Study of asphaltene effect on wax precipitation and deposition.

摘要

Wax precipitation and deposition in oil pipelines is a critical issue during crude oil production and transportation. Accurately predicting wax deposition in oil pipeline can reduce production downtime, saving millions of dollars. To study wax deposition, a laboratory flow loop was built and tested using model oil. Model oil was prepared by dissolving food grade wax into kerosene and mineral oil mixture to simulate carbon number distribution of a waxy crude oil. A dynamic wax deposition model was built and the model prediction showed excellent agreement with experimental data. It was identified that wax deposit thickness was strongly dependent on the operating conditions. The wax deposit thickness was decreased when flow rate was increased and/or when wall temperature was increased. Due to insulation effect of wax deposit, wax deposit thickness stopped increasing after approximately 12 hours. However, wax content in the deposit continued to increase due to the diffusion of wax molecules within gel deposit. Molecular diffusion induced by radial thermal gradient within the porous deposit was the main cause of aging process.;Asphaltene is one of the four major classes in crude oil with saturates, aromatics and resin. Very limited researches have been carried out on the effect of asphaltenes on wax deposition. To study the impact, Bitumen crude oil with high asphaltene content was chosen to mix with the model oil. Effect of asphaltenes on wax precipitation was investigated by measuring wax appearance temperatures (WATs) of oil mixtures using a densitometer. Asphaltene concentration in the oil was ranged from 0.01 wt% to 0.06 wt%. Results indicated WAT was firstly decreased with the existence of asphaltene, and then was increased after asphaltene concentration reached certain limitation. Cross-polarized microscopic (CPM) observation revealed that the wax crystals image was changed from needle shape to globular shape, and the waxy crystal network was no longer entangled together. Laboratory flow loop experiment was conducted using oil mixture with asphaltene concentration of 0.01 wt% and 0.03 wt%. Results showed that asphaltenes slowed down the whole deposition process and the impact increased as asphaltene concentration increased. The dynamic wax deposition model developed over-predicted deposition due to asphaltenes effect. A model with an adjustable equation was proposed to consider the impact of asphaltene in predicting wax deposition. In the equation, two unknown parameters were obtained by fitting the experimental data to the equation. After adjustment, wax deposition model results successfully predicted the wax deposition rate with asphaltene contents.;The effect of asphaltene on wax deposition was further studied for crude oil blends. Two types of crude oils (one with high asphaltene content and one with high paraffin content) were mixed together in different ratios, and were tested using a small scale wax deposition (cold finger) setup. Cold finger experimental results revealed that asphaltene affected initial wax deposition rate but the long-term wax deposition was determined by the WAT of oils. Asphaltene compound increased long-term wax deposition weight possibly due to the fact that WAT were increased as well as asphaltenes were entrapped in the deposits. In addition, thermal gradient influenced the asphaltene effect on wax deposition. Hence for field application, asphaltene effect dominates wax deposition. Commercial thermodynamic simulator, GUTS, over-predicted initial wax deposition rate of crude oil blends, thus needs to be adjusted to better predict deposition.