(418b) Controlling Aggregation In Non-Polar Environments: Applications for Energy and Enhanced Oil Production

The problem of asphaltene precipitation falls at the intersection of two important chemical engineering topics: nanomaterials for enhanced oil production applications and the fundamentals of nanoparticle and colloidal aggregation.  Furthermore, investigations of asphaltenes reveal unique electrostatic characteristics which may prove to be important in understanding and controlling asphaltene aggregation and precipitation. 

Asphaltenes, the heaviest and most aromatic component of crude oils, comprise a broad chemical class defined only by their solubility: asphaltenes are soluble in aromatic solvents such as toluene and xylene, but insoluble in light alkanes such as hexane and heptane. The precipitation process takes asphaltenes from the molecular scale in solution to a fully separated phase which can coat rock walls and metal equipment.  Asphaltene formation in reservoirs, on bulkheads and wellbores, and in pipelines can greatly impede oil production operations.  While the application of bulk aromatic solvents can facilitation the dissolution of asphaltenes to the molecular level, their use is neither economically viable nor environmentally friendly.  Mechanisms which can stabilize asphaltenes at the nanoparticle or colloidal scale would not impart full thermodynamic stability to systems containing asphaltenes, but the metastability provided could greatly enhance oil production and allow for asphaltene removal.  For instance, several polymeric dispersants have been shown to stabilize asphaltenes at the colloidal scale, through surface adsorption onto the particles. 

The phenomenon of nanoparticle and colloidal aggregation provides the key to understanding the phase separation of asphaltenes.  Unstable asphaltene systems undergo a cascading aggregation process: molecular association is quickly followed by aggregation to the nano- and micron scale, followed by aggregation to even larger length scales, sedimentation, and full separation.  Truncating this aggregation process could arrest the precipitation process entirely.  At the same time, asphaltene colloids and nanoparticles occur naturally in non-polar systems: hydrocarbon reservoirs.  Colloidal aggregation typically occurs through long-range electrostatic attraction and short-range van der Waals forces.  The control of aggregation in aqueous environments can be accomplished through surface charge functionalization of colloids to induce repulsion, or the addition of counterions in solution to screen electrostactic attractions.  However, non-polar suspension are more energetically difficult to stabilize than aqueous suspensions due to the low dielectric constant of non-polar solvents and the lack of counterions in solution to screen electrostatics interactions.  Studies in non-polar suspensions of carbon black, silica, and polytstyrene colloids with surfactant additives suggest that charging can occur even in non-polar suspensions.  While some polymeric dispersant can stabilize asphaltenes at the colloidal scale, the involvement of electrostatic interactions remains an open question.

In this study, we investigate the aggregation of colloids to the micron scale and beyond, as well as its arrest through the use of polymeric dispersants.  While some dispersants can actually dissolve asphaltenes at sufficient concentrations (~1% by weight), our measurements demonstrate that other dispersants can truncate asphaltene aggregation and stabilize colloidal asphaltenes on the sub-micron scale.  Furthermore, the dispersants which arrest aggregation do so at much smaller concentrations (~10-100 ppm by weight), showing great promise for industrial applications.  The dynamics of the truncated aggregation lend some clues as to the nature of the stabilization provided.  We measure both colloidal asphaltene aggregation rates as well as the electrophoretic mobility of the asphaltene colloids to investigate the possibility that electrostatic repulsion accounts for the stabilization of colloidal asphaltenes.