Case Study of Estimating Wax Appearance Temperature and Application to Wax Deposition Modelling
The need for cheap energy to run the world economy has propelled a lot of petroleum companies to produce oils that have high paraffin content. The paraffin, also referred to as wax, poses a problem when it forms and deposits in wellbores or pipelines carrying this oil. This flow assurance problem is observed when the wax appearance temperature (WAT) exceeds the ambient temperature, the oil cools, and wax will form resulting in a deposition. This research focuses on the sensitivity analysis of the impact of the number of pseudo components used to characterize the crude oil on the accuracy of models for estimating the wax appearance temperature and the application of WAT to model wax deposition. The WAT is the temperature at which the first wax crystal appears and is often used to measure the probability of crude oil to deposit wax relative to the reduction in pressure and temperature. In this study, the WAT of three crude oil samples is calculated using correlations. These correlations require the lumping or pseudoization of components to characterize the fluid compositions. The Whitson and Pedersen lumping schemes are utilized to calculate the coefficients of the correlations for estimating the WAT. A sensitivity analysis is carried out to evaluate the impact of pseudoization on the WAT. A tornado chart was plotted to show how sensitive the WAT is to each independent variable as they change over their allowed ranges. In a case study with data collected from the Niger Delta, the Matzain (1999), Singh et al wax deposition models and the heat-mass transfer numerical model of Stubsjoen (2013) are used to estimate the wax deposition thickness as a function of time and distance from the inlet along the pipeline. The wax deposition process is modeled using a code developed in MATLAB/Octave. The wax deposit thickness was found to increase with time. The work can be applied to estimate WAT and model wax depositional thickness to mitigate wax precipitation in crude oil pipelines.