Analysis of imbibition of n-alkanes in kerogen slits by molecular dynamics simulation for characterization of shale oil rocks

Shale oil formations contain both inorganic and organic media. The organic matter holds both free oil in the pores and dissolved oil within the kerogen molecules. The free oil flow in organic pores and the dissolved oil diffusion in kerogen molecules are coupled together. The molecular flow of free n-alkanes is an important process of shale oil accumulation and production. To study the dynamics of imbibition process of n-alkane molecules into kerogen slits, molecular dynamics(MD) simulations are conducted.Effects of slit width, temperature, and n-alkane types on the penetration speed, dynamic contact angle,and molecular conformations were analyzed. Results showed that molecular transportation of n-alkanes is dominated by molecular structure and molecular motion at this scale. The space-confinement conformational changes of molecules slow down the filling speeds in the narrow slits. The n-alkane molecules with long carbon chains require more time to undergo conformational changes. The high content of short-chain alkanes and high temperature facilitate the flow of alkane mixtures in kerogen slits. Results obtained from this study are useful for understanding the underlying nanoscale flow mechanism in shale formations.