Energy & Fuels, Vol.34, No.12, 15782-15801, 2020
Effect of Initial Cooling Temperature on Structural Behaviors of Gelled Waxy Crude Oil and Microscopic Mechanism Investigation
A comprehensive investigation of the effect of initial cooling temperature on the structural behaviors of gelled waxy crude oil is conducted, including the "temperature-dependent viscoelasticity behaviors", "time-dependent viscoelasticity behaviors", "time-dependent deformation behaviors", "structural decomposition behaviors", "thixotropic behavior", and "microstructure manifestation". Two different structural behaviors are exhibited with contrary rigid and brittle structural characters, structural regeneration performances, and cooling rate and oscillation frequency responses. The strongest elastic-dominated viscoelasticity and thixotropic behaviors occur at the initial cooling temperature near wax dissolution temperature (WDT). With increasing initial cooling temperature, a noticeable decrease in viscoelasticity and thixotropic behavior results in the clustering of opposite structural behaviors. With decreasing initial cooling temperature below WDT, although the structural behavior can be clustered with that at the initial cooling temperature near WDT, the gradual decreasing viscoelasticity behavior is presented. The direct microscopic evidence on the rheological deterioration and improvement are provided by two microscopic observation methods. Meanwhile, two different kinds of wax crystals with contrary influences on rheological behaviors are observed at initial cooling temperatures lower than the WDT. Furthermore, a hypothesis is proposed to explain the influence mechanism of initial cooling temperature lower than the WDT on the structural behaviors of waxy crude oil. In addition, different motion behaviors and mechanical performances of wax crystals are exhibited in response to the external load and impact, which further characterizes the intrinsic differences of inner structures of waxy crude oil formed at different initial cooling temperatures.