Three-dimensional fingering structures in immiscible flow at the crossover from viscous to capillary fingering

Fluid displacement experiments were performed with immiscible fluids in a packed bed of glass spheres. The three-dimensional (3D) structure of the fingering pattern was visualized by computer tomography (CT) for a range of capillary numbers (Ca) between 5.22 x 10(-7) and 1.04 x 10(-4) at the viscosity ratio of log M = -2.203 in the absence of the influence of buoyancy force. Based on the 3D CT images, the characteristics of the fingering pattern at the crossover from viscous fingering (VF) to capillary fingering (CF) were investigated quantitatively. The structure of the fingering pattern changed gradually from streak-like for VF to more compact for CF with a decrease in Ca. Associated with the crossover from VF to CF, saturation of the invading phase (IP) and the fractal number of the structure both increased. The invaded pore size and invaded throat size distributions were also estimated for all values of Ca. The invaded pore size distribution was similar for all Ca values, but a clear shift in the throat size distribution was observed with the crossover from VF to CF. (C) 2019 Elsevier Ltd. All rights reserved.