Molecular simulation of shale gas adsorption onto overmature type II model kerogen with control microporosity

ABSTRACT

We use an all-atom molecular dynamics simulation to generate the dense porous structures of overmature type II kerogen with control microporosity. The structures mimic the organic part of Barnett shale under a typical reservoir condition of 365 K and 275 bar. First, we build an atomistic model of kerogen unit using the consistent valence force field and chemical structure proposed by Ungerer and his colleagues. Second, we generate kerogen structures by gradual cooling and compression of the initial low-density random configurations of kerogen units. During the structure generation, we use a dummy particle of varying size to introduce microporosity into the kerogen structures. We systematically characterise the microporous kerogen structures by calculating the geometric pore size distribution, pore limiting diameter, maximum pore size, accessible surface area, and pore volume and by analysing the pore network accessibility. Third, we employ grand canonical Monte Carlo (GCMC) to study the adsorption of two proxies of shale gas (pure methane and mixture of 82% of methane, 12% of ethane and 6% of propane) in the kerogen structures. The shale gas adsorptions are compared with GCMC simulation of CO₂ adsorption in the kerogen structures. Hydrocarbons are modelled using the all-atom optimized potential for liquid simulations (OPLS) force field while carbon dioxide is represented by the EPM2 model. We complement the adsorption studies by exploring accessibility of pore space of the kerogen structures using molecular dynamics simulation. Finally, we introduce a mesoscale pore void into a microporous kerogen structure and probe the adsorption behaviour of the hydrocarbon mixture in such a multiscale kerogen model.

KEYWORDS:

• Barnett shale
• clay
• multiscale kerogen model
• organic matter
• pore space accessibility

Acknowledgment

The computer time was provided by the METACentrum computing facility.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work has received funding from the European Union's Horizon 2020 research and innovation programme [grant number 640979], and was also supported by the Czech Science Foundation (Project No. 16-12291S) and the Internal Grant Agency of J. E. Purkinje University (Project No. 5322 16 0003 01).