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

Abstract

One of the much debated mysteries in 1 H NMR relaxation measurements of bitumen and heavy crude oils is the departure from expected theoretical trends at high viscosities, where traditional theories of 1 H− 1 H dipole−dipole interactions predict an increase in T 1 with increasing viscosity. However, previous experiments on bitumen and heavy crude oils clearly show that T 1LM (i.e., log-mean of the T 1 distribution) becomes independent of viscosity at high viscosities; in other words, T 1LM versus viscosity approaches a plateau. We report 1 H 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 (ω 0 /2π = f 0 = 2.3 MHz ↔ 400 MHz) and find that at high viscosities (i.e., in the slow-motion regime) T 1LM plateaus to a value T 1LM> ∝ ω 0 independent of viscosity, similar to bitumen. More specifically, on a frequency-normalized scale, we find that T 1LM> × 2.3/f 0 ≃ 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 T 1LM> and T 2LM> for polymers, bitumen, and heavy crude oils can be explained by 1 H− 1 H 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 T 1LM and T 2LM , solely based on 1 H− 1 H dipole−dipole interactions. We also determine the surface relaxation components T 1S and T 2S of heptane in the polymer−heptane mixes, where the polymer acts as the "surface" for heptane. We report ratios up to T 1S /T 2S ≃ 4 and dispersion T 1S (ω 0 ) 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 1 H− 1 H dipole−dipole interactions enhanced by nanopore confinement dominate T 1S and T 2S relaxation in saturated organic-rich shales.

Elucidating the $^1$H NMR relaxation mechanism in polydisperse polymers and bitumen using measurements, MD simulations, and models

“…the signal was below the detection limit of the apparatus). The paramagnetic concentration in the polymers is at least an order of magnitude less than the estimated ≃ 1,000 ppm for Athabasca bitumen [43,70].…”

“…The large polydispersivity of the polymers make them ideal for comparing with crude-oils, which are also highly dispersed as evidenced by their wide T 2 distributions [8]. In the case of the three poly(isobutene) polymers in Table I, the viscosity fit well to the functional form η ≃ A M α w [69], with α ≃ 2.4 and A ≃ 1.07 × 10 −4 in units of (cP) and (g/mol) at ambient [43]. An illustration of a section of poly(isobutene) is shown in Fig.…”

“…We previously showed that at high viscosities, the log-mean relaxation time T 1LM for the polydisperse polymers becomes independent of viscosity and proportional to frequency T 1LM ∝ f 0 [42,43]. This behavior presents significant deviations from the traditional Bloembergen, Purcell and Pound (BPP) model for 1 H-1 H dipole-dipole relaxation of monodisperse hard-spheres [63] where T 1,BPP ∝ f 2 0 η/T is predicted at high viscosities.…”

“…In order to investigate the NMR relaxation mechanism in heavy-oils and bitumen, we previously reported a series of NMR measurements on polydisperse polymers and polymer-heptane mixes [42,43]. Polymers are known to have similar rheology as heavy oils [55], making them good models for studying the rheology of viscous fluids.…”

“…This then lead us to develop a phenomenological model based on 1 H-1 H dipole-dipole relaxation which accounts for T 1LM plateau at high viscosities by lowering the frequency exponent in the BPP model [42,43]. Lowering the frequency exponent implies a distribution in molecular correlation times of the viscous fluid, which is a similar approach to the phenomenological Cole-Davidson function [64,65] commonly used for dielectric and NMR data of glycerol [66] and monodisperse polymers [58].…”