Multi-component waxy model oil design to mimic rheologically complex gas condensate liquids

Abstract

This paper provides a detailed case study on the construction of two model oils with the purpose of being used during the DEFINE phase flow assurance strategy development process for a greenfield gas-condensate project. Details on the characterization methods used on the scarcely available well-test samples are given along with instructions on constructing the model oils. These fluids were unique with an abnormally high paraffin content, unusual (cyclic) alkane content, low wax appearance temperature, resulting in a complex rheological behavior. The real field fluids were severely volume-limited and non-destructive techniques were used as much as possible to characterize the oil. Classical methods such as High-Temperature Gas Chromatography (HTGC) and Differential Scanning Calorimetry (DSC) were used to elucidate the paraffinic species and SARA the remaining non-paraffinic species. Additionally, Nuclear Magnetic Resonance (NMR) and (Fourier-Transform Infrared Spectroscopy) FTIR were used to further speciate the crude oil components providing further clarification on composition. The characterization methods showed that the additional methods provided a better understanding of gas-condensate composition and behavior. The presence of alicyclic, aromatic, and carbonyl compounds were poorly detected with classic methods. These components can have a profound influence on the rheology of the oil and, therefore too, on gelation and/or waxing potential. NMR and FTIR are shown to be essential in this regard and offer a quantification method for drilling mud contamination. Construction of the model oils had to not just include n-paraffinic components but also iso-paraffins, alicyclics, and aromatics. Rheology results show the step by step addition of these components to the model oils and the individual influence they have on rheology and gelation behavior compared to the real field fluids. In conclusion, two prototype model oils are presented that enabled further assessment of the paraffin-related flow assurance risks to the development project including chemical selection. This is the first time such complex model oils have been constructed and reported. With the advent of more gas-condensate fields and the expense (and therefore scarcity) of obtaining liquid samples for flow assurance studies the methodology described here offers an alternative to costly sampling pre-production sampling campaigns.