Non-Polar Colloidal Asphaltenes

Postdoctoral Research

Asphaltene Precipitation

Petroleum fluids are amazingly complex materials, composed of several thousand types of compounds from the simplest short-chained alkanes to large aromatic compounds composed of many fused benzene rings. While the smallest molecules, like methane, ethane, and butane, can be easily extracted for immediate use as fuels, the components on the other end of the spectrum, called asphaltenes, cause problems not only in oil extraction and transportation, but also in the refining process.

Asphaltene precipitation occurs through a liquid-liquid phase separation process that can be triggered by changes in pressure, temperature, and composition. While asphaltenes are often stable in their native petroleum fluids, precipitation can hinder petroleum extraction and can even fully block large pipelines. Precipation conditions define asphaltenes: they are a solubility class only, being soluble in aromatics such as toluene and xylene, and insoluble in medium-chain alkanes such as hexane and heptane.

To investigate asphaltene precipitation at standard temperature and pressure in the laboratory, we combine petroleum fluids with heptane to precipitate asphaltenes. We study the bulk and colloidal properties of the resulting suspensions, and also send them through a metal pipe to assess asphaltene deposition behavior.

Asphaltene dissolution by a strong organic acid

SM-Figure3Conducting polymers like polyaniline (PANI) are widely studied for their potential use in flexible electronic devices. PANI is a semi-flexible chain composed of phenyl groups alternating with amino groups. Although PANI itself is not easily dissolved by very many solvents, it can be doped by a strong organic acid like dodeclbenzene sulfonic acid (DBSA). Depending on the level of doping, DBSA can protonate every or every other amino group on the PANI backbone, provinding a conduit for electrical charges to move through the polymer. This doping also widens the variety of solvents in which PANI can dissolve, and eases its processability in practical applications.

Asphaltenes are related to conducting polymers like PANI in the sense that they are composed of fused benzene rings with mobile pi-conjugated electrons. They also contain heteroatomic content like nitrogen, oxygen, and sulfur, whose lone electron pairs can be easily protonated. When protons are donated by strong organic acids like DBSA, the long-chained tails of the acid molecules can create solvation shells around the large and highly aromatic asphaltene molecules. While the aromatic asphaltenes are by definition highly insoluble in alkanes, the solvation shell created by DBSA can allow asphaltenes to dissolve even in traditional asphaltene prescipitants like heptane.

The figure on the left shows a cartoon representation of an asphaltene molecule, in blue. In (a), one molecule of DBSA (in red) protonoates a nitrogen heteroatom on the asphaltene. In (b), several DBSA molecules protonate the asphaltene, their 12-carbon chains creating a solvation shell around the asphaltene. By acting in this way, DBSA can enable a transition from unstable asphaltene suspensions in heptane to stable asphaltene solutions.

Electrostatic stabilization of asphaltene colloids in non-polar suspensions

CartoonStabilizationPolymeric dispersants are often used to stabilize petroleum fluids against asphaltene precipitation. While many dispersants act by dissolving asphaltenes to their molecular state, some can also stabilize asphaltenes in the form of colloids. By using static light scattering and UV-visible spectroscopy, we find that these dispersants act by adsorbing onto the surface of the asphaltene colloids, thereby preventing aggregation to an even larger size scale. In doing so, they change both the characteristic colloidal asphaltene sizes and surface charge, as measured by dynamic light scattering.

Non-polar media, having dielectric constants nearly 40 times less than that of water, present a much higher energetic barrier to ionization. For this reason, charged colloids in non-polar suspensions aggregate readily, as evidenced in asphaltene suspensions without any added dispersant. In many cases, polymeric dispersants can provide electrostatic stabilization in non-polar colloidal systems when present above their critical micelle concentrations. The non-polar tails of the dispersant can shield the micellar charges from associating with each other. In the case of non-polar asphaltenes, however, dispersants even below their critical micelle concentrations can stabilize the colloids through electrostatic repulsion. Our results indicate that the source of charge in stable non-polar colloidal suspensions is from within the asphaltenes themselves.

Aggregation & sedimentation of colloidal asphaltene suspensions

Sedimentation_smSedimentation behavior of a colloidal suspension can be used to assess stability against colloidal aggregation. Colloids in suspension will settle to the bottom of a container when the particulate material is denser than the solvent; sedimentation speed increases with the particle size squared. Stable colloids which sediment in dense suspensions leave a shock front in their wake which falls linearly with time. However, suspensions which aggregate while settling often exhibit sedimentation fronts which collapse in a non-linear fashion. The sedimentation collapse can be delayed in gelled suspensions having even a small yied stress.

Unstable asphaltene suspensions exhibit collapsing sedimentation fronts very soon after petroleum fluid is mixed with heptane. The addition of small amounts of dispersant can qualitatively change the sedimentation behavior, and suppress the fast collapse seen in the absence of dispersant. With additional dispersant, sedimentation dynamics can be delayed by orders of magnitude as compared to the dynamics observed without dispersant. Dynamic light scattering measurements confirm that the addition of dispersant simultaneously stabilizes asphaltene colloids against aggregation. Furthermore, the amount of dispersant required to stabilize the colloidal asphaltene suspensions scales with the volume fraction of asphaltenes in suspension.

This work is featured in the SoftMatterWorld Newsletter August 2010.

With acknowledgment to the Reservoir Engineering Research Institute for funding.

Publications

Previous Research

Click to view an overview of my PhD research done in the labs of Eric Dufresne and Michael Loewenberg at Yale University.Prior to attending Yale I studied calcite crystals at the air-water interface in the lab of Dave Weitz at Harvard.

Collaborations