Asphaltene Deposition on Metallic Surfaces

Abstract

The potential for asphaltene deposition in wellbores and flowlines is a major concern during design of oil production and transportation facilities, especially in deep‐water environments. Understanding the processes that control asphaltene deposition, especially the relationship between precipitation and deposition, can help to reduce the risk and cost. Stainless steel capillary tubes were used to study the influences of factors including temperature, degree of asphaltene instability, and precipitant molar volume on asphaltene deposition from mixtures of stock‐tank oils and n‐alkanes. Temperature varied from 20°C to 60°C. Pressure drop across the capillary tube was used to estimate the amount and distribution of deposit formation. Existing asphaltic particles in stock‐tank oil samples did not create deposits. Below the wax appearance temperature, intermittent pressure spikes indicated deposition of wax. Above the wax appearance temperature, deposition occurred gradually from near‐onset mixtures created by co‐injection of oil and n‐alkane precipitants. Asphaltenes flocculated by addition of higher molar volume n‐alkanes deposited more material than those flocculated by lower molar volume n‐alkanes. Examination of the deposited materials showed that they contained not only asphaltenes but also waxes. Much different depositional characteristics were observed for solutions of asphaltenes in an aromatic solvent than for the stock‐tank oil from which they were derived.