In situ observation of microscopic motions and the structure dynamic transformation of wax crystals in waxy crude oil subjected to shear

Abstract

The high-efficient development, storage and transportation of waxy crude oil has a significant meaning for stable supply of petroleum energy. The variety of complex morphology and microstructure of wax crystals promote complicated physical properties of waxy crude oil, which increases the difficulty of high-efficient use. At present, most of the existing research focuses on the quiescent structure and morphology of wax crystals. In this work, a new in situ microscopic observation system is introduced, together with the self-created multi-angle composite light source. The microscopic motions and structure dynamic transformation of wax crystals in the waxy crude oil subjected to shear has been first achieved. The new insights will enhance the understanding of the behavior of wax crystals in response to shear, revealing the gelation mechanism and helping the development of a high-efficient pour point depressant. Our outcomes reveal that the aggregation between wax crystals plays an important role throughout the entire microstructure evolution. Two motion modes of wax crystals are observed, including the synchronous motion that follows liquid hydrocarbons, and individual motions of rotation, torsion and stretch in 3-D. Three critical moments related to the microstructure evolution are first found, (I) the increasing individual aggregates instead of the free wax crystals. Correspondingly, two motion modes both exist with high activity. (II) The accomplishment of interconnection between aggregates in the 2-D plane implying the formation of the 2-D network structure. Correspondingly, the individual motions vanish and the synchronous motion begins to decrease. (III) The interlocking between wax crystals in different liquid layers increases to surpass the shear stress, implying the formation of the 3-D network structure. Correspondingly, all forms of motion cease. When the shear rate is low, it can promote aggregation. As it increases, the impact is different and depends on the inherent structure of crude oil. Two opposite effects may exist simultaneously.