Freed Denise E
Schlumberger-Doll Research, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2007 May 7;126(17):174502. doi: 10.1063/1.2723734.
Many naturally occurring fluids, such as crude oils, consist of a very large number of components. It is often of interest to determine the composition of the fluids in situ. Diffusion coefficients and nuclear magnetic resonance (NMR) relaxation times can be measured in situ and depend on the size of the molecules. It has been shown [D. E. Freed et al., Phys. Rev. Lett. 94, 067602 (2005)] that the diffusion coefficient of each component in a mixture of alkanes follows a scaling law in the chain length of that molecule and in the mean chain length of the mixture, and these relations were used to determine the chain length distribution of crude oils from NMR diffusion measurements. In this paper, the behavior of NMR relaxation times in mixtures of chain molecules is addressed. The author explains why one would expect scaling laws for the transverse and longitudinal relaxation times of mixtures of short chain molecules and mixtures of alkanes, in particular. It is shown how the power law dependence on the chain length can be calculated from the scaling laws for the translational diffusion coefficients. The author fits the literature data for NMR relaxation in binary mixtures of alkanes and finds that its dependence on chain length agrees with the theory. Lastly, it is shown how the scaling laws in the chain length and the mean chain length can be used to determine the chain length distribution in crude oils that are high in saturates. A good fit is obtained between the NMR-derived chain length distributions and the ones from gas chromatography.
许多天然存在的流体，如原油，由大量成分组成。确定流体的原位组成往往很有意义。扩散系数和核磁共振（NMR）弛豫时间可以原位测量，并且取决于分子大小。[D. E. 弗里德等人，《物理评论快报》94, 067602 (2005)] 表明，烷烃混合物中各组分的扩散系数在该分子的链长和混合物的平均链长方面遵循标度律，并且这些关系被用于根据NMR扩散测量确定原油的链长分布。本文探讨了链状分子混合物中NMR弛豫时间的行为。作者解释了为什么人们会预期短链分子混合物和烷烃混合物的横向和纵向弛豫时间存在标度律，特别是对于烷烃混合物。展示了如何根据平动扩散系数的标度律计算弛豫时间对链长的幂律依赖关系。作者对烷烃二元混合物中NMR弛豫的文献数据进行拟合，发现其对链长的依赖关系与理论相符。最后，展示了如何利用链长和平均链长的标度律来确定饱和烃含量高的原油中的链长分布。NMR得出的链长分布与气相色谱得出的链长分布之间获得了良好的拟合。
