Directional Dependence of Gas Dispersion in Carbonates and Sandstones

Abstract

This study investigates the directional dependence of gas dispersion in porous media with varying lithology and macroscopic heterogeneity. Eight rock samples, including three carbonates and five sandstones, were examined through CH₄–CO₂ core-flooding experiments at 300 psi and 65 °C. Each core was first flooded with CO₂ displacing CH₄, then the core was inverted 180° to quantify the dispersion coefficient in both core orientations. Across most samples, gas dispersion showed directional dependence, with fitted K_L values differing by up to 60%. The directional difference depended on the sequence in which the gas mixing front encountered visible heterogeneity. Carbonates exhibited higher dispersion coefficients and smaller directional differences than most sandstone samples, whereas visibly layered sandstones showed larger directional differences. Directional differences decreased with increasing permeability for homogeneous cores but showed no consistent trend in heterogeneous ones, where macroscopic structure dominated flow behavior. Rocks with pronounced macroscopic heterogeneity, such as the Nugget sandstone, experienced the greatest directional differences. These findings indicate that dispersion directionality is governed by a hierarchy of controls, in which lithology, macroscopic heterogeneity, and permeability influence the extent of gas mixing. Rock-dependent in situ fluid mixing can therefore have important implications for subsurface systems engineering in geologic gas storage, particularly in well placement and operational strategy design.