Experimental and modeling study of CO2 solubility in formation brines at in-situ conditions

Abstract

The dissolution of CO2 into formation brine, one of the dominant trapping mechanisms for geological carbon storage (GCS) in deep saline aquifers, has been widely investigated. Most experimental and modeling studies focused on the CO2 solubility in synthetic brines. However, rare studies have been performed to investigate CO2 solubility in real formation brines, especially when ion-ion interactions were considered. In this work, the CO2 solubility of seventy formation brines from major basins in China including the Junggar basin, Ordos basin, and North and South Songliao basin was measured at in-situ pressure and temperature conditions. Experimental results show that CO2 solubility has a strong relationship with pressure and total dissolved solids (TDS), while the relationship between solubility and temperature is weak. Three solubility regions were classified in terms of depth. In Regions I and III, CO2 solubility increases with depth and is independent with depth in Region II. However, the effect of salinity on CO2 solubility was observed in all regions. Additionally, a modified Duan's model used for a mixed-salt system was proposed using dataset from this study and literature. Comparisons with Duan's model for CO2 solubility in mixed-salt systems show the average absolute relative deviation equals 5.14%, which reveals a clear improvement in the modified model. In addition, the validation of the model was performed, and results show that the average absolute relative deviation before and after improvement are 12.10% and 5.11%, respectively, indicating the high accuracy of the improved model. Overall, this research improves the understanding of CO2 solubility in real formation brines and illustrates the efficiency and accuracy of the modified model, which is crucial for the evaluation of the CO2 storage capacity, fate of CO2 over storage site life for commercial GCS projects.