Spontaneous Imbibition of Fluids in Porous Vycor
Authors: Wade W.H. 
Published: December 2007

The spontaneous imbibition of alkanes and alcohols in pores of 35 radius has been studied. At a rather critical molecular size The pore volume becomes insensitive to the size of imbibing molecule. Darcy's law for transient imbibition kinetics is obeyed, fluid viscosity and surface tension are valid flow parameters, the temperature coefficient of flow rate is explainable, and surface flow is found to be minimal. All these observations indicate that fluid flow in very small pores does not differ from its macroscopic behavior.

Introduction

The study of fluid flow in porous media has been a popular but discouraging endeavor for many years. The popular has in a vital need for detailed flow information systems such as crude oil migration in porous geological formations. The discouragement lies in both the impossibility of mathematically modeling the porous medium with sufficient accuracy and the difficulty of accurately describing fluid/ fluid interfacial contours within the porous matrix. Sporadically it is argued that fluid flow may be anomalous in pores smaller than micron size. This is based on the assumption that when molecular size becomes no longer infinitesimal compared with pore dimensions, then fluid viscosity and interfacial pore dimensions, then fluid viscosity and interfacial tension become less and less tenable parameters. Moreover Darcy's law for hydrodynamic flow should fail when the mean free path in the fluid approaches pore dimensions. pore dimensions. If one adopts the view that the driving force for fluid movement is pressure gradient, one can say that there are usually two contributions thereto: purposely applied hydrostatic pressures and those purposely applied hydrostatic pressures and those pressures arising from curved liquid interfaces (with pressures arising from curved liquid interfaces (with nonzero interfacial tension) in the pores. In large pores the former usually overshadows the latter, pores the former usually overshadows the latter, whereas in small pores this situation can easily be reversed. The study to be reported on here involves various aspects of the spontaneous imbibition of fluids by porous Vycor. Porous Vycor (PV) is a leached Pyrex-type glass manufactured by the Corning Glass Works, which has been of interest primarily to researchers studying adsorption and primarily to researchers studying adsorption and capillary condensation. By value of the steepness of the desorption branches of isotherms obtained on PV, it is considered a model pore system with very PV, it is considered a model pore system with very uniform pore openings. PV has a surface area of ~120 sq m/gm as determined by N2 adsorption, and the pore-size distribution is sharply centered at 30 to 35 equivalent cylindrical radius. Such pores are extremely small by usual standards for studying fluid flow, but are of a size clearly comparable to molecular size. Except for one report by Ferguson and Wade and numerous reports of pore-size distributions, as little is known about the pore structure of PV as of any other porous material. To clearly illustrate the importance of capillary driving pressures in pores of 35 A radius one can use a straightforward application of the Laplace equation:(1)

where p is the hydrostatic pressure difference across an interface with tension a and curvature J. For a hemispherical contact angle equals zero meniscus in a 35 A cylindrical pore,(2)

For water, with a 70 dynes/cm,(3)

and it should be explicitly noted that the pressure behind the fluid interface is a negative 400 atm. Alternately, water in a PV rod would have a capillary rise of 4 kilometers. Clearly such pressures are comparable with or greater than hydrostatic pressures usually employed to stimulate fluid flow in pressures usually employed to stimulate fluid flow in more common porous media. The following series of experiments on porous Vycor will be described and discussed below. 1. The molecular accommodation to n-aliphatic alcohols and n-alkanes by the pore structure, 2. The application of Darcy's law to the transient imbibition behavior of porous Vycor in a variety of liquids,

SPEJ

P. 139