Abstract
The mechanism behind the NMR surface-relaxation times (T1S,2S) and the large T1S/T2S 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 1H NMR relaxation and diffusion of n-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 n-heptane shows a power-law dependence on the concentration of n-heptane (φC7) in the polymer matrix, consistent with Archie's model of tortuosity. We calculate the autocorrelation function G(t) for 1H-1H dipole-dipole interactions of n-heptane in the polymer matrix and use this to generate the NMR frequency (f0) dependence of T1S,2S as a function of φC7. We find that increasing molecular confinement increases the correlation time, which decreases the surface-relaxation times for n-heptane in the polymer matrix. For weak confinement (φC7 > 50 vol %), we find that T1S/T2S ≃ 1. Under strong confinement (φC7 ψ 50 vol %), we find that T1S/T2S ψ 4 increases with decreasing φC7 and that the dispersion relation T1S ∝ f0 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 1H-1H dipole-dipole interactions enhanced by organic nanopore confinement dominate the NMR response in saturated organic-rich shales.
Original language | English |
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Pages (from-to) | 3801-3810 |
Number of pages | 10 |
Journal | Journal of Physical Chemistry B |
Volume | 124 |
Issue number | 18 |
DOIs | |
State | Published - May 7 2020 |
Externally published | Yes |
Funding
We thank Chevron Energy Technology Company, the Rice University Consortium on Processes in Porous Media, and the American Chemical Society Petroleum Research Fund (No. ACS PRF 58859-ND6) for funding this work. We gratefully acknowledge the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy (No. DE-AC02-05CH11231) and the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for high-performance computer time and support. We thank the anonymous referees for their helpful comments.
Funders | Funder number |
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Chevron Energy Technology Company | |
Texas Advanced Computing Center | |
U.S. Department of Energy | DE-AC02-05CH11231 |
Office of Science | |
American Chemical Society Petroleum Research Fund | ACS PRF 58859-ND6 |
Rice University | |
National Energy Research Scientific Computing Center |