CO₂ trapping in reservoirs with fluvial architecture: Sensitivity to heterogeneity in permeability and constitutive relationship parameters for different rock types

A number of important CO₂ reservoir candidates exhibit sedimentary architecture reflecting fluvial deposition, which typically includes small-scale (decimeter to meter) textural facies among different rock types. As a result, highly contrasting spatial distribution of intrinsic permeability and constitutive relationship parameters (capillary pressure and relative permeability characteristic curves) are common for this type of reservoir. Previously, we showed that small-scale heterogeneity organized in fluvial depositional architecture controls the dynamics of a CO₂ plume during the injection and postinjection periods. This is due to (1) highly anisotropic effective permeability causing CO₂ to move in laterally, and (2) trapping by capillary pinning, which is in addition to snap-off capillary trapping. The detailed petrophysical and geological parameters of any specific reservoir are typically uncertain, which motivates studies of parameter sensitivity. The goal of this paper is to analyze the sensitivity of capillary trapping and dissolution to the variability in basic petrophysical parameters and rock composition in highly heterogeneous fluvial-type reservoirs. We show that (1) the larger the contrast in permeability between rock types the larger the CO₂ plume and the larger the rate of capillary trapping and dissolution; (2) an increase in capillary pressure contrast increases pinning trapping; and (3) the variability in the proportion between high- and low-permeability rocks (difference in connectivity of high-permeability clusters) does not affect snap-off trapping and dissolution, although it does strongly affect their spatial distributions. Though dissolution and snap-off capillary trapping may vary by up to 40% because of parameter variability, the trapping by capillary pinning may vary by an order of magnitude.