A Series of Poly(styrene-co-alkyl maleimide) As Versatile Flow Improvers for the Crude Oils with Asphaltenes and Waxes

^*Corresponding Author's E-mail: kcao@che.zju.edu.cn

This study was supported by the National Natural Science Foundation of China through Project No. 21176217.

Crude oils are complex mixtures of hydrocarbons that contain polar components such as asphaltenes and nonpolar long-chain waxes. Within the decades the massive consumption of crude oils results in the deterioration of quality for crude oil resources worldwide, indicating the increasing contents of asphaltenes and waxes in crude oils. Once the conditions such as temperature or pressure are changed in the reservoir or pipeline, asphaltenes and waxes precipitate out of crude oil matrix with the dramatic decreasing solubility. The aggregation of long-chain wax solids and asphaltenes particles can lead to deposition at the pipe walls, reducing crude oil flowing ability and potentially clogging the pipeline. Such a problem is particularly severe in cold regions and offshore deep water.

Flow improvers, which are usually polymeric additives, have been widely utilized to avert the clogging problem for the crude oils. Most of previous researches have separately focused on inhibiting asphaltene aggregation and modifying wax crystallization. Few has study the simultaneous effects on asphaltene and wax. In this work, the novel flow improvers, poly(styrene-co-octadecyl maleimide), with various molecular structures were prepared. The precursor, poly(styrene-co-maleic anhydride), was first synthesized through RAFT polymerization, by which the various structure, such as different molecular weights, alternate sequence, random sequence with different maleic anhydride contents and block structure, can be readily attained. Then the precursors were amidated with n-octadecyl amine and subsequently imidized to obtain the desired flow improvers with different imidization degrees by controlling reaction time. The influence of molecular structures of flow improvers on the flow improving effectiveness for various kinds of model crude oils have been intensively examined by the methods of rheology, cross-polarized microscopy, differential scanning calorimetry and stability analysis.

Scheme 1. The imidization processes for obtaining the designed flow improvers

For model waxy crude oils without asphaltenes, the results indicated that adding a small amount of the flow improvers can significantly reduce the crystallization temperature, the number and size of waxy crystals, and the yield stress of the model oil. Moreover, it has been found that the effectiveness of flow improvers is significantly affected by the imidization degree and maleic anhydride content. With increasing imidization degree, more maleamic acid groups can be converted into maleimide groups, which promotes the compatibility between nonpolar wax and flow improvers.

For model waxy crude oils with asphaltenes, with the addition of the flow improvers, the wax crystals become fewer, smaller, and more dispersed in the oils. Asphaltene particles are better dispersed, and the crystallization temperature and the yield stress can be reduced considerably. It is consistent with results found for waxy crude oils without asphaltenes that the crystallization temperature can be reduced with increasing imidization degree. Consistent with the preceding results, it could be seen that the addition of flow improvers decreases the number and size of wax crystals more significantly with the imidization degree increasing. However, contrary to that without asphaltenes, there is an optimal medium imidization degree for dispersing the asphaltene particles and best depressing the viscosity and yield stress of waxy crude oils with asphaltenes. With the addition of asphaltene, the model waxy oils show higher polarity than before. The flow improvers with medium imidization degree possess polar moieties such as COOH and NH, which are likely to form hydrogen bonds with the asphaltenes containing OH and NH groups, which can benefit the dispersion of asphaltenes. Furthermore, the investigation of those flow improvers for model waxy crude oil with higher content of asphaltenes is also carried out, in which the stability of asphaltene particles have been explored using Turbiscan Lab Expert.