Coupled THM Modeling of Hydroshearing Stimulation in Tight Fractured Volcanic Rock

In this study, we use the TOUGH-FLAC simulator for coupled thermo–hydro-mechanical modeling of well stimulation for an Enhanced Geothermal System (EGS) project. We analyze the potential for injection-induced fracturing and reactivation of natural fractures in a porous medium with associated permeability enhancement. Our analysis aims to understand how far the EGS reservoir may grow and how the hydroshearing process relates to system conditions. We analyze the enhanced reservoir, or hydrosheared zone, by studying the extent of the failure zone using an elasto-plastic model, and accounting for permeability changes as a function of the induced stresses. For both fully saturated and unsaturated medium cases, the results demonstrate how EGS reservoir growth depends on the initial fluid phase, and how the reservoir extent changes as a function of two critical parameters: (1) the coefficient of friction, and (2) the permeability-enhancement factor. Moreover, while well stimulation is driven by pressure exceeding the hydroshearing threshold, the modeling also demonstrates how injection-induced cooling further extends the effects of stimulation

Coupled THM Modeling of Hydroshearing Stimulation in Tight Fractured Volcanic Rock

In this study, we use the TOUGH-FLAC simulator for coupled thermo–hydro-mechanical modeling of well stimulation for an Enhanced Geothermal System (EGS) project. We analyze the potential for injection-induced fracturing and reactivation of natural fractures in a porous medium with associated permeability enhancement. Our analysis aims to understand how far the EGS reservoir may grow and how the hydroshearing process relates to system conditions. We analyze the enhanced reservoir, or hydrosheared zone, by studying the extent of the failure zone using an elasto-plastic model, and accounting for permeability changes as a function of the induced stresses. For both fully saturated and unsaturated medium cases, the results demonstrate how EGS reservoir growth depends on the initial fluid phase, and how the reservoir extent changes as a function of two critical parameters: (1) the coefficient of friction, and (2) the permeability-enhancement factor. Moreover, while well stimulation is driven by pressure exceeding the hydroshearing threshold, the modeling also demonstrates how injection-induced cooling further extends the effects of stimulation

Fracture Sustainability in Enhanced Geothermal Systems: Experimental and Modeling Constraints

Enhanced geothermal systems (EGS) offer the potential for a much larger energy source than conventional hydrothermal systems. Hot, low-permeability rocks are prevalent at depth around the world, but the challenge of extracting thermal energy depends on the ability to create and sustain open fracture networks. Laboratory experiments were conducted using a suite of selected rock cores (granite, metasediment, rhyolite ash-flow tuff, and silicified rhyolitic tuff) at relevant pressures (uniaxial loading up to 20.7 MPa and fluid pressures up to 10.3 MPa) and temperatures (150-250 °C) to evaluate the potential impacts of circulating fluids through fractured rock by monitoring changes in fracture aperture, mineralogy, permeability, and fluid chemistry. Because a fluid in disequilibrium with the rocks (deionized water) was used for these experiments, there was net dissolution of the rock sample: This increased with increasing temperature and experiment duration. Thermal-hydrological-mechanical-chemical (THMC) modeling simulations were performed for the rhyolite ash-flow tuff experiment to test the ability to predict the observed changes. These simulations were performed in two steps: A thermal-hydrological-mechanical (THM) simulation to evaluate the effects of compression of the fracture, and a thermal-hydrological-chemical (THC) simulation to evaluate the effects of hydrothermal reactions on the fracture mineralogy, porosity, and permeability. These experiments and simulations point out how differences in rock mineralogy, fluid chemistry, and geomechanical properties influence how long asperity-propped fracture apertures may be sustained. Such core-scale experiments and simulations can be used to predict EGS reservoir behavior on the field scale

Low Static Shear Modulus Along Foliation and Its Influence on the Elastic and Strength Anisotropy of Poorman Schist Rocks, Homestake Mine, South Dakota

We investigate the influence of foliation orientation and fine-scale folding on the static and dynamic elastic properties and unconfined strength of the Poorman schist. Measurements from triaxial and uniaxial laboratory experiments reveal a significant amount of variability in the static and dynamic Young’s modulus depending on the sample orientation relative to the foliation plane. Dynamic P-wave modulus and S-wave modulus are stiffer in the direction parallel to the foliation plane as expected for transversely isotropic mediums with average Thomsen parameters values 0.133 and 0.119 for epsilon and gamma, respectively. Static Young’s modulus varies significantly between 21 and 117 GPa, and a peculiar trend is observed where some foliated sample groups show an anomalous decrease in the static Young’s modulus when the symmetry axis (x3-axis) is oriented obliquely to the direction of loading. Utilizing stress and strain relationships for transversely isotropic medium, we derive the analytical expression for Young’s modulus as a function of the elastic moduli E1, E3, ν31, and G13 and sample orientation to fit the static Young’s modulus measurements. Regression of the equation to the Young’s modulus data reveals that the decrease in static Young’s modulus at oblique symmetry axis orientations is directly influenced by a low shear modulus, G13, which we attribute to shear sliding along foliation planes during static deformation that occurs as soon as the foliation is subject to shear stress. We argue that such difference between dynamic and static anisotropy is a characteristic of near-zero porosity anisotropic rocks. The uniaxial compressive strength also shows significant variability ranging from 21.9 to 194.6 MPa across the five sample locations and is the lowest when the symmetry axis is oriented 45° or 60° from the direction of loading, also a result of shear sliding along foliation planes during static deformation