Department of Chemical and Biomolecular Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States.
J Phys Chem B. 2020 May 7;124(18):3801-3810. doi: 10.1021/acs.jpcb.0c00711. Epub 2020 Apr 24.
The mechanism behind the NMR surface-relaxation times () and the large / ratio of light hydrocarbons confined in the nanopores of kerogen remains poorly understood and consequently has engendered much debate. Toward bringing a molecular-scale resolution to this problem, we present molecular dynamics (MD) simulations of H NMR relaxation and diffusion of -heptane in a polymer matrix. The high-viscosity polymer is a model for kerogen and bitumen that provides an organic "surface" for heptane. Diffusion of -heptane shows a power-law dependence on the concentration of -heptane (ϕ) in the polymer matrix, consistent with Archie's model of tortuosity. We calculate the autocorrelation function () for H-H dipole-dipole interactions of -heptane in the polymer matrix and use this to generate the NMR frequency () dependence of as a function of ϕ. We find that increasing molecular confinement increases the correlation time, which decreases the surface-relaxation times for -heptane in the polymer matrix. For weak confinement (ϕ > 50 vol %), we find that / ≃ 1. Under strong confinement (ϕ ≲ 50 vol %), we find that / ≳ 4 increases with decreasing ϕ and that the dispersion relation ∝ is consistent with previously reported measurements of polydisperse polymers and bitumen. Such frequency dependence in bitumen has been previously attributed to paramagnetism; instead, our studies suggests that H-H dipole-dipole interactions enhanced by organic nanopore confinement dominate the NMR response in saturated organic-rich shales.
核磁共振表面弛豫时间 (T1) 和轻烃在干酪根纳米孔中较大的 (/T1) 比值的背后机制仍未被充分理解，因此引发了广泛的争论。为了从分子尺度上解决这个问题，我们对 H 核磁共振弛豫和 - 庚烷在聚合物基质中的扩散进行了分子动力学 (MD) 模拟。高粘度聚合物是干酪根和沥青的模型，为庚烷提供了有机“表面”。- 庚烷的扩散显示出对聚合物基质中 - 庚烷浓度 (φ) 的幂律依赖性，与 Archie 的迂曲度模型一致。我们计算了聚合物基质中 - 庚烷的 H-H 偶极-偶极相互作用的自相关函数 ()，并使用该函数生成 NMR 频率 () 对φ的依赖关系，以作为聚合物基质中 - 庚烷的 T1 函数。我们发现，分子限制的增加会增加相关时间，从而降低聚合物基质中 - 庚烷的表面弛豫时间。对于弱限制 (φ > 50 体积%)，我们发现 / ≃ 1。对于强限制 (φ ≲ 50 体积%)，我们发现 / ≳ 4 随 φ 的减小而增加，并且弥散关系 ∝ 与先前报道的多分散聚合物和沥青的测量结果一致。沥青中的这种频率依赖性以前归因于顺磁性;相反，我们的研究表明，由有机纳米孔限制增强的 H-H 偶极-偶极相互作用主导了富含饱和有机页岩的 NMR 响应。
