Numerical simulation of wax deposition in pipelines

Crude oils are complex mixtures of hydrocarbons. Among these components, high molecular weight paraffin (waxes) and asphaltenes may cause severe flow assurance issues in production and transportation subsea pipeline systems. Wax molecules are dissolved in the crude oil, when the temperature decreases, the solubility of high molecular weight paraffins decreases rapidly, and this results in the formation of gels of complex morphology, that deposit on the cold walls of the subsea pipelines during the flow of waxy crude oils, and this is a big problem for the subsea pipelines. The cost of wax management is enormous and rapidly increasing because of increased oil production in deep-sea areas. In this work, is presented a new numerical model for predicting wax deposition in oil pipes. While the almost part of the models existing in the literature are based on a separated two-fluid description of the oil/wax-oil flow, the model proposed is based on a mixture description, where wax is considered as a suspended phase surrounded by crude oil. When wax separates in the crude oil, the mixture becomes a gel-like fluid, and this mechanism is incorporated in the flow model by a specific rheological model where the mixture viscosity depends on both the local wax concentration and its “age”. In order to simultaneously model moving regions of different “ages”, our model solves a transport equation for the ageing time. Deposition is modelled by solving wax mass balance, dissolved asphaltenes mass balance, and energy. Wax generation and asphaltenes removal are modelled as a first-order reaction coupling mass balance between the species, where the local equilibrium saturation is given as a function of the mixture temperature. In this work, is present the implementation of the model in the open-source C++ library OpenFOAM and validation of the model between some similar works. Besides a discussion of the results for wax deposition and a parametric study about the critical parameters.