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Understanding snap-off dynamics in pore–throat channels with non-circular cross-sections is crucial for subsurface applications, as most natural porous rocks exhibit complex geometrical features. The fundamental mechanism governing snap-off in non-circular pore–throat systems is identified as a curvature-gradient-driven instability, which is further modulated by geometric constraints and fluid properties.
In this study, microfluidic experiments combined with numerical simulations were conducted to investigate snap-off dynamics in constricted channels with non-circular cross-sections during drainage displacement. Three types of constricted channels with square, equilateral triangular, and four-pointed star cross-sections were fabricated using 3D printing techniques, all with a pore-to-throat size ratio of 3. Two pairs of immiscible fluids—surfactant solution with n-decane and surfactant solution with paraffin—were employed. The wetting phase (surfactant solution) initially saturated the microfluidic models, after which the non-wetting phase was injected at a constant flow rate.
As the non-wetting phase traversed the throat and entered the pore space, snap-off events occurred due to capillary-driven flows. The snap-off time and the volume of the disconnected non-wetting phase were quantified over a wide range of capillary numbers (Ca). Classical theoretical and experimental studies (Gauglitz, St. Laurent et al. 1987, Ransohoff, Gauglitz et al. 1987) suggest that above a critical capillary number, the snap-off time is independent of Ca, whereas below this threshold it is inversely proportional to Ca.
Systematic investigations in this study reveal that the transition Ca lies between 10-6~10-4. For Ca<10-6, the snap-off volume remains constant and the snap-off time decreases linearly with Ca, indicating that the static snap-off theory (Roof 1970) is applicable. For Ca>10-4, the snap-off time becomes insensitive to Ca, consistent with previous findings (Ransohoff, Gauglitz et al. 1987). Within the transition regime, the snap-off time follows a new power-law relationship with Ca. The viscosity ratio is found to have a negligible influence on snap-off dynamics.
Furthermore, numerical simulations provide detailed velocity and pressure fields within the channels, offering mechanistic support for the experimental observations. This work advances the understanding of snap-off behavior in complex porous geometries and provides valuable insights for engineering applications such as hydrocarbon recovery and CO2 sequestration.
Gauglitz, P. A., C. M. St. Laurent and C. J. Radke (1987). An Experimental Investigation of Gas-Bubble Breakup in Constricted Square Capillaries. SPE California Regional Meeting.
Ransohoff, T. C., P. A. Gauglitz and C. J. Radke (1987). "Snap-off of gas bubbles in smoothly constricted noncircular capillaries." AIChE Journal 33(5): 753-765.
Roof, J. (1970). "Snap-off of oil droplets in water-wet pores." Society of Petroleum Engineers Journal 10(01): 85-90.
| Country | China |
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