Molecular simulation of adsorption and thermodynamic properties on type II kerogen: Influence of maturity and moisture content

- Realistic kerogen models are used to simulate the adsorption capacity curves.
- The effects of the maturity and moisture on CH4 absorption capacities are quantified.
- Moisture content greater affects the adsorption capacities on high mature keorgen.
- A limit reduction of adsorption capacity is found with increasing moisture content.
- Heat of CH4 adsorption is related to the chemical compositions.

The adsorption characteristics of shale under the reservoir conditions have significant effects on the original gas in place and productivity. In this study, based on realistic kerogen models, methane (CH4) adsorption isotherms were simulated considering the influence of maturity (immature, oil window mature and postmature) and moisture content (0.6, 1.2, 1.8, and 2.4 wt.%) using molecular dynamics (MD) and Monte Carlo (MC) simulation methods. The effects of the maturity and moisture content on the CH4 adsorption capacities and thermodynamic properties were discussed. Simulation results indicate that the adsorption capacities of CH4 on dry kerogen increase with maturity but decrease with temperature. The presence of the moisture content sharply decreases the CH4 adsorption capacities and has a greater effect on the kerogen with high maturity than on that with low maturity. The reduction of the adsorption capacity decreases with increasing pressure and increases with increasing moisture content until reaching a limiting value that depends on the maturity. The heat of CH4 adsorption shows no obvious relationship with the maturity, but is mainly affected by the differences in chemical compositions (especially related to sulfur) on dry kerogen. The heats of CH4 adsorption on moist kerogens (both in immature, oil window mature and postmature) also reduce with increasing moisture content until reaching a limiting value, after which the moisture content has little effect on the heat of CH4 adsorption. Interestingly, this limiting moisture value is in accordance with the limiting value found in the water effect on CH4 adsorption capacities. This study provides a quantitative understanding of the effects of moisture content and maturity on CH4 adsorption and thermodynamic properties from a microscopic perspective.