Assessment of different methods to determine the total wax content of crude oils

Abstract

Wax formation by temperature decrease is a serious issue in the petroleum industry, since it makes the production difficult and can lead to important economic losses. As a result, a large effort has been made in order to predict the occurrence of this phenomenon as accurately as possible. Thermodynamic models are commonly used to predict the main parameters involved in the wax precipitation process. Models with wide basis require as most common input information the n-paraffin distribution and the total wax content (C20+ fraction). The aim of this work is to check the capabilities of different experimental techniques to determine the C20+ content of different crude oils. For that purpose, High Temperature Gas Chromatography (HTGC) and Differential Scanning Calorimetry (DSC) analyses (direct analysis of the raw crude oil) were used, showing reasonable agreement. Likewise, the wax precipitation of the raw crude oils was performed to obtain the direct C20+ content. Such experiments lead to clearly higher values than those obtained by HTGC and DSC analyses because of the presence of trapped crude oil in the precipitated solid. After that porosity correction carried out by 1H-NMR, DSC and sequential elution chromatography techniques, the obtained results indicate that both 1H-NMR, DSC analyses yield more consistent results. An empirical correlation was developed to determine the C20+ content from the API gravity and the pour point of the selected crude oils. The correlation was applied to a number of crude oils, obtaining a reasonable agreement between experimental and calculated C20+ values, which shows that the validity of that correlation is wider than previous unreliable equations.