Modelling of Fragmentation Process in Deformation Bands and Faults

Fragmentation in deformation bands and faults is relatively complex with multiple fracture sets breaking single grains into anisotropic splinters. An understanding of the development of deformation bands and faults and the associated mechanical and permeability evolution is important for many applications including treatment of faults in reservoir models, flow properties of faults in geothermal systems and CCS (Carbon Capture and Storage), and seismic hazards. Various mathematical models have been developed to capture rock fragmentation process at single and multi-grain scale as well as to study the hydraulics in large scale geological discontinuities. However a multi-scale approach is needed to understand the implications of the changes in mechanical properties and permeability due to small scale rock fragmentation on the large scale bands, faults and fractures. This study aims to employ an extended Discrete Element Method (DEM) approach with multi-scale aggregates to model the evolution of deformation bands in porous sandstones.

Grain failure in rocks can be caused by different loading conditions, such as compressive loading, shear displacement, thermal, hydraulic or chemical effects. In this study, simulation of the comminution  process of the polyhedral shaped grains is achieved under compressive and shear loading. Single grain fragmentation is realised with the Mohr-Coulomb approach, which is a classical failure criterion for brittle particle systems. The failure criterion is combined with the Weibull statistical distribution that captures the grain size effect. A multiple grain model is simulated with DEM combined with the Mohr-Coulomb-Weibull concept. The computations are carried out with an open-source discrete numerical modelling framework YADE. The study sheds light on the influence of initial rock properties (porosity, grain size and shape) and deformation mechanism (compaction, shearing and combinations) on the development of deformation bands and faults.