Enhancement of cold flowability for high waxy crude oil by the mildly crosslinked poly(styrene-octadecyl acrylate)

The precipitation of wax at low temperature often leads to the viscosity increase of waxy crude oil, and even causes the blockage of pipelines in severe cases. The addition of polymer additives is an effective method for inhibiting wax precipitation. In the present work, poly(styrene-octadecyl acrylate)s with low crosslinking density were prepared through microemulsion polymerization, with styrene and octadecyl acrylate as monomers and N-(hydroxyethyl) acrylamide (NMA) as the crosslinking agent. The chemical structures and molecular weights of polymers were characterized by Fourier transform infrared spectroscopy and gel permeation chromatography, respectively. The crosslinking densities (CLD) of polymers were determined in the range of 1.32 × 10−4 to 4.66 × 10−4 cm3/mol by the dissolution equilibrium method. Waxy model oils were prepared to explore the crosslinked polymers on the wax crystallization behaviors by using X-ray diffraction (XRD) approaches and isothermal crystallization kinetics (ICK). As the CLD of polymers increases to 1.99 × 10−4 cm3/mol, the crystallinity index (CI), grain size, and structural strength of the wax crystals decrease. However, as the CLD of polymers is further increased to 4.66 × 10−4cm3/mol, all of the above parameters increase, due to the deterioration of the co-crystallization ability of polymers. In the ICK tests, The Avrami index n of the model oil samples increases from 1.94 to 2.43 as the CLD increases, implying that the nucleation effect of polymers is increased. The effect of CLD on the cold flowability of Inner Mongolia crude oil was investigated by pour point test, rheology, differential scanning calorimeter (DSC), and polarized light microscopy (PLM). The results show that the pour point, viscosity, yield stress, thixotropic ring area, cumulative wax precipitation and wax crystal size of the crude oil decrease when the CLD of polymers increases to 1.99 × 10−4 cm3/mol. However, if the CLD is further increased to 4.66 × 10−4 cm3/mol, all of the parameters increase. Consequently, the mechanism for the improvement of waxy crude oil by mildly crosslinked polymer is proposed.