Asphaltene Deposition in Vertical Open Flow

Asphaltenes can precipitate and form deposits in open flow in wellbores, pipelines, and oil and gas facilities. These deposits can lead to production losses and high treatment costs; hence, deposition models are used to predict the conditions at which deposition is a risk. Asphaltene deposition at near ambient temperatures, where asphaltenes are commonly considered to precipitate as glassy particles, involves the adhesion and accumulation of asphaltene molecules, particles, and aggregates at the surface of a pipe or vessel. Several models have been developed for asphaltene deposition at these conditions which account for asphaltene aggregate size and concentration, shear conditions, fluid properties, and surface properties [1-2]. However, asphaltene deposition at the higher temperatures encountered in deep, offshore production has not been rigorously investigated. At higher temperatures, asphaltenes separate from the oil as part of a heavy liquid phase (liquid droplets) rather than as glassy particles and may deposit differently.

In a recent study, an asphaltene deposition apparatus was commissioned to investigate asphaltene deposition in horizontal laminar flow in both the glassy particle and liquid droplet regimes [3]. The apparatus consists of two transfer vessels, a co-axial nozzle-like static mixer, and a capillary tube test section, all housed in an air bath. Metered positive displacement pumps are used to displace pre-diluted bitumen and n-heptane, respectively, from the transfer vessels to the test section. In a deposition experiment, the differential pressure across a capillary tube (an indicator of asphaltene deposition) is measured during the flow period. In addition, the capillary tube is removed after each experiment to measure the mass, solvent content, and location of the deposit.

In the previous study, asphaltene deposition in horizontal laminar flow was evaluated in a 1.7 mm ID capillary tube for mixtures of bitumen and n-heptane, with n-heptane contents ranging from 65 to 90 wt%, fluid flow rates from 2 and 4 cm³/min, capillary tube lengths from 3 to 30 cm, and temperatures of 50, 90, and 130°C. It was found that, in the glassy particle regime (50 and 90°C), precipitated asphaltene particles formed a porous deposit near the inlet of the capillary tube with cycles of deposition and erosion occurring during the flow period. In the liquid droplet regime (130°C), asphaltene-rich heavy phase droplets settled and coalesced to form a continuous heavy phase layer leading to stratified flow with occasional temporary plugging.

The main objective of the current study is to determine if the deposition mechanisms change in vertical flow. The previously commissioned asphaltene deposition apparatus was modified to evaluate asphaltene deposition in laminar vertical flow in both the glassy particle regime (50°C) and the liquid droplet regime (130°C). Deposition (or multiphase flow) in a 1.7 mm ID capillary tube was assessed for a test fluid of bitumen diluted with n-heptane. Feed composition from 65 to 90 wt%, flow rates of 2, 4, and 8 cm³/min, and tube lengths from 3 to 30 cm were investigated. The preliminary results were qualitatively similar to those for horizontal flow. In the glassy particle regime, the differential pressure cycled, consistent with periods of deposition and erosion. The mass of the deposit was small and its location varied in each experiment. In the liquid droplet regime, the pressure drop increased exponentially in the first 40 min until the heavy phase occupied approximately 80% of the capillary tube volume. The flow regime up to this point was interpreted as core-annular flow with the heavy phase coating the surface of the pipe and the light phase carrying entrained liquid droplets in the core. After 40 min, low amplitude cycles of increasing and decreasing pressure drop were observed, consistent with slug flow. In summary, as observed in horizontal flow, deposition occurred in the glassy particle regime while multiphase flow was observed in the liquid droplet regime.

References

1.Vargas, F.M., Creek, J.L., Chapman, W.G. On the Development of an Asphaltene Deposition Simulator. Energy Fuels, 24, 2010, 2294-2299.

2.Eskin, D., Ratulowski, J., Akbarzadeh, K., Pan, S. Modelling Asphaltene Deposition in Turbulent Pipeline Flows. J. Chem. Eng., 89, 2011, 421-441.

3.Do, “Effect of Temperature on Asphaltene Deposition Mechanisms in Horizontal Flow,” University of Calgary, 2021.