Flow behaviour of fractured geothermal reservoir rocks under in-situ stress and temperature conditions

Enhanced Geothermal Systems (EGS) is a potential carbon-neutral form of renewable energy whereby fractured granite at depths of around 2–4 km. Such geothermal reservoirs are high-temperature rock formations located at deep underground and therefore with ultra-low permeable characteristics. Therefore, natural and artificially created rock fractures provide major flow pathways for the reservoir fluid circulation process. Evolution of permeability through such rock fractures under potentially existing extreme geothermal conditions is quite important for an effective application of EGS systems. This study therefore discusses the experimental results of a series of flow experiments conducted on artificially fractured Australian Strathbogie granite under wide range of pressure (30 MPa confining pressure, 5 MPa to 25 MPa injection pressure) and temperature (from room temperature to 250 °C) conditions using a newly developed high temperature-high pressure rock triaxial test apparatus with capability of simulating environment of EGS reservoirs. The steady state flow rates through fractures found to linearly increase with increasing injection pressure under the considered injection pressures and temperatures. Permeability along rock fractures was therefore calculated by employing cubic law and relevant temperature and pressure dependent fluid properties. According to the experimental results, both stress level and the temperature have significant influences on fracture flow characteristics of the tested granite. Increasing of temperature caused a significant non-linear increment in flow rate and permeability through fractures due to the thermally induced micro crack generation.