Pore-scale experimental investigation of capillary desaturation in fractured porous media

Establishing the capillary desaturation curve (CDC) in subsurface porous media is necessary for planning nonaqueous phase liquid (NAPL) cleanup and recovery projects. CDCs are well established in fractures and intact (unfractured) rocks, but poorly so in fractured porous media where many industrial applications reside. We constructed the CDC in fractured porous media and explored the displacement/trapping mechanisms impacting its behavior using a newly designed fractured micromodel. The micromodel consisted of four different matrix blocks, surrounded by a fracture network, to probe the effect of variable pore elements sizes. Waterflooding experiments into the oil-saturated model at different injection rates were performed to characterize the desaturation process and develop the CDC under water-wet (WW) and oil-wet (OW) conditions. Capillary number formulas under the two wettability states were systematically defined, incorporating the size of the pore elements as well as the oil viscosity. In the WW model, no distinguishable trend was noted for the CDC. Capillary suction displacement, indicated by negative pressure responses, was identified during the experiments conducted at low capillary numbers. Under OW conditions, a modified capillary-number expression was proposed to account for the respective locations of the viscous and capillary forces. The obtained CDCs exhibited uniform trends compared to those in the WW model. Oil bridges, correlated with a peak in the pressure signal, were observed within the fracture between adjacent matrices. The residual oil saturation increased in the matrix blocks located at the terminus of the formed bridges. In addition, the residual oil saturation increased in the matrix block with the largest throat size due to oil bypassing by the water phase invading from multiple locations of the fracture. Volumes of the residual oil clusters were found to depend on the matrix throat size, but not the pore size. Finally, desaturation under the discontinuous displacement mode, tested via an individual multi-rate experiment, resulted in lower brine invasion into the matrices compared to that during the continuous displacement mode. The insights gained from this study can be extended to various applications involving multiphase flow in fractured porous media.