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Viscosity effects occurring in aqueous solutions containing a combination of surfactants, polymers and salts are of considerable interest in enhanced oil recovery operations. High molecular weight polymers, which are normally employed to improve sweep efficiency, lose much of their viscosity because of mechanical degradation and other adverse conditions in the reservoir. The mixing of the surfactant slug with the polymer slug in micellar flooding may lead to phase instability and low recovery efficiency. The formation of surfactant-polymer aggregates on the other hand, might have a positive effect on recovery. Such association complexes do not lose their viscosity permanently due to shear as they are quickly rebuilt upon cessation of shear forces. The present work shows that significant viscosity increases can be obtained by combining a petroleum sulfonate surfactant with a low molecular weight polyethylene oxide. For example, an addition of Carbowax®−20M to 4% Petrostep®−420 at 1.5% NaCl salinity first results in a viscosity increase with a maximum at about 0.2% of polymer. After this, the viscosity drops sharply reaching a minimum at 0.5% of polymer. Beyond this point, the viscosity begins to rise again at a rate which is somewhat higher than that observed for the polymer solution alone. The behavior observed with Carbowax®−6000 was similar although the increase in viscosity was smaller. No discernible effects were observed with Carbowax®−600. The magnitude of the viscosity increase observed at 0.2% of polymer was found to be strongly dependent on salinity.
Viscosity effects occurring in aqueous solutions containing a combination of surfactants, polymers and salts are of considerable interest in enhanced oil recovery operations. High molecular weight polymers, which are normally employed to improve sweep efficiency, lose much of their viscosity because of mechanical degradation and other adverse conditions in the reservoir. The mixing of the surfactant slug with the polymer slug in micellar flooding may lead to phase instability and low recovery efficiency. The formation of surfactant-polymer aggregates on the other hand, might have a positive effect on recovery. Such association complexes do not lose their viscosity permanently due to shear as they are quickly rebuilt upon cessation of shear forces. The present work shows that significant viscosity increases can be obtained by combining a petroleum sulfonate surfactant with a low molecular weight polyethylene oxide. For example, an addition of Carbowax®−20M to 4% Petrostep®−420 at 1.5% NaCl salinity first results in a viscosity increase with a maximum at about 0.2% of polymer. After this, the viscosity drops sharply reaching a minimum at 0.5% of polymer. Beyond this point, the viscosity begins to rise again at a rate which is somewhat higher than that observed for the polymer solution alone. The behavior observed with Carbowax®−6000 was similar although the increase in viscosity was smaller. No discernible effects were observed with Carbowax®−600. The magnitude of the viscosity increase observed at 0.2% of polymer was found to be strongly dependent on salinity.
Summary Aqueous solutions of two petroleum sulfonate surfactants, a pure sulfonate surfactant and one mixed with an ethoxylated sulfate surfactant, were studied with polarizing microscopy, polarized light screening (PLS), and viscometry. The same sequence of phases was seen with increasing salinity for the various surfactants-i.e., transformation from an isotropic micellar solution to a lamellar liquid crystalline phase to an isotropic phase that scattered light and exhibited streaming birefringence. Addition phase that scattered light and exhibited streaming birefringence. Addition of calcium ions to the petroleum-sulfonate/ethoxylated-sulfate mixtures led to the same basic sequence of transformations, but the effect of calcium was about 11.5 times as great as that of sodium on a molar basis. Many small crystalline particles were also observed when calcium was present. A cone-and-plate viscometer was used to measure the apparent viscosity of the various solutions. A maximum in apparent viscosity was always found in the transition region between a dispersion of spherulitic liquid crystalline particles in an aqueous phase and a single liquid crystalline phase. The maximum occurred whether the transition was brought about by phase. The maximum occurred whether the transition was brought about by changes in salinity, divalent cation concentration, oil content, or other compositional variables. Apparent viscosity generally decreased with increasing temperature, although dependence on temperature was not monotonic at high salinities. Apparent viscosity decreased with increasing shear rates and was time-dependent at a given shear rate. At low salinities, apparent viscosity generally decreased with time. At high salinities, apparent viscosity frequently increased with time. Introduction The surfactant slugs injected during chemical flooding processes typically contain between 2 and 10% surfactant, the remainder being brine, short-chain alcohol. and in some cases, oil. After injection, the slugs begin to mix with reservoir fluids, including brines with appreciable contents of divalent cations. It is desirable for process design to understand system phase behavior and its relationship to viscosity in the composition range pertinent for slug formulation. One requirement a slug must meet is long-term stability with respect to macroscopic phase separation. Aqueous micellar solutions obviously satisfy this criterion, but the surfactant in such solutions is often too hydrophilic to produce the ultralow interfacial tensions (IFT's) required to displace oil. When salt is added or the composition is otherwise modified to make the surfactant less hydrophilic and capable of achieving ultralow tensions, the micellar solution is frequently converted to a lamellar liquid crystal or a stable dispersion of liquid crystal in an aqueous solution. Thus, mixtures suitable for surfactant slugs often contain liquid crystal. In previous papers, we described some general trends in the phase behavior of dilute anionic surfactant/alcohol/brine systems when composition and temperature are varied. Here we present information on the effect of divalent cations. Particularly noteworthy in both cases is the existence at some compositions of brine-rich liquid crystalline phases that are not highly viscous and hence would not cause plugging of the reservoir rock. These phases demonstrate that chemical flooding processes need not be designed to preclude formation of liquid crystals because the crystals are not invariably the extremely viscous "gels" sometimes encountered. Indeed, liquid crystals have been proposed as oil-displacing agents. The viscosity of a surfactant slug must be high enough to provide proper mobility control. It is well known that slugs sometimes provide proper mobility control. It is well known that slugs sometimes exhibit non-Newtonian, behavior and that slug viscosity can vary greatly with composition. It seems plausible that such behavior stems from the presence of liquid crystals. Indeed, some previous effort has been directed toward relating viscosity to phase previous effort has been directed toward relating viscosity to phase behavior in systems containing liquid crystals or at least to the appearance or disappearance of birefringence. However, few general guidelines for systematically controlling slug viscosity have emerged from these studies. In this paper, we present extensive viscosity measurements and relate them to the general trends in phase behavior mentioned previously. Although a similar approach was used earlier to explain the previously. Although a similar approach was used earlier to explain the effect of salinity and alcohol on viscosity, the results given here also treat the effects of temperature, divalent ions, oil, and the addition of an ethoxylated sulfate surfactant. With the information obtained, it should be possible in many cases to predict whether viscosity of particular formulations will increase or decrease upon addition of oil, divalent cations, or salt.
The surfactant and polymer slugs for the chemical recovery pilot test operated by DOE were designed to be effective at the divalent ion concentration of the injection water. Laboratory tests indicated that even if there were no intrusion by the formation brine, the slugs would experience an increase in divalent ion content on contact with the rock. It was found that with calcium present, optimal salinity of the surfactant system was reduced by 4 to 7 times the amount of calcium on an equivalent basis. The minimum IFT was raised 4-fold by 8 eq/m3(meq/L) of calcium, but the valley was much broader. The distribution of calcium indicated that it occurred primarily in the surfactant phase. Empirical equations were developed for calculating the effect on polymer viscosity of polymer concentration, salinity, and calcium. Over a range of polymer concentrations and salinities, calcium had close to 10 times more effect on viscosity than an equivalent increase in salinity. It was found that calcium from the rock played a major role in surfactant behavior, but affected polymer behavior in only a minor way.
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