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Distillation assays provide the most widely used characterization data for modeling crude oils in refinery processes but can only define a relatively small fraction of the boiling curve for heavy oils. One way to extend the distillation range for heavy oils is to lower the distillation pressure while still avoiding thermal cracking temperatures. A vacuum distillation apparatus (DVFA-I) that was previously used to measure the vapor pressure of heavy components was modified to fractionate heavy oils and bitumens. The modified apparatus (DVFA-II) is a batch distillation system without reflux operating at pressures down to approximately 0.01 Pa, compared with 100 Pa for a conventional vacuum distillation. With the standard procedure developed for DVFA-II, up to 50 wt % of a Western Canadian bitumen was distilled into eight cuts plus a residue. This is an improvement over the 26 wt % distilled by a spinning band vacuum apparatus. The cumulative weight percent distilled was repeatable to within 2.3% for a given boiling point range. The densities and molecular weights of the cuts from two fractionations increased monotonically versus the weight percent distilled and were repeatable to within 0.18 and 4.0%, respectively.

Reservoir fluids from Lake Maracaibo have reportedly caused asphaltene operational problems ranging from plugging of wellbores, pipelines and flowlines to clogging of surface facilities (Garcia et al, 2001). Production of fluids from some part of the region has been dramatically reduced due to asphaltene precipitation and deposition (Vasquez, 2010). Asphaltene and wax precipitation is a serious problem in production, transport and processing of reservoir fluids. Of particular concern are the effects of asphaltene precipitation and their potential to disrupt production due to deposition in the near-wellbore regions and production tubulars. This phenomenon is directly influenced by changes in temperature, pressure and composition. Commonly, low temperatures increase the probability of asphaltene precipitation; however, experimental studies on the fluid under study demonstrated unusual asphaltene phase behavior. This project involved experimental studies on fluid phase behavior as part of a formation damage investigation. The main challenges with fluids from the Maracaibo area are the relative high H2S content (1–3%), high reservoir temperature (270°F) and the asphaltenic nature of the crudes. In this study, the asphaltene precipitation envelope was determined using Near Infrared (NIR) Solid Detection System (SDS), High Pressure Microscope (HPM), Particle Size Analysis (PSA) and gravimetric techniques. As expected, a significant amount of asphaltene was observed to precipitate during depressurization. However, reversibility of the precipitated asphaltene was also observed below the bubble point and during re-pressurization. What was unusual about this fluid was the unconventional asphaltene precipitation onset conditions found at low temperatures. For most crude oils worldwide, asphaltene precipitation onset pressure increases at lower temperatures; however, the fluids considered in this work have shown a non typical behavior wherein the asphaltene onset pressure decreases with decreasing temperature. Such behavior was earlier presented by Ting et al, 2003 on the Maracaibo oils; however, no reports have been published since.

The applicability of simulated distillation (SimDist) for the characterization of heavy oils and bitumen is examined with a focus on the fraction of the oil corresponding to the distillation residue. To assess SimDist, atmospheric equivalent boiling points from a SimDist assay, a conventional physical distillation assay, and a deep vacuum distillation assay were compared for three heavy oil samples from different geographical regions. All of the data were obtained from the literature. The SimDist data matched the distillation data to within its reported repeatability of ±7 °C in the range of 100–400 °C but deviated progressively from the distillation data beyond 400 °C, erroneously characterizing the oil as more paraffinic. The deviation in the SimDist boiling points is attributed to calibration curves based on alkane properties and possibly to the polarization of the liquid phase of the chromatograph by the aromatic and asphaltenic components of the bitumen. A Gaussian extrapolation of the SimDist data fitted to the boiling points below 400 °C matched the boiling points from the physical distillation to within ±10 °C.

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