Study on waxy crudes characterisation and chemical inhibitor assessment

Abstract

Wax deposition brings severe challenges to the production, transportation, and storage of crude oils. Accumulation of wax can block the pipeline, lead to equipment failure, and lose production. The use of chemical inhibitors has been reported as one of the most effective measures to mitigate wax deposition in the pipeline. The inhibition efficiency depends on the effects of the inhibitor molecules on the formation, growth, agglomeration and morphology of wax crystals. The chemicals and their compositions in the inhibitor together with the crude composition determine wax inhibition performance. Many different types of chemical inhibitors have been developed. Oil industry benefits significantly from high-efficiency inhibitors. This paper reported a simple method to optimise the inhibition performance by blending different inhibitors. Firstly, a comprehensive characterisation of two naturally waxy crude oils (KSG#49 and Ex Mwambe) and a synthetic crude oil was presented. The synthetic crude oil was prepared by mixing a dead oil of 3% wt wax content with a solid wax sample obtained from the North Sea field. The variation in properties of the three oils was evaluated through cloud point measurements, pour point, wax content, SARA fractions, colloidal instability index, American Petroleum Institute gravity (APIg), and the n-Paraffin distribution. Then, the influence of four Polymer-based pour point depressants (PPDs) on wax crystallization were critically assessed. The PPDs induced morphological changes to wax crystals transformed the needle-shaped crystals into an agglomerate. The small particles dispersed in the oil matrix reduced the apparent viscosity and wax gelling properties, and the efficiency of PPDs could be affected by the SARA fraction. A higher fraction of flocculated asphaltenes provides active sites for wax crystallization. Hence it increases cloud point and interferes with the crystal inhibition mechanism. Finally, a blended PPD produces a synergistic effect on inhibition performance that effectively reduces apparent viscosity, wax appearance temperature (WAT) and pour point (PP). The improvement can be attributed to the interactions between the molecules of wax inhibitors with wax crystals. This works sheds light on new inhibitor development by blending different inhibitors to promote performance synergies.