.[1] 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 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 values. Significant differences in values were observed in uncorrected measurement between the three systems, but corrected values were very similar in all cases. Corrected 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 values may result at least in part from porous medium packing microheterogeneities that influence flow and pressure gradients in larger systems.Citation: Hou, L., L. Chen, and T. C. G. Kibbey (2012), Dynamic capillary effects in a small-volume unsaturated porous medium: Implications of sensor response and gas pressure gradients for understanding system dependencies, Water Resour.

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While cotton is largely produced for its fibers, the seed and hull byproducts are essential for agricultural economies around the world for their high oil content and wide range of uses. Cottonseed oil is often extracted by solvent extraction, a process that is of increasing concern due to its potential to impact human and ecological health. For this reason, environmentally‐benign extraction methods are increasingly being explored. The focus of this work is on aqueous surfactant‐based extraction of cottonseed collets taken directly from a conventional solvent‐based extraction facility. A propoxylated–ethoxylated extended anionic surfactant was tested for its ability to extract cottonseed oil. Results found that moderate (77%) extraction yields could be achieved by the surfactant given adequate surfactant concentration (0.5% in this case). However, unlike previous work with other oilseeds, the best extractions were achieved at moderate interfacial tensions (0.2–3.0 mN/m); surfactant formulations capable of producing ultralow interfacial tensions led to much lower extraction yields, as did water‐only extraction. Similar behavior was also observed for two different conventional surfactants, with decreasing interfacial tension values below 0.1 mN/m leading to decreasing yields for all surfactants tested.

The assumption that gas is infinitely mobile, moving without viscous pressure drops, is common in studies of unsaturated flow in porous media. The objectives of this work were to use experimental measurements to examine that assumption in systems experiencing rapid drainage and to explore the extent to which observed pressure drops could be described by conventional multiphase flow simulation tracking viscous flow in both phases. Because many published studies have used vented columns in an effort to equilibrate pore gas pressures with inlet gas, an additional objective of the work was to use experimental measurements to explore the ability of column vents to equilibrate pore gas with inlet gas during dynamic drainage. Results of the work suggest that gas pressure gradients can be significant, and that the assumption of infinite gas mobility is likely to be unsatisfactory in many systems where moderately rapid saturation change occurs. While vents have the potential to influence flow by providing additional gas inlets, experimental results of this work show almost no impact on pore gas pressures from a vent similar in size to those in other published studies. An equation developed as a part of the work suggests that the spatial slope of gas pressure with distance away from the front during dynamic drainage is proportional to the ratio of outflow Darcy velocity to saturated hydraulic conductivity for vertical columns. As such, systems with more rapid saturation change also have a greater potential to exhibit experimental artifacts related to gas pressure gradients.