Testing the frackability of granite using supercritical carbon dioxide: Insights into geothermal energy systems

Supercritical carbon dioxide (ScCO2) fracturing is an attractive method which has favourable physical and chemical characteristics for the enhancement of underground reservoir permeability, and as a replacement for the current water-based fracturing in reservoir stimulation. However, the behaviour of ScCO2 as a fracturing fluid and the reservoir response to the injection of ScCO2 remain unclear in the geothermal environment. Therefore, in this study, a series of ScCO2 fracturing tests were conducted on Harcourt granite rock specimens under different confining pressures (5-40 MPa) and temperatures (50-300 degrees C) to better understand the applicability of ScCO2 as a fracturing fluid and its influence on breakdown pressure under geothermal reservoir conditions. According to the results, the breakdown pressure under ScCO2 fracturing is lower than that for water-based fracturing. This is because ScCO2, being a low viscous fluid, has the ability to percolate into the microcracks in the reservoir rock, and the pressurisation of these micro-cracks increases the pore-pressure inside the rock matrix, reducing the effective stress required for fracture initiation. Our results reveal that the breakdown pressure increases linearly with increasing confining pressure. High-resolution CT scanning results show that the average density of the induced fracture tends to increase with the increase of confining pressure due to the sudden release of a large amount of energy under higher negative effective stress conditions (26% increase in fracture density with increase of confining pressure from 30 MPa to 40 MPa at 50 degrees C). In addition, regardless of the confining pressure, the breakdown pressure exhibits a decreasing trend with increasing temperature, which is due to the thermally-induced microstructural and mechanical deterioration of the rock specimen. The temperature increase significantly alters the fracture morphology, resulting in narrow fractures with multiple secondary branches under high temperature conditions. The results of the tortuosities of the induced fractures provide further evidence of this phenomenon. Importantly, ScCO2 has the ability to penetrate into the thermally-induced micro-cracks in the rock and link these micro-cracks to the main diametrical fracture, resulting in interconnected and complex fracture patterns under high temperature conditions. Also, the fractures tend to propagate mainly along the grain boundaries of quartz and feldspar minerals, while biotite grains exhibit a comparative resilience to fracture propagation.