Probing Multiscale Structure and Dynamics of Waxy Crude Oil by Low-Field NMR, X-ray Scattering, and Optical Microscopy

Wax deposition is one of the major concerns for waxy crude oil production and transportation. A better understanding and prediction of fluid properties related to this issue require knowledge of the medium structuration at scales ranging from nanometers (molecules) to a few micrometers (crystals). For this purpose, the behavior of a waxy crude oil in the bulk was compared to that of a model oil over a wide range of temperatures above and below the wax appearance temperature. The combined use of cross-polarized microscopy (CPM) and the implementation of innovative techniques for the field such as small- and wide-angle X-ray scattering and low-field nuclear magnetic resonance has provided a more precise idea of the structure of these two types of fluids. If the nature of the orthorhombic crystals and their lamellar shape are identical for both fluids, a very appreciable difference is highlighted in their chain axis dimensions. The n-paraffin crystal size is larger than 120 nm in the model oil. In crude oil, it is only about 1-10 nm with a long-range order in the directions perpendicular to the chain axis and a monomolecular thickness in the chain axis. Moreover, from the model oil CPM images, we observed aggregates of lamellar shape crystals. Because the model oil does not contain asphaltenes and resins, the crystals are larger and the branches divide significantly less than in crude oil, which results in a trapped liquid proton population unobservable in the model oil. All these observations give a vision of the structure of n-paraffin crystals. It is made of aggregates of relatively dense lamellae in the center and more aerated lamellae at the periphery, which split into several branches.