Application of a phenomenological model for the prediction of the thickness and position of paraffin deposits of light crude oil in production pipe

Wax deposition is an important problem in the petroleum industry because it may cause obstruction in well bores, production facilities and transportation pipelines during production. To control or avoid this problem it is necessary to predict the formation of these deposits under different operational conditions. In this work is presented an implementation of a deposition model proposed by Singh et al., (2000) and applied to a highly paraffinic Colombian crude. Solution was carried out using a CFD code and the analysis considers the behavior of operational variables like: flow rate, wall temperature and time of deposition. INTRODUCTION Wax deposition over the internal surface of pipelines is common during oil transportation under the sea, where temperatures are around 4°C. In this case, wax deposition is caused by low temperatures that lead the decreasing of the solubility of the paraffin in the crude mixture. However, it has also seen that some crude oil with high paraffinic content undergoes a decrease on the solubility under less severe conditions of temperature. However, in order to reduce or control the deposition phenomenon, it is necessary to predict the effect of operational variables and the physical and chemical properties of oil over the paraffin deposition. This study will use the model proposed by Singh et al., (2000) which has been validated experimentally and cited recently by Huang et al., (2011) and Lu et al., (2012) In their model, they consider mass diffusion as the only transport mechanism for paraffin and, therefore responsible in the formation of deposits in the pipes for a laminar flow regime. Herein, this model was applied for a particular case using a sample of highly paraffinic oil from an oil field in Colombia with API gravity between 30 and 42. The model solution is based on mass and energy balances of the fluid and wax deposit phases. These balances are coupled through with the interfacial mass balance, which takes into account the movement of the interface generated by the deposition of wax. The application of this model requires the characterization of crude oil in study in order to obtain both the physical properties and the curve of solubility of paraffinic in oil as a function of temperature, which are fundamental parameters in the model. The aim of this work was to study the wax deposition process and the influence of parameters such as the flow rate and wall temperature of the pipe over the deposit thickness for light crude oil with high paraffinic content by implementing the model of Singh et al., (2000). Pseudo steady conditions in the fluid phase are assumed during the analysis. The Finite Differences method was applied to solve the differential equation system and the solution was implemented numerically using the language programming C++. The model was validated with the experimental results reported by Singh et al., 2000 and posteriorly used to simulate the growth of the deposit as a function of flow rate. The results of this study confirm the importance of this last parameter in the paraffinic deposition under assumptions made in this model. NOMENCLATURE C [kg/m] Wax concentration Cb [kg/m] Bulk concentration of wax Cp [J/kg °C] Allowable volumetric heat generation density increase Cwall [kg/m] Wax concentration Deff [m/s] Effective diffusivity