(425h) 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.