NMR Relaxation of Polymer - Alkane Mixes, A Model System for Crude Oils
- Philip M. Singer (Rice University) | Zeliang Chen (Rice University) | Lawrence B. Alemany (Rice University) | George J. Hirasaki (Rice University) | Kairan Zhu (Xi’an Shiyou University) | Zonghai Harry Xie (Core Laboratories) | Tuan D. Vo (Core Laboratories)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 58th Annual Logging Symposium, 17-21 June, Oklahoma City, Oklahoma, USA
- Publication Date
- Document Type
- Conference Paper
- 2017. copyright held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
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- 117 since 2007
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At low viscosities, it is well known that the NMR T1 and T2 relaxation times of bulk fluids are inversely proportional to viscosity. However, one of the much debated mysteries in NMR relaxation measurements of crude oils is the departure from expected theoretical trends at high viscosities. In particular, at high viscosities traditional theories of hydrogen-hydrogen (1H-1H) dipole-dipole interactions predict an increase in T1 with increasing viscosity. Previous experiments on crude oils clearly show that T1 becomes independent of viscosity at high viscosities, in other words T1 versus viscosity approaches a plateau. Some reports have suggested that the observed T1 plateau at high viscosity is explained by the presence of asphaltenes in the crude oils, where the asphaltenes contain paramagnetic relaxation sites on the surface. However, we show that the T1 plateau is the same for model systems which do not contain paramagnetic relaxation sites on the surface. More specifically, we report a T1 plateau at high viscosity for pure polymer and polymer-heptane mixes, over a wide range of viscosities and NMR frequencies. Furthermore, on a frequency-normalized T1 versus viscosity plot, the polymer T1 plateaus to the same value as previously reported crude oils.
Our findings strongly suggest that the NMR relaxation in both crude oils and polymers can be explained by 1H-1H dipole-dipole interactions, without the need for surface paramagnetism. We then propose a new NMR relaxation model that successfully fits the viscosity and dispersion (i.e. frequency dependence) of T1 and T2 for the pure polymer and polymer-heptane mixes. We then compare our new model to previously published crude-oil data. We also determine the surface relaxation of heptane in the polymer-heptane mixes, where the polymer is considered as the surface for heptane. We then draw analogies between the surface-relaxation mechanism for heptane in polymer-alkane mixes with the surface-relaxation mechanism for hydrocarbons confined in organic matter such as the bitumen and kerogen found in organic-rich shales. Using the analogy between the two systems, we then explain the origin of the large T1/T2 >> 1 ratio and T1 dispersion previously reported in hydrocarbon-saturated organic-shales, from 1H-1H dipole-dipole interactions alone.
|File Size||3 MB||Number of Pages||18|