Visualizing and quantifying the crossover from capillary fingering to viscous fingering in a rough fracture

• Published in: Water resources research 2017-09, Vol.53 (9), p.7756-7772
• Main Authors: Chen, Yi‐Feng, Fang, Shu, Wu, Dong‐Sheng, Hu, Ran
• Format: Article
• Language: English
• Subjects: Apertures; capillary fingering; Capillary flow; Carbon dioxide; Carbon dioxide fixation; Carbon sequestration; crossover zone; Displacement; Enhanced oil recovery; Flow rates; Forces (mechanics); Fractures; Front velocity; Imaging techniques; immiscible displacement; Oil; Oil recovery; Porous media; Ratios; rough fracture; Saturation; Statistical analysis; Statistical methods; Viscosity; viscous fingering
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1002/2017WR021051

Immiscible fluid‐fluid displacement in permeable media is important in many subsurface processes, including enhanced oil recovery and geological CO2 sequestration. Controlled by capillary and viscous forces, displacement patterns of one fluid displacing another more viscous one exhibit capillary and viscous fingering, and crossover between the two. Although extensive studies investigated viscous and capillary fingering in porous media, a few studies focused on the crossover in rough fractures, and how viscous and capillary forces affect the crossover remains unclear. Using a transparent fracture‐visualization system, we studied how the two forces impact the crossover in a horizontal rough fracture. Drainage experiments of water displacing oil were conducted at seven flow rates (capillary number log10Ca ranging from −7.07 to −3.07) and four viscosity ratios (M=1/1000,1/500,1/100 and 1/50). We consistently observed lower invading fluid saturations in the crossover zone. We also proposed a phase diagram for the displacement patterns in a rough fracture that is consistent with similar studies in porous media. Based on real‐time imaging and statistical analysis of the invasion morphology, we showed that the competition between capillary and viscous forces is responsible for the saturation reduction in the crossover zone. In this zone, finger propagation toward the outlet (characteristic of viscous fingering) as well as void‐filling in the transverse/backward directions (characteristic of capillary fingering), are both suppressed. Therefore, the invading fluid tends to occupy larger apertures with higher characteristic front velocity, promoting void‐filling toward the outlet with thinner finger growth and resulting in a larger volume of defending fluid left behind. Key Points Lower invading fluid saturations were observed in the crossover zone and a phase diagram was obtained for rough fracture The saturation reduction in the crossover zone is induced by the significant suppression of both viscous and capillary fingerings In the crossover, invading fluid occupies larger aperture spaces with higher front velocity, stimulating void‐filling toward the outlet