Investigation of coupled chemo-hydro-mechanical processes with discrete element modeling
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Geological storage of CO₂ is proposed as a near-term economically viable approach to mitigate CO₂ emissions, and is an example of the coupled chemo-hydro-mechanical processes. Although CO₂ injection and enhanced oil recovery are viewed as mature technologies in the oil and gas industry, investigation of all possible implications is necessary for secure and effective long-term CO₂ storage. The injection of a large volume of CO₂ into target storage formations is usually associated with a number of geomechanical processes that are initiated at the pore scale. Therefore, a pore-scale geomechanical model, i.e. Discrete Element Method (DEM), is of great importance to better understand the underlying pore-scale processes and mechanisms that govern the large-scale CO₂ geological storage. In this work, we concentrate on several significant pore-scale coupled phenomena. Firstly, CO₂ injection into geological formations involves chemo-mechanical processes and shifts the geochemical equilibrium between the minerals and resident brine, which subsequently induces mineral-brine-CO₂ reactions and affects CO₂ storage mechanical integrity. We utilize a numerical model that couples the Discrete Element Method (DEM) and the Bonded-Particle Model (BPM) to perform simulations on synthetic rocks that mimic tested rock samples. Numerical results, in agreement with experimental evidence, show that both cement and particle dissolution significantly contribute to rock weakening in sandstones with carbonate/hematite cements and pore-filling carbonate. Secondly, reservoir compaction involves hydro-mechanical processes that induce changes in porosity and permeability, and is a significant concern for the oil and gas production. We develop a grain crushing model based on the DEM to investigate the changes in porosity and permeability under the reservoir stress path. Grain crushing is shown to be the dominant mechanism for significant changes in porosity and permeability under a high effective stress. Samples consisting of large and soft grains tend to be more readily compacted. Finally, fluid injection in the subsurface may induce fractures and is another common hydro-mechanical process. We couple the Discrete Element Method (DEM) to solve for the mechanics of a solid granular medium and the Computational Fluid Dynamics (CFD) to model fluid flow and drag forces. We validate the resolved CFD-DEM numerical model against experiments from the literature and investigate the impact of physical properties and injection parameters. This work reveals how the pore-scale processes contribute to fluid-driven fracture initiation.