Asphaltene Deposition and Heavy Phase Accumulation in a Vertical Capillary Tube

Asphaltene deposition is a long-standing flow assurance issue typically modeled as particle deposition. In recent laminar horizontal flow experiments with n -heptane diluted bitumen, it was demonstrated that the asphaltene deposition mechanism changed from particle deposition at room temperature, where asphaltenes precipitated as glassy particles, to heavy phase accumulation at 130 °C, where a liquid asphaltene-rich phase formed. The purpose of this study is to determine if and how the deposition and flow behavior change in vertical versus horizontal flow for the same fluids and conditions. As in the previous study, the mechanisms were assessed at 50 and 130 °C in laminar flow through a 1.75 mm capillary tube using mixtures of bitumen and n -heptane with n -heptane contents from 65 to 90 wt %. However, in this case, the tube was oriented vertically with an upward flow. The pressure drop was monitored during the flow period, and the mass and location of the deposit were measured at the end of each experiment. Experiments were performed for capillary tube lengths of 3 to 30 cm and flow rates of 2, 4, and 8 cm 3 /min. It was confirmed that the mechanism for flow disruption changed from particle deposition below the glass transition to heavy phase accumulation in the multiphase flow regime above the glass transition. In the glassy particle regime, porous annular deposits occurred with more rapid cycles of blockage and blowout than with horizontal flow. In the liquid droplet regime, core-annular flow occurred, with the heavy phase accumulating in the annulus and appearing to undergo cycles of annular and slug flow. The behavior was analogous to the stratified flow observed in the horizontal configuration, but with more frequent cycles. The results suggest that, in the liquid droplet regime, flow assurance may better be evaluated as a multiphase flow issue rather than a deposition issue.