**2.2 Residual trapping**

Residual trapping is a phenomenon in which CO2 is trapped in the pores of rocks by capillary force. While the CO2 is injected, it will enter the rock pore space and replace the original fluid. The difference in density between groundwater and CO2 causes an upward. Then, the groundwater re-enters the rock pore space and the wetting phase (groundwater) will replace part of the weak wetting phase (CO2). The replacement of CO2 by groundwater leads to a significant reduction in the percentage of CO2 in the rock pores, which are eventually trapped in the small pores, as shown in **Figure 3**. Thus, the isolated CO2 is trapped as a stable phase by a trapping mechanism called residual trapping or capillary trapping [15, 21].
