Thermo-Hydro-Mechanically Coupled Processes in Fractured Rocks

  • Adrian Garcia

Student thesis: Doctoral Thesis

Abstract

Energy demand is driven by increasing population and quality of life. Fractures localize mechanical deformations and fluid flow, and they impede heat flow through the rock matrix. Therefore, fractures present a challenge to both the recovery of underground energy and long-term waste disposal solutions like carbon geological storage. Fracture are planar discontinuities that form when brittle rocks. The discrete element method can model the complex micromechanics of rock failure. In this thesis we present a digital rocks analogue which is used to explore 1) the rock brittle-to-ductile transition with increased confining stress 2) the meaning of friction in intact rocks and what factors control confinement-dependent strength, 3) exhumation damage and its effect on rocks strength, and 4) multistage loading. The design, analysis and construction of a large-scale true triaxial load frame opens the door to geophysical studies on fractured rock masses. The frame can subject large cubical rock specimens (50cm × 50cm × 50cm) to boundary stresses up to 3 MPa. Auxiliary systems include active acoustic monitoring, passive acoustic emissions sensing, and high-pressure fluid injection. The evolution of P-wave velocity under anisotropic stress demonstrate the device’s capabilities. The true triaxial load-frame and the high-pressure fluid injection system are used to study hydraulic fracturing in pre-fractured media. We explore the competing influences of stress and rock mass fabric. Notably, even under extreme stress anisotropy, the fluid invades all fracture sets of our pre-fractured specimen. Fracture 5 intersections act as flow conduits and feed the invading fluid into the adjacent fractures, and local phenomena such as gouge-displacive fingering are identified. Thermal contact resistance impedes heat flow between neighboring rock blocks in fractured rocks. Contact resistance manifests as a discontinuous thermal gradient. It strongly influences the rock effective thermal conductivity and makes it sensitive to water saturation, stress, and the presence of gouge material. Finally, we conduct detailed thermal conductivity measurements on sand-silt gouge mixtures and propose physics-inspired models that accurately predict the thermal conductivity and mass density of dry and wet specimens as a function of stress and fines content.
Date of AwardJun 2020
Original languageEnglish (US)
Awarding Institution
  • Physical Science and Engineering
SupervisorJ. Carlos Santamarina (Supervisor)

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