Measurement of Air−Water Interfacial Area for Multiple Hysteretic Drainage Curves in an Unsaturated Fine Sand

Abstract

A new method has been developed to measure fluid-fluid interfacial area during multiple drainages, along with the measurement of hysteretic capillary pressure-saturation (P(c)-S) relationships in unsaturated porous media. The method makes use of an automated device which has been successfully used for rapid measurement of hysteretic P(c)-S relationships, in combination with a novel technique for interfacial area measurement. A pure anionic surfactant, sodium octylbenzene sulfonate (SOBS), is used as a surface-active tracer, and a flow-through UV spectrometer is used to monitor the real-time concentration change of SOBS solution due to adsorption to the fluid-fluid interface during drainage. The Gibbs and Langmuir adsorption equations are applied in combination with a continuous mole balance to calculate interfacial areas. Using this method, air-water interfacial area of a fine sand was measured as a function of capillary pressure and saturation during primary, secondary, and one scanning drainages to explore the influence of drying/wetting history on interfacial area. Results show that 8-20 and 12-22 cm(2)/g more air-water interface was generated in secondary and scanning drainages, respectively, than in primary drainage, with the magnitude of the difference varying as a function of saturation. An advantage of the method is that interfacial tension variations from the method itself are relatively small, typically on the order of 5 mN/m, so measured areas are not skewed by surface-tension-induced changes in interfacial area. In a measurement specifically designed to study the influence of surfactant-induced interfacial tension variations, approximately two times more interfacial area was observed for a 25 mN/m interfacial tension change, in comparison with a system with relatively constant interfacial tension. Implications of results of interfacial area measurements for hysteresis in the three-dimensional relationship between capillary pressure, saturation, and interfacial area are discussed.

Insights into the Relationships Among Capillary Pressure, Saturation, Interfacial Area and Relative Permeability Using Pore-Network Modeling

“…Hassanizadeh and Gray (1993b) conjectured that the resulting surface might be the same for drainage and imbibition, i.e., it might be devoid of hysteresis. This conjecture has been investigated in few studies using computational and experimental approaches (see, e.g., Reeves and Celia 1996;Held and Celia 2001;Chen and Kibbey 2006).…”

“…These techniques are mostly categorized under two main groups; tracers or imaging techniques. Aqueous tracers have been used by Karkare and Fort (1996), Brusseau et al (1997), Kim et al (1997), Saripalli et al (1998), Anwar (2000), Schaefer et al (2000), and Chen and Kibbey (2006), and gas tracers have been used in some other studies such as Kim et al (1999), Costanza-Robinson and Brusseau (2002), and Brusseau et al (2006). Imaging techniques have been used by Montemagno and Gray (1995), Wildenschild et al (2002), Cheng et al (2004), Culligan et al (2004), Al-Raoush and Willson (2005a,b), and Brusseau (2005, 2006).…”

“…3. There are some experimental studies where a nw -S w curves are produced (see, e.g., Cheng et al 2004;Chen and Kibbey 2006). These curves are all found to be hysteretic.…”

Measurement of Interfacial Area Production and Permeability Within Porous Media

“…This method is deficient with respect to physics in that it completely neglects the role and presence of fluidfluid interfaces and leads to hysteretic relationships. Interfaces between fluids are not only crucial quantities for interfacedependent processes, such as mass transfer between phases, but there is a growing amount of experimental evidence that shows a relationship between p c , interfacial area, and saturation (S) accounts for the observed hysteresis in the k r -S relationship by collapsing the curves upon a single surface (Culligan et al 2004, Brusseau et al 2006, Chen and Kibbey 2006, PyrakNolte et al 2008.…”

“…While many static tests have been performed to calculate relations between a wn , p c , and S, e.g. (Culligan et al 2004, Brusseau et al 2006, Chen and Kibbey 2006, Pyrak-Nolte et al 2008, there are some parameters that cannot be determined by standard, static experimental procedures. In this work, we present a combination of new techniques that overcome this discrepancy and allow for an estimation of the needed dynamic model parameters.…”