A pore-scale investigation of flow patterns during spontaneous and forced imbibition in fractured porous media

Microfractures are ubiquitous in subsurface porous media due to natural and engineering activities. Imbibition in fractured porous media affects various geological engineering applications, while their behaviors have not been sufficiently understood. This study conducts spontaneous and forced imbibition experiments at various capillary numbers in micromodels with different fracture apertures. Air and mixtures of glycerol and ethanol are the non-wetting and wetting fluids, respectively. Results identify two imbibition patterns: matrix-preferential and fracture-preferential. A global capillary number is insufficient to predict flow patterns in fractured porous media. Pore-scale interfacial behaviors influence imbibition patterns by affecting the crossflow between fracture and matrix. The concave-to-convex transition of the meniscus is found in pore invasion and introduces a transient capillary resistance inhibiting the crossflow into the matrix. Cooperative pore fillings weaken the capillary resistance and promote imbibition. The crossflow from the wetting front in the fracture to the matrix driven by capillary pressure prefers matrix imbibition at low capillary numbers. A higher capillary number increases the viscous resistance and limits crossflow, which promotes the transition from matrix- to fracture-preferential imbibition. A rise in the glycerol concentration increases the viscosity and contact angle. These features increase the viscous and capillary resistance for crossflow and facilitate fracture-preferential imbibition. A wider-fracture aperture enhances the capillary-dominant matrix-preferential imbibition by increasing the capillary pressure difference, while it also has a lower viscous resistance and causes an early transition of imbibition patterns at large capillary numbers. Results are relevant to the prediction and simulation of imbibition in fractured porous media.