Effect of Temperature on Asphaltene Deposition Mechanisms in Vertical Flow
Creators: name: Rodriguez, Maria Alejandra Gomez
Publisher: name: Graduate Studies
Year: 2023
Subjects: subject: Asphaltenes
subject: Deposition
subject: High temperature
subject: Bitumen
subject: Engineering--Chemical

description: Deposits of asphaltenes can cause significant problems in oil and gas facilities, pipelines, and wellbores. These deposits can lead to production losses and high treatment costs. To prevent deposition, models are used to predict the risk of asphaltene precipitation. When asphaltenes precipitate at near ambient temperatures, they form glassy particles that adhere to the surfaces of pipes or vessels. Several models have been developed to account for factors such as asphaltene aggregate size and concentration, shear conditions, fluid properties, and surface properties. However, asphaltene deposition at higher temperatures, such as those encountered in deep offshore production, has not yet been thoroughly researched. At higher temperatures, asphaltenes separate from crude oil as part of a heavy liquid phase (liquid droplets) 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 regime (&lt; 100°C) and the liquid droplet regime (&gt; 130°C). Asphaltene deposition in horizontal laminar flow was evaluated in a capillary tube for mixtures of bitumen and n-heptane at different flow rates and tube lengths. In the glassy particle regime, 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, 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 was to use the asphaltene deposition apparatus to determine if the deposition mechanisms change in vertical flow. In the glassy particle regime, the deposition mechanism for vertical and horizontal flow was found to be similar. Porous deposits were formed near the inlet of the capillary tube, exhibiting cycles of blockage and blowout. The deposits were localized and occupied a maximum of 40% of the tube volume, with solvent contents of 82 ±10 wt%. The frequency of the blockages was higher in vertical flow compared to horizontal flow. In the liquid droplet regime, the deposition mechanism was also found to be similar for horizontal and vertical flow. The heavy phase, with solvent contents of approximately 30 wt% in horizontal flow and about 40 wt% in vertical flow, accumulated throughout the length of the tube. In horizontal flow, the flow regime was interpreted as stratified flow of a light-phase emulsion over a heavy-viscous liquid. In vertical flow, the flow regime was interpreted to begin as dispersed flow with a gradual accumulation of heavy phase at the tube wall leading to core-annular flow. The heavy phase accumulation caused a rise in pressure drop until the holdup reached 80%, then transitioning to slug flow with cycles of pressure build-up and blowout.