Pore-Scale Investigation of Boundary and Gravity Effects on Chemical-Assisted Spontaneous Imbibition Using Microfluidics

Spontaneous imbibition is a primary recovery mechanism in unconventional reservoirs. Although core-scale experiments capture the combined effects of parameters such as interfacial tension (IFT) and permeability, the individual roles of boundary conditions and gravity remain poorly understood at the pore scale. This study employed microfluidics to directly visualize and quantify these effects during chemically assisted spontaneous imbibition. Micromodels consisting of a tight matrix adjacent to a fracture were saturated with dyed kerosene. Two boundary conditions were examined: cocurrent imbibition, with two ends open, and countercurrent imbibition, with only one side open. Experiments were conducted in vertical and horizontal orientations using water and nonionic (Tween 80) and anionic (Enordet O342) surfactants. Time-lapse imaging was used to track imbibition dynamics. Quantitative metrics, including the imbibition front velocity, perimeter-area ratio, displacement efficiency, and recovery factor, were extracted to characterize the mechanics of the process. For water-only experiments, cocurrent imbibition produced smoother displacement fronts with reduced fingering and a higher displacement efficiency than countercurrent imbibition. Similar trends were observed in most of the chemical-assisted tests. Both surfactants enhanced the final recovery and displacement efficiency by stabilizing the imbibition front and reducing residual oil saturation. Horizontal experiments exhibited higher early time front velocities but more pronounced fingering and lower displacement efficiency than vertical experiments. In the absence of gravity, long unstable fingers formed rapidly and impeded the further recovery. In vertical water imbibition, a high capillary force dominated the process, which caused rapid but unstable fingering. Conversely, the application of chemical additives reduced the IFT and transitioned the system to a gravity-dominated regime, which effectively stabilized the front in the vertical orientation. The pore-scale morphologies and recovery trends were found to be governed by the interplay of the inverse Bond number, viscous coupling, and relative permeability.