Effect of Polyethylene-Vinyl Acetate Pour Point Depressants on the Flow Behavior of Degassed Changqing Waxy Crude Oil before/after scCO_2 Extraction

摘要

For the pipeline transportation of waxy crude oil, pour point depressants (PPDs) have been developed and used to improve its flowability at low temperature. As supercritical CO, (scCO(2)) flooding becomes a common technique to improve oilfield recovery, how the sensitivity of PPD changes due to the degassed crude oil treated by CO, should be studied. In this work, the effect of scCO(2) extraction on the performance of EVA in improving the flowability of Changqing degassed crude oil is investigated by means of a self-developed scCO(2) treating equipment, SARA/HTGC analysis, SEM analysis, pour point test, rheological measurement, asphaltene precipitation test, particle size analysis, DSC test, and microscopic observation. The results show that without EVA addition, scCO(2) extraction aggravates the flowability of the crude oil at low temperature. The scCO(2) extraction extracts light components from asphaltene aggregates causing asphaltenes to precipitate, which inhibits the asphaltenes on playing a role as the natural PPD. As a result, it leads to the formation of smaller needle-like wax crystals with large amount, worsening the flowability of the crude oil. When the EVA is present, however, scCO(2) extraction favors the functioning of EVA on improving the flowability of the crude oil at low temperature. The addition of EVA can slightly enhance the solubility of wax molecules thus decreasing the crude oil WAT to some extent, while can significantly change the morphology of precipitated wax crystals from small needle-like to agglomerated clusters, thus improving the flowability of the crude oil. Because of the synergistic effect of EVA and asphaltene aggregates after scCO(2) extraction, a more compact EVA asphaltene wax ternary crystal structure will form thus further decreasing the pour point, elastic modulus, and equilibrium viscosity at low temperature.