Meaning
Dimensionless ratios quantify the relative impact of viscous forces versus surface tension forces within a porous medium or narrow channel. The capillary number represents this relationship by dividing the product of fluid velocity and dynamic viscosity by the interfacial tension. Engineers use the metric to predict the displacement efficiency of one fluid by another, particularly when oil recovery or coating processes demand specific wetting conditions.
It becomes a primary constraint once the physical dimensions of the flow path approach the scale of pore throats or thin film clearances.
Fluid Dynamics
Calculations for the capillary number rely on precise measurements of interfacial tension between two immiscible liquids. Low values indicate that surface tension dominates the interaction and tends to trap residual phases in dead-end pores. Higher values signal that viscous forces overcome the pinning effects of the interface.
Experimental data sets establish critical thresholds for transition zones where flow regimes shift from stable displacement to fingering patterns.
Operational Performance
Maintaining target velocity profiles requires constant adjustment of pump pressure to hold the capillary number within an efficient production window. Operators observe how shifts in fluid temperature alter viscosity and shift the calculated ratio accordingly. Inadequate pressure leads to fluid retention in stagnant zones, whereas excessive force consumes energy without improving the sweep efficiency.
Managers compare these results against laboratory bench tests to confirm the displacement behaviour of the production stream under reservoir conditions.
System Sensitivity
Small changes in surface tension properties create nonlinear responses in displacement patterns during injection phases. High capillary numbers often cause the interface to break up into droplets rather than moving as a uniform front. Such behavior dictates the design of chemical additives which alter the interfacial chemistry to stabilize the sweep.
Precise control over this ratio ensures the maximum recovery of hydrocarbons from porous rock formations.
