Interpretation of NMR Relaxation in Bitumen and Organic Shale Using Polymer–Heptane Mixes

One
of the much debated mysteries in 1H NMR relaxation
measurements of bitumen and heavy crude oils is the departure from
expected theoretical trends at high viscosities, where traditional
theories of 1H–1H dipole–dipole
interactions predict an increase in T1 with increasing viscosity. However, previous experiments on bitumen
and heavy crude oils clearly show that T1LM (i.e., log-mean of the T1 distribution)
becomes independent of viscosity at high viscosities; in other words, T1LM versus viscosity approaches a plateau. We
report 1H NMR data at ambient conditions on a set of pure
polymers and polymer–heptane mixes spanning a wide range of
viscosities (η = 0.39 cP ↔ 334 000 cP) and NMR
frequencies (ω0/2π = f0 = 2.3 MHz ↔ 400 MHz) and find that at high viscosities
(i.e., in the slow-motion regime) T1LM plateaus to a value T1LM> ∝
ω0 independent of viscosity, similar to bitumen.
More specifically,
on a frequency-normalized scale, we find that T1LM> × 2.3/f0 ≃
3 ms
(i.e., normalized relative to 2.3 MHz), in good agreement with bitumen
and previously reported polymers. Our findings suggest that in the
high-viscosity limit T1LM> and T2LM> for polymers, bitumen, and heavy crude
oils can be explained by 1H–1H dipole–dipole
interactions without the need to invoke surface paramagnetism. In
light of this, we propose a new relaxation model to account for the
viscosity and frequency dependences of T1LM and T2LM, solely based on 1H–1H dipole–dipole interactions. We also
determine the surface relaxation components T1S and T2S of heptane in the polymer–heptane
mixes, where the polymer acts as the “surface” for heptane.
We report ratios up to T1S/T2S ≃ 4 and dispersion T1S(ω0) for heptane in the mix, similar to previously
reported data for hydrocarbons confined in organic matter such as
bitumen and kerogen. These findings imply that 1H–1H dipole–dipole interactions enhanced by nanopore confinement
dominate T1S and T2S relaxation in saturated organic-rich shales.