Deposition of high molecular weight paraffins on the inner wall of subsea production and transportation pipelines continues to be a critical operational problem faced by the petroleum industry. The accurate prediction of wax deposition rates and deposited wax spatial distribution would be invaluable information for the design of subsea lines. A critical review of the literature conducted lead to the conclusion that there is not enough experimental evidence to determine which are the relevant mechanisms responsible for wax deposition. Based on the conclusions of the literature search, a research program was initiated with the aim of identifying the relative importance of the suggested wax deposition mechanisms namely, molecular diffusion, Brownian diffusion, shear dispersion and gravity settling. To this end, simple basic experiments were designed and complemented by numerical simulations. In these experiments, visualization of the deposition process was sought for a stagnant solution of paraffin and solvent within a cavity submitted to a transverse temperature gradient. Studies were also conducted for deposition under laminar channel flow, also submitted to a transverse temperature gradient. The visualizations studies for the deposition in the stagnant layer revealed the formation of a mushy region growing from the cooled wall formed by a network of crystals. Images with higher magnifications revealed motions of crystals not formed at wall. Detailed data of temporal and spatial distributions of the wax deposited under laminar flow conditions were obtained. Numerical simulations of the experiments conducted were developed employing molecular diffusion as the only deposition mechanism, as it is done in the vast majority of the deposition models available in the literature. The model under predicted the evolution of the deposited wax, indicating that mechanisms other than molecular diffusion may be present.
Wax deposition in production and transportation pipelines continues to be a relevant problem for the industry, particularly in offshore operations. The crude oil flows out of the reservoir at, typically, 60 oC into the production pipelines. These lines carry the oil to the platforms and from the platforms to shore. At large water depths, the ocean temperature at the bottom is of the order of 5 oC. The solubility of wax in the oil is a decreasing function of temperature. As the oil flows, it loses heat to the surrounding water. If the crude oil temperature falls below the Wax Appearance Temperature (WAT), the wax may precipitate and deposit along the inner walls of the pipeline. The accumulation of the deposited material may lead to increased pumping power, decreased flow rate or even to the total blockage of the line with loss of production and capital investment.
A significant research effort has been devoted to the understanding and modeling of the wax deposition problem [1–4]. This is a complex problem that involves several disciplines such as thermodynamics, heat transfer, mass transfer, crystal growth and fluid dynamics. An accurate prediction of the temporal and spatial distributions of the deposited wax along the pipeline would be invaluable information that would help in the design stages of the field,
