The role of injection method on residual trapping at the pore-scale in continuum-scale samples

The injection of CO$_2$ into underground reservoirs provides a long term solution for anthropogenic emissions. A variable injection method (such as ramping the flow rate up or down) provides flexibility to injection sites, and could increase trapping at the pore-scale. However, the impact of a variable injection method on the connectivity of the gas, and subsequent trapping has not been explored at the pore-scale. Here, we conduct pore-scale imaging in a continuum-scale sample to observe the role of a variable flow rate on residual trapping. We show that the injection method influences how much of the pore space is accessible to the gas, even when total volumes injected, and total flow rates remain constant. Starting at a high flow rate, then decreasing it, leads to a larger amount of the pore space accessed by the gas. Conversely, starting at a low flow rate, and increasing it, leads to a larger role of heterogeneity of the pore space. This can promote trapping efficiency because channelling of the two fluids can occur, but less gas is trapped overall. Overall, a high-to-low injection scenario is optimum for residual trapping in the pore space due to increases in pore space accessibility.