Capillary Desaturation Curves for CO2-water systems

CO2 capture and storage is one of the most important ways to reduce greenhouse emissions and thus reduce the increase of the world average temperature. Deep saline aquifers are suitable geological formations for CO2 underground storage in a permanent and safe way. Among the different trapping mechanisms, residual CO2 trapping indicates the trapping of the injected carbon dioxide inside the rock pores thanks to capillary forces. Residual CO2 saturation when the CO2 plume migrates through the aquifer cannot be predicted and must be assessed experimentally. The Capillary Desaturation Curve obtained from laboratory data is the most used plot to evaluate the residual CO2 saturation as a function of the Capillary number, expressing the ratio of viscous to capillary forces. The Capillary Desaturation Curves can also be plotted as a function of the Trapping number, which includes both the capillary number and the gravitational effect through the Bond number. In this study, the assessment of the residual CO2 saturation was performed for two rock samples, a sandstone and a carbonate, through core flooding experiments with a Relative Permeability System. Firstly, CO2 was injected to evaluate the residual water saturation; then, water was injected at increasing rates until quasi-steady state was reached. The residual CO2 saturation was calculated for each step and plotted as a function of the Capillary number. Two different definitions of the Capillary number were considered and used to calculate the Trapping number and plot the Capillary Desaturation Curve. Results confirmed that the residual CO2 saturation depends on the rock absolute permeability but also on the injection rate. As expected, when the injection rates are high, gravitational effects become negligible. The effects of solubility, salinity and contact angle were not assessed in this work because the experiments are extremely long (in the order of weeks), so further investigations would be needed to assess the impact of these parameters on the Critical Capillary number.