Formation and aging of incipient thin film wax-oil gels

Abstract

A fundamental study of the deposition and aging of a thin incipient wax-oil gel that is formed during the flow of waxy oils in cooled pipes was performed. The solubility of high molecular weight paraffins in naphthenic, aromatic or paraffinic solvents is very low and decreases rapidly with decreasing temperature. This property of the paraffins leads to the formation of gels of complex morphology that deposit on the cold walls of the subsea pipelines during the flow of waxy crudes. This deposition reduces the pipe diameter and decreases the flow capacity of the pipe. These wax-oil gels contain a large fraction of oil trapped in a 3-D network structure of the wax crystals that behaves as a porous medium. After the incipient gel is formed, wax molecules continue to diffuse into this structure, thereby increasing its wax content. A model system of wax and oil mixture was used to understand the aging process of the wax-oil gels, which hardens the wax deposit with time. To understand the physics of the aging process for incipient thin-film deposits, a series of laboratory flow loop experiments was performed. The aging process was a counterdiffusion phenomenon with a critical carbon number above which wax molecules diffuse into the gel deposit and below which oil molecules diffuse out of the deposit. The aging rate of the gel deposit depends on the oil flow rate and the wall temperature. A mathematical model developed predicted the growth and wax content of the gel deposit on externally cooled pipe walls. The theory agreed with experiments excellently for thin gels.