The impact of pressure induced flow instability on gas dispersion in porous media

Abstract

Dispersion is influenced by the complex interplay between rock heterogeneity, flow dynamics, and thermodynamic conditions. Previous studies have shown that factors like heterogeneity and injection rate affect how fluids mix and spread in geological formations. However, the role of system pressure and flow regime in shaping dispersion characteristics, particularly under unstable flow conditions, remains less understood. This study examines the effects of system pressure and flow rate on the dispersion of CO₂ and CH₄ in Indiana limestone and Silurian dolomite, two carbonate rocks with distinct pore structures. Experiments were conducted at pressures of 300, 600, and 900 psi, with flow rates of 1.69 × 10⁻⁴ m/s, 2.12 × 10⁻⁴ m/s, and 3.39 × 10⁻⁴ m/s, to evaluate how dispersion characteristics evolve under varying conditions. The results indicate that under stable flow, pressure has minimal impact on dispersion. However, under unstable flow, increasing pressure alters velocity distributions and enhances fluid mixing, leading to deviations in the dispersion coefficient beyond the effects of rock heterogeneity alone. In more homogeneous media, a threshold is observed where dispersion under unstable flow is lower relative to stable flow. These findings demonstrate that pressure amplifies dispersion primarily under unstable flow governed by the fluid density contrasts, and that heterogeneity can either enhance or dampen these effects.