distinct material properties, i.e., mineral and matrix within the rock; (iii) cohesive element method (CEM) for modelling the propagation criteria of hydraulic fractures. Results demonstrate that heterogeneity significantly plays a role in both breakdown pressure and hydraulic fracture propagation. The breakdown pressure estimated from the proposed framework is in excellent agreement with the one measured in the experiment. The simulated trajectory of hydraulic fracture is also consistent with the one obtained from laboratory experiments. These observations reveal that the newly proposed finite element modelling framework can be employed for numerical studies of fluid-driven fracture in any configuration of the heterogeneous orebody.