Abstract
Advanced and receded contact angles have beenmeasured on various high- and low-energy substrates asfunctions of microemulsion-excess phase interfacialtensions (IFT's). Many experimental difficulties peculiar to these low-tension systems caused large measurementerrors. But with this constraint and with one exception, contact angles were hysteresis-free and independent ofthe substrate.
For lower-phase microemulsions and high-energy substrates, it is proposed that the surfactant polar groupadsorbs on the solid and then a surfactant bilayer forms. This bilayer provides the effective substrate that relates to contact angle and IFT's through Young's equation.
An optimal salinity for contact angles is defined andrelated to previously introduced optimal salinities, inparticular to that associated with best oil recovery. Results suggest the optimum attainable contact anglesfor microemulsion-based oil recovery may not be 0 degrees.
Introduction
IFT and contact angle do not occur explicitly in the macroscopic equations governing multiphase flowthrough porous media, rather their impact is manifested implicitly through the relative permeability and capillary pressure functions. This dependence has been established experimentally, but there is not yet a satisfactory theoretical treatment. It is partly for this reason that it is difficult to ascribe with confidence the individualand collective effects of these two parameters and partly because contact angles measured on idealized substratesmay not accurately imitate those obtained in situ.
So far, attention has focused primarily on the roles played by independently specified IFT's and contactangles in the displacement of isolated residual oil ganglia. One conclusion of these studies is that themost favorable wettability condition for tertiary oil recovery is 100% water-wet (i.e. theta = 0 degrees)when measured through the aqueous displacing phase.
However, as we have pointed out, oil mobilization isnot the central question. Rather, from the onset of oilbank formation, the essential problem is to "maintain continuity of the flowing oil filaments to as low a saturation as possible before they rupture and are irretrievably lost." Since the mechanism of this rupture-trapping process is different from that of oil mobilization, it is quite possible (in fact likely) that the effects of contact angle and IFT also aredifferent. The only specific proposal germane to this line of inquiry has been make by Morrow.
In view of these considerations, the possibility mustbe entertained that theta = 0 degrees is not optimal fortertiary oil recovery.
Measurement of contact angles for high-tensionsystems such as liquid/vapor or nonpolar liquid/water isexacting for a variety of reasons. For example. surface preparation is critical and requires meticulous attentionto asperity, heterogeneity. chemical composition. and contamination. Fluids must be scrupulously purified orbe at least of reproducible composition.
Avoiding these pitfalls was a prime consideration inthis study. So first, a surface preparation technique was developed that guaranteed a clean smooth substrate. Second, it seemed obvious a priori that the presence of surfactant in high concentrations would completely dominate the usual laboratory contaminants. However, new difficulties attended contact angle measurements atthe low IFT's common to multiphase microemulsion systems, and these may have clouded results.
The most that can be claimed is that a start has been made toward acquiring techniques needed to measure contact angles potentially pertinent to flow of microemulsions through porous media. Some trendshave been developed, correlations made, a model proposed, and a few conclusions and conjectures outlined, but much more and much better work will be required before significant advances in understanding are made.
Terminology
It is conventional to measure contact angles through themore dense phase. Thus, for a drop of oil against air, the contact angle is measured through the oil. For a drop ofthe same oil against water, the contact angle would be measured through the water.
A similar convention holds in regard to moving interfaces; they are called advancing or receding dependingon motion of the more dense fluid with respect to substrate it has contacted. In the experiments reported here a drop is placed on a substrate previously equilibrated with cell fluid and allowed to spread. When the drop fluid is more dense than the cell fluid, contactangles obtained during spreading are advancing angles. Otherwise they are receding. Eventually, motion ceases and the contact angle adopts a constant value. This is therecorded value and, as suggested by Huh and Scriven, it is correspondingly labeled advanced or receded.
SPEJ
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