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ItemAnisotropy and microcrack propagation induced by weathering, regional stresses and topographic stresses(American Geophysical Union., 2022) Xu, Tingting ; Shen, Xianda ; Reed, Miles ; West, Nicole ; Ferrier, Ken L. ; Arson, ChloéThis paper presents a new model for anisotropic damage in bedrock under the combined influences of biotite weathering, regional stresses, and topographic stresses. We used the homogenization theory to calculate the mechanical properties of a rock representative elementary volume made of a homogeneous matrix, biotite inclusions that expand as they weather, and ellipsoidal cracks of various orientations. With this model, we conducted a series of finite element simulations in bedrock under gently rolling topography with two contrasting spatial patterns in biotite weathering rate and a range of biotite orientations. In all simulations, damage is far more sensitive to biotite weathering than to topographic or regional stresses. The spatial gradient of damage follows that of the imposed biotite weathering rate at all times. The direction of micro-cracks tends to align with that of the biotite minerals. Relative to the topographic and regional stresses imparted by the boundary conditions of the model, the stress field after 1,000 years of biotite weathering exhibits higher magnitudes, wider shear stress zones at the feet of hills, more tensile vertical stress below the hilltops, and more compressive horizontal stress concentrated in the valleys. These behaviors are similar in simulations of slowing eroding topography and static topography. Over longer periods of time (500 kyr), the combined effects or weathering and erosion result in horizontal tensile stress under the hills and vertical tensile stress under and in the hills. These simulations illustrate how this model can help elucidate the influence of mineral weathering on Critical Zone evolution.
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ItemMicro-mechanical Modeling for Rate-Dependent Behavior of Salt Rock under Cyclic Loading( 2021) Shen, Xianda ; Ding, Jihui ; Arson, Chloé ; Chester, Judith S. ; Chester, Frederick M.The dependence of rock behavior to the deformation rate is still not well understood. In salt rock, the fundamental mechanisms that drive the accumulation of irreversible deformation, the reduction of stiffness and the development of hysteresis during cyclic loading are usually attributed to intracrystalline plasticity and diffusion. We hypothesize that at low pressure and low temperature, the rate-dependent behavior of salt rock is governed by water-assisted diffusion along grain boundaries. Accordingly, a chemo-mechanical homogenization framework is proposed, in which the Representative Elementary Volume (REV) is viewed as a homogeneous polycrystalline matrix that contains sliding grain-boundary cracks. The slip is related to the mass of salt ions that diffuse along the crack surface. The rate of diffusion is calculated by a pressure solution model. The relationship between fluid inclusion-scale and REV-scale stresses and strains is established by using the Mori-Tanaka homogenization scheme. The proposed rate-dependent homogenization model is calibrated against cyclic compression tests. It is noted from the model that a lower strain rate and a larger number of sliding cracks enhances stiffness reduction and hysteresis. Thinner sliding cracks (i.e. thinner brine films) promote stiffness reduction and accelerate stress redistributions in the crack inclusions. Higher roughness angles lead to an increased difference of normal stress along the different segments of the crack plane and to a reduced diffusion path, which both amplify the reduction of stiffness and the development of hysteresis. The larger the volume fraction of the crack inclusions, the larger the REV deformation and the larger the hysteresis. Results presented in this study shed light on the mechanical behavior of salt-rock that is pertinent to the design of geological storage facilities that undergo cyclic unloading, which could help optimize the energy production cycle with low carbon emissions.
