Gas-Liquid Two-Phase Imbibition in Microscale Capillaries: Experimental Insights into a Minimum-Resistance Radius

Spontaneous imbibition is a capillarity-driven mechanism that controls multiphase transport in porous media and plays a critical role in fluid recovery from tight formations. In this study, quartz microtube imbibition experiments (100-320 μm) were performed with kerosene as the wetting fluid and air as the nonwetting fluid. A two-phase flow model that accounts for gravitational and inertial forces was developed and validated, achieving an average error reduction of 81.9% relative to the classical Lucas-Washburn (L-W) formulation. Experiments revealed a nonmonotonic dependence of imbibition time on tube radius, confirming a minimum resistance radius at which imbibition efficiency is maximized. Dimensionless analysis based on Capillary, Reynolds, and Bond numbers showed that tube radius predominantly governs imbibition dynamic, whereas interfacial tension mainly affects early stage rise velocity. The identification of the minimum resistance radius as a characteristic length scale provides a new physical basis for understanding capillary-driven transport and offers practical guidance for enhancing oil recovery in tight reservoirs. A minimum resistance radius ensures the most efficient imbibition, implying that tuning pore size distributions toward this scale could accelerate fluid uptake and improve recovery of nonwetting fluids in tight reservoirs.