Wax Crystallization in Model and Crude Oils: Linking Time-Domain Nuclear Magnetic Resonance and Calorimetry to Hydrocarbon Composition

Abstract Wax crystallization changes both the observable proton population and the mobility of the remaining liquid in crude oils. This study combines time-domain nuclear magnetic resonance (TD-NMR), microcalorimetry (μDSC), gas chromatography, and high-resolution mass spectrometry to examine these responses in n-C26/n-C36 model oils and four Brazilian crude oils. Carr−Purcell−Meiboom−Gill (CPMG) decays were corrected using liquid USP mineral oil (United States Pharmacopeia) before inverse Laplace transformation, preserving the amplitude information used to calculate an apparent rigid fraction. Samples underwent thermal conditioning at 80 °C for 30 min; TD-NMR measurements were acquired isothermally at 2 °C intervals, with cooling segments of 0.8 °C min−1. Model oils exhibited progressive signal loss without resolved multiple relaxation regimes, whereas crude oils developed additional short-T2 contributions. The TD-NMR onset estimates were 37, 49, 51, and 47 °C for oils A−D, respectively, compared with μDSC wax appearance temperatures of 37.7, 50.2, 54.5, and 49.8 °C. At 4 °C, apparent rigid contents were 2.47–7.28% by TD-NMR and estimated crystallized fractions were 2.58–7.06% by μDSC, with differences of at most 0.27 percentage point. Carbon-number profiles of the HC[H] class with double-bond equivalent zero revealed a broader high-carbon-number contribution in crude oil C, despite its lower total saturate content than oils B and D. The combined analysis links signal loss and relaxation redistribution to hydrocarbon composition and distinguishes the detection of an onset from the subsequent accumulation of crystallized material.