Factors Influencing Dynamic Nonequilibrium Effects in Drainage Processes of an Air‐Water Two‐Phase Fine Sandy Medium

Abstract

In efforts to reduce uncertainties about effects of minor changes in the pore structure of porous media on dynamic nonequilibrium effects (DNEs), we monitored water saturation‐capillary pressure relationships in both dynamic flow and static states in stepwise drainage processes of a sandy medium. The acquired data were used to detect DNE and their changes with time in each drainage step and construct new parameters to characterize DNE. Effects of factors affecting water flow—including capillary number, water saturation, rate of change of water saturation with time (△Sw/△t), and the maximum value of this rate (RSMAX)—on the magnitude of the DNE were then determined. Results show that entry pressures are considerably larger under dynamic flow conditions than under static flow conditions, and DNE only occurs when the water saturation exceeds a threshold value in a drainage process. During either a smooth drainage or drainage step, the maximum capillary pressure (PMAX) is reached before the minimum water saturation (SMIN). Moreover, in drainage steps RSMAX occurs before the maximum difference between the capillary pressures associated with the static and dynamic states (DPMAX), except when they start from the saturated state. Accordingly, DNEs in a drainage step are mainly reflected in the time between PMAX and SMIN (△t), DPMAX, the average sum of the squares of the difference between capillary pressures of the static and dynamic states (DPAVE), and the dynamic coefficient τ (which is often used to characterize DNE). Furthermore, our results indicate that the water saturation and its rate of change with time are more important parameters than the capillary number for modeling DNE in a drainage process, and we detected no clear correlation between either maximum or minimum values of τ and the timing of the most significant DNE during drainages. τ appears to characterize DNE effectively in smooth drainage processes, while △t, DPAVE, and DPMAX constitute meaningful supplementary parameters for the characterization of DNE in a drainage step of a stepwise drainage.

Watertable fluctuation-induced variability in the water retention curve: Sand column experiments

“…As can be seen, both the values of d decreases monotonically with increasing dSw/dt for drainage processes (Camps-Roach et al, 2010;Li et al, 2019;Sakaki et al, 2010). However, in our experiments,  first decreases and then increases as the fluctuation period decreases for both drying and wetting (Figures 10a,c).…”

“…Note that the trend of air-entry pressure is consistent with  since the air-entry pressure is inversely proportional to  . Li et al (2019) observed an increasing air-entry pressure with increasing flow velocity. This is because the maximum flow rate in their experiments was 2.59 × 10 -4 m/s, two orders of magnitude smaller than the average fluctuation rate of 1.9 × 10 -2 m/s in our experiment with amplitude of 0.19 m and period of 4000 s. Thus, the pore water blockage may not occur in their experiments.…”

“…To avoid air encapsulation or layering effects on the measurements, the column was filled carefully following previous studies (Li et al, 2019;Sakaki et al, 2010). First, the column was gradually filled with deionized water.…”

“…Nevertheless, the displacement pressure estimated from above fitting method was slightly larger than the air-entry pressure. Visual determination of the inflection point along the WRC is a more accurate method to obtain airentry pressure (Corey, 1994;Li et al, 2019).…”

An automated testing device for continuous measurement of the hysteretic water retention curve of granular media

“…Traditionally, the WRC has been considered a stationary or equilibrium relationship. The transient, or dynamic WRC, evolving from fast changes in suction or prescribed flow with high flow rates, has been investigated amongst others by Topp et al [46], Hassanizadeh et al [14], Zhuang [53], Milatz et al [30] and Li et al [22]. Independent findings show that different water retention curves are obtained for different rates of pore water volume change oV w ot or depending on the change rate of degree of saturation with time oS r ot .…”