The role of injection method on residual trapping: Insights into bridging scales and heterogeneity

Abstract

CO${}_2$ injection into subsurface reservoirs provides a long-term solution to anthropogenic emissions. A variable injection rate (such as ramping the flow rate up or down) provides flexibility to injection sites, and could influence the amount of residual trapping. Observations made in cm-scale samples showed that starting at a low flow rate established a flow pathway across the core at a low capillary pressure, leading to a long-term reduction in pore space utilization, as increases in flux were accommodated with little change in saturation. In this work, the scalability of these observations is evaluated by performing experiments with variable injection rates in larger samples: 5 cm diameter and 12 cm length, compared to 2.5 cm diameter and 4.5 cm length in previous work (Spurin et al., 2024). We observed that starting at a low flow rate did not lead to a long-term reduction in pore space utilization. Instead, saturation increased significantly with increased flux, leading to a higher pore space utilization than experiments where injection started with the higher flow rate. The difference in observations depending on sample size and the role of heterogeneity highlights potential uncertainties in upscaling experimental observations to field-scale applications.