Thermal-fluid coupling analysis of oil shale pyrolysis and displacement by heat-carrying supercritical carbon dioxide

Supercritical carbon dioxide (SC-CO₂) has advantages in solubility and diffusion. It is suitable for extracting organic matter in porous media as heat-carrying fluid. This paper investigates whether supercritical carbon dioxide can promote the pyrolysis of oil shale using both laboratory experiments and numerical simulations. The diffusion and velocity distribution of heat carrying supercritical carbon dioxide in oil shale are simulated by using COMSOL Multiphysics. The results show that at the pressure 7.8 mpa, while the injection temperature is 400 °C and the injection flow rate is 2.5–9 ml/s, both the axial displacement rate and the front pyrolysis rate increase with increasing injection flow rate and pyrolysis time. The pyrolysis front gradually transited from uniform pyrolysis along the whole section to the interface centralized pyrolysis along the axial displacement rate distribution. In addition, in an indoor experiment when the pressure is 7.8Mpa the temperature is 350–400 ℃, and the injection velocity is 9 ml/s of supercritical carbon dioxide, the effective recovery of shale oil increases from 36% to 97.4% with the increasing temperature and pyrolysis time. However, at the same temperature, the increasing trend of shale oil recovery decreases gradually for an extension of time. The analysis of a combined gas chromatography shows that the shale oil is secondary pyrolysis due to long-term retention in high temperature conditions and thus the increasing trend of oil recovery is gradually eased off.