Viscosity and Phase Behavior of Petroleum Sulfonate Solutions in the Liquid Crystalline Region

Abstract

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.

Videomicroscopy of Two-Phase Steady Cocurrent Flow in a Model Porous Medium

“…Liquid crystals (LC) of this type exhibit non-Newtonian behavior, with viscosity being both shear rate and time dependent (20).…”

Microemulsion to Liquid-Crystal Transition in Two Anionic Surfactant Systems

“…The apparent viscosities measured for these dispersions are comparable in magnitude with those measured for oil-free liquid-crystalline dispersions in a petroleum-sulfonate system. 6 Fig . 18 indicates that the middle-phase microemulsion is Newtonian and exhibits a viscosity of about 8.0 mPa·s [8.0 cpl.…”

“…A corresponding general pattern in the behavior of apparent viscosity has also been found. 6 The works of Ghosh and Miller 7,8 show the rich, and apparently general, phase behavior that is obtained when increasing amounts of hydrocarbon are added to the aqueous mixtures discussed. The liquid crystals generally tend to dissolve gradually as more hydrocarbon is added; and, at a water/oil ratio (WOR) of I, classic microemulsion behavior is obtained.…”