Abstract
Abstract 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.
Highlights
Several studies [1,2,3] have highlighted the potential of Enhanced Geothermal Systems (EGS) as a source of energy for generating electricity in the United States from 100 GWe up to 5,157 GWe
Creating and sustaining fracture flow pathways is critical to the long-term performance of EGS reservoirs
While there have been numerous laboratory experiments and numerical modeling of field-scale processes focused on fracture stimulation for EGS [7, 18-20, 39, 40, 4346], there has been relatively little focus on the long-term evolution of fracture permeability related to continued stress under hydrothermal conditions that could lead to changes in fracture aperture caused by compression, shear failure, mineral dissolution, and mineral precipitation
Summary
Several studies [1,2,3] have highlighted the potential of Enhanced Geothermal Systems (EGS) as a source of energy for generating electricity in the United States from 100 GWe up to 5,157 GWe. There are a number of critical technical challenges that need to be addressed to make EGS a technically and economically viable option: these include reservoir access (through improved drilling technology), reservoir creation (through improved fracture stimulation methods), and reservoir sustainability. The FORGE Roadmap [4] identifies three critical research areas for EGS: stimulation planning and design, fracture control, and reservoir management. These topics involve the stimulation, control, and sustainability of fractures needed for circulation of a working fluid to extract heat from an EGS reservoir [5]. Mixed-mode stimulation, where both shear and tensile failure of fractures occur, is another possible option for increasing fracture permeability in an EGS reservoir [7]
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