Abstract:
The formation of solid wax crystals that interlock and form a gel-like layer on the inner
wall of the pipeline greatly influences the transportation of waxy crude oil through
pipeline systems. The deposited layer grows continuously and hardens during oil
transportation. This phenomenon reduces the effective internal diameter of the pipeline
and the flow rate. In extreme cases, the deposited layer may block the crude oil pipeline
leading to permanent pipeline shutdown and loss of capital investment. In this study,
wax deposition from multiphase flow in field-scale crude oil pipeline transport systems
has been investigated numerically. The novelty of this work is to develop a mathematical
model that incorporates water-in-oil emulsions, wax precipitation kinetics, molecular
diffusion, and shear dispersion to enable accurate predictions of both the wax deposit
growth rate and aging of the deposit. The coupled nonlinear partial differential equations
governing the flow are discretized in time by a second-order semi-implicit time
discretization scheme, which is based on the Adams-Bashforth and Crank-Nicolson
methods that completely decouple the computation of the governing equations. The
resulting temporal numerical schemes are discretized in space by the bivariate spectral
collocation method, which is based on Chebyshev-Gauss-Lobatto grid points. The
resulting numerical schemes are simulated in MATLAB® software to obtain the profiles of the flow variables. The simulation results are presented in graphical and tabular forms and also discussed. The model's predictive capabilities are evaluated by investigating the impact of various flow parameters on the flow variables, wall shear stress, and heat and mass fluxes. The key findings reveal that wax deposition is significantly influenced by the intricate interplay of flow conditions, wax precipitation kinetics, and heat and mass transfer phenomena. Notably, increasing Reynolds number from 2.2361 to 3.1361 leads to at most 2.5% increase in wax deposition, while increasing mass Grashof number from 5 to 11 results in at most 2.0% reduction in wax accumulation. Moreover, increasing Weber number from 1.0 to 2.5 tend to mitigate wax deposition by at most 7.0%. In addition, the deposit thickness steadily increases during the initial phases of wax deposition, after which it reaches a steady-state value of 0.2 and maintains that value over time. A deposition model to predict the wax deposit growth and aging is proposed in this study. The research findings can help in making informed decisions on the planning of pigging operations, thermal insulation, and other remediation techniques to be applied in controlling wax deposition in field-scale crude oil pipeline systems.
