Experimental Study on CO2 Residual Trapping as a Function of the Brine Imbibition Rate

Different trapping mechanisms are responsible for confining the CO2 in deep saline aquifers. During injection, CO2 displaces brine according to a drainage process; after injection, CO2 migrates laterally and upward as a separate phase. Water displaces CO2 in an imbibition-like process, leading to the disconnection of the plume and the formation of a residual CO2 saturation. This research focuses on the dependency of CO2 residual trapping on the brine imbibition rate, reproduced by CO2 displacement through brine injection. We investigated the impact of different brine injection rates on sandstone plugs through laboratory experiments performed at pressure and temperature reservoir conditions using a relative permeameter system (RPS). First, the petrophysical properties of the dry plugs were measured. Then, the plugs were placed in the RPS and saturated with brine. After a preliminary drainage phase, during which brine was displaced by CO2 injection to reach residual water saturation, brine was injected again in the plug at a very low flow rate until the differential pressure across the plug stabilized, and the volume of displaced CO2 was measured. Subsequently, the injection flow rate was increased step by step from 0.1 to 10 cc/min, representative of fluid flow velocities at the reservoir scale during CO2 disposal. Each step was applied until a new stabilization of the differential pressure across the plug was reached, and the volume of the displaced CO2 was measured. The residual CO2 saturation was calculated using the sample porosity and the cumulative volume of the displaced CO2. The results of the experiments are presented using the Capillary Desaturation Curves, displaying the residual CO2 saturation as a function of the capillary number, which in turn depends on the injection flow rate. The results obtained in this study show that the CO2 residual saturation decreases as the flow rate of the displacing brine increases. This is likely due to the transition from a purely capillary-dominated flow to a mixed flow regime. In the literature, data from experimental tests show the dependency of residual saturation on the injection flow rate for oil-water and methane-water systems. However, to the best of the authors’ knowledge, these are the first experimental results describing the Capillary Desaturation Curves for a brine-CO2 system under high-pressure, high-temperature conditions.