Wax deposition and aging in flowlines from irreversible thermodynamics

The development of waxy crude oil and some gas condensate fields can lead to serious operational problems ecause of solidification of the paraffin components of the fluid in flowlines. Many numerical models in the literature predict the thickness of the wax deposit. However, most of these models assume that the wax-oil (gel) deposit has a constant wax content. In this work, we analyze wax deposition in laminar flow regime to predict the thickness and the composiion of the gel layer as a function of position and time. The wax-oil gel region is considered as a porous medium. The velocity field and the pressure drop are calculated from the Navier-Stokes equation in the liquid region and from a combined Darcy-type equation and the Navier-Stokes equation in the gel region. The wax amount is estimated as a result of a decrease in fluid temperature below the wax appearance temperatuure (WAT), counterdiffusion processes from thermal and molecular diffusions, and radial convection which occurs because of nonuniform gel layer thickness. We compare predicted results from our model with several experimental data from the literature. The results which are in agreement with data cannot be predicted by formulations in which chain rule is used to replace concentration gradient with temperature gradient in the molecular diffusion expression.