Effect of Thermal Treatment Temperature on the Flowability and Wax Deposition Characteristics of Changqing Waxy Crude Oil

Abstract

The influence of thermal treatment temperature on the flowability and wax deposition characteristics of Changqing waxy crude oil was researched in detail through pour point/rheological tests, a cylindrical Couette wax deposition experimental device, DSC analyses, asphaltenes stability tests, and microscopic observations. It is found that the flowability of the crude oil can be greatly improved through increasing the thermal treatment temperature. Meanwhile, the wax deposition rate of the crude oil can be outstandingly inhibited with the increase of thermal treatment temperature from 50 to 70 °C. Moreover, the flow regime can also influence the wax deposition characteristics. Under cold flow regime (22 °C/12 °C), the structure of formed wax deposits is homogeneous while the aging of the wax deposits is not obvious. Under hot flow regime (30 °C/20 °C), the aging of the wax deposits is obvious, but a heterogeneous two-layer structure exists in the formed wax deposits at the thermal treatment temperatures 50 and 60 °C, with a hard/thin bottom layer and a relatively soft/thick surface layer. With the increase of thermal treatment temperature to 70 °C, the two-layer wax deposit structure cannot be identified due to the extremely thin wax deposit thickness. Increasing the thermal treatment temperature can disperse and activate the asphaltenes in the crude oil; the activated asphaltenes have stronger interactions with waxes and then decrease the WAT of the oil and change the precipitated wax crystals’ morphology further, hence dramatically improving the crude oil flowability and inhibiting wax deposition. Under hot flow regime, the two-layer wax deposit structure is mainly triggered through the diffusion of wax molecules and asphaltenes from the bulk oil into the wax deposits. Under cold flow regime, the asphaltenes have already coprecipitated with wax molecules into big wax flocs, which is difficult to diffuse from the bulk oil into the already existed wax deposits. Meanwhile, the aggravated flowability of bulk oil under cold flow regime further hinders the diffusion of wax molecules into the interior of the wax deposit. Therefore, the two-layer wax deposit structure cannot be found under cold flow regime.

Effect of Initial Cooling Temperature on Structural Behaviors of Gelled Waxy Crude Oil and Microscopic Mechanism Investigation

“…The reason for the extremely different morphology and structure of wax crystals is attributed to the participation of resins and asphaltenes in different states. At the initial cooling temperature of 60 °C, although the wax crystals can be nearly totally dissolved, the resins and asphaltenes have not been fully activated and dispersed , and thus they cannot fully participate in the growth of wax crystals and take the effect of improving the structure of wax crystals. In contrast, when the initial cooling temperature is higher than 65 °C (e.g., 70 °C), the resins and asphaltenes have been fully activated and dispersed and can participate in the improved growth of wax crystal structure.…”

“…Although the focuses of the abovementioned documents are different, they definitely suggest that the initial cooling temperature is highly relevant to the flow and structural behaviors of waxy crude oils. Most research studies focused on viscosity, yield stress, WPT (WAT), and condensation point (or pour point), which are the common characteristic parameters representing the low-temperature flowability of waxy crude oil. ,− ,− With the development of the understanding of low-temperature gel system characteristics of waxy crude oil, some scholars conducted investigations on the viscoelastic behaviors of waxy crude oil. ,,, Despite this, the detailed probes on the effect of initial cooling temperature on the comprehensive structural behaviors of gelled waxy crude oil are still scarce. In recent years, with improving measurement technology and increasing interest in the structural characters of gelled waxy crude oil, new measurement methods and technologies were used to achieve the new outcomes related to the structural characters of gelled waxy crude oil.…”

“…Zhao et al 10 conducted a series of heat treatments and shear test experiments for rheological properties of waxy crude oil, which show that the thermal treatment temperature has certain influences on waxy crude oil at lowtemperature rheology. Zhu et al 11 carried out research on the effect of heat treatment temperature on the flowability and wax deposition characteristics of Changqing waxy crude oil by pour point/rheological tests, wax deposition experimental device, differential scanning calorimetry (DSC) analysis, asphaltene stability tests, and wax microscopic observations. The study shows that the increase in heat treatment temperature can disperse and activate asphaltenes, which makes the interaction between asphaltenes and waxes stronger, thus reducing the wax appearance temperature (WAT) of oil and further changing the morphology of wax crystals.…”

“…1,3−8,12−14 With the development of the understanding of low-temperature gel system characteristics of waxy crude oil, some scholars conducted investigations on the viscoelastic behaviors of waxy crude oil. 2,9,11,15 Despite this, the detailed probes on the effect of initial cooling temperature on the comprehensive structural behaviors of gelled waxy crude oil are still scarce. In recent years, with improving measurement technology and increasing interest in the structural characters of gelled waxy crude oil, new measurement methods and technologies were used to achieve the new outcomes related to the structural characters of gelled waxy crude oil.…”

Experimental and theoretical study on wax deposition and the application on a heat insulated crude oil pipeline in Northeast China

“…In the process of waxy crude oil pipeline transportation, the formation of wax deposition could reduce the effective conveying area of the pipeline, which decreases the conveying capacity of pipeline, enhances the energy consumption, and even causes huge safety hazards and economic losses by blocking pipeline [3]. Therefore, it is of great significant to predict the wax deposition rate, thickness and wax content of deposition during the transportation process of waxy crude oil through pipelines for the purpose of formulating wax removal plan [4].…”