Contact Angles for Equilibrated Microemulsion Systems
- Ronald L. Reed (Exxon Production Research Co.) | Robert N. Healy (Exxon Production Research Co.)
- Document ID
- Society of Petroleum Engineers
- Society of Petroleum Engineers Journal
- Publication Date
- June 1984
- Document Type
- Journal Paper
- 342 - 350
- 1984. Society of Petroleum Engineers
- 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.3.2 Multiphase Flow, 4.1.2 Separation and Treating, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.2.1 Phase Behavior and PVT Measurements, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc)
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Advanced and receded contact angles have been measured on various high- and low-energy substrates as functions of microemulsion-excess phase interfacial tensions (IFT's). Many experimental difficulties peculiar to these low-tension systems caused large measurement errors. But with this constraint and with one exception, contact angles were hysteresis-free and independent of the substrate.
For lower-phase microemulsions and high-energy substrates, it is proposed that the surfactant polar group adsorbs 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 and related to previously introduced optimal salinities, in particular to that associated with best oil recovery. Results suggest the optimum attainable contact angles for microemulsion-based oil recovery may not be 0 degrees.
IFT and contact angle do not occur explicitly in the macroscopic equations governing multiphase flow through 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 individual and collective effects of these two parameters and partly because contact angles measured on idealized substrates may not accurately imitate those obtained in situ.
So far, attention has focused primarily on the roles played by independently specified IFT's and contact angles in the displacement of isolated residual oil ganglia. One conclusion of these studies is that the most 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 is not the central question. Rather, from the onset of oil bank 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 are different. The only specific proposal germane to this line of inquiry has been make by Morrow.
In view of these considerations, the possibility must be entertained that theta = 0 degrees is not optimal for tertiary oil recovery.
Measurement of contact angles for high-tension systems such as liquid/vapor or nonpolar liquid/water is exacting for a variety of reasons. For example. surface preparation is critical and requires meticulous attention to asperity, heterogeneity. chemical composition. and contamination. Fluids must be scrupulously purified or be at least of reproducible composition.
Avoiding these pitfalls was a prime consideration in this 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 at the 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 trends have 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.
It is conventional to measure contact angles through the more dense phase. Thus, for a drop of oil against air, the contact angle is measured through the oil. For a drop of the 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 depending on 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, contact angles obtained during spreading are advancing angles. Otherwise they are receding. Eventually, motion ceases and the contact angle adopts a constant value. This is the recorded value and , as suggested by Huh and Scriven, it is correspondingly labeled advanced or receded.
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