Exploration of shale reservoirs is controlled by the insoluble sedimentary organic component, kerogen. Its thermal maturation determines the production window and is characterized by chemical and textural changes. Understanding the influence of kerogen structure on interfacial phenomena may support in-field reservoir evaluation. Here, nuclear magnetic resonance (NMR) relaxometry data, equivalent to well logging, are correlated with structural properties of suspended particles of kerogen and reference materials. In NMR, the alignment of specific nuclei (i.e. 1H) with an external magnetic field (B0) is manipulated using radio-frequency pulses. The rate of atomic re-equilibration with B0 comprises characteristic relaxation times - longitudinal (T1) and transverse (T2). The latter is more sensitive to changes in translational molecular motion and is commonly used in relaxometry studies. A solid-fluid affinity parameter, surface relaxivity (ρ2), is derived from 1/T2 data and can be used by petrophysicists to study reservoir properties. Here, type-II kerogens were characterized via surface and bulk analyses, serving as a base for NMR data interpretation. The data set comprised of carbon hybridization ratios, heteroatom content, and textural properties. Reference samples include copolymers with known heteroatom (O, N) fractions and moieties observed in kerogen. NMR data were collected for different solvents in a Bruker Minispec mq-20 (0.47 T, 20 MHz). Trends in ρ2 of polymers for water and n-decane highlighted their wetting nature. A positive relation was observed for ρ2,w and heteroatom content, associated with strong intermolecular interactions, and a respective decrease of ρ2,d. For kerogen, heteroatoms displayed a noticeable relaxation enhancement for acetone and toluene. In contrast, ρ2 for water and n-decane seemed to fluctuate. Observed trends seem to be influenced by different kerogen properties, including free radicals. The discussed results elucidate the sensitivity of relaxometry data to surface-solvent interactions and suggest careful interpretation of NMR well logging maps.

Studies on interfacial interactions of carbonaceous materials via nuclear magnetic resonance relaxometry

ACS Spring 2022

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