High-velocity non-Darcy flow under pore-throat deformation during cyclic injection–production in underground gas storage

Abstract

Underground gas storage reservoirs exhibit pronounced stress sensitivity and non-Darcy flow during cyclic high-velocity injection–production processes, while the presence of irreducible water further complicates the underlying flow mechanisms. A coupled numerical simulation framework integrating stress evolution, pore structure evolution, and fluid flow behavior was established to systematically investigate the pore-scale flow behavior in digital rocks. An elastoplastic mechanical model was introduced to characterize irreversible pore-throat deformation induced by cyclic stress loading. High-velocity gas flow was simulated by directly solving the Navier–Stokes equations, and the volume-of-fluid method was used to reproduce the distribution of initial water. Results show that cyclic stress loading continuously compressed pore throats and reduced pore connectivity, resulting in a cumulative permeability decreases by approximately 24%, while stress sensitivity gradually weakened with increasing cycle number. As flow velocity increased, the Reynolds number (Re) progressively increased and flow transitioned from viscous-dominated to inertia-dominated regimes. When Re < 10−1, flow behavior followed Darcy's law, whereas at Re > 10−1, the pressure gradient increased nonlinearly with flow velocity, indicating significant enhancement of non-Darcy flow. Capillary forces caused initial water phase to remain preferentially in small pore throats and dead-end pores, increasing the Forchheimer coefficient by approximately 2.7–4.6 times and strengthening the dominant flow pathways. The coupled evolution of stress-induced pore structure deformation and initial water retention jointly controlled the deterioration of gas-flow capacity and the enhancement of non-Darcy flow behavior, providing theoretical support for optimizing underground gas storage operations.