Combining molecular simulation and experiment to prove micropore distribution controls methane adsorption in kerogens

Abstract

Combining molecular simulation and experimental results provides a more detailed understanding of gas sorption in kerogens than either approach in isolation. Porosity and chemical functionality are the main differences between kerogens affecting the methane adsorption, whereas which one is the key control has not been stated clearly. Molecular dynamic (MD) simulations with matrix and slit pore models in conjunction with Grand Canonical Monte Carlo (GCMC) simulations have been combined with experimental results from isolated Type II kerogens to identify the controlling factors for methane adsorption. The experimentally determined micropore volumes (Vmicro) and equilibrium methane adsorption capacities (Qm) of isolated kerogens (10–75 mm3/g TOC and 21.3–75.8 mg/g TOC, respectively) are in a comparable range with the simulation results for over mature kerogens (19–261 mm3/g TOC, and 36.5–148 mg/g TOC). However, the higher values from simulations are due to a combination of larger inaccessible microporosity for methane, and the largest interconnecting pore necks around 2 nm considered in simulation being larger than the average neck size in the isolated kerogens. Both the experimental and simulation results indicate the major contributor to Type I (a) and I (b) isotherms are smaller (< 1 nm) and larger micropores (1–2 nm), respectively. The methane adsorption capacity of the kerogen matrix increases with increased maturity and micropore volume, with a positive correlation between Vmicro and Qm observed. MD results at 25 and 100 °C showed that methane only has affinity with certain oxygen, sulfur, and nitrogen functional groups at very low pressure (<1.6 bar at 25 °C, <0.8 bar at 100 °C), and the affinity becomes much weaker at higher pressures with no significant differences among the functional groups considered. Moreover, the similar heats of adsorption (23.2, 23.1, 23.5, 22.8 KJ/mol) of methane with kerogens of different maturity confirm that the differences in surface functionality have a negligible effect on methane adsorption. Therefore, micropore volume in kerogens is the key control for methane adsorption.