The Role of Particle Packing in Modeling Rock Mechanical Behavior using Discrete Elements

Introduction Both particle packing and particle contact parameters play important roles in determining the mechanical behavior of a discrete particle assembly. One common approach to calibrate discrete element models (DEM) of rock is to hold particle packing constant while contact parameters are changed to match observed macroscopic mechanical behavior. This often leads to non-unique material calibrations. In other words, the mechanical properties of the DEM assembly depend on the packing method. Another problem with this technique is that the parameters that describe contact interactions of particles are not directly observable in the laboratory. An opposite approach is to suggest that since we can actually observe rock texture and packing in rocks, why not mimic this in the model and keep contact parameters constant? The best method probably lies somewhere between these two end-members as suggested by mineralogical observations (i.e. not all grains have the same mechanical properties) and textural observations (i.e. our inability to capture minute details of grain shapes) of rocks (Kranz, 1983). In this context, the purpose of this paper is to gain an understanding of how different particle packing and textures influence the mechanical response of discrete assemblies. One of our hypotheses is that a more physically based calibration of a material may ultimately result in a more unique set of contact parameters. We performed numerical experiments using (1) varying clusters made up of a finite number of particles to represent unique shapes, and (2) using two different assembly generation algorithms to pack them. These issues of the competing roles of particle packing and contact parameters become even more important when modeling coupled processes such as fluid flow and mechanical behavior. This is because of the obvious dependence of fluid flow on the material packing geometry.