Morphological evolution of thick wax deposits during aging

Abstract

The presence of waxes in crude oil can lead to the formation of wax deposits on the walls of cold subsea pipelines, which restricts flow and can lead to plugging of the pipelines. This problem has recently become more significant as the production wells move further offshore causing the oil to be cooled below the cloud point before reaching shore. Wax deposition was studied in the laboratory under conditions simulating the deposition in a subsea pipeline. Wax deposition is initiated by the precipitation of wax directly on the pipe wall and the formation of a network of' wax crystals (wax-oil gel) with significant amounts of oil trapped in it. Radial diffusion of the wax molecules from the bulk solution to the gel deposit causes it to simultaneously grow and age with time. The wax molecules diffusing into the gel deposit precipitate near the interface resulting in a faster aging rate of the deposit new the interface than that near the wall. This nonuniform aging of the wax deposit causes the evolution of complex morphologies of the wax deposits. The diffusion of wax molecules into the gel matrix was analyzed theoretically during the growth of the wax deposit. This mathematical model predicted the radial variation of the morphology of the wax deposit observed in the laboratory flow loop experiments along with the deposit thickness as a function of time.