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ItemMicro-mechanical Modeling for Rate-Dependent Behavior of Salt Rock under Cyclic Loading(Georgia Institute of Technology, 2020) Shen, Xianda ; Ding, Jihui ; Arson, Chloé ; Chester, Judith S. ; Chester, Frederick M.The dependence of rock behavior on the deformation rate is still not well understood. In salt rock, the fundamental mechanisms that drive the accumulation of irreversible deformation, the reduction of stiffness and the development of hysteresis during cyclic loading are usually attributed to intracrystalline plasticity and diffusion. We hypothesize that at low pressure and low temperature, the rate-dependent behavior of salt rock is governed by water-assisted diffusion along grain boundaries. Accordingly, a chemo-mechanical homogenization framework is proposed, in which the Representative Elementary Volume (REV) is viewed as a homogeneous polycrystalline matrix that contains sliding grain-boundary cracks. The slip is related to the mass of salt ions that diffuse along the crack surface. The relationship between fluid inclusion-scale and REV-scale stresses and strains is established by using the Mori-Tanaka homogenization scheme. It is noted from the model that a lower strain rate and a larger number of sliding cracks enhance stiffness reduction and hysteresis. Thinner sliding cracks (i.e. thinner brine films) promote stiffness reduction and accelerate stress redistributions. The larger the volume fraction of the crack inclusions, the larger the REV deformation and the larger the hysteresis. Results presented in this study shed light on the mechanical behavior of salt-rock that is pertinent to the design of geological storage facilities that undergo cyclic unloading, which could help optimize the energy production cycle with low carbon emissions.
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ItemFabric evolution and crack propagation in salt during consolidation and cyclic compression tests( 2020) Shen, Xianda ; Ding, Jihui ; Lordkipanidze, Ilia ; Arson, Chloé ; Chester, Judith ; Chester, FrederickIt is of great interest to describe and quantify the evolution of microstructure for a better understanding of rock deformation processes. In this study, 2D microstructure images of salt rock are analyzed at several stages of consolidation tests and cyclic compression tests to quantify the evolution of the magnitude and orientation of solidity, coordination, local solid volume fraction and crack volume. In both the consolidation and the cyclic compression tests, the deformation of aggregates achieved by grain rearrangement is greater than that achieved by the deformation of an individual grain. In the consolidation tests, the aggregates are rearranged into horizontal layers of coordinated grains, the orientation distribution of grain indentations is quasi-uniform, and the size of the pores reduces and becomes more uniformly distributed. As a result, salt rock microstructure becomes more homogeneous. The increase of local solid volume fraction in the lateral direction is correlated with an increase of the oedometer modulus. In the cyclic compression tests, grain-to-grain contact areas decrease due to the redistribution of grains and the propagation of intergranular cracks. Aggregates are reorganized into columns of coordinated grains. Intergranular opening-mode cracks tend to develop in the axial direction, while intergranular shear-mode cracks propagate preferentially in the lateral direction. The lateral components of the fabric tensors of coordination and local solid volume fraction decrease, which results in an increase of the Poisson's ratio. The fabric descriptors used in this work allow a better quanti cation and understanding of halite deformation processes and can be used in other types of rocks encountering similar deformation mechanisms.
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ItemMechanisms of anisotropy in salt rock upon micro-crack propagation( 2020) Shen, Xianda ; Arson, Chloé ; Ding, Jihui ; Chester, Frederick M. ; Chester, Judith S.
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ItemMineral weathering and bedrock weakening: Modeling microscale bedrock damage under biotite weathering( 2019) Shen, Xianda ; Arson, Chloé ; Ferrier, Ken L. ; West, Nicole ; Dai, ShengBedrock 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.
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ItemSimulation of salt cavity healing based on a micro-macro model of pressure-solution(Geological Society, United Kingdom, 2019-01) Shen, Xianda ; Arson, ChloéCO₂ storage in salt rock is simulated with the Finite Element Method (FEM), assuming constant gas pressure. The initial state is determined by simulating cavity excavation with a Continuum Damage Mechanics (CDM) model. A micro-macro healing mechanics model is proposed to understand the time-dependent behavior of halite during the storage phase. Salt is viewed as an assembly of porous spherical inclusions that contain three orthogonal planes of discontinuity. Eshelby’s self-consistent theory is employed to homogenize the distribution of stresses and strains of the inclusions at the scale of a Representative Elementary Volume (REV). Pressure solution results in inclusion deformation, considered as eigenstrain, and in inclusion stiffness changes. The micro-macro healing model is calibrated against Spiers’ oedometer test results, with uniformly distributed contact plane orientations. 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 at the cavity wall occur within the five first days of storage and the damage is reduced by only 2%. These conclusions still need to be confirmed by simulations that account for changes of gas temperature and pressure over time. For now, the proposed modeling framework can be applied to optimize crushed salt back filling materials and can be extended to other self-healing materials.
