Phase transition and fluid backflow during the non-injection period in the Ordos CCS project, China

Abstract

CO2 phase transition and fluid interaction in the wellbore-reservoir system after shut-in are two critical phenomena, which are crucial to predicting the long-term fate of CO2 in a deep saline aquifer storage project. In this study, a wellbore-reservoir coupled numerical model is developed based on the Shenhua Ordos Carbon Capture and Storage (CCS) project in China. This model can cope with the CO2 phase transition and is designed to investigate fluid behavior during the non-injection period. The simulation results show that the liquid-phase CO2 above a depth of ∼300 m within the wellbore will transit to the gas phase during the non-injection period. The changes in the pressure and temperature associated with the phase transition are too small to be a safety hazard. Moreover, the injected CO2 will flow back to the wellbore and even flow into the top injection layer with the maximum permeability during the non-injection period; for a sufficiently long non-injection period, the formation water will also flow back. This process may redistribute the injected CO2 in the multi-layered reservoir. The parameter sensitivity analysis indicates that the permeability distribution of the reservoir has the most profound effect on the reservoir backflow process. These results provide an in-depth understanding of the fluid behavior within the wellbore-reservoir system after shut-in and offer theoretical guidelines for similar CCS project operations.