Influence of CO₂ Treatment Pressure on the Chemical Composition and Rheological Properties of Degassed Waxy Crude Oil

With the gradual application of enhanced oil recovery by CO₂ (CO₂-EOR), the rheological behavior of produced fluid is altered due to CO₂ dissolution and degassing. This work focuses on the composition, physical properties, gelation and yield characteristics, and viscosity-temperature properties of crude oil containing paraffinic wax after CO₂ treatment. Special attention is given to the effect of the phase state of CO₂. It is found that the contents of light hydrocarbons (C₉-) and liquid paraffin (C₉-C₁₆) decrease by 1.24% and 0.89%, respectively, while the contents of paraffin and microcrystalline wax increase by 1.90% and 1.71%, respectively, as the treatment pressure is increased to 25 MPa from atmospheric pressure. Besides, the content of the saturates decreases by 5.31%, while that of the aromatics, resins, and asphaltenes increases by 2.83%, 1.83%, and 0.65%, respectively, due to the extraction effect of CO₂ on light hydrocarbons. In terms of wax precipitation characteristics, the wax appearance temperature (WAT) and wax precipitation content (WPC) increase with increasing pressure. Particularly, when the pressure increases from 5 to 15 MPa, the WAT and WPC increase by 4 °C and 1.27%, respectively, obviously greater than the other conditions due to the phase transition of CO₂. Moreover, the morphology of the wax crystals becomes tinier after supercritical CO₂ (scCO₂) treatment because of the increase in the polarity of the crude oil. Likewise, considering the gelation and yield characteristics, the storage modulus, gelation temperature, and yield stress increase more obviously than the other pressure change conditions as the pressure increases from 5 to 15 MPa. All of the above structural enhancements are owing to the effect of CO₂ treatment, especially scCO₂ treatment, on the content and morphology of the precipitated wax crystals. Last, the apparent viscosity/viscosity also increases with the treatment pressure. The viscosity increases by 1.1 to 2.0 times with the pressure increasing from 5 to 15 MPa, again obviously greater than the other pressure change conditions due to the interior structural changes of the crude oil induced by scCO₂ treatment..