Dynamics and Wettability of Oil and Water in Oil Shales

Abstract

It is critical to probe in situ the dynamics and wettability of oil, water, and gas trapped in the complex microstructure of oil-shale rocks. However, usual techniques cannot separate these fluids in shale rocks. Here, we use multifrequency and multidimensional nuclear magnetic relaxation (NMR) techniques for probing these dynamics. The frequency dispersion behaviors of the longitudinal relaxation rates 1/T 1 for oil and water confined in shales are interpreted through a relaxation model showing one-dimensional (oil) and two-dimensional (2D) (water) diffusing phases confined within the organic kerogen and mineral layers, respectively. We probe the average hopping and residence times of these fluids at pore surfaces and assign signals to water and oil at both organic and mineral pore surfaces for characterizing their local wettability. This allows interpreting our 2D T 1 −T 2 correlation spectra that could be made down-hole, thus giving an invaluable tool for investigating oil and gas recovery on these important porous rocks.

Explicit calculation of nuclear-magnetic-resonance relaxation rates in small pores to elucidate molecular-scale fluid dynamics

“…The bulk oil correlation time τ b lies in the range 20-40 ps, consistent with, but slightly longer than, typical pure alkanes (15 ps) [13]. The T 1,oil /T 2,oil ratio, which ranges from 5 to 10 experimentally [6], is found to be a strong function of τ with τ = 0.1µs corresponding to T 1,oil /T 2,oil ≈ 5 and τ = 0.5µs to T 1,oil /T 2,oil ≈ 10. This result suggests that the T 1,oil /T 2,oil ratio might provide a direct measure of surface affinity.…”

“…is the gyromagnetic ratio for the paramagnetic impurity (proton) and S = ions identified as the dominant paramagnetic species by electron spin resonance [6]. Fits are undertaken by varying τ b , τ and τ d with fit quality assessed using a simple least-squares measure.…”

“…This model therefore simplifies the complex intra-pore dynamics of real fluids to three characteristic time constants,τ b , τ and τ d . Aspects of this general model, henceforth referred to as the 3τ model, are widely used throughout literature [1][2][3][4][5][6][7][8].Nuclear magnetic resonance (NMR) relaxation analysis is a uniquely powerful tool to access molecular correlation times of fluids in porous media [1][2][3][4][5][6][7][8][9]. It is rivalled only by small-angle scattering techniques, especially with neutrons, but has the advantage of being widely available using laboratory-scale equipment.…”

“…Nuclear magnetic resonance (NMR) relaxation analysis is a uniquely powerful tool to access molecular correlation times of fluids in porous media [1][2][3][4][5][6][7][8][9]. It is rivalled only by small-angle scattering techniques, especially with neutrons, but has the advantage of being widely available using laboratory-scale equipment.…”

“…Several models have been proposed (for example, [1,2,6,7,10]) but that which builds most successfully on the general dynamics of the model illustrated in Fig. 1 in terms of fitting experimental data is due to Korb and co-workers [3][4][5][6]10].…”