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ItemAn Isotropic Self-Consistent Homogenization Scheme For Chemo-Mechanical Healing Driven By Pressure Solution In Halite(Georgia Institute of Technology, 2018) Arson, Chloé ; Shen, XiandaMechanical healing is the process by which a damaged material recovers mechanical stiffness and strength. Pressure solution is a very effective healing mechanism, common in crystalline media. Chemical reactions initiate at the location of microstructure defects, which would be very difficult to account for in a homogenization scheme that separates the solid and the pore phases, as is classically the case. Here, we propose a novel chemo-mechanical homogenization model in which the inclusion is not a grain, but rather, a space that contains a pore and discontinuities, where chemical processes take place. Mass and energy balance equations are rigorously established to predict the chemical eigenstrain of each inclusion, which, added to the elastic deformation, provides the microstrain of each inclusion. From there, Hill's inclusion-matrix interaction law is used to upscale strains and stresses at the scale of a Representative Elementary Volume (REV). The model was calibrated against experimental results published in the literature for salt rock. Subsequent sensitivity analyses show that in samples with same porosity but with inclusions that have different initial void sizes, inclusions with larger voids have a negligible healing rate and they are slowing down the overall healing rate of the REV. The highest healing rate is reached in samples with uniformly distributed void sizes. In addition, the healing rate increases with the initial porosity, but the final porosity change does not depend on the initial porosity of the sample. Principal stresses of higher magnitude are noted in the inclusions that are part of REVs of high initial porosity. In specimens with smaller inclusions (i.e., smaller grains), principal stresses are more widely distributed in magnitude and the healing rate is higher. The proposed homogenization method paves the way to many future developments for upscaling chemo-mechanical processes in heterogeneous media, and can be used to design self-healing materials.
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ItemAnalysis of microstructure, deformation and permeability of salt/sand mixtures during creep(Georgia Institute of Technology, 2017-07) Shen, Xianda ; Zhu, Cheng ; Arson, ChloéThe impact of impurities on salt healing properties is studied through creep tests performed on brine saturated granular salt with various quartz contents. Quartz grains act as shields that reduce dissolution at salt grain contacts and decrease the creep rate. Non-smooth creep curves are obtained for specimens with 50% quartz contents, due to sequential pore collapse. Micro-CT images acquired before creep, after creep, and after unloading, show that pore-to-pore distances decrease with quartz contents and that the creep rate decreases as the mean area of the salt grain contacts increases. Based on grain scale thermodynamic models, we show that creep deformation is controlled by diffusion - not dissolution-precipitation. Permeability evolution is less sensitive to porosity than to void radius and spacing, which control pore connectivity. The proposed modeling framework can be used in any crystalline material to relate microscopic reaction rates to macroscopic deformation rates.
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ItemNumerical Study of Thermo-Mechanical Effects on the Viscous Damage Behavior of Rock Salt Caverns(Georgia Institute of Technology, 2017-06) Cheng, Zhu ; Shen, Xianda ; Arson, Chloé ; Pouya, AhmadUnderground cavities in rock salt have received increased attention for the storage of oil, gas, and compressed air energy. In this study, the transition between secondary and tertiary creep in salt is determined by a micro-macro model: The initiation of grain breakage is correlated with the acceleration of viscoplastic deformation rate and with the initiation of damage at the macroscopic scale. Salt stiffness decreases when macroscopic damage increases, which allows predicting the evolution of the damage zone around salt caverns used for geological storage. After implementing the phenomenological model into the Finite Element Method (FEM) program POROFIS, two thermo-mechanical coupled stress paths are simulated to analyze stress concentrations and viscous damage around a 650-m-deep cavern in axisymmetric conditions. Numerical results indicate that, despite the pressurization or depressurization-induced temperature variation, internal gas temperature always tends to approach the primary surrounding rock mass value. The viscous deformation induced by thermo-mechanical couplings significantly affects the original stress field at the cavern wall and induces high damage at the most concave sections of the cavern. Results reveal the significant influences of idle time, gas pressure range, and injection and withdrawal cycles on stress, strain and temperature distributions in the vicinity of the cavern. More analyses are needed to confirm the influence of thermo-mechanical cycles of pressurization and depressurization, and to design long-term cavern operations.