Browsing by Author "Healy, David"

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    A new anisotropic poroelasticity model to describe damage accumulation during cyclic triaxial loading of rock
    (2022) Lyakhovsky, Vladimir; Panteleev, Ivan; Shalev, Eyal; Browning, John; Mitchell, Thomas; Healy, David; Meredith, Philip G.
    Crustal rocks undergo repeated cycles of stress over time. In complex tectonic environments where stresses may evolve both spatially and temporally, such as volcanoes or active fault zones, these rocks may experience not only cyclic loading and unloading, but also rotation and/or reorientation of stresses. In such situations, any resulting crack distributions form sequentially and may therefore be highly anisotropic. Thus, the tectonic history of the crust as recorded in deformed rocks may include evidence for complex stress paths, encompassing different magnitudes and orientations. Despite this, the ways in which variations in principal stresses influence the evolution of anisotropic crack distributions remain poorly constrained. In this work, we build on the previous non-linear anisotropic damage rheology model by presenting a newly developed poroelastic rheological model which accounts for both coupled anisotropic damage and porosity evolution. The new model shares the main features of previously developed anisotropic damage and scalar poroelastic damage models, including the ability to simulate the entire yield curve through a single formulation. In the new model, the yield condition is defined in terms of invariants of the strain tensor, and so the new formulation operates with directional yield conditions (different values for each principal direction) depending on the damage tensor and triaxial loading conditions. This allows us to discern evolving yield conditions for each principal stress direction and fit the measured amounts of accumulated damage from previous loading cycles. Coupling between anisotropic damage and anisotropic compaction along with the damage-dependent yield condition produces a reasonable fit to the experimentally obtained stress–strain curves. Furthermore, the simulated time-dependent cumulative damage is well correlated with experimentally observed acoustic emissions during cyclic loading in different directions. As such, we are able to recreate many of the features of the experimentally observed directional 3-D Kaiser ‘damage memory’ effect.
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    Fluid flow in the Nevados de Chillán Geothermal System as an example of fractured reservoir, Southern Andes
    (2024) Arancibia Hernández, Gloria Cecilia; Mura Toledo, Valentina Rossana; López Contreras, Camila Andrea; Oyarzo Cespedes, Isa Paz Belen; Browning, John; Healy, David; Maza, Santiago; Morata, Diego
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    Non-linear anisotropic damage rheology model: Theory and experimental verification
    (2021) Panteleev, Ivan; Lyakhovsky, Vladimir; Browning, John; Meredith, Philip G.; Healy, David; Mitchell, Thomas M.
    We extend the isotropic non-linear damage rheology model with a scalar damage parameter to a more complex formulation that accounts for anisotropic damage growth under true triaxial loading. The model takes account of both the anisotropy of elastic properties (associated with textural rock structure) and the stress- and damage-induced anisotropy (associated with loading). The scalar, isotropic model is modified by assuming orthotropic symmetry and introducing a second-order damage tensor, the principal values of which describe damage in three orthogonal directions associated with the orientations of the principal loading axes. Different damage components, accumulated under true triaxial loading conditions, allows us to reproduce both stress-strain curves and damage- and stress-induced seismic wave velocity anisotropy. The suggested model generalization includes a non-classical energy term similar to the isotropic non-linear scalar damage model, which allows accounting for the abrupt change in the effective elastic moduli upon stress reversal. For calibration and verification of the model parameters, we use experimental stress-strain curves from deformation of dry sandstone under both conventional and true triaxial stress conditions. Cubic samples were deformed in three orthogonal directions with independently controlled stress paths. To characterize crack damage, changes in ultrasonic P-wave velocities in the three principal directions were measured, together with the bulk acoustic emission output. The parameters of the developed model were constrained using the conventional triaxial test data, and provided good fits to the stress-strain curves and P-wave velocity variations in the three orthogonal directions. Numerical simulation of the true triaxial test data demonstrates that the anisotropic damage rheology model adequately describes both non-linear stress-strain behavior and P-wave velocity variations in the tested Darley Dale sandstone.
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    Structural control on the Southern Andean Nevados de Chillán Geothermal System
    (2025) Mura, Valentina; Arancibia Hernández, Gloria Cecilia; Browning, John; Healy, David; López Contreras, Camila Andrea; Morata, Diego; Maza, Santiago; Cardona, Carlos
    Detailed structural analysis from representative outcrops is necessary to characterize geothermal reservoir dynamics. Here, we estimate fracture density and intensity, as well as the dimensional properties of individual fault and fracture sets in basement rocks of the Nevados de Chillán Geothermal System. We identified several important structural features that could be responsible for controlling local fluid flow; the high-angle sinistral Las Trancas Fault as well as a series of low-angle reverse faults within the Las Termas-Olla de Mote Fault system. Most fractures identified strike either NE-SW, NNE-SSW, and NNW-SSE. Analysis of fault-slip data, supported by seismicity, indicates the presence of a main transtensional regime with subhorizontal NE-trending σ1. Structures sub-parallel to the present-day local maximum horizontal stress show significant dilation tendencies, whilst NW-SE fractures are less prone to dilation. NE and E-W high angle faults could be primary conduits facilitating the upward migration of hot fluids from reservoirs within crystalline and fractured rocks. The fracture length distribution was analysed using power law, negative exponential, and log-normal distribution. The power law with a scaling exponent of about −3 provides the best fit to the data. This study advances our understanding of the structural control of the geothermal reservoir and its associated fracture-controlled fluid circulation and thereby improves the prospectivity in the region by quantifying the optimum fracture sets for fluid flow.