Nuclear Magnetic Resonance Dispersion of Distributions as a Probe of Aggregation in Crude Oils

Abstract

We present a new technique-nuclear magnetic resonance dispersion of distributions (NMRDD)-and we demonstrate that it is a particularly sensitive probe of aggregation in asphaltene-rich crude oils. The technique is based on the measurement of the NMR relaxation distributions at different magnetic fields, i.e., at different proton Larmor frequencies. Because the NMR relaxation rates are controlled by molecular fluctuations at the Larmor frequency, these measurements probe the presence of aggregates that tumble at characteristic rates near that frequency. Our experimental results span the Larmor frequencies from 10 kHz to 40 MHz, which is the range relevant for rotation times of asphaltene and resin aggregates in crude oil. The presence of large aggregates is evident by visual inspection of the data, and aggregate sizes for small and medium size aggregates of the order of several nanometers can be extracted without recourse to any specific models of asphaltene interactions. The technique is noninvasive and nondestructive and can be used for monitoring and study of the onset of aggregation at the preflocculation stage in unstable crude oils.

Non-Exponential 1H and 2H NMR Relaxation and Self-Diffusion in Asphaltene-Maltene Solutions

“…This difference in the T 1 /T 2 ratio persists in a solution of chloroform-d and 3 vol% of the tracer molecule (benzene-f 6 and octane-f 18 ) containing 10 wt% asphaltenes [13], extracted from the previous investigated asphaltenic oil. Furthermore, a strong dispersion of T 1 at Larmor frequencies between 10 6 Hz and 10 8 Hz was observed for the aromatic molecules in the asphaltenic oil, which is in accordance with the T 1 dispersion of the maltene mixtures present in crude oils containing asphaltenes [14][15][16]. In contrast, a significantly weaker dispersion is observed for octane-f 18 in the asphaltenic oil as well as for the aromatic molecules in the asphaltene-free oil [11].…”

Fluorine tracers for the identification of molecular interaction with porous asphaltene aggregates in crude oil

“…An attempt to fit the data by the formalism detailed in [22] was unsuccessful. A comparably strong power-law dependence in crude oils has been reported in [8,24] and was interpreted by a model involving the tumbling of finite-sized asphaltene aggregates. However, qualitatively comparable results were found for a number of bitumen samples, one of which is shown in Fig.…”

“…Recently, the maltene content trapped in precipitated asphaltene has been described [7]. Even in the solution state, asphaltene aggregates can be considered as porous structures and the molecular dynamics of maltenes in confined spaces will be affected similarly to conventional, solid porous media [8], with maltenes being in exchange between the confinement and the free phase.…”

NMR relaxometry and diffusometry in characterizing structural, interfacial and colloidal properties of heavy oils and oil sands

“…Zielinski and Hürlimann demonstrated that T 1 relaxation time profiles determined on crude oils can provide information on the tumbling rates of asphaltene aggregates in heavy oils, which is considered to be a useful precursor to flocculation [97]. The new approach, known as NMR dispersion of distributions (NMRDD) involves measuring the T 1 relaxation profiles as a function of the Larmor frequency of the protons.…”