A comprehensive assessment for the structural change of waxy crude oils induced by pour point depressant beneficiation

Abstract

In the petroleum industry, the solid paraffins that precipitate out from a waxy crude oil complicate the low-temperature flow characteristics, causing major difficulties in pipeline transportation. Pour point depressants (PPDs) are often utilized to improve the flow characteristics of crude oils by modifying the morphology and structure of wax crystals, becoming a great technical and economic interest. The improving mechanism of PPDs on the structure of gelled waxy crude oil can be attributed to the nucleation, co-crystallization, and adsorption effect to the precipitated wax crystals, preventing the waxy structure from growing further and forming a volume spanning network. Many previous works about the PPDs’ effect have merely focused on the pour point, apparent viscosity, and yield strength depressing ability, while other structural properties have rarely been comprehensively evaluated. The objective of this study is to comprehensively reveal the change of the wax particle structure in oils benefitted with PPDs. The structure of gelled oils doped with PPDs was found to become less elastic and more susceptible to shear perturbation with a lower linear deformation range of 0.42% and a longer relaxed time scale of 3000 s. Large amplitude oscillatory shear (LAOS) tests show that the nonlinear viscoelastic behaviors can be weakened by PPDs, whereas the third harmonic value obtained by Fourier-transform analysis increases after adding PPDs, indicating that the sensitivity to applied load are strengthened. Meanwhile, the value of yield strain increases to about 8.7%, and the irreversible plastic deformation zone becomes relatively broader (brittleness index I increases to 70.9), which mean a more ductile fracture induced by PPDs would occur when yielding. Although the additive PPDs weaken the structural strength and the non-Newtonian characteristics (kinetic index of n1 increases to 0.8), the characteristic of thixotropy is obviously strengthened, and the doped crude oils exhibit a faster structural breakdown process after yielding. All these novel changes for structural features explored in this paper provide theoretical guidance into understanding the practical engineering problems of accelerated process for the pipeline restart flow benefitted with PPDs.