Impacts of kerogen type and thermal maturity on methane and water adsorption isotherms: A molecular simulation approach

IS de Araujo and A Jagadisan and Z Heidari, FUEL, 352, 128944 (2023).

DOI: 10.1016/j.fuel.2023.128944

The molecular structure of kerogen is known to control the type, amount of petroleum generated as well as its interaction with reservoir fluids. The adsorption behavior of kerogen with respect to reservoir fluids can enhance our understanding of kerogen wetting properties. Therefore, quantifying the governing factors controlling the adsorption of water and methane in organic pores is critical in understanding fluid flow and adsorption phenomena in organic-rich mudrocks. In this paper, we perform molecular simulations (MS) and quantify water and methane adsorption isotherms on kerogen pore surfaces for the purpose of investigating the impact of kerogen molecular structure on kerogen-fluid interfacial interaction. To realistically represent kerogen structures, we use different kerogen molecular models to emulate kerogen of different types and thermal maturity. The kerogen molecules are condensed to form a porous kerogen structure. The Grand Canonical Monte Carlo simulation (GCMC) method is applied to calculate adsorption isotherms of methane and water molecules in kerogen nanopores. We compute the adsorption behavior of water and methane molecules on type I, II and III kerogen samples. The molecular simulations of increasing thermal maturity from IIA to IID at low pressures (less than 10 MPa) and temperature of 300 K in type II kerogen resulted in increasing methane adsorption capacity. At 5 MPa, the methane adsorption relatively increased 81% from IIA to IID. The adsorption capacity of methane is also shown to increase from type IA to IIA to IIIA. Similarly to methane, water adsorption capacity is found to increase from type IA to IIA to IIIA. However, water adsorption capacity on kerogen of different thermal maturity increases from IIB to IIA. Kerogen structures of types IID and IIC show very similar adsorption capacity nearly overlapping at four different pressures. However, water adsorption on IID and IIC kerogen is still found to be smaller compared to IIA and IIB. The outcomes of this work contribute to improving the understanding of the impact of kerogen molecular structure on adsorption of fluids on kerogen surfaces. Moreover, it can provide insights into fluid mobility assessments in organic-rich mudrocks by providing information about the adsorbed gas content on kerogen surfaces.

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