Experimental study on supercritical carbon dioxide convective heat transfer in fractured rocks

Abstract

This study developed an innovative flow-heat transfer experimental system that can cover the operating range of subcritical to supercritical pressures (up to 60 MPa) and accurately simulate real geological conditions. Utilizing supercritical CO₂'s superior thermophysical properties, we conducted a series of well - designed convective heat transfer experiments within a smooth/artificial fracture of Sichuan outcrop rocks. The key parameters influencing the heat transfer, such as rock properties, morphology of fracture, injection flow, and the initial temperature of rock sample, were measured and analyzed. A new correlation formula was developed, showing a 25 % deviation from classical models but achieving a 4.37 % mean absolute error against experimental data. Test results show that the high temperature heat energy of nearly 60–70 °C can be recovered from the sandstone reservoir, which is higher 10 °C than that of the carbonate rock. The initial temperature of the sample directly affects the fluid physical property, and lead to change of heat transfer performance and production. Under the same initial temperature, the higher flow rate, the faster the production temperature drops. Due to the neglect of thermodynamic expansion effect in traditional fluid mechanics analysis, the influence of thermodynamic expansion on flow acceleration is deduced from the measurement of experimental temperature and pressure gradient, and a modified continuous equation including acceleration term is proposed to describe the flow process more accurately. It provides theoretical basis and technical support for optimizing geothermal energy storage system, and helps to improve energy utilization efficiency and reduce carbon emissions.