The role of chemical compaction in the evolution of permeability and strength in granular aggregates

Dissolution and precipitation of mineral constituents may have a significant influence on the evolution of the mechanical and transport properties of granular aggregates pushed far from equilibrium. They are major porosity-altering processes that operate in many sedimentary rocks. In addition, they may control the build-up and release of fluid pressures in sedimentary basins and along fault zones. Understanding these physicochemical processes is critical in determining the diagenetic and deformational history of rocks and their potential as hydrocarbon reservoirs. Grain intergrowths accelerated by chemical and stress effects increase compaction, reduce porosity and permeability and may augment strength and stiffness. Precipitated mineral matter may similarly occlude pore throats and alter the capillary and permeability of the aggregate. In this work, we explore the magnitude of these effects by representing grain-grain bonding and intergrowth during compaction and fluid circulation using a granular mechanics model (PFC). Grain intergrowth is accommodated by effective dissolution at grain contact points: temperature and local stress control dissolution rate relative to a critical stress that initiates dissolution. Grain-grain contacts are represented by contact stiffness in parallel with a variable rate damping connection to represent creep intergrowth effects. The redistribution of mineral matter is accommodated by diffusion and subsequent precipitation. Diffusion transports dissolved mater from the interface to the pore space, and then precipitates mineral at the less-stressed surface of the grains. Precipitation rate is indexed through aqueous concentration relative to equilibrium concentration through a rate constant.