Introduction

Production of heavy oils and paraffinic crude reserves often results in the deposition of organic solids, typically waxes or asphaltenes. The organic deposits can reduce the productivity of the reservoir as well as foul piping and surface equipment. Current chemical and mechanical methods for treating deposition are only partially effective partly because the deposition process is poorly understood.

A joint research program investigating asphaltene deposition is now underway at the University of Calgary, the University of Alberta and DB Robinson Research Ltd of Edmonton. The various steps of the deposition process (precipitation, aggregation, surface contact and adhesion) are to be investigated under both static and flowing conditions. A key component of the project is an apparatus for non-intrusively measuring asphaltene deposition under flowing conditions using x-ray tomography. This flow-loop apparatus will be designed and constructed with COURSE funding.

ASPHALTENE DEPOSITION

Asphaltenes are a solubility class and are usually defined as the fraction of a crude oil that precipitates in an aliphatic solvent (typically n-pentane or n-heptane) yet remains soluble in toluene. Asphaltenes are the most aromatic and polar fraction of crude oil and have the largest heteroatom and metal content. They consist of a variety of molecular species with molar masses of at least 1,000 g/mol (1).

Asphaltenes appear to self-associate on a molecular level even in aromatic solvents. The degree of association depends on the composition, temperature and likely the pressure of the system. The average size of the associated asphaltenes ranges from 2,000 to 10,000 g/mol or approximately 2 to 6 molecules per aggregate (2).

Asphaltenes can also precipitate upon a change in temperature, pressure or composition. The asphaltenes appear to precipitate as small "primary" particles which rapidly aggregate into macro-particles. The size of the primary particles is unknown but is likely in the order of a few microns based on visual observations. The size of the aggregated asphaltenes depends on the solven temperature and pressure but is in the order of several hundred microns (3).

Solubilized asphaltenes can adsorb directly onto hydrophilic surfaces probably through interactions with heteroatom functional groups (4). Hence, adsorption can be significant in the reservoir. Direct adsorption is less likely on hydrophobic surfaces such as metals. Deposition on pipes and surface facilities more likely requires the precipitation of asphaltenes, the formation of aggregates and adhesion of aggregates to equipment surfaces.

To understand and effectively prevent or treat asphaltene deposition, it is desirable to investigate each step of the deposition process, particularly under the flowing conditions where deposition normally occurs. In this way, potential treatments can be designed for particular steps in the deposition process.

Our research is focused on deposition in pipelines. First, the precipitation, aggregation and adhesion of asphaltenes will be investigated at static conditions in order to identify critical deposition factors and to design the flow-loop apparatus. Then, the flow-loop will be employed to assess deposition under flowing conditions and to gather data for deposition models.

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