Molecular dynamics simulation of liquid alkane occurrence state in pores and slits of shale organic matter

Molecular dynamics simulation was used to study the occurrence state of liquid alkane in pores and slits of shale organic matter. On the basis of OPLS (Optimized Potentials for Liquid Simulation) force field, the alkane densities under different pressures and temperatures were calculated; the comparison with experimental results validated the effectiveness of this approach. With n-heptane as an example, the basic occurrence behaviors of alkanes in the pores and slits of organic matter were analyzed under formation conditions, and the effects of slit aperture, thermal maturity of organic matter, carbon chain length and isomers were also discussed. Results show that: (1) The density distribution of the alkanes across the pores and slits is not uniform, but presents a periodic fluctuation; (2) A “solid-like” alkane layer will form in the vicinity of the solid surface, and its density approximates to 1.9−2.7 times as large as that of the bulk-fluid; (3) Multiple adsorption layers are always shown for liquid alkanes and the thickness of each layer is 0.48 nm; the total number of adsorbed layers is influenced by the slit aperture and fluid composition. Finally, using this approach, the proportion of adsorbed-phase (18.2%) is determined for oil in an organic matter slit.