Investigation of temperature- and pressure-dependent flow characteristics of supercritical carbon dioxide- induced fractures in Harcourt granite: application to CO₂-based enhanced geothermal systems

The use of supercritical carbon dioxide (ScCO₂) as the working fluid in enhanced geothermal systems (EGSs) replacing water has attracted attention recently, due to the favourable properties of ScCO₂ in terms of deep geothermal reservoir stimulation, heat transmission and transport properties, and long-term reservoir integrity. However, deep geothermal reservoir stimulation using ScCO₂, and the flow behaviour of ScCO₂-stimulated reservoirs under the extreme reservoir conditions prevailing in deep underground are poorly understood to date. Therefore, the aim of this study is to investigate thermo-hydro-mechanical and chemical (THMC) flow characteristics of ScCO₂-induced fractures under extreme reservoir conditions. A series of flow-through experiments was conducted on ScCO₂-fractured Harcourt granite specimens under a range of confining pressures and temperatures from 10–60 MPa, and 25–250ºC, respectively. Importantly, the permeability of ScCO₂-fractured rock specimens with multiple fractures with secondary branches, is more than three orders greater than that of water-fractured rock specimens with single plane fractures. Approximately 90% of fracture permeability is reduced with the increase of confining pressure from 10MPa to 50MPa due to the stress-induced fracture closure. However, the temperature influences fracture flow characteristics significantly. The increase of temperature up to 100°C results in 80% reduced fracture permeability due to thermally-induced fracture closure as a result of rock volume expansion. A 5–20% increase in fracture permeability results with the further increase of temperature from 100°C. The gradual increase in fracture permeability occurs due to the coupled hydro-thermo-mechanical and chemical behaviour of the fractured rock specimen. In addition, the enhanced mobility of water at high-temperature conditions in terms of viscosity reduces flow resistance and therefore increases flowability along fractures at high temperatures. The chemical analysis shows that the enhanced dissolution of rock minerals with temperature increase, especially quartz influences in increasing the fracture aperture and permeability. Also, minerals dissolution dominates the precipitation of secondary minerals during fluid flow through fractured rock media in the temperature range of 50–250°C.