Dynamic capillary effects in a small-volume unsaturated porous medium: Implications of sensor response and gas pressure gradients for understanding system dependencies

Rate dependencies in system properties observed during nonsteady state unsaturated and multiphase flow are often referred to as dynamic capillary effects. One widely studied dynamic capillary effect is the apparent dependence of measured capillary pressure on the rate of saturation change. While this phenomenon has been observed for over four decades, a clear picture of the source of the phenomenon and its true magnitude remains elusive. Furthermore, reported dependencies on system properties and state variables have been contradictory. The focus of this work was on quantifying the relationship between measured capillary pressure and rate of saturation change using a small volume system with highly characterized fluid-selective microsensors. Experimental measurements in three systems were used to calculate the dynamic capillary coefficient tau as a function of saturation during drainage. Corrections for sensor response and flow-induced gas pressure gradients were applied to explore how these potential artifacts would impact measured tau values. Significant differences in tau values were observed in uncorrected measurement between the three systems, but corrected values were very similar in all cases. Corrected tau values were found to be on the order of 10(3) Pa s or less-one to two orders of magnitude lower than the uncorrected values, and two or more orders of magnitude lower than most published values for similar porous medium/fluid combinations. Because of the small size of the experimental system used, results suggest that at the representative elementary volume (REV) scale, the dependence of measured capillary pressure on the rate of saturation change may not be as significant as previously thought for unsaturated systems. It is hypothesized that the larger magnitude of some previously reported tau values may result at least in part from porous medium packing microheterogeneities that influence flow and pressure gradients in larger systems.