Hypergravity effects on immiscible two-phase displacement processes via CFD simulation and an analytical bi-channel model

Gravity significantly influences two-phase displacement processes in porous media such as enhanced oil recovery and CO2 geological storage, yet its quantitative evaluation has been limited. This study establishes a three-dimensional phase diagram with the third axis as modified bound number ($\log \left(B_m\right)$), distinguishing two gravity-dominant regions (gravitational fingering and gravitational stable displacement) from viscous fingering, stable displacement, and capillary fingering. The diagram is based on nondimensional analysis of fingering criterion derived from a simplified model that quantifies the effects of gravity, viscous force, and capillary force. A threshold velocity is identified, separating stable displacement from fingering under the competition of gravity and viscous force. Additionally, a series of CFD simulations were conducted in isotropic porous media incorporating various gravity and velocity. The simulation results aligned with analytical findings: gravitational effect in the opposite direction as displacement ($({\rho }_i-{\rho }_d)g<0$) favored the transition from fingering to stable displacement, increased displacement efficiency and facilitated the increasing tendency of injection pressure; while gravitational effect in the same direction as displacement ($({\rho }_i-{\rho }_d)g>0$) did the opposite. The systematical investigation in this work deepened the understanding of two-phase displacement involving gravity, serving as reference for fluidic manipulation and development of EOR or CO2 storage.