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The surface excess of sodium dodecyl sulfate (SDS) in aqueous solutions of SDS and the polymer poly(vinylpyrrolidone) (PVP) has been measured as a function of SDS and PVP concentrations using neutron reflection. Below the critical aggregation concentration (CAC) the adsorption of SDS is increased by the presence of PVP, indicating that the two components interact cooperatively at the surface. Between the CAC and the critical micelle concentration (CMC) of the surfactant there is a slight depletion of SDS from the surface. Comparison of coverages determined by neutron reflection with those from earlier radiotracer work indicates that, in the higher concentration range, PVP is bound to the surfactant layer, creating a region from which surfactant is depleted, which is further evidence for a strong polymer/surfactant interaction at the surface. Comparison of the effect of added PVP on the surface tension with the neutron reflection measurements indicates that, even below the CAC, the surfactant complexes to the polymer to some extent in the bulk solution. There are no measurable effects of the polymer on the thickness of the surfactant layer at any concentration. There is an indication that at the surface the surfactant is slightly displaced outward from water on addition of polymer, but accurate structural determination of the mixed layer proved too difficult to be certain of this result.

Summary Increasing flooding-solution viscosity with polymers provides a favorable mobility ratio compared with brine flooding and hence improves volumetric sweep efficiency. Flooding with a polymer solution exhibiting elastic properties has been reported to increase displacement efficiency, resulting in a sustained doubling of the recovery enhancement compared with the use of conventional viscous-polymer flooding (Wang et al. 2011). Flooding with viscoelastic-polymer solutions is claimed also to increase recovery more than expected from changes in capillary number alone (Wang et al. 2010). This increase in displacement efficiency by viscoelastic polymers is reported to occur because of changes in the steady-state-flow profile and enhancements in oil stripping and thread formation. However, within the industry there are doubts that a genuine effect is observed, or that improvements in displacement efficiency occur with field-applicable flow regimes (Vermolen et al. 2014). In this study, we demonstrate that flooding with viscoelastic-polymer solutions can indeed increase recovery more than expected from changes in capillary number. We show a mechanism of fluctuations in flow at low Reynolds number by which viscoelastic-polymer solutions provide improvements in displacement efficiency. The mechanism, known as elastic turbulence, is an effect previously unrecognized in this context. We demonstrate that the effect may be obtained at field-relevant flow rates. Furthermore, this underlying mechanism explains both the enhanced capillary-desaturation curves and the observation of apparent flow thickening (Delshad et al. 2008; Seright et al. 2011) for these viscoelastic solutions in porous media. The work contrasts experiments on flow and recovery by use of viscous and viscoelastic-polymer solutions. The circumstances under which viscoelasticity is beneficial are demonstrated. The findings are applicable to the design of formulations for enhanced oil recovery (EOR) by polymer flooding. A combination of coreflooding, micromodel flow, and rheometric studies is presented. The results include single-phase and multiphase floods in sandstone cores. Polymer solutions are viscoelastic [partially hydrolyzed polyacrylamide (HPAM)] or viscous (xanthan). The effects of molecular weight, flow rate, and concentration of the HPAMs are described. The data lead us to suggest a mechanism that may be used to explain the observations of improved displacement efficiency and why the improvement is not seen for all viscoelastic-polymer floods.

A rheo-optical device outfitted with a Peltier temperature control for rapid temperature changes has been constructed that allows simultaneous measurement of the optical rotation of light and the controlled-stress rheology. Optical rotation provides a direct in situ assessment of the extent of triple helix reversion in a gelatin solution undergoing physical gelation in the rheometer. Thermal gelation of gelatin was monitored over a wide range of concentrations and temperatures. Assuming dynamic scaling theory applies, viscosity data below the gel point were used to evaluate the gel point and determine the value of the viscosity exponent. Above the gel point, creep-recovery experiments are used to measure the shear modulus and determine the dynamic scaling elastic modulus exponent. During thermal gelation, the time-dependent optical rotation shows an initial rapid growth region where new helices are formed, followed by a slower growth region involving helix lengthening. For cases where the gel point occurs before the helix reversion slows appreciably, the viscosity and modulus exponents are found to depend on gelatin concentration, but not on temperature. However, anomalous exponents are measured using the same methods at higher temperatures, where the helix reversion slows appreciably before the gel point is reached. These results suggest that extreme caution must be used in evaluating dynamic exponents from any physical gelation process. The observed concentration dependences of the dynamic scaling exponents are discussed in terms of chain overlap and entanglement. For gelatin gelation, the plethora of different, reported percolation exponents in the literature are rationalized.

Single-phase flows of viscoelastic polymer solutions in both microfluidic devices and rock cores exhibit apparent flow thickening. We demonstrate that this thickening occurs above a critical Deborah number corresponding to the onset of spatio-temporal fluctuations. These fluctuations are observed to occur over a broad range of spatial and temporal scales consistent with elastic turbulence. The fluctuations provide a previously unreported mechanism for enhancing the displacement of a second, capillary trapped, immiscible phase.

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The spin-probe detected polarity index H(25 °C) of SDS micelles decreases linearly with the number of inserted sugar-based nonionic surfactant molecules. This decrease is interpreted as being due to the expulsion of water molecules by the sugar groups from the polar shell surrounding the hydrocarbon core of the dodecyl micelle. Employing the geometrical model described in the companion paper immediately preceding this work, the effective volume of water expulsion is found to be similar to the volume of the sugar groups after taking into account that the OH bonds of the sugar groups also contribute to the polarity index H(25 °C). The estimate of the hydration of pure SDS micelles as a function of their aggregation number from these studies with the spin probe 16 doxylstearic acid methyl ester is similar to that with 5 doxylstearic acid methyl ester. This confirms that both spin probes are located similarly in the polar shell.

Properties of new nonionic surfactants are reported. The amphiphiles possess two n-alkyl chains and two glucamide head-groups and have the formula (C n H2n+1)2C[CH2NHCO(CHOH)4CH2OH]2 with n = 5−9 (abbreviated here as di-(Cn-Glu)). For di-(C7-Glu) in the concentration range 0.4−8%, an upper critical solution temperature (UCST) is observed; this is unusual behavior for a nonionic surfactant. Nonetheless these materials behave like classic surfactants with regard to changes in liquid crystal phases, surface tensions, and critical micelle concentrations (cmc's) as a function of chain length n. Small-angle neutron scattering (SANS) data indicate the presence of anisotropic micelles, cylindrical for di-(C5-, 6-, and 7-Glu) but discoidal for di-(C8-Glu). There is apparently little effect of temperature (25−70 °C) on cmc or micelle structures.