Evolution of permeability in heterogeneous granular aggregates during chemical compaction: Granular mechanics models

Abstract

We develop a granular mechanics model to represent the process of chemical compaction within granular media. This model represents the serial processes of stress‐enhanced dissolution, mass diffusion along the fluid film separating grain boundaries, and then mass ejection into the pore space, where it may then be either reprecipitated onto the pore wall or removed by fluid advection in an open system. This process is controlled by the evolution of intergranular effective stress and mass concentrations in the fluid either in the water film or the pore space. The evolution of intergranular stress is followed by a granular mechanics model that rigorously couples the magnitude of the grain‐grain contact stress to determine the time history of stress‐driven dissolution. Pore fluid concentration provides a feedback to intergranular dissolution, halting intergranular compaction as fluid concentrations approach aqueous saturation. Importantly, chemical feedbacks onto the mechanical system and mechanical feedbacks on the chemical system are rigorously accommodated. Compaction is halted either by the amelioration of the driving stress (through the growth of the contact area) or by the saturation of the fluid in the pore space. This model is used to explore the influence of heterogeneous assemblages of particles on the rate and ultimate magnitude of compaction and the resulting evolution of permeability. Specifically, we explore the influence of heterogeneity in granular packs by using heterogeneous distributions of particles (linear, Gaussian, or bimodal). Results indicate that porosity and permeability decrease with compaction. Small particles dominate this dissolution‐mediated process and accelerate compaction; this is true both for homogeneous distributions of small particles and for coarser aggregates containing a fraction of smaller particles. Overall, compaction is also greater for finer aggregates resulting from the larger deformation required, reaching the critical stress that will halt compaction. This feature is due to the necessary redistribution of intergranular stresses that are shed from point contacts in active chemical dissolution onto those not deforming.