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

The injection of CO2 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 low flow rate led to a lower gas saturation at breakthrough. Once a pathway was established across the sample, increasing the flow rate did not improve gas saturation significantly, as the increase in flux was accommodated by the connected pathway across the sample. Starting at a high flow rate led to a higher pore space utilization, which is optimal for CO2 storage. Overall the high to low injection scenario led to more residual trapping.