Experimental Investigation of the Rheological Properties of a Typical Waxy Crude Oil Treated with Supercritical CO<sub>2</sub> and the Stability Change in Its Emulsion

The application of supercritical CO<sub>2</sub> (scCO<sub>2</sub>)-flooding technology is increasing worldwide. After a waxy crude oil is treated with scCO<sub>2</sub>, its composition and rheological properties are changed and the stability of its emulsion is accordingly altered as well, thus affecting the dewatering process in gathering stations. To probe the specific changes occurring in waxy crude oil and its emulsion after scCO<sub>2</sub> flooding, a scCO<sub>2</sub> treatment device was first designed to simulate the reservoir conditions. Next, the composition changes in the waxy crude oil caused by the scCO<sub>2</sub> treatment were studied, and the changes in the rheological properties, such as the pour point, viscosity, yield stress, and wax precipitation characteristics, caused by the composition variation were analyzed. Then, the stability of the degassed crude oil emulsion and the corresponding interfacial properties were examined. Finally, the demulsification characteristics of the emulsion under the action of a demulsifier were tested. The results reveal a content increase in the heavy components, such as asphaltenes, resins, and high-carbon-number hydrocarbons, in the waxy crude oil as a result of the scCO<sub>2</sub> extraction of the light components. This results in increases in the pour point, wax appearance temperature, and abnormal point as well as the growth of the apparent viscosity and yield stress. As a result of the structural enhancement of the interface, as reflected by the dilational modulus increase in the interface, the stability of the scCO<sub>2</sub>-treated waxy crude oil emulsion is strengthened and its demulsification efficiency is reduced after dosing with the same demulsifier. All of these changes could unfavorably influence the safe transportation of the produced fluid in scCO<sub>2</sub>-flooding oil fields and the subsequent dewatering process.