SAS Environmental Services Ltd.

Microemulsions for the oil & gas industry


Courtesy of SAS Environmental Services Ltd.

The S urf acta nt Te ch no log ie s (STL) Group has developed a revolutionary set of new* chemical materials specialising in mcroemulsions technology based on optimised surfactant design bee also SCM, November 2004, pages 32-36). This has overcome some of the drawbacks of conventional chemistries in terms of rational rrofecular design, surfactant chemistry, phase behavbur and physico-chemical modes of action.

Now, Surface Active Solutions (SAS), the company's safes and marketing arm for the oil and gas industry, is poised to use this technobgy to revolutionise the way in which this industry manages its day-to-day operations, with particular relevance to operating and production efficiencies and waste management opera-tbns.

A microemulsbn is defined as a system of oil, water and amphiphife (surfactant) that is a single-phase, optically isotropic and thermody-namically stable liquid solution. Microemulsions usually behave like Newtonian fluids; their viscosity can be comparable to that of water, even at high droplet concentrations.

Mcroemulsion systems are clearly distinguished from emulsbns. They are thermodynamically stable and sett-organising. As such, they do not require an input of energy to form, unlike emulsion systems. This is due to the specif re mo fee ular design such that virtually all of the surfactant is located at the oil-water interlace, providing very effefent mofecular packing parameters of microemul-sion-forming surfactants at that interface.

This is not the case in emulsbns, where molecular packing is not efficient and the surfactant only partially coats the interface, leaving gaps. The uncoated surfaces are therefore directly exposed to the continuous phase. This is thermodynamically unfavourable; the dropfets aggregate by coafescing at their exposed surfaces, increasing the surface area/volume ratio and hence minimising oil-water contact. The outcome of extensive dropfet coafes-cence is therefore bulk-phase separation.

In microemulsions, there are no uncoated surfaces and thus there is no driving force for coafescence and phase separatbn. The rate of phase separation of an emulsion can actually be relatively sbw; emulsions can be created which only compfetely phase separate after several weeks or even months.

Nevertheless, the fundamental distinctbn between emulsion and microemulsbn systems remains. Indeed, the interesting properties of microemulsbns, both in general and in the context of the specific applications described here, come from the the rmodynamic properties arising from total separatbn of oil and water arising from interracial close-packing of the surfactant molecufes.

The relative oil and water domains that form in microemulsbn systems are usually so small Gn the region of 10-20 nm or less in diameter) that they do not scatter light. In emulsion systems, the structures are large enough to scatter light and, as such, they appear as cloudy colbidal solutions in comparison.

In essence, emulsion-forming surfactants can break down and disperse oil into aqueous phases, whereas O/W microemuIsioreform-ing surfactants have the ability to take things one step further and actually solubilise oil into water, almost behaving as a true solutbn.

As well as gross physcal differences, which can be determined by visual examination (microemulsions show no tendency to phase separate and are usually optically transparent, whereas emulsions are opalescent or turbid and inevitably phase separate), the two can be distinguished by measuring the surface tension at the oil-water interface.

The surface tension at plain oil-water interlaces is typically of the order of 50 mNrrr1. Emulsbns formed by mixing oil, water and ordinary' Q.e. no n-microemu Is ion-forming) surfactants are typically characterised by interracial surface tensbns of the order of 20-50 mNrrr1; microemulsions are characterised by far lower surface tensions (or ultra-low interracial surface tensbns) typically bebw 20 mNm'' and can be of the order of lO^-IO'6 mNm'1, these latter values reflecting the absence of direct oil-water contact.

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