Investigation of Asphaltene Deposition in Horizontal Flow Above and Below the Heavy-Phase Glass Transition

Abstract

Asphaltene deposition is a longstanding flow assurance issue and has been much investigated at near-ambient temperatures where asphaltenes typically precipitate as glassy particles. However, there are few data available at higher temperatures where an asphaltene-rich phase may come out of solution as liquid droplets. An apparatus was designed and commissioned to investigate deposition mechanisms over a range of temperatures in horizontal laminar flow using a test fluid of bitumen diluted with n-heptane. The pressure drop through a 1.75 mm I.D. capillary tube was monitored for indications of deposition during the flow period, and the tube was removed at the end of each experiment to measure the mass and location of the deposit. Asphaltene deposition was assessed considering the following variables: capillary tube lengths from 3 to 30 cm, n-heptane contents in the feed from 65 to 90 wt %, fluid flow rates of 2 and 4 cm3/min, and temperatures of 50, 90, and 130 °C. In the glassy particle regime (50 and 90 °C), a highly porous localized deposit formed near the inlet of the capillary tube. Cycles of deposition and erosion were observed during the flow period. However, in the liquid droplet regime (130 °C), periodically unstable stratified flow was observed. The heavy-phase holdup at the peak pressure drop was ∼88% of the capillary tube volume. The n-heptane content of the heavy phase was consistent with equilibrium heavy-phase compositions reported in the literature. The results suggest that some assumptions made in the models developed for asphaltene particle deposition may not apply in high-temperature applications where the asphaltene-rich phase separates as liquid droplets.