Pore-scale gravity–capillary–viscous competition during drainage: A microfluidics approach

In this study, microfluidic experiments and theoretical analysis are conducted to elucidate how gravitational, viscous, and capillary forces compete to control drainage dynamics and pore-scale interfacial behavior in porous media. A tilted microfluidic platform with precise angle adjustment to modulate the magnitude and direction of gravitational acceleration is used to perform drainage experiments on water – oil and gas – oil displacement pairs under seven gravity conditions and six viscosity ratios. The results reveal that in the capillary-fingering regime, gravity aligned with the flow promotes backward/transverse meniscus motion that stabilizes the front, while counter-flow gravity accelerates longitudinal interfaces, triggering a transition to gravitational fingering. In the crossover zone, gravity aligned with the flow reduces the frontal velocity and viscous resistance, producing a compact tip-splitting morphology. In the viscous-fingering regime, gravity influences pattern evolution without regime transitions, with stabilization dominating below a critical velocity. Quantitative analysis of Haines jumps and the associated energy conversion and dissipation in the capillary–gravitational forces-dominated regime indicates that gravity alters the entry capillary pressure threshold. Increasing gravity in the flow direction shortens the buildup stage preceding Haines jumps, thereby increasing the frequency of these jumps. This induces a more compact displacement morphology, with the net efficiency, denoted as the ratio of external work converted to surface energy, decreasing from 63% to 46% for gas displacing oil and from 50% to 45% for water flooding as gravity increases from 0.5 g to 1 g . These findings bridge pore-scale interface dynamics and reservoir-scale flows to optimize subsurface processes.