EFFECT OF CATIONIC SURFACTANTS ON ORGANIC LIQUID WATER CAPILLARY-PRESSURE SATURATION RELATIONSHIPS

Many solutes, either naturally occurring or introduced, are surface ac tive and sorb preferentially at the interfaces of subsurface systems. In multiphase systems, the sorption of surfactants affects the capilla ry pressure-saturation relationships, fundamental constitutive relatio nships in the modeling of multiphase flow. In this study, the impact o f surfactant sorption on capillary pressure relationships for organic liquid-waters systems was demonstrated by qualitatively correlating me asurements of sorption and zeta potential, with interfacial tension an d contact angle and, in turn, quantitatively relating these measuremen ts to changes in capillary pressure-saturation relationships for o-xyl ene-water-quartz systems containing a cationic surfactant, cetyltrimet hylammonium bromide (CTAB). The results show that the sorption of CTAB causes the naturally hydrophilic system to become hydrophobic, as evi denced by a change in the contact angle from about 10 degrees-15 degre es to 155 degrees or 180 degrees, depending on the pH. This change in hydrophilicity is reflected in the zeta potential of the system which goes from negative to positive as the aqueous phase CTAB concentration increases. The spontaneous imbibition capillary pressure-saturation r elationship is more sensitive to the sorption of CTAB than the drainag e relationship. To predict the observed changes in both capillary pres sure-saturation relationships, a modified form of Leverett's function was used where roughness and curvature corrections were incorporated i nto the intrinsic contact angle to give an operational contact angle. A comparison of the measured and predicted capillary pressure-saturati on relationships showed reasonable agreement.