Impact of Sub-core-scale Heterogeneity on Meter-Scale Flow and Brine Displacement in Drainage by CO₂

Studies of CO₂-brine two-phase flow are predominantly carried out on core samples or reservoir-scale domains. In this work we investigate intermediate-scale flow considering a cubic rock with 1 m³ volume incorporating sub-core (mm)-scale heterogeneity. First, a coreflooding experiment is analyzed to estimate subcore permeability spatial distribution and the results are used as input into the meter-scale model without any upscaling required. High-resolution numerical simulations are conducted to model CO₂ injection into a brine saturated rock. We find significant differences between the meter-scale rock simulation and core experiment saturation distributions resulting from highly influential boundary effects on the core scale and pronounced gravity impact on the meter scale. A detailed analysis of flow and brine trapping in rock simulations is presented considering different flow models and permeability structures. It is shown that gravity and capillary heterogeneity have substantial impact on the saturation in the deeper regions, which exhibit sufficiently low capillary pressure, and these regions also show increased trapping of brine. Two trapping mechanisms are discussed: Type I which is a result of entry pressure considerations and Type II, which occurs due to lack of CO₂ connectivity. The two types of trapping are found to be correlated to the permeability structure. A formula is presented relating between trapping and permeability statistical parameters, that is, variance of log-permeability and correlation length scale, as well as to the Bond number characterizing the ratio between gravity and capillary effects.