(169br) Effect of Maturity on the NMR Relaxation of Kerogen Using MD Simulations

Maturation studies of organic-rich source rocks are integral to evaluating unconventional hydrocarbon reservoir formations, including gas shale and oil shale. Kerogen, the predominant organic macromolecular polymer within these source rocks, holds significant importance in unconventional formation evaluation. Kerogen-rich gas shale reservoirs play a pivotal role during the energy transition, serving as a clean-burning energy source and a possible sink for geological carbon capture and storage (CCUS).

We previously showed how atomistic molecular dynamic (MD) simulations can elucidate the underlying physics behind the ¹H NMR relaxation of bulk fluids [1-4] and fluids under organic nanoconfinement [5-6], without resorting to the traditional assumptions of BPP theory. More specifically, we used MD simulations to compute the ¹H-¹H dipole-dipole autocorrelation function, which successfully predicted the ¹H NMR relaxation dispersion (i.e., frequency dependence) of fluids, without any relaxation models and without any adjustable parameters in the interpretation of the simulations.

In this work we extend our MD simulation technique to study the transverse ¹H NMR relaxation (T_2G) of the solid organic matrix itself, i.e., kerogen, with varying thermal maturity. The MD simulations are used to quantify both the intramolecular and intermolecular components of T_2G relaxation by computing the ¹H-¹H dipole-dipole second moment (SM) of the kerogen molecules with varying thermal maturity. We find a strong correlation between SM and H/C (elemental ratio) of kerogen as a function of maturity, which we use to predict T_2G (and T₁) relaxation versus H/C. We find a consistent trend between the simulated T_2G (and T₁) versus H/C, and the trend found from NMR measurements of Type II-S organic rich chalk as a function of maturity using a solid-echo pulse sequence to detect the solid kerogen [7].

Illustration of the uploaded figures:

(a) Schematic of the simulation details for Type IIC Kerogen as an example. 8 Type IIC Kerogen molecules are packed to conduct the MD simulation.

(b),(c) MD simulation of the intra-molecular $G_R(t)$ (left) and inter-molecular $G_T(t)$ auto-correlation functions for Type IA, IIA, IIB, IIC, IID, and IIIA Kerogen.

(d) Computed the intra and inter square root of second moment in kHz of Type IA, IIA, IIB, IIC, IID, and IIIA Kerogen as a function of H/C using the autocorrelation function through the MD simulation. As a comparison, the results for benzene (aromatic) and heptane (aliphatic) are also shown.

(e) The predicted longitudinal relaxation rate (R1 = 1/T1) by MD simulation and Plateau model (red) and compared the experimental R1 of Type IIS organic-rich chalk (blue) as a function of H/C. The consistent trend is observed between the simulated T2G (and T1) versus H/C, and the trend found from NMR measurements of Type II-S organic rich chalk as a function of maturity using a solid-echo pulse sequence to detect the solid kerogen.

References:

1. Singer et.al, J. Magn. Reson. 2017, 277, 15–24, DOI: 10.1016/j.jmr.2017.02.001
2. Singer et.al., J. Chem. Phys. 2018, 148, 164507, DOI: 10.1063/1.5023240
3. Singer et.al, J. Chem. Phys. 2018, 148, 204504, DOI: 10.1063/1.5027097
4. Asthagiri, et.al, J. Phys. Chem. B 2020, 124, 10802–10810, DOI: 10.1021/acs.jpcb.0c08078
5. Parambathu et.al, J. Phys. Chem. B 2020, 124, 3801–3810, DOI: 10.1021/acs.jpcb.0c00711
6. Parambathu et.al, J. Phys. Chem. Lett. 2023, 14, 4, 1059–1065, DOI: 10.1021/acs.jpclett.2c03699
7. Liu et.al, Fuel 2024, 367, 131378, DOI: 10.1016/j.fuel.2024.131378