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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 …