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Cornell University

Geomechanics for energy and the environment

Finite Element simulation of geological storage

A self-consistent inclusion-matrix model was formulated to homogenize the viscoplastic deformation of halite polycrystals and predict the number of broken grains in a Representative Elementary Volume of salt. This micro-macro modeling framework was used to simulate creep tests under various axial stresses and calculate the critical viscoplastic strain at which grain breakage is expected to occur. We used the critical viscoplastic deformation as a yield criterion to control the transition between secondary and tertiary creep in a phenomenological viscoplastic model implemented in a Finite Element Method program. Simulations of cavern depressurization indicate that a strain-dependent damage evolution law is more suitable than a stress-dependent damage evolution law, because it avoids high damage concentrations and allows capturing the formation of a damaged zone around the cavity.

Next, we simulated carbon dioxide storage in salt rock, assuming constant gas pressure.The initial state is determined by simulating cavity excavation with a Continuum Damage Mechanics (CDM) model.Storage was simulated by using a micro macro healing mechanics model. FEM simulations show that independent of salt diffusion properties, healing is limited by stress redistributions that occur around the cavity during pressure solution. In standard geological storage conditions, the displacements of the cavity occur within the five first days of storage and the damage is reduced by only 2%.

Finite Element simulation of soil formation by granite bedrock weathering

Bedrock weakening is of wide interest because it influences landscape evolution, chemical weathering, and subsurface hydrology. A longstanding hypothesis states that bedrock weakening is driven by chemical weathering of minerals like biotite, which expand as they weather and create stresses sufficient to fracture rock. Here we build on recent advances in rock damage mechanics to develop a model for the influence of multi-mineral chemical weathering on bedrock damage, which is defined as the reduction in bedrock stiffness. We use biotite chemical weathering as an example application of this model to explore how the abundance, aspect ratio, and orientation affect the time-dependent evolution of bedrock damage during biotite chemical weathering. Our simulations suggest that biotite abundance and aspect ratio have a profound effect on the evolution of bedrock damage during biotite chemical weathering. These characteristics exert particularly strong influences on the timing of the onset of damage, which occurs earlier under higher biotite abundances and smaller biotite aspect ratios. Biotite orientation, by contrast, exerts a relatively weak influence on damage. Our simulations further show that damage development is strongly influenced by the boundary conditions, with damage initiating earlier under laterally confined boundaries than under unconfined boundaries. These simulations suggest that relatively minor differences in biotite populations can drive significant differences in the progression of rock weakening. This highlights the need for observations of biotite abundance, aspect ratio, and orientation at the mineral and field scales, and motivates efforts to upscale this microscale model to investigate the evolution of the macroscale fracture network. We are currently coupling a continuum mechanics model of anisotropic damage induced by weathering with a cohesive zone model in order to examine whether or not weathering damage can deviate the trajectory of fractures induced by field stresses.