Modelling Faults as Fracture Swarms to Introduce Multi-Scale Heterogeneity

Faults modify the geomechanical and hydrogeological properties of their host rock. Consequently, proper characterization of faults is relevant to a wide range of sectors including energy, storage and sequestration, mining and geotechnical. Although faults may be conceptualized as simple surfaces when considered at their full scale, a detailed view reveals a heterogeneous structure with damage zones and anastomosing fault cores. Consequently, simulations of whole fault systems can achieve reasonable results by modelling the faults as simple surfaces, but where model applications require a higher resolution another approach is required to introduce out-of-plane heterogeneities. This paper discusses two methods that achieve this by simulating faults as swarms of fractures located around midplanes, including a novel method that can readily scale to very large networks. Crucially, both methods can use midplanes generated as large-scale discrete fracture networks (DFNs) to produce meter scale DFNs with aspects controlled by kilometer scale features. This offers two benefits: a single model of the fault system can be applied across a wider range of scales, and the intensity distribution of the DFNs better simulate naturally occurring fracture networks. Both geomechanical and hydrogeological models can benefit from this approach. 1. INTRODUCTION The impact of faults on the geomechanical and hydrogeological properties of their host rock is a widely studied topic, with implications for a wide range of sectors including energy, storage and sequestration, mining and civil engineering (e.g. Fossen, 2010). As numerical modelling continues to become a more prevalent tool for understanding the subsurface, the numerical characterization of faults becomes increasingly important. Although faults may be considered near planar features without a discrete thickness when conceptualized at their full scale (a concept subsequently referred to as a fault midplane), a view of a fault at a smaller scale reveals a heterogeneous internal structure, both in the local geometry of the features comprising the fault core and the geomechanical and hydrogeological properties that arise (Aaltonen et al., 2016; Caine et al., 1996; Choi et al., 2016; de Joussineau, 2023; Faulkner et al., 2008; Fossen, 2010; Gabrielsen & Braathen, 2014; Kim et al., 2004; Nordbäck & Mattila, 2018; Wang et al., 2023). Consequently, simulations of fault systems at a scale that encompasses whole faults can achieve reasonable results by modelling the faults as simple surfaces. However, if the model also needs to simulate faults at a meter scale, a single planar surface may not adequately represent observed geological features such as damage zones and anastomosing fault cores.