Article pubs.acs.org/EF
Method for Estimating Oil Viscosity via Dielectric Spectroscopy W. H. Hunter Woodward,*,† Hagar Zohar,‡ Robbyn Prange,‡ Rakesh Srivastava,† David Brennan,‡ Suraj Deshmukh,‡ and Jeff Mitchell‡ †
Analytical Sciences, and ‡Formulation Science, The Dow Chemical Company, Midland, Michigan 48667, United States ABSTRACT: The physical properties of bitumen (i.e., asphalt) are a topic of consistent interest in the fields of oil, gas, and mining. In this study, the relationship between bitumen viscosity and the rotation of asphaltene clusters was empirically explored via broadband dielectric spectroscopy (BDS). Cluster size was found to be independent of the concentration, and BDS was found to be a reliable means to predict viscosity in neat bitumen and concentrated solutions. Because of the simplicity and portability of modern impedance analyzers, this technique offers a unique method for estimating a wide range of oil viscosities in the field (from 105 cP) and even offers the possibility of in situ oil viscosity monitoring in pipelines or oil-lubricated mechanical systems.
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INTRODUCTION Canada’s reserve of extra-heavy oil or bitumen is the third largest source of petroleum in the world at 175 billion barrels.1 Because of the remote location, bitumen is transported from reserve locations (e.g., western Canada) to refineries (e.g., Chicago) via ambient temperature pipelines. The viscosity of bitumen does not allow for neat transport, however, and requires viscosity reduction through solvent addition. Bitumen is diluted with 20−40% natural gas condensate (NGC) to reduce viscosity, enable effective transport, and meet pipeline viscosity specifications.2 Because of diluent cost, recycle loops, and potential supply issues, there is significant motivation to reduce NGC levels necessary to transport bitumen. This is difficult, however, because the composition of bitumen is somewhat debated in modern literature, and its predicted physical properties could be dependent upon one or more of several proposed models. It is important, therefore, to understand which analytical techniques can be used to examine and quantify the properties of bitumen. Here, the use of broadband dielectric spectroscopy (BDS) to quickly estimate the viscosity of even highly viscous bitumen was explored. The observed correlation between dielectric relaxation phenomena and viscosity supports the colloidal model for the physical description of bitumen. Like any crude oil, bitumen is composed of saturates, aromatics, resins, and asphaltenes (i.e., SARA). The ratio of SARA fractions in bitumen varies widely based on the source, but the SARA fractions are approximately 20, 35, 30, and 15 wt %, respectively.3 The composition results in an American Petroleum Institute (API) gravity below 10° and a viscosity on the order of 105 cP.4 The high viscosity of bitumen has been attributed to interactions between the asphaltene molecules/ aggregates. It is still somewhat debated whether asphaltenes give rise to colloidal particles via π−π stacking5 or a supermolecular assembly6 and whether resins/maltenes play a significant role in aggregate formation,7−9 although the colloidal model is favored in the literature. According to the modified Yen model, bitumen is a colloidal system where asphaltenes (4−10 molecules) are dispersed in the continuous phase as nanoaggregates with interiors of π−π © 2014 American Chemical Society
stacked fused aromatic ring systems and exteriors of peripheral alkane substituents (diameters of ∼2 nm).10,11 At higher concentrations (2−5 kg/m3), these nanoaggregates can coalesce further to form clusters (diameters of
